Formal Total Synthesis of Amphidinolide e

Article abstract of DOI:10.1021/acs.joc.7b01973

A formal total synthesis of the cytotoxic macrolide amphidinolide E is reported. The strategic steps are three Julia-Kocienski reactions (J-K), for the formation of the C5-C6, C9-C10, and C17-C18 double bonds, a Suzuki-Molander C21-C22 bond formation reaction, and a Kita-Trost macrolactonization. The "instability" of the two dienic systems and of the stereocenter at C2 (allylic methine, α to the carboxy group) and the protecting groups at C17-OH and C18-OH have posed difficult challenges. Each Julia-Kocienski olefination has been systematically optimized to provide the highest possible E/Z ratios.

Full text of DOI:10.1021/acs.joc.7b01973

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Article
Formal Total Synthesis of Amphidinolide E
Lluís Bosch, Laura Mola, Elena Petit, Mar Saladrigas, Jorge Esteban, Anna M Costa, and Jaume Vilarrasa
J. Org. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.joc.7b01973 • Publication Date (Web): 28 Sep 2017
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Formal Total Synthesis of Amphidinolide E
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Lluís Bosch, Laura Mola, Elena Petit, Mar Saladrigas, Jorge Esteban, Anna M. Costa,*and Jaume Vilarrasa*
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Organic Chemistry Section, Facultat de Química, Universitat de Barcelona, Diagonal 645, 08028 Barcelona, Catalonia, Spain
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 Supporting Information
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J–K
10
17
J–K
O
OH
18
21
10
Br
X
O
26
C10–C21
18
9
21
22
26
HO
HO
O
O
9
KF3B
C22–C26
OPG
O
O
1
O
5
6
1
amphidinolide E (1)
J–K
C1–C9
ABSTRACT: A formal total synthesis of the cytotoxic macrolide amphidinolide E is reported. The strategic steps are three Julia–
Kocienski reactions (J–K), for the formation of the C5–C6, C9–C10, and C17–C18 double bonds, a Suzuki–Molander C21–C22 bond
formation reaction, and a Kita–Trost macrolactonization. The "instability" of the two dienic systems and of the stereocenter at C2
(allylic methine, α to the carboxy group) and the protecting groups at C17-OH and C18-OH have posed difficult challenges. Each Julia–
Kocienski olefination has been systematically optimized to provide the highest possible E/Z ratios.
INTRODUCTION
The amphidinolides are a family of complex macrolides isolated from cultured marine dinoflagellates of the genus Amphidinium
sp., which live off the coasts of Okinawa Island (Japan).1 Some years ago we engaged in a research program directed towards the
total synthesis and elucidation of the biological mechanism of action of several members of this family of natural products.2 As
part of this research effort we now report a formal total synthesis of amphidinolide E (1), a cytotoxic 19-membered macrolactone
with an embedded tetrahydrofuran ring, eight stereocenters, two conjugated dienes and a unique side chain. The isolation of 1
was reported by Kobayashi et al. in 19903a while its absolute stereochemistry was determined in 20023b and was later confirmed
by two total syntheses from the groups of Lee4 and Roush.5 The preparation of several fragments has also been reported by other
authors.6 The strategy of Lee et al.4 involved the formation of the oxolane (tetrahydrofuran) ring by a radical cyclization
(formation of the C13–C14 bond), an enyne Ru-catalyzed reaction with 2-methyl-1,4-pentadiene to generate the C22–C23 double
bond, and a final macrolactonization step. The strategy of Va and Roush5 was based in the formation of the oxolane ring of 1 by
an annulation reaction of an aldehyde (C5/6–C13), arising from tartaric acid, and an allylsilane (C14–C21/29) with a terminal
triple bond between C21 and C29, arising from L-glyceraldehyde; the C5–C6 double bond was installed by means of a RCM, but
the requirement of a conjugate diene C1–C5/6 forced the authors to protect it as a Fe(CO)3 complex in the previous step. Part of
the side chain (C22–C26) was added in the last step by means of a Stille coupling, after the RCM reaction.
