An expeditious total synthesis of kalkitoxins: Determination of the absolute stereostructure of natural kalkitoxin

Tetrahedron 60 (2004) 6859–6880

An expeditious total synthesis of kalkitoxins: determination of the

absolute stereostructure of natural kalkitoxin^q

†

Fumiaki Yokokawa,^aToshinobu Asano,^aTatsufumi Okino,^b,cWilliam H. Gerwick^c

,

and Takayuki Shioiri^d

,

*

^aGraduate School of Pharmaceutical Sciences, Nagoya City University, Tanabe-dori, Mizuho-ku, Nagoya 467-8603, Japan

^bGraduate School of Environmental Earth Science, Hokkaido University, N10, W5, Sapporo 060-0810, Japan

^cCollege of Pharmacy, Oregon State University, Corvallis, Oregon 97331, USA

^dGraduate School of Environmental and Human Sciences, Meijo University, Shiogamaguchi, Tempaku, Nagoya 468-8502, Japan

Received 22 February 2004; revised 14 May 2004; accepted 3 June 2004

Abstract—Kalkitoxin, a potent neurotoxin isolated from the marine cyanobacteria Lyngbya majuscula, and its congeners (1–7) were

efﬁciently synthesized utilizing Hruby’s diastereoselective 1,4-addition and the Wipf’s oxazoline-thiazoline conversion as key steps. These

synthetic efforts in combination with spectral studies of natural kalkitoxin clearly determined the absolute stereostructure of kalkitoxin to be

7.

1. Introduction

blocker of the voltage sensitive Na^þchannel in mouse

neuro-2a cells (EC₅₀1 nM).

This paper describes an expeditious total synthesis of

kalkitoxin, a novel potent neurotoxin, and its congeners via

a highly convergent and versatile synthetic strategy, which

clearly determined the absolute stereostructure of natural

kalkitoxin. Kalkitoxin was isolated from the marine

cyanobacterium Lyngbya majuscula aided by bioassay-

guided fractionation using the brine shrimp and gold ﬁsh

toxicity assays.^1,2Kalkitoxin was revealed to be strongly

ichthyotoxic to the common goldﬁsh (Carassius auratus,

LC₅₀700 nM), potently brine shrimp toxic (Artemia salina,

LC₅₀170 nM), and potently inhibited cell division in a

fertilized sea urchin embryo assay (IC₅₀,25 nM), while in a

primary cell culture of rat neurons kalkitoxin displayed an

exceptional level of neurotoxicity (LC₅₀3.86 nM) and its

effects were inhibitable with NMDA receptor antagonists.³

In addition, kalkitoxin was highly active in an inﬂammatory

disease model which measured IL-1b-induced sPLA₂

secretion from HepG2 cells (IC₅₀27 nM). Furthermore,

preliminary evidence suggests that kalkitoxin is a potent

Kalkitoxin is a lipoamide containing four methyl groups on

the carbon chain and possessing ﬁve stereogenic centers, an

N-methylamide, and a thiazoline ring. Free rotation around

the chain precluded the NOE assignment of the structure,

and the N-methylamide function causes restricted rotation

resulting in a complex pattern in its NMR spectra. These

structural characteristics precluded the determination of

absolute stereostructure. In continuation of our interests on

the total synthesis of biologically active aquatic natural

products,^4,5kalkitoxin’s potent biological activity and

limited natural availability prompted us to synthesize this

unique molecule and its congeners and to determine its

absolute conﬁguration. Our synthetic efforts supplied

kalkitoxin and its congeners (1–7), culminating in the

determination of the absolute stereostructure of kalkitoxin

to be 7 (Fig. 1).^1,6

2. Synthetic strategy

^qSupplementary data associated with this article can be found in the online

version, at doi: 10.1016/j.tet.2004.06.014

When we initiated the synthetic studies of kalkitoxin, the

stereochemical structure was still undetermined, which led

us to adopt a highly ﬂexible synthetic route to kalkitoxins

having any possible stereostructure. As shown in Scheme 1,

the whole molecule of kalkitoxin was divided into three

building blocks 8–10, of which the left fragment 8 is

commercially available and the right fragment 10 would be

Keywords: Kalkitoxin; Marine cyanobacteria; Total synthesis; Absolute

conﬁguration; Stereoselective 1,4-addition.

*

Corresponding author. Tel./fax: þ81-52-832-1555;

e-mail address: shioiri@ccmfs.meijo-u.ac.jp

Present address: Tsukuba Research Institute, Novartis Pharma K. K.,

Ohkubo 8, Tsukuba, Ibaraki 300-2611, Japan.

†

doi:10.1016/j.tet.2004.06.014

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F. Yokokawa et al. / Tetrahedron 60 (2004) 6859–6880

Figure 1.

Scheme 1.

synthesized from serine (13). The central fragment 9 would

be synthesized from methyl 3-hydroxy-2-methylpropionate

(12) by asymmetric crotylation followed by deoxygenation

of the resulting alcohol 11. The stereogenic center at C7

would be introduced by 1,4-addition of the chiral oxa-

zolidinone after coupling with the left fragment 8. Since the

thiazoline ring was thought to be fragile during synthesis, its

construction should be carried out at the ﬁnal stage of the

synthesis. In addition, it was thought to be preferable that

the starting materials adopted should be available in a

stereochemically clear form and the reactions chosen should

proceed in a highly stereoselective manner. According to

this strategy, we launched the total synthesis of the

kalkitoxins.

3. Results and discussion

We ﬁrst started the synthesis of (3R,7R,8S,10R,2⁰S)-

kalkitoxin (1) to establish the facile synthetic route toward

kalkitoxins. The known⁷hexenol 15, prepared from methyl

(R)-3-hydroxy-2-methylpropionate (14) in 4 steps, was

converted to the diol 16 by hydroboration with 9-BBN

under sonication conditions⁸followed by oxidation, shown

F. Yokokawa et al. / Tetrahedron 60 (2004) 6859–6880

6861

Scheme 2.

in Scheme 2. After protection of the primary hydroxyl

function with TBDPSCl (^tBu(C₆H₅)₂SiCl), the secondary

hydroxyl group of 17 was deoxygenated via the thio-

carbonate 18 according to the Barton–McCombie

procedure⁹by thiocarbonylation and then radical reduction.

The deoxygenated product 19 was treated with TBAF

(Bu₄N^þF²) to give the alcohol 20, which was converted to

the azide 21 under various conditions, shown in Table 1.

The ordinary two-step process of the mesylation and then

azidation afforded the desired azide 21 in 88% yield. The

one step procedure utilizing DPPA ((C₆H₅O)₂P(O)N₃) and

DBU (1,8-diazabicyclo[5.4.0]unde-7-ene)¹⁰sluggishly

proceeded at room temperature and required heating for a

longer time. The use of p-NO₂-DPPA (p-NO₂–

(C₆H₄O)₂P(O)N₃)¹¹in place of DPPA accelerated the

reaction and shortened the reaction time to give the azide

21 in good yield.

The azide 21 thus obtained underwent catalytic hydrogen-

ation over Lindlar catalyst¹²to give the corresponding

amine, which was coupled with (S)-2-methylbutyric acid

(22), the left fragment of kalkitoxins, by use of DEPC

((EtO)₂P(O)CN)¹³in the presence of triethylamine to

produce the amide 23 in excellent yield. After the

N-methylation, catalytic removal of the benzyl group from

24 afforded the alcohol 25. The conversion of 25 to the

(E)-enimide 27a was smoothly accomplished by the

Parikh–Doering oxidation¹⁴followed by the Horner–

Wadsworth–Emmons reaction with the oxazolidinone 26

derived from (R)-phenylglycine.¹⁵

The next problem to overcome was the stereoselective

introduction of the methyl group at the C7 position¹⁶by 1,4-

addition. Hruby and co-workers¹⁷already revealed that the

1,4-addition of a methyl group into the optically active a,b-

unsaturated acyl-4-phenyloxazolidinone by use of a com-

bination of methyl magnesium bromide and cuprous

bromide·dimethyl sulﬁde proceeded with high diastereo-

selectivity, and Romo and co-workers¹⁵utilized this method

for the total synthesis of (2)-pateamine A. High stereo-

selectivity is explained by the ﬁxed conformation due to the

chelation of magnesium by the two carbonyl groups of the

oxazolidinone and enimide, and by the attack of the methyl

group from the opposite side of the phenyl group in the

oxazolidinone, as shown in Figure 2.

Table 1. One-pot azidation of using DPPA (C₆H₅O)₂P(O)N₃or p-NO₂-

DPPA (p-NO₂C₆H₄O)₂P(O)N₃

Entry

Reagent

(equiv.)

Base

(equiv.)

Reaction conditions

Yield

(%)

1

2

3

4

DPPA (1.5)

DBU (1.5) 0 8C, 1.5 h; rt, 21 h

DBU (1.5) 0 8C, 1.5 h; 65 8C, 21 h

23^a

97

79

81

p-NO₂-DPPA (1.5) DBU (1.5) 0 8C, 1 h; rt, 7 h

p-NO₂-DPPA (1.5) DBU (1.5) 0 8C, 1 h; 65 8C, 5 h

a

Since both enantiomers of the 5-phenyloxazolidinone are

Diphenyl phosphate was obtained in 63% yield.

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F. Yokokawa et al. / Tetrahedron 60 (2004) 6859–6880

cuprous bromide·dimethyl sulﬁde, giving the (7R)-oxa-

zolidinone 30a quantitatively with complete stereo-

selectivity. Removal of the oxazolidinone moiety with

alkaline hydrogen peroxide afforded the carboxylic acid

31a.²⁰

Coupling of the acid 31a with (R)-2-amino-3-butenol

hydrochloride (32)²¹smoothly proceeded by use of

EDCI·HCl (Me₂N(CH₂)₃–NvCvN–Et) to give the

amide 33a, which was converted to the oxazoline 34a

with the Burgess reagent (Et₃N^þSO₂N²CO₂Me),²²as

shown in Scheme 4. Transformation of the oxazoline ring

in 34a to the thiazoline ring in kalkitoxin (1) was

accomplished according to the method of Wipf.²³Thus,

treatment of the oxazoline 34a with hydrogen sulﬁde led to

ring-opening to give the thioamide 35a, which underwent

the recyclization with the Burgess reagent to give kalkitoxin

Figure 2.

easily available, their proper use will produce both

enantiomers different at the C7 methyl function. This

ﬂexibility led us to adopt the Hruby’s diastereoselective 1,4-

addition method (Scheme 3).^18,19Thus, the 1,4-addition

reaction to the oxazolidinone 27a was carried out under

analogous conditions using methyl magnesium bromide and

Scheme 3.

Scheme 4.

F. Yokokawa et al. / Tetrahedron 60 (2004) 6859–6880

6863

1 having (3R,7R,8S,10R,2⁰S)-conﬁguration in 14% yield.

Replacement of the Burgess reagent with DAST

(Et₂NSF₃)²⁴increased the yield to 84%.

During the investigation of the above synthetic works, the

Oregon group led by Gerwick determined the absolute

conﬁguration of the C3 position to be R ^1,25by obtaining

cysteinic acid through ozonolysis and then acid hydrolysis

of natural kalkitoxin and by identiﬁcation of L-conﬁgur-

ation through Marfey’s analysis.²⁶The relative stereo-

chemistry of the three chiral centers within the aliphatic

chain of kalkitoxin was also suggested to be 7R ^p, 8S ^p, and

10S ^pby J-based conﬁguration analysis²⁷using the E.COSY

NMR pulse sequence, HSQMBC,²⁸and a cryoprobe NMR

technology (see Supplementary data). Although the

limited amoun₀t of natural kalkitoxin precluded determi-

nation of the C 2 stereochemistry, the above stereostructure

studies reduced the total number of stereochemical

possibilities to four: (3R,7R,8S,10S,2⁰S), (3R,7R,8S,

10S,2⁰R), (3R,7S,8R,10R,2⁰S), or (3R,7S,8R,10R,2⁰R).

Analogously, the enimide 27b was synthesized by the

oxidation of the alcohol 25 and then the Horner–

Wadsworth–Emmons reaction with the (S)-phosphonate

ent-26. The diastereoselective 1,4-addition of the methyl

group to 27b, followed by alkaline removal of the chiral

auxiliary afforded the (7S)-carboxylic acid 31b, which was

coupled with 32 to give the amide 33b. Analogous

transformation of the amide 33b as above produced

kalkitoxin 2 with (3R,7S,8S,10R,2⁰S)-conﬁguration, as

shown in Scheme 5.

Although the kalkitoxin 1 exhibited a similar speciﬁc

rotation, [a]_D¼þ15.5, to that of natural kalkitoxin,

[a]_D¼þ16, the ¹³C NMR spectra were slightly different

from each other and, especially, the difference of 0.12–

0.19 ppm was observed at the C10, C11, and N-methyl

carbon signals. The ¹H NMR spectrum of 1 was also

different at the chemical shift of the C10, C11, and C12

positions. The speciﬁc rotation of the kalkitoxin 2 was

þ49.6, and its ¹³C NMR spectrum differed from that of the

natural one at the signals of the C6 (d 3.45 ppm) and C9 (d

2.63 ppm) positions. In addition, the ¹H NMR spectrum was

also not identical. These spectral features clearly showed

that the synthesized kalkitoxins 1 and 2 have different

stereochemistries from those of the natural product.

To determine the absolute conﬁguration of natural kalki-

toxin, the four kalkitoxins having possible conﬁgurations

were synthesized. Since the 2-methylbutyric acid part is

introduced at an early stage of our synthetic strategy

(Scheme 2), efforts were required to synthesize kalkitoxins

isomeric at the C2⁰position. Thus the C2⁰conﬁguration of

the intermediates was ﬁxed to be S. Instead, both (3R)- and

(3S)-isomers were synthesized because the thiazoline ring

was introduced at the ﬁnal stage of the synthesis. This

strategy would furnish kalkitoxin having natural conﬁgur-

ation or its antipode. Thus the (3R,7R,8S,10S,2⁰S),

(3S,7R,8S,10S,2⁰S) (corresponding to the antipode of the

Scheme 5.

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F. Yokokawa et al. / Tetrahedron 60 (2004) 6859–6880

(3R,7S,8R,10R,₀2⁰R)-isomer), (3R,7S,8R,10R,2⁰S), and

(3S,7S,8R,10R,2 S)-isomers (corresponding to the antipode

of the (3R, 7R, 8S,10S,2⁰R)-isomer) were synthesized by use

of analogous method, as shown in Schemes 6–8.

synthesis of 1 and 2 smoothly and stereoselectively afforded

the carboxylic acid 45 utilizing (S)-2-methylbutyric acid

(22) and the phosphonate 26. Coupling of the acid 45 with

(R)- and (S)-2-amino-3-buten-1-ol hydrochlorides (32 and

ent-32), followed by treatment with DAST, respectively,

produced 46a and 46b, from which the thiazoline ring was

constructed to give (3R,7R,8S,10S,2⁰S)-kalkitoxin (3) and its

(3S)-isomer (4), respectively.

Starting from (R)-3-hydroxy-2-methylpropionic acid (14),

the alcohol 36 with the TBDPS function was prepared

according to the literature.²⁹Removal of the hydroxyl group

of 36 was carried out by tosylation followed by hydride

reduction to give the deoxygenated compound 37, as shown

in Scheme 6. Sequence of the reactions analogous to the

Analogously, the alcohol ent-36 was prepared from the ester

14.³⁰Analogous reaction sequences as above afforded

Scheme 6.

F. Yokokawa et al. / Tetrahedron 60 (2004) 6859–6880

6865

Scheme 7.

(3R,7S,8R,10R,2⁰S)-kalkitoxin (5) and its (3S)-isomer (6), as

shown in Scheme 7.

(3R,7R,8S,10S,2⁰R)-(þ)-kalkitoxin (7) was synthesized

through the same strategy, as shown in Scheme 8. This

kalkitoxin was essentially identical to natural compound.

Thus, the absolute stereochemistry of natural kalkitoxin was

fully determined by this total synthesis in addition to

spectral studies.

A comparison of the ¹³C NMR spectra of these synthesized

four kalkitoxins 3–6 with that of natural kalkitoxin is shown

in Figure 3. Close similarity was observed in the

(3S,7S,8R,10R,2⁰S)-isomer 6: the mean difference was

0.006 ppm and the maximal difference was 0.014 ppm.

But its speciﬁc rotation showed 27.5 (c 0.8, CHCl₃) while

that of natural kalkitoxin was þ16 (c 0.07, CHCl₃). The CD

spectra of both compounds exhibited the reverse Cotton

4. Biological activity

With seven kalkitoxins in hand, toxicity against brine

shrimp (Artemia salina) was measured. Synthetic kalkitoxin

7 showed strong toxicity (LC₅₀170 nM) which was the

same as the natural material. Interestingly, the synthesized

enantiomer 6 of kalkitoxin was 50 times less potent, LC₅₀

9300 nM. The isomer 3 with unnatural 2⁰S conﬁguration, the

isomer 1 with unnatural 2⁰S and 10R conﬁgurations, and the

1

effect (see Supplementary data). The H NMR spectra of

6 and natural one were almost identical each other. These

results indicated that 6 was the antipode of natural

kalkitoxin.

Utilizing

(R)-2-methylbutyric

acid

(ent-22),

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F. Yokokawa et al. / Tetrahedron 60 (2004) 6859–6880

Scheme 8.

isomer 4 with unnatural 2⁰S and 3S conﬁgurations were

3–10 times less potent: LC₅₀550 nM for 3, 1100 nM for 1,

and 1700 nM for 4. The conﬁgurations at the C7, C8, and

C10 also affected the toxicity, which decreased as

increasing the number of the reversed conﬁgurations. The

isomers 2 and 5 were almost inactive.

In conclusion, we have succeeded in the total synthesis of

Figure 3. Comparison of ¹³C NMR spectral difference (in DMSO-d₆at 25 8C) of natural and synthetic kalkitoxins.

F. Yokokawa et al. / Tetrahedron 60 (2004) 6859–6880

6867

kalkitoxin and its congeners, which clearly elucidated the

absolute conﬁguration of this new marine neurotoxin. Our

synthesis is straightforward, efﬁcient, and suitable for large

scale production, which will be useful for the detailed

investigation of the biological properties of kalkitoxin.

via cannula. After 10 min, the cooling bath was removed,

and the reaction mixture was allowed to warm to room

temperature. Cold (0 8C) water was then added. After the

mixture was stirred for 10 min, the layers were separated,

and the aqueous layer was extracted with ether (£3). The

organic extracts were combined and washed successively

with 1 M aqueous KHSO₄, saturated aqueous NaHCO₃, and

brine. The solution was dried (MgSO₄), ﬁltered, and

concentrated to give the crude tosylate as a pale yellow

oil (3.10 g). This intermediate was used in the next reaction

without further puriﬁcation.

5. Experimental

5.1. General

Melting points were measured on a YANACO melting point

apparatus (hot plate) and are uncorrected. Infrared (IR)

spectra were recorded on a SHIMADZU FT IR-8100

spectrometer. Optical rotations were measured on a DIP-

1000 digital polarimeter with a sodium lamp (l¼589 nm, D

line) and are reported as follows: [a]^T_D(c g/100 ml, solvent).

¹H NMR spectra were recorded on a JEOL EX-270

(270 MHz) spectrometer, unless otherwise stated. Chemical

shifts are reported in ppm from tetramethylsilane as the

internal standard. Data are reported as follows: chemical

shift, integration, multiplicity (s¼singlet, d¼doublet, t¼

triplet, q¼quartet, bd¼broad, m¼multiplet), coupling con-

stants (Hz), and assignment. Kalkitoxin numbering is used

for assignments on all intermediates. ¹³C NMR spectra were

recorded on a JEOL EX-270 (67.8 MHz) spectrometer with

complete proton decoupling. Chemical shifts are reported in

ppm from tetramethylsilane, deuterochloroform (d_C

77.0 ppm) or d⁶-dimethylsulfoxide (d_C39.5 ppm) with

solvents as the internal standard. Analytical thin layer

chromatography was performed on Merck Art. 5715,

Kieselgel 60F₂₅₄/0.25 mm thickness plates. Visualization

was accomplished with UV light, phosphomolybdic acid, or

ninhydrin solution followed by heating. Preparative thin

layer chromatography was performed on Merck Art. 5744,

Kiselgel 60F₂₅₄/0.5 mm thickness plates. Elementary

analysis (Anal) and high resolution mass spectra (HRMS)

were done at the Analytical Facility at Nagoya City

University.