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Our retrosynthetic analysis of 1 is different, although it shares with the strategy of Lee et al.4 the macrolactonization step and one
Julia–Kocienski reaction (henceforward, J–K reaction) to create the C9–C10 double bond. As shown in Scheme 1, our retrosynthesis
disconnects the molecule into three fragments: the southern fragment (C1‒C9), the northern fragment (C10‒C21) and the side chain
(C22‒C26).6d The C10–C21 fragment can be accessed through a J–K reaction between aldehyde C10–C17 and sulfone C18–C21,
followed by a Sharpless dihydroxylation of the newly formed double bond. Another J–K reaction was also chosen to assemble the
C1–C9 fragment, by reaction of aldehyde C6–C9, derived from tartaric acid, and sulfone C1–C5, prepared using a procedure
described previously by some of us.6c Although there are several possibilities to link the appropriate fragments of such a demanding
structure with two conjugate dienes and two additional double bonds, we planned to apply three J–K reactions, which would require
a fine tuning of the reaction conditions to overcome the difficulties that undoubtedly would appear.
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Scheme 1. Retrosynthetic Analysis of Amphidinolide E (1)
17
O
J–K
Sharpless
10
O
OH
10
O
X
C10–C17
Br
21
Julia–Kocienski
10
X
Suzuki–Molander
18
HO
13
Br
21
OH
PT
S
O
O
26
22
18
26
O
21
HO
C18–C21
O
KF3B
O
9
29
macrolactonization
side chain
5
1
O
O
O
HO
J–K
1
OPG
amphidinolide E (1)
OPG
SO2PT
9
O
6
5
1
O
OPG
O
C6–C9
C1–C5
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RESULTS AND DISCUSSION
Preparation of the southern fragment (C1‒C9). For the synthesis of fragment C1–C9 we envisaged the formation of the C5–C6
bond via a J–K reaction. This transformation has been a useful tool for the construction of 1,3-dienes present in complex natural
products.7 These moieties can either be accessed by a J–K reaction between an aliphatic sulfone and an α,β-unsaturated aldehyde or
that between an allylic sulfone and an aliphatic aldehyde. Both alternatives have been widely employed, with the former consistently
providing a new E double bond, whereas the diastereoselectivity of the reaction of allylic sulfones is uncertain,7 although a recent
study has helped rationalize the experimental outcome of these reactions.8 We initially explored the first strategy, with discouraging
results.9 Thus, we then focused on the reaction of sulfone C1–C5 (PG = TBDPS) with aldehyde C6–C9 (PG = TES).
As shown in Scheme 2, the reduction of dimethyl (S,S)-tartrate (2) to the corresponding diol, followed by monoprotection as
triethylsilyl (TES) ether (3)10 and oxidation with the Dess–Martin periodinane (DMP), gave the desired aldehyde 4 (fragment C6–C9).
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Scheme 2. Preparation of Aldehyde C6–C9
4
5
6
7
8
OTES
OH
OTES
TESCl
NaH
COOMe
COOMe
O
O
LiAlH4
DMP
9
O
O
O
O
NaHCO3
CH2Cl2
0 ˚C–rt,1.5 h
THF
0 ˚C, 0.5 h
THF
rt, 3 h
O
9
6
10
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2
4
60%
3
90%
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Alcohol 5, previously prepared by us via a Michael addition–elimination reaction of a chiral enolate to ethyl 3-iodoacrylate,6c was
the starting point for the preparation of the next fragment (sulfone C1–C5 in Scheme 1, see 9 in Scheme 3). A Mitsunobu reaction with
1-phenyl-1H-tetrazole-5-thiol (henceforward, PT–SH or PTSH)11 converted the alcohol into thioether 6. Elimination of the chiral
auxiliary with NaBH4 in THF–H2O and protection of the resulting alcohol 7 as TBDPS ether 8 proceeded uneventfully.
Oxidation of this allylic thioether under standard conditions (10 equiv H2O2, 20 mol % of heptamolybdate ion) afforded poor
yields of the desired sulfone (9) with isolation of substantial amounts of 9a, a mixture of known syn and anti allylic alcohols in a
quite similar ratio (1H NMR spectrum), arising from a [2,3]-sigmatropic rearrangement of the intermediate allylic sulfoxide and
subsequent hydrolysis of the allyl sulfenate. This is a known undesired reaction in the preparation of allylic heteroaryl sulfones.12 To
increase the oxidation rate of the allylic sulfoxide to sulfone, relative to the [2,3]-sigmatropic rearrangement, both the equivalents of
H2O2 and catalyst were raised. In our case, with 30 equiv of H2O2 and 40 mol % of heptamolybdate ion, the desired sulfone was
obtained in 88% yield, with minimal formation of alcohol 9a (9:1 9/9a ratio). Attempts to further improve this ratio by increasing
the amount of reagents, by very slowly adding 8 in EtOH to the aqueous yellow solution of (NH4)6Mo7O24/H2O2, by lowering the
temperature, and/or by changing the medium acidity were not successful.
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Scheme 3. Synthesis of Sulfone 9
PTSH
Ph3P/DEAD
O
O
O
O
NaBH4
PTS
PTS
HO
N
N
O
O
OH
THF, rt, 1 h
THF–H2O
7
0 ˚C–rt, 3 h
Bn
Bn
5
6
91%
89%
CH2Cl2
rt, 1 h
TBDPSCl
imidazole
H2O2
(NH4)6Mo7O24
96%
PT
S
O
OH
rearrang.
H2O
OTBDPS
PTS
OTBDPS
OTBDPS
EtOH, rt, 3 h
8
9a
10 equiv H2O2, 20 mol % heptamolybdate ion: 9/9a, 7:3
30 equiv H2O2, 40 mol % heptamolybdate ion: 9/9a, 9:1
5
1
PT = 1-phenyltetrazol-5-yl
PT
OTBDPS
S
O
O
9
(88%)
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We were now ready to explore the J–K reaction between sulfone 9 and aldehyde 4 for the formation of the C5–C6 bond (Scheme 4).
Our first attempts (KHMDS, DMF, from –65 ºC to rt, 18 h) afforded complex reaction mixtures. We soon noted that the TES group was
being cleaved under the reaction conditions and that shortening the reaction time from 18 h to 4 h avoided this side reaction and furnished
the desired (3E,5E)-diene 10, together with the (3E,5Z)-diene and an unexpected isomer in a 57:17:26 ratio. Characterization of this
unexpected product was hampered by the impossibility to separate it completely from the (3E,5E)- and (3E,5Z)-isomers by flash column
chromatography. However, by comparison of the 1H NMR spectra of pure samples of these isomers with reaction mixtures, we eventually
identified it as the (3Z,5E)-diene.13 This isomer can arise from E to Z isomerization of the allylic sulfone anion during the reaction.14
After some experimentation we could maximize the yield of (3E,5E)-10 using an excess of base (2.5 equiv of solid KHMDS)
and aldehyde (1.2 equiv) in the presence of 18-crown-6 (which is known to increase the formation of the E isomer),15 under
Barbier-like conditions, in DMF below –40 ºC. In this way, the product was obtained in good yield as an 82:13:5 mixture of
diastereoisomers. An attempt using KH, instead of KHMDS, also afforded an 80% isolated yield but of a 75:8:17 mixture. Attempts
to isomerize these diastereomeric mixtures, aimed at increasing the ratio of the E,E-isomer, with I2 and with PdCl2(NCCH3)2), were
unsuccessful.
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Scheme 4. Formation of the C5–C6 Double Bond
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Because of the difficulty of completely separating the different diastereomers of 10, the mixture was treated with ethanol and
pyridinium p-toluenesulfonate (PPTS) to furnish the expected alcohols 11 (see Scheme 5). At this stage, the E,E-isomer (ca. 65% overall
yield for the J–K olefination and the deprotection step) could be isolated by flash chromatography; ca. 10% of the so-called (3E,5Z)-11 in
Scheme 5 was also separated. Oxidation of (3E,5E)-11 with DMP allowed us to isolate the desired aldehyde 12 (stereopure) in excellent
yield (see Supporting Information for the comparison of the main 1H NMR data and the Experimental Section for 13C NMR spectra).