To a solution of the above tosylate in THF (25 mL) under

argon at room temperature was added LiAlH₄(650 mg,

17.1 mmol). The resulting mixture was heated to reﬂux for

1.5 h. After the mixture was cooled to 0 8C, the reaction was

quenched with cold brine (added dropwise). After the

mixture was stirred for 10 min, the layers were separated,

and the aqueous layer was extracted with ether (£2). The

combined organic extracts were dried (MgSO₄), ﬁltered,

and concentrated. The residue was puriﬁed by silica gel

column

chromatography

(BW-820MH,

hexane/

ether¼50:1–20:1) to afford the desired product 37 as a

colorless oil (1.30 g, 63%): [a]²_D⁶¼þ1.6 (c 1.1, CHCl₃); IR

neat

max

n

cm²¹2959, 1472, 1428, 1113, 1086; ¹H NMR

(270 MHz, TMS/CDCl₃) d 0.92 (3H, d, J¼1.5 Hz, C₈–

CH₃), 0.94 (3H, d, J¼1.5 Hz, C₁₀–CH₃), 1.07 (s, 9H,

(CH₃)₃C), 1.28–1.41 (2H, m, C₉–CH₂), 1.67–1.78 (1H, m,

C₈–CH), 2.09–2.19 (1H, m, C₁₀–CH), 3.39–3.56 (2H, m,

CH₂O), 4.83–4.89 (2H, m, C₁₂–CH₂), 5.60–5.73 (1H,

m, C₁₁–CH), 7.37–7.41 (6H, m, ArH), 7.65–7.67 (4H, m,

ArH); ¹³C NMR (67.8 MHz, CHCl₃/CDCl₃) d 17.5, 17.9,

20.3, 27.0, 27.6, 33.3, 35.3, 40.3, 68.7, 112.1, 127.5, 127.7,

129.3, 129.4, 133.9, 134.0, 135.5, 135.6, 145.1. HRMS (EI)

m/z Calcd for C₂₀H₂₅OSi: 309.1675 (M^þ2t-Bu). Found:

309.1688.

5.1.2. General procedure B for the synthesis of (3S,5S)-

3,5-dimethyl-6-(tert-butyldiphenylsilyloxy)hexanol (38)

and its stereoisomers. To a solution of the silyloxyhexene

37 (1.28 g, 3.49 mmol) in THF (17 mL) was added

9-borabicyclononane dimer (1.70 g, 6.97 mmol). The

resulting solution was stirred for 10 min, and then placed

in a water bath and sonicated for 40 min. Aqueous NaOH

solution (4 M, 3.5 mL) and 30% aqueous H₂O₂(3.5 mL)

were added sequentially at 25 8C. The resulting mixture

was diluted with water and extracted with CHCl₃(£3). The

combined organic extracts were dried (Na₂SO₄), ﬁltered,

and concentrated. The residue was puriﬁed by silica gel

column chromatography (BW-200, hexane/EtOAc¼5:1) to

Solvents for extraction and chromatography were reagent

grade. Liquid chromatography was performed with forced

ﬂow (ﬂash chromatography of the indicated solvent mixture

on silica gel BW-820MH or BW-200 (Fuji Silysia Co.)).

Tetrahydrofuran (THF) was distilled from sodium metal/

benzophenone ketyl. Dichloromethane (CH₂Cl₂), methanol

(MeOH), and acetonitrile (CH₃CN) were distilled from

calcium hydride. N,N-Dimethylformamide (DMF) was

˚

dried over 4 A molecular sieves. Triethylamine was dried

over potassium hydroxide. All other commercially obtained

reagents were used as received.

afford the desired product 38 as a colorless oil (955 mg,

neat

71%): [a]²_D⁴¼210.6 (c 1.0, CHCl₃); IR n

cm²¹3346,

max

2959, 1472, 1428, 1389, 1113, 1092, 1071; ¹H NMR

(270 MHz, TMS/CDCl₃) d 0.85 (3H, d, J¼6.3 Hz, C₈–

CH₃), 0.89 (3H, d, J¼6.6 Hz, C₁₀–CH₃), 1.05 (s, 9H,

(CH₃)₃C), 1.18–1.28 (1H, m, C₁₁–CH), 1.33–1.46 (1H, m,

C₈–CH), 1.48–1.62 (3H, m, C₉–CH₂, C₁₁–CH), 1.68–

1.79 (1H, m, C₁₁–CH), 3.39–3.51 (2H, m, CH₂O), 3.57–

3.71 (2H, m, CH₂OH), 7.34–7.44 (6H, m, ArH), 7.65–7.67

(4H, m, ArH); ¹³C NMR (67.8 MHz, CHCl₃/CDCl₃) d 16.7,

19.4, 19.5, 26.8, 27.0, 33.2, 40.8, 40.9, 61.1, 69.5, 127.5,

129.4, 133.9, 135.5. HRMS (EI) m/z Calcd for C₂₀H₂₇O₂Si:

327.1780 (M^þ2t-Bu). Found: 327.1782.

General procedures A–M were described for

(3R,7R,8S,10S,2⁰R)-kalkitoxin (7)

5.1.1. General procedure A for the synthesis of (2S,4R)-

2,4-dimethyl-1-(tert-butyldiphenylsilyloxy)-5-hexene

(37) and its stereoisomers. To a solution of the alcohol 36²⁹

(2.16 g, 5.65 mmol) in THF (15 mL) under argon at 278 8C

was added n-BuLi (4.1 mL, 6.21 mmol, 1.5 M in hexane)

dropwise via syringe. The solution was stirred at 278 8C for

30 min, then a solution of p-toluenesulfonyl chloride (1.3 g,

6.78 mmol) in THF (3 mL plus 1 mL–2 rinse) was added

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F. Yokokawa et al. / Tetrahedron 60 (2004) 6859–6880

5.1.3. General procedure C for the synthesis of (2S,4S)-6-

azido-2,4-dimethyl-1-(tert-butyldiphenylsilyloxy)hexane

(39) and its stereoisomers. (a) MsCl–NaN₃method. To a

solution of the alcohol 38 (873 mg, 2.27 mmol) in CH₂Cl₂

(7 mL) was added triethylamine (0.47 mL, 3.39 mmol),

methanesulfonyl chloride (0.23 mL, 2.97 mmol), DMAP

(10 mg, 0.082 mmol) at 0 8C. After 15 min, the cooling bath

was removed, and the reaction mixture was stirred at room

temperature for 1 h. The mixture was diluted with ether, and

washed with 1 M aqueous KHSO₄, water, saturated aqueous

NaHCO₃, water, and brine. The organic layer was dried

(MgSO₄), ﬁltered, and concentrated to give the crude

mesylate as a colorless oil (1.102 g). This intermediate was

used in the next reaction without further puriﬁcation.

was successively added diethyl phosphorocyanidate

(0.32 mL, 2.11 mmol) and triethylamine (0.52 mL,

3.75 mmol). After 1 h, the cooling bath was removed, and

the reaction mixture was stirred at room temperature for

1.5 h. The mixture was diluted with EtOAc, and washed

with 1 M aqueous KHSO₄, water, saturated aqueous

NaHCO₃, water, and brine. The organic layer was dried

(Na₂SO₄), ﬁltered, and concentrated. The residue was

puriﬁed by silica gel column chromatography (BW-

820MH, hexane/EtOAc¼4:1) to afford the desired product

ent-47 as a pale yellow oil (826 mg, 93%): [a]²_D⁶¼215.1 (c

neat

max

1.2, CHCl₃); IR n

1462, 1428, 1387, 1237, 1113, 1092; H NMR (270 MHz,

cm²¹3295 (bd), 2963, 1644, 1553,

1

0

TMS/CDCl₃) d 0.84–0.91 (9H, m, C₃–CH₃, C₈–CH₃,

C₁₀–CH₃), 1.05 (9H, s, (CH₃)₃C), 1.10 (3H, d, J¼6.8 Hz,

0 0

C₂–CH₃), 1.19–1.80 (8H, m, C₃–CH₂, C₉–CH₂, C₁₀–CH,

To a solution of the above mesylate in DMF (7 mL) at room

temperature was added sodium azide (738 mg, 11.4 mmol).

The resulting mixture was diluted with ether, and washed

with water (£2) and brine. The organic layer was dried

(MgSO₄), ﬁltered, and concentrated. The residue was

puriﬁed by silica gel column chromatography (BW-

820 MH, hexane/ether¼30:1) to afford the desired product

39 as a colorless oil (807 mg, 87%).

0

C₁₁–CH₂), 1.96–2.09 (1H, m, C₂–CH), 3.12–3.36 (2H, m,

C₁₂–CH₂), 3.38–3.50 (2H, m, CH₂O), 5.30 (1H, bd-s, NH),

7.33–7.45 (6H, m, ArH), 7.64–7.67 (4H, m, ArH); ¹³C

NMR (67.8 MHz, CHCl₃/CDCl₃) d 12.0, 16.7, 17.6, 19.3,

19.4, 26.9, 27.4, 27.9, 33.2, 37.4, 37.7, 40.7, 43.3, 69.4,

127.4, 129.4, 133.9, 134.0, 135.5, 176.1. HRMS (EI) m/z

Calcd for C₂₅H₃₆NO₂Si (M^þ2t-Bu): 410.2516. Found:

410.2556.

(b) p-NO₂-DPPA method. To a solution of the alcohol 38

(42.7 mg, 0.111 mmol) in DMF (0.4 mL) was added bis(p-

nitrophenyl)phosphorazidate (p-NO₂-DPPA) (61 mg,

0.166 mmol) and DBU (25 mL, 0.166 mmol) at 0 8C.

After 1.5 h, the cooling bath was removed and the reaction

mixture was stirred at room temperature for 20 h. The

mixture was diluted with EtOAc, and washed with water

(£2), 1 M aqueous KHSO₄, and brine. The organic layer

was dried (Na₂SO₄), ﬁltered, and concentrated. The residue

was puriﬁed by silica gel column chromatography (BW-

200, hexane/ether¼20:1) to afford the desired product 39 as

5.1.5. General procedure E for the synthesis of (2R)-N-

methyl-N-[(3S,5S)-dimethyl-6-(tert-butyldiphenylsilyl-

oxy)-hex-1-yl]-2-methylbutyramide (ent-48) and its

stereoisomers. To a solution of the amide ent-47

(565 mg, 1.21 mmol) in THF (7 mL) under argon at

278 8C was added n-BuLi (0.94 mL, 1.41 mmol, 1.5 M in

hexane) dropwise via syringe. The solution was stirred at

278 8C for 20 min, then MeI (0.3 mL, 4.82 mmol) was

added. After 10 min, the cooling bath was removed, and the

reaction mixture was stirred at room temperature for 1 h.

Saturated aqueous NH₄Cl was then added. After dilution

with EtOAc and water, the organic layer was separated, and

washed with brine. The organic layer was dried (Na₂SO₄),

ﬁltered, and concentrated. The residue was puriﬁed by silica

gel column chromatography (BW-820MH, hexane/

EtOAc¼6:1) to afford the desired product ent-48 as a pale

brown oil (480 mg, 83%): [a]²_D⁴¼218.8 (c 1.1, CHCl₃); IR

a colorless oil (33.8 mg, 74%): [a]²_D⁵¼28.8 (c 1.0, CHCl₃);

neat

max

1

IR n

cm²¹2980, 2095, 1472, 1428, 1389, 1262, 1113,

1092; H NMR (270 MHz, TMS/CDCl₃) d 0.86 (3H, d,

J¼6.3 Hz, C₈–CH₃), 0.89 (3H, d, J¼6.6 Hz, C₁₀–CH₃),

1.07 (9H, s, (CH₃)₃C), 1.19–1.31 (1H, m, C₁₀–CH), 1.33–

1.49 (1H, m, C8–CH), 1.50–1.58 (3H, m, C9–CH₂, C11–

CH), 1.67–1.78 (1H, m, C11–CH), 3.18–3.34 (2H, m,

C12–CH2)88, ₃.4₁–3.52 (2H, m, C7–CH2), 7.35–7.45

(6H, m, ArH), 7.65–7.68 (4H, m, ArH); ¹³C NMR

(67.8 MHz, CHCl3/CDCl3) d 16.6, 19.2, 19.4, 26.9, 27.7,

33.2, 36.6, 40.5, 49.5, 69.4, 127.5, 129.4, 133.9, 135.5.

HRMS (EI) m/z Calcd for C₂₀H₂₆N₃OSi: 352.1845 (M^þ2t-

Bu). Found: 352.1843.

cm²¹2930, 1646, 1472, 1464, 1428, 1113, 1090; H

neat

max

1

n

NMR (270 MHz, TMS/CDCl₃), both rotamers unless stated

0

otherwise, d 0.82–0.92 (9H, m, C₃–CH₃, C₈–CH₃, C₁₀–

CH₃), 1.05 (9H, s, 1 rotamer, (CH₃)₃C), 1.06 (9H, s, 1

0

rotamer, (CH₃)₃C), 1.05–1.11 (3H, m, C₂–CH₃), 1.16–

0

1.57 (4H, m, C₃–CH₂, C₈–CH, C₁₀–CH), 1.58–1.80 (4H,

0

m, C₉–CH₂, C₁₁–CH₂), 2.44–2.61 (1H, m, C₂–CH), 2.90

5.1.4. General procedure D for the synthesis of (2R)-N-

[(3S,5S)-3,5-dimethyl-6-(tert-butylsilyloxy)-hex-1-yl]-2-

methylbutyramide (ent-47) and its stereoisomers. To a

solution of the azide 39 (774 mg, 1.89 mmol) in EtOAc

(7 mL) was added 5% Pd on carbon (100 mg) at room

temperature. The black slurry was stirred under 1 atm H₂for

1.5 h. The reaction mixture was ﬁltered through a pad of

celite (EtOAc rinse) and the ﬁltrate was concentrated to give

the crude amine as a pale brown oil (799 mg). This

intermediate was used in the next reaction without further

puriﬁcation.

(3H, s, 1 rotamer, N–CH₃), 2.98 (3H, s, 1 rotamer, N–CH₃),

3.19–3.50 (4H, m, C₁₂–CH₂, CH₂O), 7.32–7.44 (6H, m,

ArH), 7.63–7.67 (4H, m, ArH); ¹³C NMR (67.8 MHz,

CHCl₃/CDCl₃), both rotamers, d 12.1, 12.2, 16.6, 17.2, 17.9,

19.3, 19.4, 26.9, 27.1, 27.5, 28.1, 33.2, 33.7, 35.2, 37.0,

37.2, 37.4, 40.6, 40.7, 46.2, 48.0, 69.4, 69.5, 127.4, 129.3,

129.4, 133.9, 134.0, 135.5, 175.9, 176.2. HRMS (EI) m/z

Calcd for C₂₆H₃₈NO₂Si (M^þ2t-Bu): 424.2672. Found:

424.2703.

5.1.6. General procedure F for the synthesis of (2R)-N-

methyl-N-((3S,5S)-3,5-dimethyl-6-hydroxy-hex-1-yl)-2-

methylbutyramide (ent-49) and its stereoisomers. To a

solution of the N-methylamide ent-48 (446 mg,

To a solution of the above amine and (R)-2-methylbutyric

acid (ent-22) (0.24 mL, 2.10 mmol) in DMF (6 mL) at 0 8C

F. Yokokawa et al. / Tetrahedron 60 (2004) 6859–6880

6869

0.926 mmol) in THF (5 mL) was added TBAF (610 mg,

2.33 mmol) at 0 8C. After 30 min, the cooling bath was

removed, and the reaction mixture was stirred at room

temperature for 2 h. The reaction mixture was diluted with

EtOAc, and washed with water (£2) and brine. The organic

layer was dried (Na₂SO₄), ﬁltered, and concentrated. The

residue was puriﬁed by silica gel column chromatography

(BW-820MH, hexane/EtOAc¼6:1) to afford the desired

To a solution of the (R)-phosphonate 26 (265 mg,

0.776 mmol) in THF (2 mL) under argon was added sodium

bis(trimethysilyl)amide solution (0.62 mL, 0.621 mmol,

1.0 M in THF) at 0 8C. After 5 min, the cooling bath was

removed, and the reaction mixture was stirred at room

temperature. After 45 min, a solution of crude aldehyde in

THF (0.8 mL plus 0.3 mL £2 rinse) was added via cannula

at 0 8C. After 2 h, pH 7 buffer was added and the mixture

was diluted with EtOAc, washed successively with 1 M

aqueous KHSO₄, water, saturated aqueous NaHCO₃, and

brine. The organic layer was dried (Na₂SO₄), ﬁltered, and

concentrated. The residue was puriﬁed by silica gel column

chromatography (BW-200, hexane/EtOAc¼2:1–1:1) to

product ent-49 as a pale yellow oil (211 mg, 94%):

[a]²_D⁴¼238.4 (c 1.0, CHCl₃); IR n

cm²¹3432(bd),

2963, 1626, 1464, 1415, 1379, 1048; H NMR (270 MHz,

neat

max

1

TMS/CDCl₃), both rotamers unless stated otherwise, d

0

0.85–0.94 (9H, m, C₃–CH₃, C₈–CH₃, C₁₀–CH₃), 1.08

0

(3H, d, J¼6.3 Hz, 1 rotamer, C₂–CH₃), 1.11 (3H, d,

afford the desired product 54 as a colorless oil (195 mg,

J¼6.3 Hz, 1 rotamer, C₂–CH₃), 1.15–1.28 (2H, m, C₉–

88%): [a]²_D⁵¼251.9 (c 1.0, CHCl₃); IR n

cm²¹2964,

CHCl₃

max

0

CH₂), 1.29–1.50 (4H, m, C₃–CH₂, C₁₁–CH₂), 1.51–1.94

1779, 1688, 1634, 1456, 1383, 1362, 1329, 1200, 1103,

1082; ¹H NMR (270 MHz, DMSO/DMSO-d₆), both

rotamers unless stated otherwise, d 0.73–0.89 (9H, m,

0

(3H, m, C₈–CH, C₁₀–CH, OH), 2.49–2.66 (1H, m, C₂–

CH), 2.92 (3H, s, 1 rotamer, N–CH₃), 3.24–3.52 (4H, m,

C₁₂–CH₂, CH₂O); ¹³C NMR (67.8 MHz, CHCl₃/CDCl₃),

both rotamers, d 12.0, 12.2, 16.3, 16.4, 17.1, 17.8, 19.2,

19.4, 27.0, 27.4, 27.9, 28.0, 33.1, 33.7, 35.0, 35.3, 37.0,

37.1, 37.4, 40.4, 40.6, 46.1, 48.0, 68.6, 68.7, 176.0, 176.3.

HRMS (EI) m/z Calcd for C₁₄H₂₉NO₂: 243.2199. Found:

243.2207.

0

C₃–CH₃, C₈–CH₃, C₁₀–CH₃), 0.97 (3H, d, J¼6.2 Hz, 1

0

rotamer, C₂–CH₃), 0.99 (3H, d, J¼6.2 Hz, 1 rotamer, C₂–

CH₃), 1.12–1.38 (5H, m, C₉–CH₂, C₁₀–CH, C₁₁–CH₂),

0

1.39–1.61 (2H, m, C₃–CH₂), 2.40–2.66 (2H, m, C₂–CH,

C₈–H), 2.78 (3H, s, 1 rotamer, N–CH₃), 2.92 (3H, s, 1

rotamer, N–CH₃), 3.19–3.42 (2H, m, C₁₂–CH₂), 4.18 (1H,

dd, J¼3.6, 8.6 Hz, CH₂O), 4.75 (1H, t, J¼8.6 Hz, Ar–CH),

5.50 (1H, dd, J¼3.6, 8.6 Hz, CH₂O), 6.82 (1H, dd, J¼7.7,

15.4 Hz, C₆–CH), 7.16 (1H, d, J¼15.4 Hz, C₇–CH), 7.27–

7.40 (5H, m, ArH); ¹³C NMR (67.8 MHz, DMSO/DMSO-

d₆), the major rotamer, d 11.7, 17.2, 19.1, 19.6, 26.6, 27.5,

33.4, 33.8, 34.5, 36.2, 39.4, 42.7, 57.0, 70.1, 118.5, 125.7,

127.8, 128.6, 139.7, 153.5, 155.5, 163.6, 174.6. HRMS (EI)

m/z Calcd for C₂₅H₃₆N₂O₄: 428.2675. Found: 428.2676.

5.1.7. (4R)-3-[(Diethylphosphoro)-acetyl]-4-phenyl-2-

oxazolidinone (26). This compound was prepared accord-

ing to the published procedure (see Ref. 15).

To a ﬂask equipped with a reﬂux condenser and charged

with

(R)-3-(bromoacetyl)-4-phenyl-2-oxazolidinone

(2.02 g, 7.11 mmol) was added triethyl phosphite (2.6 mL,

14.2 mmol), and the mixture was heated to 100 8C. After

3 h, the reaction mixture was cooled to room temperature

and then puriﬁed by silica gel column chromatography

(BW-820MH, CHCl₃/MeOH¼50:1) to afford the desired

5.1.9. General procedure H for the synthesis of (4R)-4-

phenyl-3-[(3R,4S,6S)-3,4,6-trimethyl-8-((2R)-N-methyl-

2-methylbutyramido)-octanoyl]-2-oxazolidinone (55)

and its stereoisomers. To a slurry of copper (I) bromide-

dimethylsulﬁde complex (290 mg, 1.39 mmol) in THF

(2.4 mL) was added dimethylsulﬁde (1.6 mL) under argon,

and cooled to 278 8C. Methyl magnesium bromide

(2.0 mL, 1.87 mmol, 0.93 M in THF) was slowly added.

After 20 min, the mixture was warmed to 0 8C for 20 min

and then recooled to 278 8C before being transferred via

cannula to a cooled (278 8C) solution of the enimide 54

(239 mg, 0.559 mmol) in THF (1.4 mL) and CH₂Cl₂

(0.7 mL). After 30 min, the mixture was warmed to

230 8C over 1 h and stirred at this temperature for 1 h.

Phosphate buffer (pH 7) was added to the mixture, which

was diluted with EtOAc, and washed with 1 M aqueous

KHSO₄(£2), water, and brine. The organic layer was dried

(Na₂SO₄), ﬁltered, and concentrated. The residue was

puriﬁed by silica gel column chromatography (BW-200,

hexane/Acetone¼3:1) to afford the desired product 55 as a

(R)-phosphonate 26 as a pale orange oil (1.72g, 71%):

[a]²_D⁵¼261.6 (c 2.5, CHCl₃); IR n

cm²¹2986, 1779,

neat

max

1

1705, 1392, 1330, 1260, 1208, 1163; H NMR (270 MHz,

TMS/CDCl₃) d 1.28 (6H, app q, J¼7.0 Hz, CH₃CH₂), 3.68–

3.89 (2H, m, CH₂), 4.06–4.17 (4H, m, CH₃CH₂), 4.28 (1H,

dd, J¼3.9, 8.8 Hz, CH₂O), 4.71 (1H, t, J¼8.8 Hz, Ar–CH),

5.46 (1H, dd, J¼3.9, 8.8 Hz, CH₂O), 7.29–7.41 (5H, m,

ArH); ¹³C NMR (67.8 MHz, CHCl₃/CDCl₃) d 16.3

(Jp¼2.5 Hz), 34.5 (Jp¼13.2 Hz), 57.9, 62.7 (Jp¼3.4 Hz),

69.8, 125.9, 128.6, 129.0, 153.4, 164.2 (Jp¼6.7 Hz). HRMS

(EI) m/z Calcd for C₁₅H₂₀NO₆P: 341.1028. Found: 341.1020.

5.1.8. General procedure G for the synthesis of (4R)-

phenyl-3-[(4R,6S)-4,6-dimethyl-8-((2R)-N-methyl-2-

methylbutyramido)-(E)-2-octenoyl]-2-oxazolidinone

(54) and its stereoisomers. To a solution of the alcohol ent-

49 (126 mg, 0.517 mmol) in CH₂Cl₂(3 mL) was added

DMSO (1 mL), triethylamine (0.36 mL), sulfur trioxide-

pyridine complex (420 mg, 2.59 mmol) at 0 8C. After

10 min, the cooling bath was removed, and the reaction

mixture was stirred at room temperature for 50 min. After

addition of ice water, the mixture was diluted with EtOAc,

and washed with 1 M aqueous KHSO₄, water, and brine.

The organic layer was dried (Na₂SO₄), ﬁltered, and

concentrated to give the crude aldehyde as a colorless oil

(191 mg). This intermediate was used in the next reaction

without further puriﬁcation.

colorless oil (238 mg, 96%): [a]²_D⁶¼258.6 (c 1.0, CHCl₃);

CHCl₃

max

IR n

1198, 1082; H NMR (270 MHz, DMSO/DMSO-d₆), both

cm²¹2965, 1781, 1705, 1634, 1456, 1385, 1325,

1

rotamers unless stated otherwise, d 0.74–0.81 (12H, m,

0

C₃–CH₃, C₈–CH₃, C₁₀–CH₃), 0.94 (3H, d, J¼6.8 Hz, 1

0

rotamer, C₂–CH₃), 0.97 (3H, d, J¼6.8 Hz, 1 rotamer, C₂–

CH₃),1.00–1.11 (2H, m, C₉–CH₂), 1.13–1.59 (6H, m,

0

C₃–CH₂, C₈–CH, C₁₀–CH, C₁₁–CH₂), 1.82–1.96 (1H, m,

0

C₇–CH), 2.54–2.83 (3H, m, C₂–CH, C₆–CH₂), 2.78 (3H,

s, 1 rotamer, N–CH₃), 2.95 (3H, s, 1 rotamer, N–CH₃),

6870

F. Yokokawa et al. / Tetrahedron 60 (2004) 6859–6880

3.21–3.38 (2H, m, C₁₂–CH₂), 4.14 (1H, dd, J¼3.6, 8.7 Hz,

CH₂O), 4.72 (1H, t, J¼8.7 Hz, Ar–CH), 5.46 (1H, dd,

J¼3.6, 8.7 Hz, CH₂O), 7.26–7.40 (5H, m, ArH); ¹³C NMR

(67.8 MHz, DMSO/DMSO-d₆), the major rotamer, d 11.7,

16.1, 17.1, 18.9, 26.6, 27.4, 33.5, 34.1, 34.7, 36.2, 36.7,

38.2, 39.4, 44.9, 69.9, 125.7, 127.9, 128.7, 140.0, 153.7,

171.9, 174.8. HRMS (EI) m/z Calcd for C₂₆H₄₀N₂O₄:

444.2988. Found: 444.2997.

rotamers unless stated otherwise, d 0.74–0.86 (12H, m,

0

C₃–CH₃, C₈–CH₃, C₁₀–CH₃), 0.95 (3H, d, J¼7.1 Hz, 1

0

rotamer, C₂–CH₃), 0.97 (3H, d, J¼7.1 Hz, 1 rotamer,

0

C₂–CH₃), 1.00–1.14 (2H, m, C₉–CH₂), 1.15–1.61 (6H, m,

0

C₃–CH₂, C₈–CH, C₁₀–CH, C₁₁–CH₂), 1.68–2.01 (2H, m,

C₆–CH,C₇–CH), 2.02–2.15 (1H, m, C₆–CH), 2.54–2.71

0

(1H, m, C₂–CH), 2.79 (3H, s, 1 rotamer, N–CH₃), 2.97

(3H, s, 1 rotamer, N–CH₃), 3.27–3.45 (4H, m, C₄–CH₂,

C₁₂–CH₂), 4.26–4.39 (1H, m, C₄–OH), 4.66–4.75 (1H, m,

C₃–CH), 5.05 (1H, d, J¼11.7 Hz, C₁–CH), 5.10 (1H, d,

J¼17.0 Hz, C₁–CH), 5.83 (1H, ddd, J¼6.4, 11.7, 17.0 Hz,

C₂–CH); ¹³C NMR (67.8 MHz, DMSO/DMSO-d₆), the

major rotamer, d 11.7, 16.0, 16.1, 17.1, 19.1, 26.7, 27.5,

33.1, 33.7, 34.7, 35.0, 36.2, 36.8, 39.4, 44.9, 52.7, 63.4,

114.7, 137.2, 171.5, 174.8. HRMS (EI) m/z Calcd for

C₂₁H₄₀N₂O₃: 368.3039. Found: 368.3035.

5.1.10. General procedure I for the synthesis of

(3R,4S,6S)-3,4,6-trimethyl-8-((2R)-N-methyl-2-methyl-

butyramido)-octanoic acid (56) and its stereoisomers. To

a solution of the imide 55 (203 mg, 0.457 mmol) in THF–

water (4:1, 1.9 mL) was added 30% aqueous H₂O₂

(0.26 mL), followed by the addition of 0.5 M aqueous

LiOH (2.7 mL) at 0 8C. After 30 min, the mixture was

stirred at room temperature for 10 h. After dilution with

water, the aqueous layer was acidiﬁed by the addition of 1 N

aqueous HCl and extracted with EtOAc (£3). The combined

organic extracts were washed with brine. The organic layer

was dried (Na₂SO₄), ﬁltered, and concentrated. The residue

was puriﬁed by silica gel column chromatography (BW-

200, hexane/EtOAc¼1:1) to afford the desired product 56 as

5.1.12. General procedure K for the synthesis of (4R)-4-

ethenyl-2-[(2R,3S,5S)-2,3,5-trimethyl-7-((2R)-N-methyl-

2-methylbutyramido)-heptyl]oxazoline (58) and its

stereoisomers. To a solution of the amide 57 (75.3 mg,

0.204 mmol) in CH₂Cl₂(2 mL) was added DAST (30 mL,

0.224 mmol) at 220 8C under argon. After 30 min, 4 M

aqueous NH₃was added and then the mixture was diluted

with water. The aqueous layer was extracted with EtOAc

(£3) and washed with brine. The organic layer was dried

(Na₂SO₄), ﬁltered, and concentrated. The residue was

puriﬁed by silica gel column chromatography (BW-200,

hexane/EtOAc¼1:3) to afford the desired product 58 as a

colorless oil (57.8 mg, 81%): [a]²_D⁶¼þ18.6 (c 0.76, CHCl₃);

a colorless oil (132 mg, 97%): [a]²_D⁶¼240.9 (c 1.0, CHCl₃);

CHCl₃

max

IR n

cm²¹3154 (bd), 2964, 1713, 1615, 1462, 1406,

1383, 1250, 1194; ¹H NMR (270 MHz, DMSO/DMSO-d₆),

both rotamers unless stated otherwise, d 0.74–0.84 (12H, m,

0

C₃–CH₃, C₈–CH₃, C₁₀–CH₃), 0.94 (3H, d, J¼7.1 Hz, 1

0

rotamer, C₂–CH₃), 0.97 (3H, d, J¼7.1 Hz, 1 rotamer,

0

C₂–CH₃), 1.01–1.15 (2H, m, C₉–CH₂), 1.16–1.61 (6H, m,

CHCl₃

max

1

0

C₃–CH₂, C₈–CH, C₁₀–CH, C₁₁–CH₂), 1.74–2.00 (2H, m,

C₆–CH, C₇–CH), 2.16–2.27 (1H, m, C₆–CH), 2.54–2.68

0

(1H, m, C₂–CH), 2.79 (3H, s, 1 rotamer, N–CH₃), 2.96

IR n

cm²¹2963, 1666, 1646, 1456, 1381, 1196; H

NMR (270 MHz, DMSO/DMSO-d₆), both rotamers unless

0

stated otherwise, d 0.75–0.86 (12H, m, C₃–CH₃, C₈–CH₃,

0

(3H, s, 1 rotamer, N–CH₃), 3.21–3.35 (2H, m, C₁₂–CH₂),

12.0 (1H, bd-s, COOH); ¹³C NMR (67.8 MHz, DMSO/

DMSO-d₆), the major rotamer, d 11.7, 16.0, 16.4, 17.1, 19.1,

26.7, 27.5, 33.6, 34.6, 34.7, 36.2, 36.7, 37.8, 39.4, 44.9,

174.4, 174.8. HRMS (EI) m/z Calcd for C₁₇H₃₃NO₃:

299.2460. Found: 299.2440.

C₁₀–CH₃), 0.94 (3H, d, J¼7.1 Hz, 1 rotamer, C₂–CH₃),

0

0.97 (3H, d, J¼7.1 Hz, 1 rotamer, C₂–CH₃), 1.00–1.14

0

(2H, m, C₉–CH₂), 1.15–1.61 (6H, m, C₃–CH₂, C₈–CH,

C₁₀–CH, C₁₁–CH₂), 1.74–1.89 (1H, bd-m, C₇–CH), 1.90–

2.03 (1H, m, C₆–CH), 2.17–2.28 (1H, m, C₆–CH), 2.54–

0

2.68 (1H, m, C₂–CH), 2.79 (3H, s, 1 rotamer, N–CH₃),

2.96 (3H, s, 1 rotamer, N–CH₃), 3.25–3.35 (2H, m, C₁₂–

CH₂), 3.84 (1H, app t, J¼8.0 Hz, C₄–CH), 4.31 (1H, app t,

J¼9.1 Hz, C₄–CH), 4.48–4.59 (1H, m, C₃–CH), 5.07 (1H,

d, J¼10.5 Hz, C₁–CH), 5.18(1H, d, J¼17.2 Hz, C₁–CH),

5.80 (1H, ddd, J¼6.8, 10.5, 17.2 Hz, C₂–CH); ¹³C NMR

(67.8 MHz, DMSO/DMSO-d₆), the major rotamer, d 11.7,

15.9, 16.2, 17.1, 19.1, 26.6, 27.4, 31.1, 33.5, 34.7, 35.3,

36.7, 39.4, 44.9, 67.4, 71.1, 115.3, 139.1, 166.6, 174.8.

HRMS (EI) m/z Calcd for C₂₁H₃₈N₂O₂: 350.2933. Found:

350.2922.

5.1.11. General procedure J for the synthesis of N-((2R)-

3-butenol-2-yl)-[(3R,4S,6S)-trimethyl-8-((2R)-N-methyl-

2-methylbutyramido)]octanamide (57) and its stereo-

isomers. (2R)-N-Boc-amino-3-butenol 32²¹(68.0 mg,

0.361 mmol) was dissolved in 4 N HCl–EtOAc at 0 8C.

After 15 min, the mixture was warmed to room temperature.

The solution was concentrated after 1 h, and the residue was

azeotropically concentrated with toluene (₀£4). The resulting

residue and the carboxylic acid (2 R)-56 (72.1 mg,

0.241 mmol) were dissolved in CH₂Cl₂(1.2 mL), and

i-Pr₂NEt (0.10 mL, 0.60 mmol) and then DMAP (15 mg,

0.120 mmol) were added. EDCI·HCl (138 mg, 0.722 mmol)

was added after 10 min and the reaction mixture was stirred

for 13 h. After dilution with water, the aqueous layer was

acidiﬁed by the addition of 1 N aqueous HCl and extracted

with EtOAc (£4). The combined organic extracts were

washed with brine. The organic layer was dried (Na₂SO₄),

ﬁltered, and concentrated. The residue was puriﬁed by silica

5.1.13. General procedure L for the synthesis of N-((2R)-

3-butenol-2-yl)-[(3R,4S,6S)-trimethyl-8-((2R)-N-methyl-

2-methylbutyramido)]octanethioamide (59) and its

stereoisomers. The oxazoline (2⁰R)-58 (44.2 mg,

0.126 mmol) was dissolved in MeOH–triethylamine (1:1,

2 mL, saturated with H₂S), and stirred at room temperature

for 12 h. The mixture was concentrated and the residue was

puriﬁed by silica gel column chromatography (BW-200,

hexane/EtOAc¼6:1) to afford the desired product 59 as a

gel

column

chromatography

(BW-200,

CHCl₃/

MeOH¼20:1) to afford the desired product 57 as a colorless

pale yellow oil (35.6 mg, 73%): [a]²_D⁵¼þ9.6 (c 0.44,

oil (85.8 mg, 97%): [a]²_D⁶¼210.8 (c 0.84, CHCl₃); IR

CHCl₃); IR n

cm²¹3340 (bd), 2964, 1622, 1464,

CHCl₃

max

CHCl₃

max

n

1252, 1082; H NMR (270 MHz, DMSO/DMSO-d₆), both

cm²¹3304, 2964, 1717, 1651, 1539, 1464, 1381,

1415, 1381, 1217, 1080; ¹H NMR (270 MHz, DMSO/

DMSO-d₆), both rotamers unless stated otherwise, d

1

F. Yokokawa et al. / Tetrahedron 60 (2004) 6859–6880

6871

0

0.73–0.87 (12H, m, C₃–CH₃, C₈–CH₃, C₁₀–CH₃), 0.95

m), 4.75 (1H, dd, J¼7.8, 7.5 Hz), 5.01 (1H, d, J¼10.3 Hz),

5.24 (1H, ddd, J¼17.2, 1.6, 1.6 Hz), 5.85 (1H, ddd, J¼17.2,

10.3, 6.1 Hz); ¹³C NMR (DMSO-d₆, 100 MHz) d 11.69

(C-4⁰), 16.0 (C-13), 16.0 (C-14), 17.13 (C-5⁰), 19.07 (C-15),

26.64 (C-3⁰), 27.47₀(C-10), 33.40 (C-8), 34.70 (C-16), 34.92

(C-11), 36.20 (C-2 ), 36.68 (C-7), 37.49 (C-6), 37.85 (C-4),

39.4 (C-9), 44.88 (C-12), 77.91 (C-3), 115.24 (C-1), 137.96

(C-2), 169.18 (C-5), 174.79 (C-1⁰); ¹³C NMR (benzene-d₆,

125 MHz, from HSQC and HSQMBC data sets) d 12.4

(C-4⁰), 16.4 (C-13), 16.4 (C-14), 17.8 (C-5⁰), 19.5 (C-15),

27.8 (C-3⁰), 28.3 (C-10), 34.4 (C-8), 34.5 (C-16), 36.0

(C-11), 37.5 (C-7), 37.6 (C-2⁰), 38.6 (C-6), 38.9 (C-4), 40.3

(C-9), 46.0 (C-12), 79.2 (C-3), 115.3 (C-1), 138.3 (C-2),

170.2 (C-5), 175.5 (C-1⁰).

0

(3H, d, J¼7.1 Hz, 1 rotamer, C₂–CH₃), 0.97 (3H, d,

0

J¼7.1 Hz, 1 rotamer, C₂–CH₃), 1.04–1.16 (2H, m,

0

C₉–CH₂), 1.17–1.62 (6H, m, C₃–CH₂, C₈–CH,

C₁₀–CH, C₁₁–CH₂), 2.05–2.28 (1H, bd-m, C₇–CH),

2.38–2.51 (2H, m, C₆–CH₂), 2.54–2.73 (1H, m,

0

C₂–CH), 2.79 (3H, s, 1 rotamer, N–CH₃), 2.98 (3H, s, 1

rotamer, N–CH₃), 3.23–3.35 (2H, m, C₁₂–CH₂), 3.44–

3.58 (2H, m, C₄–CH₂), 4.88 (1H, t, J¼5.6 Hz, C₃–CH),

5.05–5.21 (3H, m, C₁–CH₂, C₄–OH), 5.85 (1H, ddd,

J¼6.4, 11.4, 16.5 Hz, C₂–CH); ¹³C NMR (67.8 MHz,

DMSO/DMSO-d₆), the major rotamer, d 11.7, 15.1, 16.2,

17.1, 19.1, 26.7, 27.4, 33.1, 33.7, 34.7, 36.2, 36.6, 38.0,

39.4, 44.9, 59.0, 67.4, 115.9, 135.1, 174.8, 203.8. HRMS

(EI) m/z Calcd for C₂₁H₄₀N₂O₂S: 384.2810. Found:

384.2825.

Description of tertiary amide isomers in the NMR spectra of

kalkitoxin. Complication in the NMR-based strategy for

structure elucidation of kalkitoxin resulted from a 3:2 ratio

5.1.14. General procedure M for the synthesis of

(3R,7R,8S,10S,2⁰R)-kalkitoxin (7) and its stereoisomers.