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Scheme 5. Isolation of 11 and Preparation of Aldehyde 12 (Fragment C1–C9)
6
7
8
9
OH
O
DMP
NaHCO3
O
O
O
O
TBDPSO
TBDPSO
CH2Cl2
rt, 1 h
OTES
(3E,5E)-11
(65% overall)
12
10
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O
O
92%
TBDPSO
(C1–C9)
EtOH, PPTS
7
5
CH2Cl2
rt, 18 h
3
HO
OTBDPS
(3E,5Z)-11
10
O
O
(3E,5E)/(3E,5Z)/(3Z,5E)
82:13:5
(10% overall)
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Synthesis of the northern fragment (C10‒C21). The synthesis of the C10–C21 fragment via a J–K reaction between sulfone
C10–C17 and aldehyde C18–C21 was described by some of us several years ago.6d In practice, we had obtained the tetrahydrofuran
derivative (oxolane C10–C17) from a butyrolactone (the allylsilyloxy derivative shown in Scheme 6, top) via an intramolecular
allylation of the TiCl4-generated oxocarbenium ion, which afforded the desired 2,5-disubstituted oxolane in 70% yield as a 85:15
cis/trans mixture.16 Although reliable, this route presented several drawbacks when gram-scale amounts of C10–C17 were needed.
The intramolecular allylation16 required working under high-dilution conditions (5·10–3 M) and the separation of the cis/trans
diastereomers was extremely tedious. Thus, we decided to build up the C10–C17 fragment via alternative routes (Scheme 6, bottom).
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Scheme 6. Alternative Disconnections of Fragment C10–C21
O
Si
O
O
ref 6d
&
16
Br
Br
10
17 SO2PT
O
+
O
13
C10–C17
C18–C21
C18–C21
OH
17
10
O
Br
21
OPG
HO
17
O
PT
10
+
SO2
O
C10–C21
OPG
this work
C10–C17
O
OBn
10
17
O
OH
OPG
OPG
OMs
O
17
17
10
13
13
OBn
OTBS
OBn
OTBS
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First, the synthesis of aldehyde C10–C17 began with 4-penten-1-ol, protected as its TBDPS derivative (13), which was epoxidized
using Berkessel’s catalyst (Scheme 7). We prepared it from Ti(OiPr)4 and cis-salalen according to the procedure reported.17a,b We
obtained the desired epoxide 14 in 90% yield with 96:4 e.r. after 3 days of reaction by using 18 equiv of H2O2. The new catalyst of
Berkessel et al.,17c an analog with a binaphthyl substituent (see Scheme 7, bottom), afforded a better yield (98%) and identical enantio-
purity with 12 equiv of H2O2 within 2 days. Copper(I)-catalyzed ring opening of epoxide 14 with allylmagnesium bromide furnished
alcohol 15, which was ready for a second epoxidation, this time using ent-cis-salalen, which we prepared as well. The epoxide cyclized
in situ, to provide oxolane 16 as a ≥85 : ≤1 : 7 : 7 mixture of stereoisomers, as determined by chiral HPLC, by comparison with a
reference mixture of (±)-cis and (±)-trans isomers, prepared independently by epoxidation of the double bond with m-CPBA. Despite the
excellent performance of these asymmetric epoxidation catalysts, the cis/trans ratio of the final product was practically identical to that
obtained by means of the previous route. To date, no efforts have been made to improve the stereoselectivity of the last step(s), e.g. by
protecting the OH group before the second epoxidation. Finally, the Swern oxidation (DMSO/ClCOCOCl, CH2Cl2, –78 ˚C, then Et3N,
from –78 ˚C to 0 ˚C)18 of 16 afforded the desired fragment C10–C17 (aldehyde 17) in 4 steps from 13 and an overall yield of 50%.19
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Scheme 7. Syntheses of 17 (Fragment C10–C17)
MgCl
H2O2
O
OH
cis-salalen (30 mol%)
Ti(OiPr)4 (30 mol%)
ClCH2CH2Cl, rt, 3 d
CuCl, THF
–50 ˚C, 30 min
OTBDPS
OTBDPS
OTBDPS
14
13
15
88%
(96:4 e.r.)