To a solution of thioamide 59 (27.9 mg, 0.0725 mmol) in

CH₂Cl₂(0.7 mL) was added DAST (11 mL, 0.0797 mmol)

at 220 8C under argon. After 30 min, 4 M aqueous NH₃was

added and then the mixture was diluted with water. The

aqueous layer was extracted with EtOAc (£3) and washed

with brine. The organic layer was dried (Na₂SO₄), ﬁltered,

and concentrated. The residue was puriﬁed by silica gel

column chromatography (BW-200, hexane/EtOAc¼1:1) to

afford the synthetic (3R,7R,8S,10S,2⁰R)-kalkitoxin (7) as a

1

of tertiary amide isomers in its room temperature H NMR

(DMSO-d₆, 298 K). This required that structure elucidation

of kalkitoxin 7 be based on a combination of data sets for

experiments run at both 298 and 340 K. The origin of the

‘twinning’of signals was conﬁrmed by observing alterations

in the signal ratios as a function of different NMR solvents

(ratio between E and Z oleﬁn isomers ranged from a low of

0.5:1.0 in benzene-d₆to a high of 1.58:1.0 in acetone-d₆)

and temperature. At relatively high temperature (340 K)

these ‘twinned’ signals coalesced to singlets; however, the

value of this elevated temperature experiment was compro-

mised because, for reasons that we do not understand,

pale yellow oil (21.6 mg, 81%): [a]²_D⁶¼þ7.0 (c 1.0, CHCl₃);

CHCl₃

max

IR n

cm²¹2963, 2930, 2874, 1646, 1464, 1412, 1381,

1082; ¹H NMR (270 MHz, DMSO/DMSO-d₆), both

rotamers unless stated otherwise, d 0.75–0.88 (12H, m,

several ¹³C NMR signals and H–¹³C NMR correlations

1

that should have been present were lost.

0

C₃–CH₃, C₈–CH₃, C₁₀–CH₃), 0.95 (3H, d, J¼7.1 Hz,

0

C₂–CH₃), 0.97 (3H, d, J¼7.1 Hz, C₂–CH₃), 1.01–1.16

HRMS (EI) m/z obs. [M]^þ366.2696 (15.9, 0.9 mmu dev. for

C₂₁H₃₈N₂OS); HR-EIMS cleavages between C6–C7 in

kalkitoxin obs. m/s 240.2329 for [C₁₅H₃₀NO]^þ(25%,

0.2 mmu dev.) and m/z 127.0459 for [C₆H₉NS]^þ(26%,

0.3 mmu dev.); cleavage C7–C8 obs. m/z 154.0683 for

[C₈H₁₂NS]^þ(100%, 0.8 mmu dev.).

0

(2H, m, C₉–CH₂), 1.17–1.62 (6H, m, C₃–CH₂, C₈–CH,

C₁₀–CH, C₁₁–CH₂), 1.73–1.89 (1H, bd–m, C₇–CH),

2.17–2.30 (1H, m, C₆–CH₂), 2.40–2.47 (1H, m, C₆–

0

CH₂), 2.54–2.69 (1H, m, C₂–CH), 2.79 (3H, s, 1 rotamer,

N–CH₃), 2.97 (3H, s, 1 rotamer, N–CH₃), 3.02 (1H, dd,

J¼8.4, 11.0 Hz, C₄–CH), 3.25–3.39 (2H, m, C₁₂–CH₂),

3.48 (1H, dd, J¼8.4, 11.0 Hz, C₄–CH), 4.84–4.97 (1H, m,

C₃–CH), 5.10 (1H, d, J¼10.4 Hz, C₁–CH), 5.23 (1H, d,

J¼17.1 Hz, C₁–CH), 5.90 (1H, ddd, J¼6.4, 10.4, 17.1 Hz,

C₂–CH); ¹³C NMR (67.8 MHz, DMSO/DMSO-d₆), the

major rotamer, d 11.70, 16.02(CH₃£2), 17.14, 19.08, 26.64,

27.47, 33.39, 34.70, 34.92, 36.20, 36.68, 37.49, 37.85,

39.40, 44.88, 77.90, 115.27, 137.97, 169.20, 174.80. HRMS

(EI) m/z Calcd for C₂₁H₃₈N₂OS: 366.2705. Found:

366.2722.

5.1.16. (2S)-N-[(3S,5S)-3,5-Dimethyl-6-(tert-butylsilyl-

oxy)-hex-1-yl]-2-methylbutyramide (40). According to

general procedure D (using (S)-2-methylbutyric acid 22),

the azide 39 (774 mg, 1.89 mmol) provided the amide 40 as

a pale yellow oil (826 mg, 93%): [a]²_D⁵¼23.7 (c 1.1,

CHCl₃); IR nⁿ_m^e_a^a_x^tcm²¹3295, 2930, 1644, 1553, 1462, 1428,

1387, 1237, 1113, 1094; ¹H NMR (270 MHz, TMS/CDCl₃)

0

d 0.82–0.91 (9H, m, C₃–CH₃, C₈–CH₃, C₁₀–CH₃), 1.05

0

(9H, s, (CH₃)₃C), 1.11 (3H, d, J¼6.9 Hz, C₂–CH₃), 1.18–

0

1.79 (8H, m, C₃–CH₂, C₉–CH₂, C₁₀–CH, C₁₁–CH₂),

0

1.95–2.09 (1H, m, C₂–CH), 3.16–3.31 (2H, m, C₁₂–CH₂),

5.1.15. Natural kalkitoxin. Pure natural kalkitoxin showed

the following physicochemical data: [a]²_D⁵¼þ16 (c 0.07,

CHCl₃); CD c 0.022, EtOH l_ext226 nm (D1þ4.75), 207.8

(0.0) and see Figure 6 in Supplementary data; IR

(CHCl₃) 2961, 2928, 2880, 1643, 1464, 1086, 1410,

1380 cm²¹; UV (MeOH) l_max250 nm (1¼2600); ¹H

NMR (benzene-d₆, 500 MHz) d 0.76 (3H, d, J¼6.8 Hz),

0.85 (3H, d, J¼6.1 Hz), 0.88 (3H, d, J¼7.5 Hz), 0.95 (d, 3H,

J¼6.8 Hz), 1.02 (1H, m), 1.10 (1H, m), 1.10 (3H, d,

J¼6.7 Hz), 1.24 (1H, m), 1.34 (1H, m), 1.38 (1H, m), 1.39

(1H, m), 1.54 (1H, m), 1.87 (1H, m), 2.05 (1H, m), 2.28 (1H,

m), 2.31 (1H, m), 2.43 (3H, s), 2.55 (1H, m), 2.72 (1H, dd,

J¼10.7, 8.4 Hz), 2.94 (1H, dd, J¼10.5, 8.8 Hz), 3.35 (2H,

3.37–3.51 (2H, m, CH₂O), 5.27 (1H, bd-s, NH), 7.35–7.42

(6H, m, ArH), 7.63–7.67 (4H, m, ArH); ¹³C NMR

(67.8 MHz, CHCl₃/CDCl₃) d 12.0, 16.7, 17.7, 19.3, 19.4,

26.9, 27.4, 27.9, 33.2, 37.4, 37.7, 40.7, 43.3, 69.4, 127.4,

129.4, 133.9, 134.0, 135.5, 176.0. HRMS (EI) m/z Calcd for

C₂₅H₃₆NO₂Si: 410.2515 (M^þ2t-Bu). Found: 410.2491.

5.1.17. (2S)-N-Methyl-N-[(3S,5S)-dimethyl-6-(tert-butyl-

diphenylsilyloxy)-hex-1-yl]-2-methylbutyramide (41).

According to general procedure E, the amide 40 (565 mg,

1.21 mmol) provided the N-methylamide 41 as a pale brown

oil (480 mg, 83%): [a]²_D⁴¼þ3.0 (c 1.0, CHCl₃); IR n

neat

max

6872

F. Yokokawa et al. / Tetrahedron 60 (2004) 6859–6880

cm²¹2961, 1646, 1472, 1464, 1428, 1113, 1090; ¹H NMR

(270 MHz, TMS/CDCl₃), both rotamers unless stated

0

otherwise, d 0.82–0.93 (9H, m, C₃–CH₃, C₈–CH₃, C₁₀–

CH₃), 1.05 (9H, s, 1 rotamer, (CH₃)₃C), 1.06 (9H, s, 1

colorless oil (87 mg, 97%): [a]D²⁵232.8 (c 1.5, CHCl3);

IR n maxneat cm21 2930, 1782, 1705, 1636, 1458, 1385,

1325, 1198, 1082; ¹H NMR (270 MHz, DMSO/DMSO-d₆),

both rotamers unless stated otherwise, d 0.74–0.81 (12H, m,

0

rotamer, (CH₃)₃C), 1.07–1.12 (3H, m, C₂–CH₃), 1.19–

0

1.56 (4H, m, C₃–CH₂, C₈–CH, C₁₀–CH), 1.57–1.80 (4H,

0

m, C₉–CH₂, C₁₁–CH₂), 2.45–2.61 (1H, m, C₂–CH), 2.90

0

C₃–CH₃, C₈–CH₃, C₁₀–CH₃), 0.94 (3H, d, J¼6.6 Hz, 1

0

rotamer, C₂–CH₃), 0.97 (3H, d, J¼6.6 Hz, 1 rotamer, C₂–

CH₃), 1.00–1.08 (2H, m, C₉–CH₂), 1.15–1.58 (6H, m,

0

C₃–CH₂, C₈–CH, C₁₀–CH, C₁₁–CH₂), 1.82–1.96 (1H, m,

(3H, s, 1 rotamer, N–CH3), 2.97 (3H, s, 1 rotamer, N–

CH₃), 3.20–3.34 (2H, m, C₁₂–CH₂), 3.35–3.52 (2H, m,

CH₂O), 7.36–7.45 (6H, m, ArH), 7.62–7.67 (4H, m, ArH);

¹³C NMR (67.8 MHz, CHCl₃/CDCl₃), both rotamers, d

12.1, 12.2, 16.6, 17.2, 17.9, 19.3, 19.4, 26.9, 27.1, 27.5,

28.0, 28.1, 33.2, 33.3, 33.7, 35.2, 37.0, 37.2, 37.4, 40.6,

40.7, 46.0, 48.0, 69.4, 69.5, 127.4, 129.3, 129.4, 133.8,

133.9, 134.0, 135.5, 175.9, 176.2. HRMS (EI) m/z Calcd for

C₂₆H₃₈NO₂Si: 424.2672 (M^þ2t-Bu). Found: 424.2654.

0

C₇–CH), 2.50–2.76 (3H, m, C₂–CH, C₆–CH₂), 2.78 (3H,

s, 1 rotamer, N–CH₃), 2.95 (3H, s, 1 rotamer, N–CH₃),

3.15–3.42 (2H, m, C₁₂–CH₂), 4.14 (1H, dd, J¼8.7, 3.5 Hz,

CH₂O), 4.72 (1H, t, J¼8.7 Hz, Ar–CH), 5.46 (1H, dd,

J¼8.7, 3.5 Hz, CH₂O), 7.26–7.40 (5H, m, ArH); ¹³C NMR

(67.8 MHz, DMSO/DMSO-d₆), the major rotamer, d 11.6,

16.1, 17.1, 18.8, 26.7, 27.3, 33.5, 34.1, 34.6, 36.2, 36.8,

38.2, 39.4, 44.7, 57.0, 69.9, 125.7, 127.9, 128.7, 140.0,

153.7, 171.8, 174.8. HRMS (EI) m/z Calcd for C₂₆H₄₀N₂O₄:

444.2988. Found: 444.2990.

5.1.18. (2S)-N-Methyl-N-((3S,5S)-3,5-dimethyl-6-

hydroxy-hex-1-yl)-2-methylbutyramide (42). According

to general procedure F, the N-methylamide 41 (446 mg,

5.1.21. (3R,4S,6S)-3,4,6-Trimethyl-8-((2S)-N-methyl-2-

methylbutyramido)octanoic acid (45). According to

general procedure I, the imide 44 (212 mg, 0.476 mmol)

0.926 mmol) provided the alcohol 42 as a pale yellow oil

(211 mg, 94%): [a]_D²⁴¼þ6.0 (c 1.0, CHCl₃); IR n

cm²¹

neat

max

3432, 2928, 1626, 1464, 1414, 1379, 1048; ¹H NMR

(270 MHz, TMS/CDCl₃), both rotamers unless stated

provided the carboxylic acid 45 as a colorless oil (126 mg,

88%): [a]²_D⁵¼22.9 (c 1.1, CHCl₃); IR n

cm²¹3346

1

CHCl₃

max

0

otherwise, d 0.85–0.93 (9H, m, C₃–CH₃, C₈–CH₃,

0

C₁₀–CH₃), 1.07–1.14 (3H, m, C₂–CH₃), 1.15–1.27 (2H,

(bd), 2964, 1732, 1634, 1404, 1383, 1252, 1190; H NMR

(270 MHz, DMSO/DMSO-d₆), both rotamers unless stated

0

m, C₉–CH₂), 1.28–1.62 (4H, m, C₃–CH₂, C₁₁–CH₂),

1.63–1.80 (2H, m, C₈–CH, C₁₀–CH), 2.46–2.64 (1H, m,

0

C₂–CH), 2.92 (3H, s, 1 rotamer, N–CH₃), 3.01 (3H, s, 1

rotamer, N–CH₃), 3.20–3.56 (4H, m, C₁₂–CH₂, CH₂O);

¹³C NMR (67.8 MHz, CHCl₃/CDCl₃), both rotamers, d

12.0, 12.2, 16.2, 16.3, 17.1, 17.8, 19.2, 19.4, 27.1, 27.4,

27.8, 28.0, 33.1, 33.7, 35.0, 35.2, 37.0, 37.1, 37.3, 40.4,

40.6, 46.0, 48.1, 68.6, 68.7, 176.0, 176.3. HRMS (EI) m/z

Calcd for C₁₄H₂₉NO₂: 243.2199. Found: 243.2203.

0

otherwise, d 0.74–0.84 (12H, m, C₃–CH₃, C₈–CH₃, C₁₀–

0

CH₃), 0.94 (3H, d, J¼6.9 Hz, 1 rotamer, C₂–CH₃), 0.97

0

(3H, d, J¼6.9 Hz, 1 rotamer, C₂–CH₃), 1.01–1.14 (2H, m,

0

C₉–CH₂), 1.16–1.60 (6H, m, C₃–CH₂, C₈–CH, C₁₀–CH,

C₁₁–CH₂), 1.74–1.99 (2H, m, C₆–CH, C₇–CH), 2.15–

0

2.27 (1H, m, C₆–CH), 2.52–2.70 (1H, m, C₂–CH), 2.79

(3H, s, 1 rotamer, N–CH₃), 2.96 (3H, s, 1 rotamer, N–CH₃),

3.12–3.25 (2H, m, C₁₂–CH₂), 10.20 (1H, bd-s, COOH);

¹³C NMR (67.8 MHz, DMSO/DMSO-d₆), the major

rotamer, d 11.7, 16.0, 16.5, 17.2, 19.0, 26.7, 27.4, 33.7,

34.6, 34.9, 36.2, 36.8, 37.8, 39.4, 44.7, 174.1, 174.5. HRMS

(EI) m/z Calcd for C₁₇H₃₃NO₃: 299.2460. Found: 299.2458.

5.1.19. (4R)-Phenyl-3-[(4R,6S)-4,6-dimethyl-8-((2S)-N-

methyl-2-methylbutyramido)-(E)-2-octenoyl]-2-oxa-

zolidinone (43). According to general procedure G, the

alcohol 42 (204 mg, 0.838 mmol) provided the enimide 43

as a colorless oil (333 mg, 93%): [a]²_D⁴¼227.5 (c 1.1,

CHCl₃); IR nⁿ_m^e_a^a_x^tcm²¹2980, 1779, 1688, 1634, 1456, 1383,

1362, 1329, 1200, 1103, 1082; ¹H NMR (270 MHz, DMSO/

DMSO-d₆), both rotamers unless stated otherwise, d 0.67–

5.1.22. (4R)-4-Ethenyl-2-[(2R,3S,5S)-2,3,5-trimethyl-7-

((2S)-N-methyl-2-methylbutyramido)-heptyl]oxazoline

(46a). According to general procedures J and K, the

carboxylic acid 45 (79.3 mg, 0.265 mmol) provided the

oxazoline 46a as a colorless oil (45.1 mg, 70%, 2 steps):

cm²¹2963, 1660,

CHCl₃

max

[a]²_D⁵¼þ53.0 (c 1.0, CHCl₃); IR n

0

0.88 (9H, m, C₃–CH₃, C₈–CH₃, C₁₀–CH₃), 0.92–1.00

(3H, m, C₂–CH₃), 1.09–1.37 (5H, m, C₉–CH₂, C₁₀–CH,

0

C₁₁–CH₂), 1.39–1.68 (2H, m, C₃–CH₂), 2.42–2.63 (2H,

1646, 1464, 1381, 1196; ¹H NMR (270 MHz, DMSO/

DMSO-d₆), both rotamers unless stated otherwise, d 0.75–

0

m, C₂–CH, C₈–CH), 2.78 (3H, s, 1 rotamer, N–CH₃), 2.90

0

0.86 (12H, m, C₃–CH₃, C₈–CH₃, C₁₀–CH₃), 0.94 (3H, d,

0

(3H, s, 1 rotamer, N–CH₃), 3.06–3.51 (2H, m, C₁₂–CH₂),

4.18 (1H, dd, J¼8.5, 3.3 Hz, CH₂O), 4.75 (1H, t, J¼8.5 Hz,

Ar–CH), 5.50 (1H, dd, J¼8.5, 3.3 Hz, CH₂O), 6.82 (1H, dd,

J¼15.4, 7.8 Hz, C₆–CH), 7.16 (1H, d, J¼15.4 Hz, C₇–CH),

7.28–7.38 (5H, m, ArH); ¹³C NMR (67.8 MHz, DMSO/

DMSO-d₆), the major rotamer, d 11.6, 17.1, 19.1, 19.5, 26.7,

27.3, 33.2, 33.8, 34.4, 36.2, 39.4, 42.8, 57.1, 70.1, 118.9,

125.9, 128.0, 128.8, 139.7, 153.8, 155.7, 163.8, 174.8.

HRMS (EI) m/z Calcd for C₂₅H₃₆N₂O₄: 428.2675. Found:

428.2673.

J¼6.8 Hz, 1 rotamer, C₂–CH₃), 0.97 (3H, d, J¼6.8 Hz, 1

0

rotamer, C₂–CH₃), 1.00–1.14 (2H, m, C₉–CH₂), 1.15–

0

1.62 (6H, m, C₃–CH₂, C₈–CH, C₁₀–CH, C₁₁–CH₂),

1.71–1.89 (1H, m, C₇–CH), 1.90–2.08 (1H, m, C₆–CH),

0

2.15–2.30 (1H, m, C₆–CH), 2.55–2.69 (1H, m, C₂–CH),

2.79 (3H, s, 1 rotamer, N–CH₃), 2.96 (3H, s, 1 rotamer,

N–CH₃), 3.12–3.37 (2H, m, C₁₂–CH₂), 3.84 (1H, app t,

J¼7.9 Hz, C₄–CH), 4.31 (1H, app t, J¼8.9 Hz, C₄–CH),

4.45–4.61 (1H, m, C₃–CH), 5.07 (1H, d, J¼10.2 Hz, C₁–

CH), 5.17 (1H, d, J¼16.8 Hz, C₁–CH), 5.80 (1H, ddd,

J¼17.1, 10.4, 6.6 Hz, C₂–CH); 13C NMR (67.8 MHz,

DMSO/DMSO-d6), the major rotamer, d 11.7, 16.3, 17.2,

19.0, 26.7, 27.3, 31.1, 33.5, 34.8, 35.3, 36.2, 36.7, 39.4,

44.7, 67.4, 71.1, 115.1, 138.9, 166.4, 174.5. HRMS (EI) m/z

Calcd for C₂₁H₃₈N₂O₂: 350.2933. Found: 350.2944.

5.1.20. (4R)-4-Phenyl-3-[(3R,4S,6S)-3,4,6-trimethyl-8-

((2S)-N-methyl-2-methylbutyramido)-octanoyl]-2-oxa-

zolidinone (44). According to general procedure H, the

enimide 43 (86 mg, 0.201 mmol) provided the imide 44 as a

F. Yokokawa et al. / Tetrahedron 60 (2004) 6859–6880

6873

5.1.23. (3R,7R,8S,10S,2⁰S)-Kalkitoxin (3). According to

general procedures L and M, the oxazoline 46a (42.9 mg,

0.122 mmol) provided the synthetic (3R,7R,8S,10S,2⁰S)-

(1H, m, C₆–CH), 2.55–2.72 (1H, m, C₂–CH), 2.79 (3H, s,

1 rotamer, N–CH₃), 2.96 (3H, s, 1 rotamer, N–CH₃), 3.03

(1H, dd, J¼7.9, 10.4 Hz, C₄–CH), 3.12–3.29 (2H, m, C₁₂–

CH₂), 3.48 (1H, dd, J¼7.9, 10.4 Hz, C₄–CH), 4.86–5.00

(1H, m, C₃–CH), 5.10 (1H, d, J¼10.2 Hz, C₁–CH), 5.22

(1H, d, J¼17.2 Hz, C₁–CH), 5.90 (1H, ddd, J¼6.6, 10.2,

17.2 Hz, C₂–CH); ¹³C NMR (67.8 MHz, DMSO/DMSO-

d₆), the major rotamer, d 11.66, 16.00 (CH₃£2), 17.13,

18.97, 26.69, 27.35, 33.43, 34.60, 34.84, 36.20, 36.60,

37.48, 37.87, 39.4, 44.71, 77.89, 115.22, 138.00, 169.23,

174.80. HRMS (EI) m/z Calcd for C₂₁H₃₈N₂OS: 366.2705.

Found: 366.2694.

0

kalkitoxin (3) as a pale yellow oil (24.2 mg, 57%, 2 steps):

cm²¹2963, 2930,

CHCl₃

max

[a]²_D⁵¼þ39.7 (c 0.88, CHCl₃); IR n

2874, 1646, 1412, 1381, 1082; ¹H NMR (270MHz, DMSO/

DMSO-d₆), both rotamers unless stated otherwise, d 0.75–

0

0.88 (12H, m, C₃–CH₃, C₈–CH₃, C₁₀–CH₃), 0.95 (3H, d,

0

J¼7.0 Hz, 1 rotamer, C₂–CH₃), 0.97 (3H, d, J¼7.0 Hz, 1

0

rotamer, C₂–CH₃), 1.01–1.17 (2H, m, C₉–CH₂), 1.19–

0

1.64 (6H, m, C₃–CH₂, C₈–CH, C₁₀–CH, C₁₁–CH₂), 1.71–

1.90 (1H, bd-m, C₇–CH), 2.16–2.30 (1H, m, C₆–CH₂),

0

2.39–2.46 (1H, m, C₆–CH₂), 2.54–2.71 (1H, m, C₂–CH),

2.79 (3H, s, 1 rotamer, N–CH₃), 2.96 (3H, s, 1 rotamer,

N–CH₃), 3.02 (1H, dd, J¼8.4, 10.9 Hz, C₄–CH), 3.12–

3.30 (2H, m, C₁₂–CH₂), 3.48 (1H, dd, J¼8.4, 10.9 Hz,

C₄–CH), 4.83–4.97 (1H, m, C₃–CH), 5.10 (1H, d,

J¼10.2 Hz, C₁–CH), 5.23 (1H, d, J¼17.1 Hz, C₁–CH),

5.90 (1H, ddd, J¼6.3, 10.2, 17.1 Hz, C₂–CH); ¹³C NMR

(67.8MHz, DMSO/DMSO-d₆), the major rotamer, d 11.65,

16.08 (CH₃£2), 17.12, 18.99, 26.68, 27.33, 33.40, 34.59,

34.84, 36.19, 36.69, 37.48, 37.85, 39.40, 44.69, 77.90,

115.27, 137.97, 169.21, 174.79. HRMS (EI) m/z Calcd for

C₂₁H₃₈N₂OS: 366.2705. Found: 366.2722.

5.1.26. (2R,4S)-2,4-Dimethyl-1-(tert-butyldiphenylsilyl-

oxy)-5-hexene (ent-37). According to general procedure

A, the alcohol ent-36³⁰(5.37 g, 14.04 mmol) provided the

silyloxyhexene ent-37 as a pale yellow oil (2.95 g, 57%):

neat

[a]²_D⁵¼21.1 (c 1.0, CHCl₃); IR n

cm²¹2957, 1428,

max

1113, 804; ¹H NMR (270 MHz, TMS/CDCl₃) d 0.92 (3H, d,

J¼1.6 Hz, C₈–CH₃), 0.94 (3H, d, J¼1.6 Hz, C₁₀–CH₃),

1.07 (9H, s, (CH₃)₃C), 1.27–1.43 (2H, m, C₉–CH₂), 1.66–

1.78 (1H, m, C₈–CH), 2.09–2.19 (1H, m, C₁₀–CH),

3.39–3.56 (2H, m, CH₂O), 4.83–4.89 (2H, m, C₁₂–CH₂),

5.60–5.73 (1H, m, C₁₁–CH), 7.35–7.41 (6H, m, ArH),

7.65–7.67 (4H, m, ArH); ¹³C NMR (67.8 MHz, CHCl₃/

CDCl₃) d 17.5, 17.9, 20.3, 27.0, 27.6, 33.3, 35.3, 40.3, 68.7,

112.1, 127.5, 127.7, 129.3, 129.4, 133.9, 134.0, 135.5,

135.6, 145.1. HRMS (EI) m/z Calcd for C₂₀H₂₅OSi:

309.1675 (M^þ2t-Bu). Found: 309.1688.

5.1.24. (4S)-4-Ethenyl-2-[(2R,3S,5S)-2,3,5-trimethyl-7-

((2S)-N-methyl-2-methylbutyramido-heptyl)]oxazoline

(46b). According to general procedure J (using (2S)-N-Boc-

amino-3-butenol ent-32) and procedure K, the carboxylic

acid 45 (69.9 mg, 0.233 mmol) provided the oxazoline 46b

as a colorless oil (64.4 mg, 80%, 2 steps): [a]²_D⁶¼262.7 (c

5.1.27. (3R,5R)-3,5-Dimethyl-6-(tert-butyldiphenylsilyl-

oxy)hexanol (ent-38). According to general procedure B,

the silyloxyhexene ent-37 (2.95 g, 8.05 mmol) provided the

alcohol ent-38 as a colorless oil (2.10 g, 68%): [a]²_D⁴¼þ11.2

(c 1.2, CHCl₃); IR n_m^ne_a^a_x^tcm²¹3346, 2859, 1472, 1428, 1389,

1113, 1091, 1071; ¹H NMR (270 MHz, TMS/CDCl₃) d 0.85

(3H, d, J¼6.4 Hz, C₈–CH₃), 0.89 (3H, d, J¼6.6 Hz, C₁₀–

CH₃), 1.05 (9H, s, (CH₃)₃C), 1.18–1.28 (1H, m, C₁₁–CH),

1.33–1.46 (1H, m, C₈–CH), 1.49–1.62 (3H, m, C₉–CH₂,

C₁₁–CH), 1.68–1.78 (1H, m, C₁₁–CH), 3.39–3.51 (2H, m,

CH₂O), 3.58–3.70 (2H, m, CH₂OH), 7.35–7.44 (6H, m,

ArH), 7.64–7.67 (4H, m, ArH); ¹³C NMR (67.8 MHz,

CHCl₃/CDCl₃) d 16.7, 19.4, 19.5, 26.8, 26.9, 33.2, 40.7,

40.8, 60.9, 69.4, 127.4, 129.4, 133.9, 135.5. HRMS (EI) m/z

Calcd for C₂₀H₂₇O₂Si: 327.1781 (M^þ2t-Bu). Found:

327.1782.

CHCl₃

max

0.19, CHCl₃); IR n

cm²¹2968, 1666, 1646, 1464,

1381, 1196; ¹H NMR (270 MHz, DMSO-d₆), both rotamers

0

unless stated otherwise, d 0.75–0.85 (12H, m, C₃–CH₃,

C₈–CH₃, C₁₀–CH₃), 0.94 (3H, d, J¼6.8 Hz, 1 rotamer,

0

C₂–CH₃), 0.97 (3H, d, J¼6.8 Hz, 1 rotamer, C₂–CH₃),

0

1.00–1.15 (2H, m, C₉–CH₂), 1.16–1.62 (6H, m, C₃–CH₂,

C₈–CH, C₁₀–CH, C₁₁–CH₂), 1.70–1.88 (1H, bd-m,

C₇–CH), 1.90–2.03 (1H, m, C₆–CH), 2.16–2.29 (1H, m,

0

C₆–CH), 2.54–2.71 (1H, m, C₂–CH), 2.79 (3H, s, 1

rotamer, N–CH₃), 2.95 (3H, s, 1 rotamer, N–CH₃), 3.10–

3.24 (2H, m, C₁₂–CH₂), 3.84 (1H, app t, J¼7.8 Hz,

C₄–CH), 4.31 (1H, app t, J¼8.6 Hz, C₄–CH), 4.46–4.61

(1H, m, C₃–CH), 5.07 (1H, d, J¼10.2 Hz, C₁–CH), 5.17

(1H, d, J¼17.2 Hz, C₁–CH), 5.79 (1H, ddd, J¼6.4, 10.2,

17.2 Hz, C₂–CH); ¹³C NMR (67.8 MHz, DMSO-d₆), the

major rotamer, d 11.6, 15.9, 16.2, 17.1, 18.9, 26.7, 27.3,

31.0, 33.5, 34.6, 35.2, 36.2, 36.7, 39.4, 44.7, 67.4, 71.1,

115.2, 139.1, 166.6, 174.8. HRMS (EI) m/z Calcd for

C₂₁H₃₈N₂O₂: 350.2933. Found: 350.2943.

5.1.28. (2R,4R)-6-Azido-2,4-dimethyl-1-(tert-butyl-

diphenylsilyloxy)hexane (ent-39). According to general

procedure C, the alcohol ent-38 (1.90 g, 4.95 mmol)

provided the azide ent-39 as a colorless oil (1.93 g, 95%):

cm²¹2959, 2095,

5.1.25. (3S,7R,8S,10S,2⁰S)-Kalkitoxin (4). According to

general procedures L and M, the oxazoline 46b (60.7 mg,

0.173 mmol) provided the synthetic (3S,7R,8S,10S,2⁰S)-

[a]²_D⁴¼þ8.5 (c 1.0, CHCl₃); IR n

neat

max

1

1472, 1428, 1389, 1262, 1113, 1092; H NMR (270 MHz,

TMS/CDCl₃) d 0.85 (3H, d, J¼6.4 Hz, C₈–CH₃), 0.89 (3H,

d, J¼6.8 Hz, C₁₀–CH₃), 1.05 (9H, s, (CH₃)₃C), 1.19–1.30

(1H, m, C₁₀–CH), 1.33–1.49 (1H, m, C₈–CH), 1.50–1.58

(3H, m, C₉–CH₂, C₁₁–CH), 1.67–1.77 (1H, m, C₁₁–CH),

3.17–3.33 (2H, m, C₁₂–CH₂), 3.40–3.50 (2H, m, C₇–

CH₂), 7.35–7.45 (6H, m, ArH), 7.64–7.67 (4H, m, ArH);

¹³C NMR (67.8 MHz, CHCl₃/CDCl₃) d 16.6, 19.2, 19.4,

26.9, 27.7, 33.2, 36.5, 40.5, 49.5, 69.4, 127.5, 129.4, 133.9,

135.5. HRMS (EI) m/z Calcd for C₂₀H₂₆N₃OSi: 352.1845

(M^þ2t-Bu). Found: 352.1835.

kalkitoxin (4) as a pale yellow oil (40.4 mg, 66%, 2 steps):

[a]²_D⁵¼246.1 (c 0.81, CHCl₃); IR n

cm²¹2963, 2930,

2874, 1646, 1464, 1412, 1381, 1082; H NMR (270 MHz,

CHCl₃

max

1

DMSO/DMSO-d₆), both rotamers unless stated otherwise, d

0

0.75–0.88 (12H, m, C₃–CH₃, C₈–CH₃, C₁₀–CH₃), 0.94

0

(3H, d, J¼6.7 Hz, C₂–CH₃), 0.97 (3H, d, J¼6.7 Hz, C₂–

CH₃), 1.01–1.17 (2H, m, C₉–CH₂), 1.18–1.65 (6H, m,

0

C₃–CH₂, C₈–CH, C₁₀–CH, C₁₁–CH₂), 1.71–1.90 (1H,

bd–m, C₇–CH), 2.18–2.32 (1H, m, C₆–CH), 2.39–2.46

6874

F. Yokokawa et al. / Tetrahedron 60 (2004) 6859–6880

5.1.29. (2S)-N-[(3R,5R)-3,5-Dimethyl-6-(tert-butylsilyl-

oxy)-hex-1-yl]-2-methylbutyramide (47). According to

general procedure D (using (S)-2-methylbutyric acid 22),

the azide ent-39 (1.87 g, 4.57 mmol) provided the amide 47

as a pale yellow oil (1.84 g, 86%): [a]²_D⁴¼þ13.8 (c 1.0,

CHCl₃); IR nⁿ_m^e_a^a_x^tcm²¹3325, 2963, 1644, 1553, 1462, 1427,

1389, 1237, 1113, 1094; ¹H NMR (270 MHz, TMS/CDCl₃)

(S)-phosphonate ent-26), the alcohol 49 (115 mg,

0.472 mmol) provided the enimide 50 as a colorless oil

(147 mg, 73%): [a]²_D⁵¼þ52.8 (c 1.0, CHCl₃); IR n_m^ne_a^a_x^tcm²¹

2966, 1779, 1688, 1634, 1458, 1383, 1329, 1200, 1103,

1082; H NMR (270 MHz, DMSO/DMSO-d₆), both rota-

0

mers unless stated otherwise, d 0.76–0.88 (9H, m, C₃–

1

0

CH₃, C₈–CH₃, C₁₀–CH₃), 0.95–0.98 (3H, m, C₂–CH₃),

1.15–1.39 (5H, m, C₉–CH₂, C₁₀–CH, C₁₁–CH₂), 1.40–

0

d 0.84–0.91 (9H, m, C₃–CH₃, C₈–CH₃, C₁₀–CH₃), 1.05

0

(9H, s, (CH₃)₃C), 1.10 (3H, d, J¼6.9 Hz, C₂–CH₃), 1.18–

1.53 (2H, m, C₃–CH₂), 2.39–2.61 (2H, m, C₂–CH, C₈–

CH), 2.78 (3H, s, 1 rotamer, N–CH₃), 2.92 (3H, s, 1

rotamer, N–CH₃), 3.20–3.42 (2H, m, C₁₂–CH₂), 4.17 (1H,

dd, J¼3.4, 8.6 Hz, CH₂O), 4.75 (1H, t, J¼8.6 Hz, Ar–CH),

5.50 (1H, dd, J¼3.4, 8.6 Hz, CH₂O), 6.83 (1H, dd, J¼15.4,

7.5 Hz, C₆–CH), 7.16 (1H, d, J¼15.4 Hz, C₇–CH), 7.31–

7.38 (5H, m, ArH); ¹³C NMR (67.8 MHz, DMSO/DMSO-

d₆), the major rotamer, d 11.8, 17.2, 19.2, 19.6, 26.6, 27.5,

33.4, 33.8, 34.5, 36.2, 39.4, 42.7, 57.1, 70.1, 118.5, 125.7,

127.8, 128.6, 139.7, 153.6, 155.5, 163.6, 174.7. HRMS (EI)

m/z Calcd for C₂₅H₃₆N₂O₄: 428.2675. Found: 428.2673.

0

1.76 (8H, m, C₃–CH₂, C₉–CH₂, C₁₀–CH, C₁₁–CH₂),

0

1.98–2.09 (1H, m, C₂–CH), 3.16–3.33 (2H, m, C₁₂–CH₂),

3.38–3.52 (2H, m, CH₂O), 5.27 (1H, bd-s, NH), 7.35–7.45

(6H, m, ArH), 7.63–7.67 (4H, m, ArH); ¹³C NMR

(67.8 MHz, CHCl₃/CDCl₃) d 12.0, 16.7, 17.6, 19.3, 19.4,

26.9, 27.4, 27.9, 33.2, 37.4, 37.7, 40.7, 43.3, 69.4, 127.4,

129.4, 133.9, 135.5, 176.0. HRMS (EI) m/z Calcd for

C₂₅H₃₆NO₂Si: 410.2515 (M^þ2t-Bu). Found: 410.2537.

5.1.30. (2S)-N-Methyl-N-[(3R,5R)-dimethyl-6-(tert-

butyldiphenylsilyloxy)-hex-1-yl]-2-methylbutyramide

(48). According to general procedure E, the amide 47

(1.83 g, 3.91 mmol) provided the N-methylamide 48 as a

pale brown oil (1.76 g, 93%): [a]²_D⁵¼þ17.8 (c 1.0, CHCl₃);

IR n_m^ne_a^a_x^tcm²¹2930, 1646, 1472, 1464, 1428, 1113, 1090; ¹H

NMR (270 MHz, TMS/CDCl₃), both rotamers unless stated

5.1.33. (4S)-4-Phenyl-3-[(3S,4R,6R)-3,4,6-trimethyl-8-

((2S)-N-methyl-2-methylbutyramido)-octanoyl]-2-oxa-

zolidinone (51). According to general procedure H, the

enimide 50 (172 mg, 0.402 mmol) provided the imide 51 as

a colorless oil (167 mg, 93%): [a]²_D⁴¼þ55.9 (c 1.0, CHCl₃);

IR n

1198, 1136; H NMR (270 MHz, DMSO/DMSO-d₆), both

cm²¹2930, 1782, 1705, 1634, 1458, 1385, 1327,

max

neat

0

otherwise, d 0.83–0.90 (9H, m, C₃–CH₃, C₈–CH₃, C₁₀–

1

CH₃), 1.05 (9H, s, 1 rotamer, (CH₃)₃C), 1.06 (9H, s, 1

0

rotamer, (CH₃)₃C), 1.07 (3H, d, J¼6.7 Hz, 1 rotamer, C₂–

rotamers unless stated otherwise, d 0.74–0.81 (12H, m,

0

CH₃), 1.10 (3H, d, J¼6.7 Hz, 1 rotamer, C₂–CH₃), 1.19–

C₃–CH₃, C₈–CH₃, C₁₀–CH₃), 0.94 (3H, d, J¼6.8 Hz, 1

0

1.55 (4H, m, C₃–CH₂, C₈–CH, C₁₀–CH), 1.58–1.75 (4H,

0

rotamer, C₂–CH₃), 0.96 (3H, d, J¼6.8 Hz, 1 rotamer, C₂–

0

m, C₉–CH₂, C₁₁–CH₂), 2.49–2.61 (1H, m, C₂–CH), 2.90

CH₃), 1.03–1.11 (2H, m, C₉–CH₂), 1.15–1.53 (6H, m,

0

C₃–CH₂, C₈–CH, C₁₀–CH, C₁₁–CH₂), 1.84–1.95 (1H, m,

(3H, s, 1 rotamer, N–CH₃), 2.98 (3H, s, 1 rotamer, N–CH₃),

3.22–3.51 (4H, m, C₁₂–CH₂, CH₂O), 7.35–7.44 (6H, m,

ArH), 7.64–7.66 (4H, m, ArH); ¹³C NMR (67.8 MHz,

CHCl₃/CDCl₃), both rotamers, d 12.1, 12.2, 16.6, 17.2, 17.9,

19.3, 19.4, 26.9, 27.1, 27.5, 28.1, 33.1, 33.2, 33.7, 35.2, 36.9,

37.2, 37.4, 40.6, 40.7, 46.1, 48.0, 69.3, 69.5, 127.4, 129.3,

129.4,133.8,133.9,135.4,175.9,176.2. HRMS(EI)m/zCalcd

for C₂₆H₃₈NO₂Si: 424.2672 (M^þ2t-Bu). Found: 424.2675.