90%
ent-cis-salalen (10 mol%)
Ti(OiPr)4 (10 mol%) H2O2
ClCH2CH2Cl, rt, 5 d
O
OH
Swern
O
O
17
O
OH
10
TBDPS
76%
88%
O
OTBDPS
OTBDPS
17
16
N
NH
N
NH
OH HO
Ph
OH HO
cis-salalen
binaphthyl analog
cis-salalen
1. (+)-Ipc2B
OH
MsCl
Et3N
O
Et2O, –78 ˚C, 1 h
2. H2O2/NaOH, rt, 5 h
95%
OBn
OTBS
OBn
OTBS
TBAF
THF, rt
CH2Cl2
0 ˚C, 2 h
19
18
97%
98%
H2
TBDPSCl
1. 9-BBN
Swern
96%
17
O
Pd/C
EtOH
imidazole
DMF
2. H2O2
NaOH
OBn
20
90%
98%
91%
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Another route, simpler but longer, was also examined, in order to avoid the cumbersome separation of the cis/trans mixture.
Starting from chiroblock 18, a known compound,20 the sequence of standard high-yielding reactions shown in Scheme 7 (bottom)
allowed us to obtain aldehyde 17 in a stereopure condition.
6
7
Fragment C18–C21 was synthesized from a known alcohol, 21.21 Preparation and isolation of 1-phenyltetrazol-5-yl sulfide 22
and its sulfone 23 did not pose any problem in this relatively simple case (Scheme 8).
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9
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11
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Scheme 8. Preparation of Sulfone 23
H2O2
Mo(VI)
PTSH
Ph3P/DEAD
18
Br
Br
21
Br
PT
PT
HO
S
S
THF, rt, 1 h
96%
EtOH, rt, 15 h
96%
O
O
21
23
22
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The optimization of the J–K coupling between this sulfone and aldehyde 17 (fragment C10–C17) is shown in Table 1, which
summarizes around 25 experiments. The use of LiHMDS or NaHMDS in DMF, THF, or THF/HMPA (entries 1–4) was not
encouraging. We confirmed the known fact6d,7,22 that polar coordinating solvents and large counter-ions, such as K+, favor the
formation of the E stereoisomers (entries 5 and 6). Addition of HMPA6d was detrimental to yield (entry 7). Use of solid KHMDS
in DMF (entry 8) was not advantageous, probably due to solubility problems. We tried to further improve the outcome of the
reaction by adding 18-crown-6 as an additive15 (entry 9), but no advantages were noted in this particular case. Finally, an
increase of the reaction scale allowed us to lower the amount of sulfone (entry 11) and to achieve the best yield.
Table 1. Optimization of the Julia–Kocienski Reaction between 23 and 17
O
17
base
O
O
Br
+
23
21
10
solvent
–65 ˚C to rt
18 h
18
OTBDPS
OTBDPS
17
24
yield of
24 (%)
entry base (equiv)
equiv of 23
solvent/additive
E/Za
1
2
LiHMDS in THF (2.0)
NaHMDS in THF (1.1)
NaHMDS in THF (1.1)
NaHMDS in THF (2.0)
KHMDS in toluene (1.2)
KHMDS in toluene (2.0)
KHMDS in toluene (2.0)
solid KHMDS (2.0)
2.1
1.2
1.2
2.1
1.1
2.1
2.1
2.1
2.1
1.6
DMF
THF
62
55
26
60
54
74
65
69
72
80d
83:17
50:50
89:11
75:25
95:5
3
DMF
4
THF/HMPA
DMF
5
6
DMF
95:5b
7
DMF/HMPA
95:5
8
DMF
92:8
9
10d
KHMDS in toluene (2.0)
KHMDS in toluene (1.5)
DMF/18-crown-6c
DMF
96:4
96:4
a E/Z ratios were determined by 1H NMR (400 MHz). Reactions with 0.10–0.15 mmol of 17, unless
otherwise indicated. Isolated yields of the E/Z mixtures are referred to 17, which is the substrate more
b
"expensive" and difficult to recover. A modified Julia reaction, with the benzothiazol-2-yl group
instead of the 1-phenyltetrazol-5-yl group, gave a similar yield but lower stereoselectivity (87:13
E/Z). c With 2.0 equiv of 18-crown-6. d At 0.73-mmol scale.
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With oxolane 24 in our hands, we completed the synthesis of the northern fragment as shown in Scheme 9. Sharpless
asymmetric dihydroxylation of 24 with AD-mix-β provided syn-diol 25 and its diastereomeric syn-diol in a 93:7 ratio.