0

C₇–CH), 2.50–2.74 (3H, m, C₂–CH, C₆–CH₂), 2.78 (3H,

s, 1 rotamer, N–CH₃), 2.95 (3H, s, 1 rotamer, N–CH₃),

3.22–3.38 (2H, m, C₁₂–CH₂), 4.14 (1H, dd, J¼8.7, 3.4 Hz,

CH₂O), 4.72 (1H, t, J¼8.7 Hz, Ar–CH), 5.46 (1H, dd,

J¼8.7, 3.4 Hz, CH₂O), 7.26–7.40 (5H, m, ArH); ¹³C NMR

(67.8 MHz, DMSO/DMSO-d₆), the major rotamer, d 11.7,

16.1, 17.1, 18.8, 18.9, 26.6, 27.5, 33.5, 34.1, 34.9, 36.2,

36.7, 38.2, 39.4, 44.9, 56.9, 69.8, 125.5, 127.7, 128.5, 139.7,

153.5, 171.6, 174.5. HRMS (EI) m/z Calcd for C₂₆H₄₀N₂O₄:

444.2988. Found: 444.2983.

5.1.31. (2S)-N-Methyl-N-((3R,5R)-3,5-dimethyl-6-

hydroxy-hex-1-yl)-2-methylbutyramide (49). According

to general procedure F, the N-methylamide 48 (1.72 g,

3.57 mmol) provided the alcohol 49 as a pale yellow oil

(804 mg, 93%): [a]²_D⁴¼þ35.2 (c 1.0, CHCl₃); IR n_m^ne_a^a_x^tcm²¹

3432, 2928, 1626, 1464, 1414, 1379, 1048; ¹H NMR

(270 MHz, TMS/CDCl₃), both rotamers unless stated

5.1.34. (3S,4R,6R)-3,4,6-Trimethyl-8-((2S)-N-methyl-2-

methylbutyramido)octanoic acid (52). According to

general procedure I, the imide 51 (167 mg, 0.375 mmol)

provided the carboxylic acid 52 as a colorless oil (102 mg,

cm²¹3218,

91%): [a]²_D⁵¼þ38.4 (c 1.0, CHCl₃); IR n

neat

max

0

otherwise, d 0.85–0.93 (9H, m, C₃–CH₃, C₈–CH₃, C₁₀–

2964, 1728, 1634, 1456, 1404, 1381, 1246, 1192, 1084; ¹H

NMR (270 MHz, DMSO/DMSO-d₆), both rotamers unless

0

CH₃), 1.08 (3H, d, J¼6.3 Hz, 1 rotamer, C₂–CH₃), 1.11

0

(3H, d, J¼6.3 Hz, 1 rotamer, C₂–CH₃), 1.14–1.29 (2H, m,

0

C₉–CH₂), 1.30–1.60 (4H, m, C₃–CH₂, C₁₁–CH₂), 1.61–

0

stated otherwise, d 0.77–0.82 (12H, m, C₃–CH₃, C₈–CH₃,

0

1.80 (2H, m, C₈–CH, C₁₀–CH), 2.51–2.63 (1H, m, C₂–

0

C₁₀–CH₃), 0.94 (3H, d, J¼6.8 Hz, 1 rotamer, C₂–CH₃),

0

CH), 2.93 (3H, s, 1 rotamer, N–CH₃), 3.01 (3H, s, 1

rotamer, N–CH₃), 3.18–3.45 (4H, m, C₁₂–CH₂, CH₂O);

¹³C NMR (67.8 MHz, CHCl₃/CDCl₃), both rotamers, d

12.0, 12.2, 16.3, 16.4, 17.1, 17.8, 19.2, 19.4, 27.1, 27.4,

27.9, 28.0, 33.1, 33.7, 35.0, 35.3, 37.0, 37.2, 37.4, 40.4,

40.6, 46.1, 48.0, 68.6, 68.7, 176.0, 176.3. HRMS (EI) m/z

Calcd for C₁₄H₂₉NO₂: 243.2199. Found: 243.2204.

0.97 (3H, d, J¼6.8 Hz, 1 rotamer, C₂–CH₃), 1.04–1.09

0

(2H, m, C₉–CH₂), 1.15–1.60 (6H, m, C₃–CH₂, C₈–CH,

C₁₀–CH, C₁₁–CH₂), 1.76–1.95 (2H, m, C₆–CH, C₇–CH),

0

2.17–2.26 (1H, m, C₆–CH), 2.54–2.69 (1H, m, C₂–CH),

2.79 (3H, s, 1 rotamer, N–CH₃), 2.96 (3H, s, 1 rotamer,

N–CH₃), 3.23–3.33 (2H, m, C₁₂–CH₂), 12.0 (1H, bd-s,

COOH); ¹³C NMR (67.8 MHz, DMSO/DMSO-d₆), the

major rotamer, d 11.7, 16.0, 16.4, 17.2, 19.1, 26.7, 27.5,

33.7, 34.6, 34.7, 36.2, 36.7, 37.8, 39.4, 44.9, 174.1, 174.5.

HRMS (EI) m/z Calcd for C₁₇H₃₃NO₃: 299.2460. Found:

299.2450.

5.1.32. (4S)-Phenyl-3-[(4S,6R)-4,6-dimethyl-8-((2S)-N-

methyl-2-methylbutyramido)-(E)-2-octenoyl]-2-oxa-

zolidinone (50). According to general procedure G (using

F. Yokokawa et al. / Tetrahedron 60 (2004) 6859–6880

6875

5.1.35. (4S)-4-Ethenyl-2-[(2S,3R,5R)-2,3,5-trimethyl-7-

((2S)-N-methyl-2-methylbutyramido-heptyl)]oxazoline

(53b). According to general procedure J (using (2S)-N-Boc-

amino-3-butenol ent-32) and procedure K, the carboxylic

acid 52 (61.2 mg, 0.204 mmol) provided the oxazoline 53b

CH, C₁₁–CH₂), 1.71–1.89 (1H, m, C₇–CH), 1.90–2.05

(1H, m, C₆–CH), 2.16–2.28 (1H, m, C₆–CH), 2.54–2.69

0

(1H, m, C₂–CH), 2.79 (3H, s, 1 rotamer, N–CH₃), 2.96

(3H, s, 1 rotamer, N–CH₃), 3.24–3.34 (2H, m, C₁₂–CH₂),

3.84 (1H, app t, J¼8.1 Hz, C₄–CH), 4.31 (1H, app t,

J¼8.4 Hz, C₄–CH), 4.46–4.60 (1H, m, C₃–CH), 5.07 (1H,

d, J¼10.4 Hz, C₁–CH), 5.17 (1H, d, J¼17.1 Hz, C₁–CH),

5.80 (1H, ddd, J¼17.1, 10.6, 6.6 Hz, C1–CH); ¹³C NMR

(67.8 MHz, DMSO/DMSO-d₆), the major rotamer, d 11.7,

15.9, 16.2, 17.2, 19.1, 26.7, 27.5, 31.1, 33.5, 34.9, 35.2, 36.2,

36.7, 44.9, 67.4, 71.1, 115.1, 138.9, 166.3, 174.5. HRMS (EI)

m/z Calcd for C₂₁H₃₈N₂O₂: 350.2933. Found: 350.2934.

as a colorless oil (56.0 mg, 82%, 2 steps): [a]²_D⁶¼213.7 (c

CHCl₃

max

0.11, CHCl₃); IR n

cm²¹2961, 1667, 1647, 1464,

1379, 1196; H NMR (270 MHz, DMSO/DMSO-d₆), both

1

rotamers unless stated otherwise, d 0.75–0.86 (12H, m,

0

C₃–CH₃, C₈–CH₃, C₁₀–CH₃), 0.94 (3H, d, J¼7.3 Hz, 1

0

rotamer, C₂–CH₃), 0.97 (3H, d, J¼7.3 Hz, 1 rotamer, C₂–

CH₃), 1.00–1.14 (2H, m, C₉–CH₂), 1.15–1.61 (6H, m,

0

C₃–CH₂, C₈–CH, C₁₀–CH, C₁₁–CH₂), 1.73–1.89 (1H,

bd-m, C₇–CH), 1.90–2.05 (1H, m, C₆–CH), 2.16–2.29

0

(1H, m, C₆–CH), 2.54–2.69 (1H, m, C₂–CH), 2.79 (3H, s,

5.1.38. (3R,7S,8R,10R,2⁰S)-Kalkitoxin (5). According to

general procedures L and M, the oxazoline 53a (48.4 mg,

0.138 mmol) provided the synthetic (3R,7S,8R,10R,2⁰S)-

1 rotamer, N–CH₃), 2.96 (3H, s, 1 rotamer, N–CH₃), 3.24–

3.32 (2H, m, C₁₂–CH₂), 3.84 (1H, app t, J¼8.1 Hz, C₄–

CH), 4.31 (1H, app t, J¼8.4 Hz, C₄–CH), 4.47–4.60 (1H,

m, C₃–CH), 5.07 (1H, d, J¼10.2 Hz, C₁–CH), 5.18 (1H, d,

J¼17.1 Hz, C₁–CH), 5.80 (1H, ddd, J¼6.6, 10.2, 17.1 Hz,

C₂–CH); ¹³C NMR (67.8 MHz, DMSO/DMSO-d₆), the

major rotamer, d 11.7, 15.9, 16.2, 17.1, 19.1, 26.6, 27.4,

31.1, 33.5, 34.7, 35.3, 36.2, 36.7, 39.4, 44.9, 67.4, 71.1,

115.3, 139.1, 166.6, 174.8. HRMS (EI) m/z Calcd for

C₂₁H₃₈N₂O₂: 350.2933. Found: 350.2940.

kalkitoxin (5) as a pale yellow oil (33.6 mg, 70%, 2 steps):

[a]²_D⁶¼þ77.2 (c 0.83, CHCl₃); IR n

cm²¹2963, 2930,

2876, 1640, 1464, 1412, 1381, 1082; H NMR (270 MHz,

CHCl₃

max

1

DMSO/DMSO-d₆), both rotamers unless stated otherwise, d

0

0.75–0.88 (12H, m, C₃–CH₃, C₈–CH₃, C₁₀–CH₃), 0.95

0

(3H, d, J¼7.1 Hz, C₂–CH₃), 0.97 (3H, d, J¼7.1 Hz, C₂–

CH₃), 1.01–1.17 (2H, m, C₉–CH₂), 1.18–1.63 (6H, m,

0

C₃–CH₂, C₈–CH, C₁₀–CH, C₁₁–CH₂), 1.72–1.90 (1H,

bd-m, C₇–CH), 2.18–2.31 (1H, m, C₆–CH), 2.38–2.46

0

(1H, m, C₆–CH), 2.54–2.70 (1H, m, C₂–CH), 2.79 (3H, s,

5.1.36. (3S,7S,8R,10R,2⁰S)-Kalkitoxin (6). According to

general procedures L and M, the oxazoline 53b (54.1 mg,

0.154 mmol) provided the synthetic (3S,7S,8R,10R,2⁰S)-

1 rotamer, N–CH₃), 2.97 (3H, s, 1 rotamer, N–CH₃), 3.03

(1H, dd, J¼8.3, 10.9 Hz, C₄–CH), 3.25–3.36 (2H, m, C₁₂–

CH₂), 3.48 (1H, dd, J¼8.3, 10.9 Hz, C₄–CH), 4.85–4.98

(1H, m, C₃–CH), 5.10 (1H, d, J¼10.4 Hz, C₁–CH), 5.23

(1H, d, J¼17.1 Hz, C₁–CH), 5.90 (1H, ddd, J¼6.6, 10.4,

17.1 Hz, C₂–CH); ¹³C NMR (67.8 MHz, DMSO/DMSO-

d₆), the major rotamer, d 11.70, 15.98 (CH₃£2), 17.13,

19.04, 26.64, 27.47, 33.41, 34.71, 34.92, 36.20, 36.66,

37.47, 37.86, 39.40, 44.90, 77.88, 115.21, 138.00, 169.21,

174.80. HRMS (EI) m/z Calcd for C₂₁H₃₈N₂OS: 366.2705.

Found: 366.2710.

kalkitoxin (6) as a pale yellow oil (33.7 mg, 61%, 2 steps):

[a]²_D⁶¼27.5 (c 0.80, CHCl₃); IR n

cm²¹2963, 2928,

2874, 1646, 1464, 1412, 1381, 1082; H NMR (270 MHz,

CHCl₃

max

1

DMSO/DMSO-d₆), both rotamers unless stated otherwise, d

0

0.75–0.88 (12H, m, C₃–CH₃, C₈–CH₃, C₁₀–CH₃), 0.95

0

(3H, d, J¼7.1 Hz, C₂–CH₃), 0.97 (3H, d, J¼7.1 Hz, C₂–

CH₃), 1.01–1.16 (2H, m, C₉–CH₂), 1.17–1.62 (6H, m,

0

C₃–CH₂, C₈–CH, C₁₀–CH, C₁₁–CH₂), 1.73–1.89 (1H,

bd-m, C₇–CH), 2.17–2.30 (1H, m, C₆–CH), 2.40–2.47

0

(1H, m, C₆–CH), 2.55–2.68 (1H, m, C₂–CH), 2.79 (3H, s,

5.1.39. (2R,3R,4S)-1-Benzyloxy-2,4-dimethyl-3,6-hexa-

nediol (16). To a solution of the benzyloxyhexene 15⁷

(101 mg, 0.431 mmol) in THF (2 mL) was added 9-bora-

bicyclononane dimer (322 mg, 1.29 mmol). The resulting

solution was stirred for 10 min, and then placed in a water

bath and sonicated for 40 min. Aqueous NaOH solution

(4 N, 1 mL) and 30% aqueous H₂O₂(1 mL) were added

sequentially at 25 8C. The resulting mixture was diluted

with water and extracted with EtOAc (£2). The combined

organic extracts were washed with 1 M aqueous KHSO₄and

brine. The organic layer was dried (Na₂SO₄), ﬁltered, and

concentrated. The residue was puriﬁed by silica gel column

chromatography (BW-200, hexane/EtOAc¼1:1) to afford

the desired product 16 as a colorless oil (99 mg, 91%):

[a]²_D⁴¼þ7.9 (c 1.1, CHCl₃); IR n_m^ne_a^a_x^tcm²¹3367 (bd), 2875,

1454, 1363, 1089; ¹H NMR (270 MHz, TMS/CDCl₃) d 0.86

(3H, d, J¼6.7 Hz, C₈–CH₃), 0.98 (3H, d, J¼6.7 Hz, C₁₀–

CH₃), 1.54–1.96 (4H, m, C₇–CH₂, C₈–CH, C₁₀–CH),

3.49–3.64 (4H, m, CH₂O, CH₂OH), 3.71–3.79 (1H, m, C₉–

CH), 4.49 (1H, d, J¼11.9 Hz, Ar–CH₂), 4.54 (1H, d,

J¼11.9 Hz, Ar–CH₂), 7.28–7.38 (5H, m, ArH); ¹³C NMR

(67.8 MHz, CHCl₃/CDCl₃) d 9.5, 17.5, 34.7, 34.9, 37.9,

61.2, 73.5, 76.5, 78.5, 127.5, 127.7, 128.4, 137.8. Anal.

Calcd for C₁₅H₂₄O₃: C, 71.39; H, 9.59. Found: C, 71.14; H,

9.66.

1 rotamer, N–CH₃), 2.97 (3H, s, 1 rotamer, N–CH₃), 3.02

(1H, dd, J¼8.0, 10.9 Hz, C₄–CH), 3.25–3.39 (2H, m, C₁₂–

CH₂), 3.48 (1H, dd, J¼8.0, 10.9 Hz, C₄–CH), 4.85–4.97

(1H, m, C₃–CH), 5.10 (1H, d, J¼10.4 Hz, C₁–CH), 5.23

(1H, d, J¼17.1 Hz, C₁–CH), 5.90 (1H, ddd, J¼6.4, 10.4,

17.1 Hz, C₂–CH); ¹³C NMR (67.8 MHz, DMSO/DMSO-

d₆), the major rotamer, d 11.69, 16.01(CH₃£2), 17.13,

19.08, 26.63, 27.47, 33.38, 34.70, 34.91, 36.19, 36.67,

37.48, 37.84, 39.40, 44.87, 77.90, 115.25, 137.96, 169.18,

174.80. HRMS (EI) m/z Calcd for C₂₁H₃₈N₂OS: 366.2705.

Found: 366.2693.

5.1.37. (4R)-4-Ethenyl-2-[(2S,3R,5R)-2,3,5-trimethyl-7-

((2S)-N-methyl-2-methylbutyramido-heptyl)]oxazoline

(53a). According to general procedures J and K, the

carboxylic acid 52 (57.8 mg, 0.193 mmol) provided the

oxazoline 53a as

[a]²_D⁵¼þ95.4 (c 0.89, CHCl₃); IR n

a colorless oil (52.0 mg, 82%):

cm²¹2964,

CHCl₃

max

1660, 1645, 1464, 1381, 1196; ¹H NMR (270 MHz,

DMSO/DMSO-d₆), both rotamers unless stated otherwise,

0

d 0.75–0.86 (12H, m, C₃–CH₃, C₈–CH₃, C₁₀–CH₃), 0.94

0

(3H, d, J¼7.0 Hz, 1 rotamer, C₂–CH₃), 0.97 (3H, d,

0

J¼7.0 Hz, 1 rotamer, C₂–CH₃), 1.00–1.15 (2H, m,

0

C₉–CH₂), 1.16–1.61 (6H, m, C₃–CH₂, C₈–CH, C₁₀–

6876

F. Yokokawa et al. / Tetrahedron 60 (2004) 6859–6880

5.1.40. (2R,3R,4S)-1-Benzyloxy-2,4-dimethyl-6-(tert-

butyldiphenylsilyloxy)-3-hexanol (17). To a solution of

the diol 16 (1.08 g, 4.29 mmol) in DMF (20 mL) was added

tert-butyldiphenylsilyl chloride (1.3 mL, 4.72 mmol),

imidazole (585 mg, 8.58 mmol) at 0 8C. After 5 min, the

cooling bath was removed, and the reaction mixture was

stirred at room temperature for 2 h. The mixture was diluted

with ether, and washed with water (£5), 1 M aqueous

KHSO₄, and brine. The organic layer was dried (MgSO₄),

ﬁltered, and concentrated. The residue was puriﬁed by silica

gel column chromatography (BW-820MH, hexane/

EtOAc¼9:1–4:1–2:1) to afford the desired product 17 as

a colorless oil (1.65 g, 78%): [a]²_D⁵¼þ3.9 (c 1.1, CHCl₃); IR

colorless oil (128 mg, 68%): [a]²_D⁶¼23.3 (c 1.0, CHCl₃); IR

cm²¹2857, 1495, 1427, 1361, 1207, 1111; H NMR

neat

max

1

n

(270 MHz, TMS/CDCl₃) d 0.80 (3H, d, J¼6.6 Hz, C₈–

CH₃), 0.89 (3H, d, J¼6.6 Hz, C₁₀–CH₃), 1.04 (s, 9H,

(CH₃)₃C), 1.10–1.62 (4H, m, C₇–CH₂,C₉–CH₂), 1.65–

1.95 (2H, m, C₈–CH, C₁₀–CH), 3.17–3.32 (2H, m, CH₂O),

3.62–3.74 (2H, m, CH₂O), 4.46 (1H, d, J¼12.2 Hz, Ar–

CH₂), 4.51 (1H, d, J¼12.2 Hz, Ar–CH₂), 7.27–7.69 (15H,

m, ArH); ¹³C NMR (67.8 MHz, CHCl₃/CDCl₃) d 17.2, 19.6,

19.7, 26.8, 27.2, 31.2, 40.9, 41.4, 62.4, 73.3, 76.9, 127.7,

127.8, 127.9, 128.6, 129.8, 134.5, 135.9, 139.2. Anal. Calcd

for C₃₁H₄₂O₂Si: C, 78.43; H, 8.92. Found: C, 78.37; H, 8.90.

neat

max

1

n

cm²¹3453 (bd), 2858, 1496, 1427, 1111, 1089; H

5.1.43. (3R,5S)-6-Benzyloxy-3,5-dimethylhexanol (20).

To a solution of the deoxyganated product 19 (90 mg,

0.189 mmol) in THF (1 mL) was added TBAF (100 mg,

0.380 mmol) at 0 8C. After 15 min, the cooling bath was

removed, and the reaction mixture was stirred at room

temperature for 35 min. The reaction mixture was diluted

with EtOAc, and washed with water (£2) and brine. The

organic layer was dried (Na₂SO₄), ﬁltered, and concen-

trated. The residue was puriﬁed by silica gel column

chromatography (BW-200, hexane/EtOAc¼4:1) to afford

NMR (270 MHz, TMS/CDCl₃) d 0.81 (3H, d, J¼6.9 Hz,

C₈–CH₃), 0.95 (3H, d, J¼6.9 Hz, C₁₀–CH₃), 1.05 (s, 9H,

(CH₃)₃C), 1.45–2.04 (4H, m, C₇–CH₂, C₈–CH, C₁₀–CH),

3.46–3.82 (5H, m, CH₂O £2, C₉–CH), 4.51 (2H, s, Ar–

CH₂), 7.27–7.73 (15H, m, ArH); ¹³C NMR (67.8 MHz,

CHCl₃/CDCl₃) d 9.8, 16.5, 19.1, 26.8, 33.7, 35.3, 35.8, 62.3,

73.3, 75.0, 76.8, 127.5, 127.6, 127.7, 128.3, 128.4, 129.6,

133.6, 134.8, 135.6, 138.4. Anal. Calcd for C₃₁H₄₂O₃Si: C,

75.87; H, 8.63. Found: C, 75.70; H, 8.66.

the desired product 20 as a colorless oil (39 mg, 87%):

neat

max

5.1.41. (2R,3R,4S)-O-Phenyl-3-[1-benzyloxy-2,4-

dimethyl-6-(tert-butyldiphenylsilyloxy)-hexyl]thiocar-

bonate (18). To a solution of the alcohol 17 (297 mg,

0.606 mmol) in THF (19 mL) under argon at 278 8C was

added n-BuLi (0.46 mL, 0.727 mmol, 1.6 M in hexane)

dropwise via syringe. The solution was stirred at 278 8C for

5 min, then phenyl chlorothionoformate (0.10 mL,

0.727 mmol) was added dropwise. After 15 min, the cooling

bath was removed, and the reaction mixture was allowed to

warm to room temperature over 1 h. Cold (0 8C) water was

then added. The mixture was diluted with ether and washed

with 1 M aqueous KHSO₄, saturated aqueous NaHCO₃, and

brine. The solution was dried (MgSO₄), ﬁltered, and

concentrated. The residue was puriﬁed by silica gel column

chromatography (BW-200, hexane/EtOAc¼12:1) to afford

[a]²_D⁶¼211.0 (c 1.0, CHCl₃); IR n

cm²¹3368 (bd),

1

2928, 1454, 1377, 1096, 1074; H NMR (270 MHz, TMS/

CDCl₃) d 0.88 (3H, d, J¼6.7 Hz, C₈–CH₃), 0.91 (3H, d,

J¼6.6 Hz, C₁₀–CH₃), 1.05–1.36 (2H, m, C₉–CH₂), 1.36–

1.65 (3H, m, C₇–CH₂, OH), 1.82–1.95 (1H, m, C₁₀–CH),

3.24 (1H, dd, J¼6.6, 8.9 Hz, CH₂O), 3.30 (1H, dd, J¼6.6,

8.9 Hz, CH₂O), 3.62–3.69 (2H, m, CH₂OH), 4.50 (2H, s,

Ar–CH₂), 7.27–7.35 (5H, m, ArH); ¹³C NMR (67.8 MHz,

CHCl₃/CDCl₃) d 16.9, 19.3, 26.6, 30.8, 40.6, 41.2, 60.9,

72.9, 76.5, 127.4, 127.5, 128.3, 138.7. Anal. Calcd for

C₁₅H₂₄O₂: C, 76.23; H, 10.24. Found: C, 75.94; H, 10.43.

5.1.44. (2S,4R)-5-Azido-1-benzyloxy-2,4-dimethylhexane

(21). To a solution of the alcohol 20 (1.04 g, 4.40 mmol) in

CH₂Cl₂(20 mL) was added triethylamine (1.2 mL,

the desired product 18 as a yellow oil (330 mg, 87%):

8.8 mmol),

methanesulfonyl

chloride

(0.51 mL,

[a]²_D⁵¼29.6 (c 1.0, CHCl₃); IR n

cm²¹2858, 1740,

6.6 mmol), DMAP (16 mg, 0.132 mmol) at 0 8C. After

15 min, the cooling bath was removed, and the reaction

mixture was stirred at room temperature for 11 h. The

mixture was diluted with EtOAc, and washed with 1 M

aqueous KHSO₄, saturated aqueous NaHCO₃, and brine.

The organic layer was dried (Na₂SO₄), ﬁltered, and

concentrated to give the crude mesylate as a pale yellow

oil (1.38 g). This intermediate was used in the next reaction

without further puriﬁcation.

neat

max

1489, 1282, 1197, 1111; ¹H NMR (270 MHz, TMS/CDCl₃)

d 0.90 (3H, d, J¼6.7 Hz, C₈–CH₃), 1.01 (3H, d, J¼6.7 Hz,

C₁₀–CH₃), 1.06 (s, 9H, (CH₃)₃C), 1.25–1.45 (1H, m,

C₈–CH), 1.82–1.98 (1H, m, C₁₀–CH), 2.12–2.30 (2H, m,

C₇–CH₂), 3.34–3.47 (2H, m, CH₂O), 3.63–3.77 (2H,

m, CH₂O), 4.43 (1H, d, J¼11.6 Hz, Ar–CH₂), 4.51 (1H, d,

J¼11.6 Hz, Ar–CH₂), 5.52 (1H, dd, J¼4.3, 6.9 Hz, C₉–

CH), 7.00–7.68 (20H, m, ArH); ¹³C NMR (67.8 MHz,

CHCl₃/CDCl₃) d 11.7, 15.9, 19.2, 26.9, 31.8, 34.5, 35.6,

61.7, 72.6, 73.3, 90.1, 121.9, 126.3, 127.5, 127.6, 127.7,

128.3, 129.4, 129.6, 133.8, 133.9, 135.5, 138.3, 153.4,

195.6. Anal. Calcd for C₃₈H₄₆O₄SSi: C, 72.80; H, 7.40.

Found: C, 72.62; H, 7.32.

To a solution of the above mesylate in DMF (15 mL) at

room temperature was added sodium azide (855 mg,

13.2 mmol), and then the mixture was heated to 50 8C.

After 2 h, the resulting mixture was diluted with ether, and

washed with water (£5) and brine. The organic layer was

dried (MgSO₄), ﬁltered, and concentrated. The residue was

puriﬁed by silica gel column chromatography (BW-820MH,

hexane/EtOAc¼12:1) to afford the desired product 21 as a

colorless oil (1.01 mg, 88%): [a]²_D⁶¼28.2 (c 1.0, CHCl₃);

5.1.42. (2S,4R)-1-Benzyloxy-6-(tert-butyldiphenylsilyl-

oxy)-2,4-dimethylhexane (19). Under argon, the thio-

carbonate 18 (247 mg, 0.394 mmol) was dissolved in

n-Bu₃SnH (1 mL) and AIBN (3 mg) was added. The

mixture was heated to 100 8C. After 20 min, the reaction

mixture was cooled to room temperature and then puriﬁed

by silica gel column chromatography (BW-200, hexane–

hexane/Et₂O¼20:1) to afford the desired product 19 as a

neat

max

1

IR n

cm²¹2925, 2096, 1728, 1454, 1265, 1099; H

NMR (270 MHz, TMS/CDCl₃) d 0.88 (3H, d, J¼7.3 Hz,

C₈–CH₃), 0.91 (3H, d, J¼6.9 Hz, C₁₀–CH₃), 1.03–1.31

(2H, m, C₉–CH₂), 1.35–1.72 (3H, m, C₇–CH₂, C₈–CH),

F. Yokokawa et al. / Tetrahedron 60 (2004) 6859–6880

6877

1.79–1.95 (1H, m, C₁₀–CH), 3.20–3.34 (4H, m, CH₂O,

CH₂N₃), 4.50 (2H, s, Ar–CH₂), 7.33 (5H, s, ArH); ¹³C

NMR (67.8 MHz, CHCl₃/CDCl₃) d 16.8, 18.9, 27.5, 30.8,

36.5, 40.8, 49.4, 72.9, 76.5, 127.4, 127.5, 128.3, 138.7.

Anal. Calcd for C₁₅H₂₃N₃O: C, 68.93; H, 8.87; N, 16.08.

Found: C, 69.01; H, 8.83; N, 15.77.

3.47 (4H, m, C₁₂–CH₂, CH₂O), 4.44 (2H, s, Ar–CH2),

7.22–7.37 (5H, m, ArH); ¹³C NMR (67.8 MHz, CHCl₃/

CDCl₃), both rotamers, d 11.6, 11.8, 16.7, 17.1, 17.8, 19.1,

26.7, 27.0, 27.1, 27.3, 30.3, 33.1, 34.4, 34.5, 36.1, 36.2,

36.4, 40.6, 41.1, 44.7, 47.1, 71.9, 75.6, 75.7, 127.3, 128.2,

128.6, 138.7, 174.8, 175.0. Anal. Calcd for C₂₁H₃₅NO₂: C,

75.63; H, 10.58; N, 4.20. Found: C, 75.45; H, 10.65; N, 4.23.

5.1.45. (2S)-N-((3R,5S)-3,5-Dimethyl-6-benzyloxy-

hexyl)-2-methylbutyramide (23). To a solution of the

azide 21 (248 mg, 0.950 mmol) in EtOH (10 mL) was added

Lindlar catalyst (5% Pd on carbon/Pb–CaCO₃) (100 mg) at

room temperature. The black slurry was stirred under 1 atm

H₂for 1 h. The reaction mixture was ﬁltered through a pad

of celite (EtOAc rinse) and the ﬁltrate was concentrated to

give the crude amine as a yellow oil (228 mg). This

intermediate was used in the next reaction without further

puriﬁcation.

5.1.47. (2S)-N-Methyl-N-((3R,5S)-3,5-dimethyl)-6-hexa-

nol (25). To a solution of the amide 24 (91 mg,

0.269 mmol) in EtOH (1.5 mL) was added 5% Pd on

carbon (40 mg) at room temperature. The black slurry was

stirred under 1 atm H₂for 1 h. The reaction mixture was

ﬁltered through a pad of celite (EtOAc rinse) and the ﬁltrate

was concentrated. The residue was puriﬁed by silica gel

column chromatography (BW-8280 MH, hexane/

EtOAc¼1:1–EtOAc) to afford the desired product 25 as a

colorless oil (67 mg, quant.): [a]²_D⁹¼þ8.2 (c 1.0, CHCl₃); IR

CHCl₃

max

To a solution of the above amine and (S)-2-methylbutyric

acid (22) (0.11 mL, 0.97 mmol) in DMF (2 mL) at 0 8C was

successively added diethyl phosphorocyanidate (0.18 mL,

1.07 mmol) and triethylamine (0.27 mL, 1.94 mmol). After

15 min, the cooling bath was removed, and the reaction

mixture was stirred at room temperature for 4 h. The

mixture was diluted with EtOAc, and washed with water

(£5) and brine. The organic layer was dried (Na₂SO₄),

ﬁltered, and concentrated. The residue was puriﬁed by silica

gel column chromatography (BW-820MH, hexane/

EtOAc¼2:1) to afford the desired product 23 as a pale

yellow oil (298 mg, 97%): [a]²_D⁵¼21.1 (c 1.0, CHCl₃); IR

nⁿ_m^e_a^a_x^tcm²¹3296, 2874, 1645, 1556, 1454, 1377, 1236, 1101;

¹H NMR (270 MHz, TMS/CDCl₃) d 0.86–0.91 (9H, m,

n

cm²¹3410 (bd), 2874, 1628, 1464, 1416, 1379,

1045; ¹H NMR (270 MHz, TMS/CDCl₃), both rotamers

0

unless stated otherwise, d 0.76–0.87 (9H, m, C₃–CH₃, C₈–

0

CH₃, C₁₀–CH₃), 0.93–0.99 (3H, m, C₂–CH₃), 1.07–1.62

0

(8H, m, C₉–CH₂, C₃–CH₂, C₁₁–CH₂, C₈–CH, C₁₀–CH),

0

2.51–2.67 (1H, m, C₂–CH), 2.79 (3H, s, 1 rotamer,

N–CH₃), 2.96 (3H, s, 1 rotamer, N–CH₃), 3.12–3.49 (4H,

m, C₁₂–CH₂, CH₂O), 4.33–4.41 (1H, bd-m, OH); ¹³C NMR

(67.8 MHz, CHCl₃/CDCl₃), both rotamers, d 11.6, 11.8,

16.4, 16.5, 17.1, 17.8, 19.2, 26.7, 27.0, 27.2, 27.4, 32.8,

33.1, 34.6, 34.7, 36.1, 36.2, 36.6, 40.4, 40.8, 44.8, 47.2,

66.8, 66.9, 174.8, 175.0. Anal. Calcd for C₁₄H₂₉NO₂: C,

69.09; H, 12.01; N, 5.75. Found: C, 68.81; H, 12.07; N, 5.66.

0

C₈–CH₃, C₁₀–CH₃, C₃–CH₃), 1.11(3H, d, J¼6.6 Hz, C₂–

5.1.48. (4R)-Phenyl-3-[(4R,6R)-4,6-dimethyl-8-((2S)-N-

methyl-2-methylbutyramido)-(E)-2-octenoyl]-2-oxa-

zolidinone (27a). According to general procedure G, the

alcohol 25 (67 mg, 0.275 mmol) provided the enimide 27a

CH₃), 1.19–1.70 (7H, m, C₉–CH₂, C₁₀–CH, C₁₁–CH₂),

0

1.82–1.91(1H, m, C₈–CH), 1.99–2.07 (1H, m, C₂–CH),

3.23–3.32 (4H, m, CH₂O, C₁₂–CH₂), 4.50 (2H, s, Ar–

CH₂), 5.50 (1H, bd-s, NH), 7.27–7.35 (5H, s, ArH); ¹³C

NMR (67.8 MHz, CHCl₃/CDCl₃) d 12.2, 17.2, 17.9, 19.5,

27.6, 28.1, 31.1, 37.6, 37.9, 41.3, 43.6, 73.3, 76.9, 127.7,

127.8, 128.6, 139.0, 176.6. Anal. Calcd for C₂₀H₃₃NO₂: C,

75.19; H, 10.41; N, 4.38. Found: C, 75.11; H, 10.28; N, 4.37.

as a colorless oil (104 mg, 85%): [a]²_D⁵¼230.7 (c 1.1,

CHCl₃

max

CHCl₃); IR n

cm²¹2964, 1770, 1687, 1633, 1456,

1383, 1197; H NMR (270 MHz, DMSO/DMSO-d₆), both

1

0

rotamers unless stated otherwise, d 0.65–0.87 (9H, m, C₃–

0

CH₃, C₈–CH₃, C₁₀–CH₃), 0.91–0.99 (3H, m, C₂–CH₃),

1.13–1.38 (5H, m, C₉–CH₂, C₁₀–CH, C₁₁–CH₂), 1.39–

0

5.1.46. (2S)-N-Methyl-N-((3R,5S)-3,5-dimethyl-6-benzyl-

oxy-hexyl)-2-methylbutyramide (24). To a solution of the

amide 23 (108 mg, 0.333 mmol) in THF (1.5 mL) under

argon at 0 8C was added NaH (133 mg, 3.33 mmol). The

solution was stirred at 278 8C for 20 min, then CH₃I

(0.3 mL, 4.82 mmol) was added. After 10 min, the cooling

bath was removed, and the reaction mixture was stirred at

room temperature. After 20 h, water was added to the

mixture, which was diluted with EtOAc, and washed with

water, 1 M aqueous KHSO₄, and brine. The organic layer

was dried (Na₂SO₄), ﬁltered, and concentrated. The residue

was puriﬁed by silica gel column chromatography (BW-

820MH, hexane/EtOAc¼2:1) to afford the desired product

24 as a pale yellow oil (106 mg, 94%): [a]²_D⁸¼þ9.9 (c 1.0,

CHCl₃); IR nⁿ_m^e_a^a_x^tcm²¹2874, 1643, 1454, 1410, 1375, 1099;

¹H NMR (270 MHz, TMS/CDCl₃), both rotamers unless

1.58 (2H, m, C₃–CH₂), 2.39–2.60 (2H, m, C₂–CH, C₈–

H), 2.77 (3H, s, 1 rotamer, N–CH₃), 2.89 (3H, s, 1 rotamer,

N–CH₃), 3.05–3.16 (2H, m, 1 rotamer, C₁₂–CH₂), 3.18–

3.35 (2H, m, 1 rotamer, C₁₂–CH₂), 4.16 (1H, dd, J¼3.6,

8.7 Hz, CH₂O), 4.75 (1H, t, J¼8.7 Hz, Ar–CH), 5.49 (1H,

dd, J¼3.6, 8.7 Hz, CH₂O), 6.81 (1H, ddd, J¼2.3, 7.9,

15.3 Hz, C₆–CH), 7.15 (1H, d, J¼15.3 Hz, C₇–CH), 7.27–

7.40 (5H, m, ArH); ¹³C NMR (67.8 MHz, DMSO/DMSO-

d₆), the major rotamer, d 11.5, 17.1, 19.1, 19.5, 26.7, 27.4,

33.2, 33.8, 34.4, 36.2, 39.4, 42.8, 57.1, 70.1, 118.7, 125.9,

128.0, 128.8, 139.9, 153.7, 155.6, 163.8, 174.8. Anal. Calcd

For C₂₅H₃₆N₂O₄: C, 70.06; H, 8.47; N, 6.54. Found: C,

69.92; H, 8.74; N, 6.34.

5.1.49. (4R)-4-Phenyl-3-[(3R,4S,6R)-3,4,6-trimethyl-8-

((2S)-N-methyl-2-methylbutyramido)-octanoyl]-2-oxa-

zolidinone (30a). According to general procedure H, the

enimide 27a (37 mg, 0.0864 mmol) provided the imide 30a

as a colorless oil (41 mg, quant.): [a]²_D⁵¼235.2 (c 1.0,

CHCl₃); IR nⁿ_m^e_a^a_x^tcm²¹2963, 1782, 1705, 1639, 1456, 1385,

0

stated otherwise, d 0.75–0.87 (9H, m, C₃–CH₃, C₈–CH₃,

0

C₁₀–CH₃), 0.93–0.98 (3H, m, C₂–CH₃), 1.06–1.57 (7H,

0

m, C₃–CH₂, C₉–CH₂, C₈–CH, C₁₁–CH₂), 1.71–1.85 (1H,

0

m, C₁₀–CH), 2.51–2.67 (1H, m, C₂–CH), 2.78 (3H, s, 1

rotamer, N–CH₃), 2.93 (3H, s, 1 rotamer, N–CH₃), 3.12–

1

1323, 1197; H NMR (270 MHz, DMSO/DMSO-d₆), both

6878

F. Yokokawa et al. / Tetrahedron 60 (2004) 6859–6880

rotamers unless stated otherwise, d 0.71–0.82 (12H, m,

0

d, J¼10.2 Hz, C₁–CH), 5.17 (1H, d, J¼16.8 Hz, C₁–CH),

5.80 (1H, ddd, J¼17.2, 10.6, 6.9 Hz, C₂–CH); ¹³C NMR

(67.8 MHz, DMSO/DMSO-d₆), the major rotamer, d 11.6,

16.0, 16.3, 17.1, 19.0, 26.7, 27.3, 31.1, 33.5, 34.8, 35.3,

36.2, 36.8, 39.4, 44.7, 67.4, 115.3, 139.1, 166.6, 174.8.

HRMS (EI) m/z Calcd for C₂₁H₃₈N₂O₂: 350.2933. Found:

350.2935.