Diastereomerically pure 25 was isolated in 85% yield after flash column chromatography. Protection of the 1,2-diol of 25 as the p-
methoxybenzyl acetal (26, ca. 1:1 mixture of epimers) and selective cleavage of this acetal with DIBALH afforded 27 in excellent
yield. The resulting alcohol was protected as its MOM ether (with methoxymethyl chloride, NaI, and diisopropylethylamine) and the
primary O–TBDPS group was cleaved with TBAF.23 Conversion to the sulfone was carried out as in the preceding examples. The
synthesis of the required C10–C27 fragment, 28, was completed by deprotection of the PMB ether with 2,3-dichloro-5,6-dicyano-1,4-
benzoquinone (DDQ) and water.
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Scheme 9. Completion of the Synthesis of the Northern Fragment
OH
AD-mix-β
4-MeOC6H4CH(OMe)2
O
O
O
24
PMP
O
Br
TsOH (cat.)
CH2Cl2, rt, 1 h
Br
CH3SO2NH2
tBuOH–H2O 1:1
rt, 20 h
HO
OTBDPS
OTBDPS
25
26
88%
85%
DIBALH, CH2Cl2
_78 ˚C, 2 h
90%
OH
DDQ
H2O
OMOM
H2O2
PTSH
TBAF
THF
MOMCl
O
O
10
Br
21
Br
CH2Cl2
rt, 0.5 h
Mo(VI) Ph3P/DEAD
EtOH, THF, rt, 1 h rt, 2 h
rt, 15 h
DIPEA, NaI
CH3CN
HO
PMBO
OTBDPS
SO2PT
50 ˚C, 3 h
28
27
95%
90%
99%
90%
90%
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The end-game. With the C1–C9 and C10–C21 fragments in our hands, as well as with the side chain (C22–C26),6d we were
ready to tackle the final steps of the synthesis of amphidinolide E. After considering different approaches,24 we decided to first form
the C9–C10 bond by means of a J–K reaction. Initially, we attempted the J–K reaction of sulfone 28 with aldehyde 12, hoping to
avoid the protection of the C18 alcohol (using ≥ 2 equiv of base). However, all the conditions tested afforded poor yields and
diastereoselectivities of the desired product.
Therefore, 28 was protected as 29. We systematically examined its J–K reaction with aldehyde 12 (Scheme 10). Much to our
dismay, conversions below 30% with loss of 12 were noted in the first attempts with KHMDS/18-crown-6.25 This J–K reaction,
with two highly functionalized fragments, was really more challenging than the previous ones. As we observed in blank experiments
that sulfone 29 survived under the conditions of Scheme 10, we used an excess of it (2 equiv) and of KHMDS and 18-crown-6 (1.7
equiv, both). In this way, olefin 30 was isolated as a single stereoisomer in 68% yield. Most of the excess of 29 could be recovered.
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Scheme 10. Union of the Three Fragments
4
5
6
7
8
9
OMOM
OMOM
10
9
O
O
18
Br
KHMDS
Br
O
O
RO
12
TBDPSO
+
DMF
18-crown-6
–65 ˚C to rt
12 h
SO2PT
OTBDPS
29, R = TBDPS
10
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TBDPSOTf
2,6-lutidine
CH2Cl2, rt, 15 h
30
83%
(89% brsm)
22
68%
26
Pd(OAc)2, Ph3P
Cs2CO3
THF–H2O 10:1
70 ˚C, 4 h
KF3B
(exc.)
28, R = H
31
95%
26
OMOM
OMOM
O
O
TBAF
21
22
O
O
O
TBDPSO
TBDPSO
THF, rt
10 h
1
O
OTBDPS
OH
97%
33
32
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With such a high-MW polyfunctional substrate, the Suzuki–Molander reaction of 30 with organotrifluoroborate 316d gave no
conversion under standard conditions [5 mol % Pd(OAc)2, 10 mol % Ph3P, 3 equiv Cs2CO3].26 However, an excellent yield of 32
was eventually obtained (95%, Scheme 10) when both the amount of catalyst and phosphine was increased and an excess of 31 (2.5
equiv) was used. Selective cleavage of the TBDPS ether of the primary alcohol of 32 with TBAF afforded 33 in excellent yield,
without touching at all the more crowded O–TBDPS bond.