C₃–CH₃, C₈–CH₃, C₁₀–CH₃), 0.94 (3H, d, J¼6.6 Hz, 1

0

rotamer, C₂–CH₃), 0.96 (3H, d, J¼6.6 Hz, 1 rotamer, C₂–

CH₃), 1.02–1.12 (2H, m, C₉–CH₂), 1.16–1.59 (6H, m,

0

C₃–CH₂, C₈–CH, C₁₀–CH, C₁₁–CH₂), 1.82–1.98 (1H, m,

0

C₇–CH), 2.50–2.76 (3H, m, C₂–CH, C₆–CH₂), 2.78 (3H,

s, 1 rotamer, N–CH₃), 2.95 (3H, s, 1 rotamer, N–CH₃),

3.11–3.42 (2H, m, C₁₂–CH₂), 4.13 (1H, dd, J¼8.6, 3.6 Hz,

CH₂O), 4.72 (1H, t, J¼8.6 Hz, Ar–CH), 5.46 (1H, dd,

J¼8.6, 3.6 Hz, CH₂O), 7.26–7.41 (5H, m, ArH); ¹³C NMR

(67.8 MHz, DMSO/DMSO-d₆), the major rotamer, d 11.9,

16.4, 17.4, 18.0, 19.1, 27.0, 27.6, 33.8, 34.4, 34.9, 36.4,

36.5, 37.1, 39.4, 45.0, 57.3, 70.2, 126.0, 128.2, 129.0, 140.3,

154.0, 172.1, 175.1. Anal. Calcd For C₂₆H₄₀N₂O₄z or

1/5H₂O: C, 69.67; H, 9.09; N, 6.25. Found: C, 69.81; H,

9.14; N, 6.24.

5.1.52. (3R,7R,8S,10R,2⁰S)-Kalkitoxin (1). According to

general procedures L and M, the oxazoline 34a (37.0 mg,

0.105 mmol) provided the synthetic (3R,7R,8S,10R,2⁰S)-

kalkitoxin (1) as a pale yellow oil (18.9 mg, 50%, 2 steps):

[a]²_D⁷¼þ15.5 (c 0.75, CHCl₃); IR n

cm²¹2963, 2928,

2874, 1646, 1464, 1412, 1379, 1084; H NMR (270MHz,

CHCl₃

max

1

DMSO/DMSO-d₆), both rotamers unless stated otherwise, d

0

0.76–0.86 (12H, m, C₃–CH₃, C₈–CH₃, C₁₀–CH₃), 0.94

0

(3H, d, J¼6.9 Hz, 1 rotamer, C₂–CH₃), 0.97 (3H, d,

0

5.1.50. (3R,4S,6R)-3,4,6-Trimethyl-8-((2S)-N-methyl-2-

methylbutyramido)octanoic acid (31a). According to

general procedure I, the imide 30a (84 mg, 0.189 mmol)

provided the carboxylic acid 31a as a colorless oil (50 mg,

88%): [a]²_D⁵¼23.9 (c 1.1, CHCl₃); IR n_m^ne_a^a_x^tcm²¹3278 (bd),

2964, 1728, 1634, 1456, 1379, 1247, 1184, 1102; ¹H NMR

(270 MHz, DMSO/DMSO-d₆), both rotamers unless stated

J¼6.9 Hz, 1 rotamer, C₂–CH₃), 1.01–1.15 (2H, m, C₉–

0

CH₂), 1.17–1.63 (6H, m, C₃–CH₂, C₈–CH, C₁₀–CH, C₁₁–

CH₂), 1.71–1.90 (1H, bd–m, C₇–CH), 2.17–2.29 (1H, m,

C₆–CH₂), 2.39–2.46 (1H, m, C₆–CH₂), 2.53–2.71 (1H, m,

0

C₂–CH), 2.79 (3H, s, 1 rotamer, N–CH₃), 2.96 (3H, s, 1

rotamer, N–CH₃), 3.02 (1H, dd, J¼8.3, 11.2 Hz, C₄–CH),

3.11–3.31 (2H, m, C₁₂–CH₂), 3.45 (1H, dd, J¼8.3,

11.2 Hz, C₄–CH), 4.84–4.96 (1H, m, C₃–CH), 5.10 (1H,

d, J¼10.9 Hz, C₁–CH), 5.23 (1H, d, J¼16.8 Hz, C₁–CH),

5.90 (1H, ddd, J¼6.6, 10.9, 16.8 Hz, C₂–CH); ¹³C NMR

(67.8MHz, DMSO/DMSO-d₆), the major rotamer, d 11.65,

16.07 (CH₃£2), 17.11, 19.00, 26.69, 27.35, 33.43, 34.59,

34.85, 36.19, 36.70, 37.49, 37.87, 39.39, 44.69, 77.90,

115.26, 137.95, 169.24, 174.81. HRMS (EI) m/z Calcd for

C₂₁H₃₈N₂OS: 366.2705. Found: 366.2715.

0

otherwise, d 0.73–0.86 (12H, m, C₃–CH₃, C₈–CH₃, C₁₀–

0

CH₃), 0.94 (3H, d, J¼7.1 Hz, 1 rotamer, C₂–CH₃), 0.97

0

(3H, d, J¼7.1 Hz, 1 rotamer, C₂–CH₃), 1.02–1.12 (2H, m,

0

C₉–CH₂), 1.17–1.59 (6H, m, C₃–CH₂, C₈–CH, C₁₀–CH,

C₁₁–CH₂), 1.72–1.98 (2H, m, C₆–CH, C₇–CH), 2.16–

0

2.27 (1H, m, C₆–CH), 2.52–2.71 (1H, m, C₂–CH), 2.79

(3H, s, 1 rotamer, N–CH₃), 2.96 (3H, s, 1 rotamer, N–CH₃),

3.24–3.42 (2H, m, C₁₂–CH₂), 12.0 (1H, bd-s, COOH); ¹³

C

NMR (67.8 MHz, DMSO/DMSO-d₆), the major rotamer, d

11.6, 15.9, 16.4, 17.1, 19.0, 26.7, 27.4, 33.7, 34.6, 34.8,

36.2, 36.8, 37.8, 39.4, 44.7, 174.4, 174.8. Anal. Calcd For

C₁₇H₃₃NO₃z or 1/4H₂O: C, 67.18; H, 11.11; N, 4.61. Found:

C, 67.16; H, 11.20; N, 4.63.

5.1.53. (4S)-Phenyl-3-[(4R,6R)-4,6-dimethyl-8-((2S)-N-

methyl-2-methylbutyramido)-(E)-2-octenoyl]-2-oxa-

zolidinone (27b). According to general procedure G (using

(S)-phosphonate ent-26), the alcohol 25 (131 mg,

0.538 mmol) provided the enimide 27b as a colorless oil

(188 mg, 82%): [a]_D²⁴¼þ93.9 (c 1.0, CHCl₃); IR n

CHCl₃

max

5.1.51. (4R)-4-Ethenyl-2-[(2R,3S,5R)-2,3,5-trimethyl-7-

((2S)-N-methyl-2-methylbutyramido)-heptyl]oxazoline

(34a). According to general procedure J, the carboxylic acid

31a (60.1 mg, 0.201 mmol) provided the amide 33a as a

colorless oil (68.4 mg, 92%), which was directly used for

the next step. To a solution of amide (61.0 mg, 0.166 mmol)

in THF (1 mL) was added Burgess reagent (83 mg,

0.662 mmol) under argon. The mixture was heated to

70 8C. After 2 h, the reaction mixture was cooled to room

temperature and concentrated. The residue was puriﬁed by

silica gel column chromatography (BW-200, hexane/

EtOAc¼1:3) to afford the desired oxazoline 34a as a

colorless oil (37.8 mg, 65%): [a]²_D⁸¼þ41.6 (c 0.68, CHCl₃);

cm²¹2960, 1779, 1688, 1634, 1458, 1383, 1200; ¹H NMR

(270 MHz, DMSO/DMSO-d₆), both rotamers unless stated

0

otherwise, d 0.72–0.91 (9H, m, C₃–CH₃, C₈–CH₃, C₁₀–

0

CH₃), 0.92–1.01 (3H, m, C₂–CH₃), 1.15–1.38 (5H, m,

0

C₉–CH₂, C₁₀–CH, C₁₁–CH₂), 1.39–1.61 (2H, m, C₃–

0

CH₂), 2.40–2.66 (2H, m, C₂–CH, C₈–CH), 2.79 (3H, s, 1

rotamer, N–CH₃), 2.95 (3H, s, 1 rotamer, N–CH₃), 3.15–

3.42 (2H, m, C₁₂–CH₂), 4.18 (1H, dd, J¼3.4, 8.7 Hz,

CH₂O), 4.76 (1H, t, J¼8.7 Hz, Ar–CH), 5.51 (1H, dd,

J¼3.4, 8.7 Hz, CH₂O), 6.83 (1H, dd, J¼15.5, 7.9 Hz, C₆–

CH), 7.15 (1H, d, J¼15.5 Hz, C₇–CH), 7.27–7.41 (5H, m,

ArH); ¹³C NMR (67.8 MHz, DMSO/DMSO-d₆), the major

rotamer, d 11.6, 17.1, 18.9, 19.5, 26.7, 27.5, 33.1, 33.8, 34.5,

36.2, 42.7, 57.1, 70.1, 118.5, 125.9, 128.0, 128.8, 139.8,

153.8, 155.8, 163.9, 175.0. HRMS (EI) m/z Calcd for

C₂₅H₃₆N₂O₄: 428.2675. Found: 428.2670.

CHCl₃

max

1

IR n

cm²¹2968, 1667, 1644, 1464, 1379, 1196; H

NMR (270 MHz, DMSO/DMSO-d₆), both rotamers unless

0

stated otherwise, d 0.75–0.86 (12H, m, C₃–CH₃, C₈–CH₃,

0

C₁₀–CH₃), 0.94 (3H, d, J¼6.9 Hz, 1 rotamer, C₂–CH₃),

0

0.97 (3H, d, J¼6.9 Hz, 1 rotamer, C₂–CH₃), 1.00–1.15

0

(2H, m, C₉–CH₂), 1.16–1.62 (6H, m, C₃–CH₂, C₈–CH,

5.1.54. (4S)-4-Phenyl-3-[(3S,4S,6R)-3,4,6-trimethyl-8-

((2S)-N-methyl-2-methylbutyramido)-octanoyl]-2-oxa-

zolidinone (30b). According to general procedure H, the

enimide 27b (259 mg, 0.604 mmol) provided the imide 30b

as a colorless oil (246 mg, 92%): [a]²_D⁷¼þ39.2 (c 1.0,

CHCl₃); IR nⁿ_m^e_a^a_x^tcm²¹2964, 1782, 1705, 1639, 1385, 1242,

1198; ¹H NMR (270 MHz, DMSO/DMSO-d₆), both

C₁₀–CH, C₁₁–CH₂), 1.71–1.88 (1H, m, C₇–CH), 1.89–

2.03 (1H, m, C₆–CH), 2.15–2.27 (1H, m, C₆–CH), 2.56–

0

2.72 (1H, m, C₂–CH), 2.79 (3H, s, 1 rotamer, N–CH₃),

2.96 (3H, s, 1 rotamer, N–CH₃), 3.15–3.25 (2H, m, C₁₂–

CH₂), 3.84 (1H, app t, J¼7.9 Hz, C₄–CH), 4.31 (1H, app t,

J¼8.3 Hz, C₄–CH), 4.47–4.60 (1H, m, C₃–CH), 5.07 (1H,

F. Yokokawa et al. / Tetrahedron 60 (2004) 6859–6880

6879

rotamers unless stated otherwise, d 0.68–0.86 (12H, m,

0

5.80 (1H, ddd, J¼16.8, 10.6, 6.6 Hz, C₂–CH); ¹³C NMR

(67.8 MHz, DMSO/DMSO-d₆), the major rotamer, d 11.6,

14.2, 17.1, 19.2 (CH₃£2), 26.7, 26.7, 27.2, 32.6, 33.1, 34.2,

34.5, 36.1, 36.2, 39.4, 44.5, 67.5, 71.1, 115.3, 139.1, 166.5,

174.8. HRMS (EI) m/z Calcd for C₂₁H₃₈N₂O₂: 350.2933.

Found: 350.2930.

C₃–CH₃, C₈–CH₃, C₁₀–CH₃), 0.94 (3H, d, J¼6.1 Hz, 1

0

rotamer, C₂–CH₃), 0.96 (3H, d, J¼6.1 Hz, 1 rotamer, C₂–

CH₃), 1.02–1.13 (2H, m, C₉–CH₂), 1.14–1.61 (6H, m,

0

C₃–CH₂, C₈–CH, C₁₀–CH, C₁₁–CH₂), 1.84–2.00 (1H, m,

0

C₇–CH), 2.50–2.70 (1H, m, C₂–CH), 2.72–2.90 (2H, m,

C₆–CH₂), 2.77 (3H, s, 1 rotamer, N–CH₃), 2.93 (3H, s, 1

rotamer, N–CH₃), 3.10–3.49 (2H, m, C₁₂–CH₂), 4.13 (1H,

dd, J¼8.7, 3.6 Hz, CH₂O), 4.72 (1H, t, J¼8.7 Hz, Ar–CH),

5.46 (1H, dd, J¼8.7, 3.6 Hz, CH₂O), 7.26–7.39 (5H, m,

ArH); ¹³C NMR (67.8 MHz, DMSO/DMSO-d₆), the major

rotamer, d 11.6, 14.1, 17.1, 17.8, 19.2, 26.7, 27.3, 32.8, 33.1,

33.3, 34.5, 36.1, 36.2, 39.4, 44.6, 57.0, 70.0, 125.7, 127.9,

128.8, 140.0, 153.7, 171.7, 174.8. HRMS (EI) m/z Calcd for

C₂₆H₄₀N₂O₄: 444.2988. Found: 444.2996.

5.1.57. (3R,7S,8S,10R,2⁰S)-Kalkitoxin (2). According to

general procedures L and M, the oxazoline 34b (43.5 mg,

0.124 mmol) provided the synthetic (3R,7S,8S,10R,2⁰S)-

kalkitoxin (2) as a pale yellow oil (22.9 mg, 53%, 2 steps):

[a]²_D⁶¼þ49.6 (c 0.64, CHCl₃); IR n

cm²¹2963, 2929,

2874, 1646, 1464, 1412, 1381, 1082; H NMR (270MHz,

CHCl₃

max

1

DMSO/DMSO-d₆), both rotamers unless stated otherwise, d

0

0.71–0.87 (12H, m, C₃–CH₃, C₈–CH₃, C₁₀–CH₃), 0.94

0

(3H, d, J¼6.9 Hz, 1 rotamer, C₂–CH₃), 0.97 (3H, d,

0

5.1.55. (3S,4S,6R)-3,4,6-Trimethyl-8-((2S)-N-methyl-2-

methylbutyramido)octanoic acid (31b). According to

general procedure I, the imide 30b (217 mg, 0.489 mmol)

J¼6.9 Hz, 1 rotamer, C₂–CH₃), 1.01–1.18 (2H, m, C₉–

0

CH₂), 1.19–1.67 (6H, m, C₃–CH₂, C₈–CH, C₁₀–CH, C₁₁–

CH₂), 1.72–1.90 (1H, bd–m, C₇–CH), 2.22–2.37 (1H, m,

C₆–CH₂), 2.38–2.49 (1H, m, C₆–CH₂), 2.55–2.71 (1H, m,

provided the carboxylic acid 31b as a colorless oil (124 mg,

85%): [a]²_D⁵¼þ8.6 (c 1.0, CHCl₃); IR n

CHCl₃

max

0

3250 (bd),

C₂–CH), 2.79 (3H, s, 1 rotamer, N–CH₃), 2.96 (3H, s, 1

2964, 1724, 1614, 1464, 1383; ¹H NMR (270 MHz, DMSO/

DMSO-d₆), both rotamers unless stated otherwise, d 0.70–

rotamer, N–CH₃), 3.03 (1H, dd, J¼8.3, 10.2 Hz, C₄–CH),

3.09–3.25 (2H, m, C₁₂–CH₂), 3.48 (1H, dd, J¼8.3,

10.2 Hz, C₄–CH), 4.86–4.99 (1H, m, C₃–CH), 5.10 (1H,

d, J¼10.3 Hz, C₁–CH), 5.23 (1H, d, J¼17.0 Hz, C₁–CH),

5.90 (1H, ddd, J¼6.3, 10.3, 17.0 Hz, C₂–CH); ¹³C NMR

(67.8MHz, DMSO/DMSO-d₆), the major rotamer, d 11.99,

14.40 (CH₃£2), 17.47, 19.63, 27.01, 27.53, 32.89, 34.52,

34.89, 36.52 (CH £2), 38.90, 40.94, 42.03, 44.89, 78.22,

115.60, 138.28, 169.42, 175.11. HRMS (EI) m/z Calcd for

C₂₁H₃₈N₂OS: 366.2705. Found: 366.2706.

0

0.89 (12H, m, C₃–CH₃, C₈–CH₃, C₁₀–CH₃), 0.95 (3H, d,

0

J¼6.8 Hz, 1 rotamer, C₂–CH₃), 0.97 (3H, d, J¼6.8 Hz, 1

0

rotamer, C₂–CH₃), 1.03–1.15 (2H, m, C₉–CH₂), 1.16–

0

1.61 (6H, m, C₃–CH₂, C₈–CH, C₁₀–CH, C₁₁–CH₂),

1.77–1.91 (1H, m, C₇–CH), 1.93–2.09 (1H, m, C₆–CH),

0

2.13–2.27 (1H, m, C₆–CH), 2.54–2.60 (1H, m, C₂–CH),

2.79 (3H, s, 1 rotamer, N–CH₃), 2.96 (3H, s, 1 rotamer,

N–CH₃), 3.10–3.52 (2H, m, C₁₂–CH₂), 12.0 (1H, bd-s,

COOH); ¹³C NMR (67.8 MHz, DMSO/DMSO-d₆), the

major rotamer, d 11.6, 14.3, 14.5, 17.1, 19.2, 26.7, 27.2,

33.0, 33.7, 33.8, 34.3, 34.5, 36.2, 39.4, 44.6, 174.2, 174.8.

HRMS (EI) m/z Calcd for C₁₇H₃₃NO₃: 299.2460. Found:

299.2462.

Acknowledgements

Work in Nagoya was partially supported by Grants-in-Aid

from the Ministry of Education, Culture, Sports, Science

and Technology, Japan (to F. Y. and T. S.) and Nagoya City

University (to F. Y.). Work in Oregon was supported by

NIH GM 86354 and the MFBS Center at OSU (ES03850),

and a JSPS fellowship to T. O.

5.1.56. (4R)-4-Ethenyl-2-[(2S,3S,5R)-2,3,5-trimethyl-7-

((2S)-N-methyl-2-methylbutyramido)-heptyl]oxazoline

(34b). According to general procedure J, the carboxylic acid

31b (78.0 mg, 0.260 mmol) provided the amide 33b as a

colorless oil (72.5 mg, 76%), which was directly used for

the next step. To a solution of the amide 33b (70.4 mg,

0.191 mmol) in THF (1.5 mL) was added Burgess reagent

(97 mg, 0.764 mmol) under argon. The mixture was heated

to 70 8C. After 1.5 h, the reaction mixture was cooled to

room temperature and concentrated. The residue was

puriﬁed by silica gel column chromatography (BW-200,

hexane/EtOAc¼1:3) to afford the desired oxazoline 34b as a

colorless oil (44.1 mg, 66%): [a]²_D⁵¼þ63.5 (c 0.98, CHCl₃);

References and notes

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CHCl₃

max

1

IR n

cm²¹2963, 1663, 1646, 1458, 1379, 1196; H

NMR (270 MHz, DMSO/DMSO-d₆), both rotamers unless

stated otherwise, d 0.73–0.89 (12H, m, C₃–CH₃, C₈–CH₃,

0

C₁₀–CH₃), 0.94 (3H, d, J¼7.3 Hz, 1 rotamer, C₂–CH₃),

0.97 (3H, d, J¼7.3 Hz, 1 rotamer, C₂–CH₃), 1.01–1.16

(2H, m, C₉–CH₂), 1.17–1.63 (6H, m, C₃–CH₂,C₈–CH,

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F. Yokokawa et al. / Tetrahedron 60 (2004) 6859–6880

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Article Doi

An expeditious total synthesis of kalkitoxins: Determination of the absolute stereostructure of natural kalkitoxin

DOI: 10.1016/j.tet.2004.06.014

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