The next step was the oxidation of the hydroxy group of 33 to the corresponding carboxylic acid, a step that Lee et al. described
as painfully difficult in his report of the total synthesis of amphidinolide E,4 where oxidation with DMP of a substrate very similar to
33 (with O-TIPS at C18 instead of O-TBDPS) caused scrambling of the NMR signals of the side-chain region; however, they
accomplished the oxidation of this alcohol to the corresponding aldehyde with 2-iodoxybenzoic acid (IBX), also used by
Mohapatra6e in a simpler molecule lacking the side-chain diene. In our hands, all attempts at oxidation with IBX of a similar
substrate (with O-TES at C18 instead of O-TBDPS) affected the internal diene NMR signals. This forced us to reevaluate the
oxidation of this sensitive molecule, using DMP (stored under vacuum in a desiccator over KOH pellets) in the presence of 2,6-
lutidine or NaHCO3, because we suspected that the acidic medium—2 mol of AcOH are obviously generated per mol of DMP—
and/or the reagent impurities initiated the undesired reactions. Our studies with model compounds—intermediates and byproducts—
were promising. Finally, when we treated 33 with DMP and NaHCO3 (finely powdered and dried over P4O10), the desired aldehyde
was cleanly obtained (Scheme 11), without byproducts arising from the epoxidation of conjugate dienes. This aldehyde was
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immediately oxidized to the corresponding carboxylic acid, 34, in 91% overall yield; to ensure that the diene moieties were not
affected in any way we did not only add an excess of 2-methyl-2-butene but also isoprene as trapping reagents for HOCl.
Cleavage of the TBDPS ether of 34 was more complicated than expected. This is probably due to the steric hindrance around the
protected C18 secondary alcohol, which is surrounded by two secondary stereocenters, and hence it is sterically more crowded than
standard secondary alcohols. Using 5 equiv of TBAF at rt the reaction progressed very slowly. After addition of up to 8 equiv of TBAF in
several portions to a solution of 34 in THF at 50 ºC allowed us to obtain sufficient amounts of seco-acid 35 (Scheme 11), but the
conversion was still incomplete. A parallel sequence with the TBS analog of 34 (also prepared from 28 successfully) gave only rise to
slightly better deprotection percentages. In principle, the separation of 34 and 35 by chromatography, followed by subjecting again
the recovered starting material, 34, to the deprotection reaction, should solve the issue, but the protecting groups of the polar
hydroxy acid 35 proved to be sensitive to silica gel and/or the eluents, with formation of byproducts during each attempted
purification.
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____________________________________________________________________________
Scheme 11. Preparation of seco-Acid 35
i) DMP, NaHCO3
CH2Cl2, rt, 1 h
OMOM
OMOM
O
O
TBAF (exc.)
33
18
HO
18
O
O
O
O
TBDPSO
ii) NaClO2, H2O
NaH2PO4
THF, 50 ˚C
6 h
COOH
COOH
35
tBuOH, 1 h
7
50% conversion
90% brsm
34
91%
1
___________________________________________________________________________
Compound 35 showed the expected HRMS(ESI–) and their NMR spectra agreed with those reported.4 Copies of these spectra
are included in the Supporting Information section. This constitutes a formal total synthesis of amphidinolide E (1), since 35 had
been converted into 1 in 44% yield by Lee et al.4 by means of the Kita–Trost macrolactonization,24b followed by the removal of the
protecting groups.
The macrolactonization should not be carried out in the presence of DMAP (e.g., by the Shiina method),24a due to the partial
epimerization that can occur at C2 and the probable migration of the double bonds at C3 and C5. Thus, it was wise to rely also
upon the Kita–Trost procedure, which uses HC≡C–OEt and a Lewis acid, [RuCl2(p-cymene)]2, for the activation of the carboxyl
group, and a Brönsted acid (CSA) for the cyclization. As mentioned in the preceding paragraph, this approach had been already
1
tested.4 In our hands, according to the H NMR spectrum and HRMS of the crude product, the macrocyclization did partially
work in variable yields (20–40% in different attempts with few mg of 35) but was accompanied by open byproducts, such as the
17,18-dihydroxy acid and the 17,18-O-methylene carboxylic acid lacking the 7,8-O-isopropylidene acetal, which means an
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unfortunate loss of valuable material in the penultimate step of a challenging total synthesis. Our plans for the future involve the
improvement of the troublesome antepenultimate and penultimate steps of this total synthesis—the cleavage of a crowded silyl
ether and the macrolactonization of a substrate that is especially sensitive to bases and acids, respectively—, which may include a
change of tactics (PGs) and/or the recovery of open byproducts and their cyclization. We need a few mg of 1 (and of their
derivatives and stereoisomers) for the determination of the mechanism of action of this cytotoxic natural product, as we did with
other amphidinolides.2b,d
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CONCLUSIONS
The synthetic challenges inherent to the two conjugate dienes of amphidinolide E (1) and its C2 stereocenter (allylic and α to the
carboxy group) have been overcome by means of three Julia–Kocienski olefinations and a Suzuki–Molander reaction. In fact, the synthesis
of 1 has turned out to be the most demanding among those the senior author has been engaged in the past twenty years. In our opinion, this
adds more value to the total syntheses accomplished by the groups of Lee and Roush years ago. Our strategy, designed independently years
ago as well, has no special advantages with regard to the number of steps (our longest linear sequence involves 25 steps to seco-acid 35)
and overall yield (ca. 7%), but during this long journey we have gained insight into and improved several venerable reactions. The
antepenultimate step (the cleavage of the sterically crowded O–Si bond located at C18) and the penultimate step (a CSA-mediated
macrolactonization, which also involves the OH at C18) are problematic and would require further efforts before scaling up the process.
As known, the J–K reaction is an outstanding method for the stereoselective formation of double bonds, but sometimes, with
polyfunctional substrates such as those dealt with here, gives rise to unpredictable outcomes (often quite poor).22 Moreover, to complete the
reaction, a relative excess of one or another reactant must be added, which introduces further complications (including a higher probability
of self-condensations of heteroaryl sulfones and/or aldol reactions). Because of the dependence of yield and selectivity on the features of
each sulfone and aldehyde to be coupled, a general procedure cannot be recommended but, by optimizing the reaction conditions in each
case, we have disclosed technical details and tricks that may be useful in other cases:
(i) We have confirmed the tendency of potassium salts of PT–SO2CH2R to chiefly afford double bonds of E configuration, when the
main solvent is DMF. We also confirmed that addition of 18-crown-6 may be advantageous,15 regarding yields and E/Z ratios. This was
the case for the formation of the C5–C6 double bond in Scheme 4 (allylic sulfone vs. α-alkoxy aldehyde) and for the formation of the
C9–C10 double bond in Scheme 10 (multifunctional sulfone vs. multifunctional α-alkoxy aldehyde), but it was not necessary for the
formation of the C17–C18 double bond (Table 1, simpler substrates).
(ii) Allylic sulfones can be prepared in excellent yields, with only small amounts of byproducts arising from the rearrangement of
the intermediate allylic sulfoxides, under the conditions of Scheme 3, while the partial E-to-Z isomerizations of the allyl sulfone moiety
during the J–K reaction course can be reduced at minimum under the conditions of Scheme 4.
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(iii) If the metallic salt of anion PT–SO2CH–R is stable under the reaction conditions, that is, if the sulfone can be recovered
unchanged after the reaction and final neutralization—a fact that can be easily proved by a blank experiment with very few mg of the
substrate—, it is better to use a large molar excess of PT–SO2CH–R/PT–SO2CH2R with regard to the aldehyde (often chiral, usually
enolizable in the presence of a strong base), as we did in the complex case of Scheme 10.
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EXPERIMENTAL SECTION
General Methods. Unless specified otherwise, all starting materials and reagents were obtained from commercial suppliers and used without
further purification. All reactions were conducted in oven-dried glassware, under N2 or Ar, with anhydrous solvents, which were dried and distilled
before use according to standard procedures. Solvents used for isolation of products and chromatography were glass distilled. Analytical thin-layer
chromatography (TLC) was performed on 0.25 mm silica gel plates (F254); retention factors (Rf) are approximate. Flash column chromatography
was performed on silica gel (35–70 µm). Yields were determined after purification of the desired compound by flash column chromatography on
silica gel and removal of last traces of solvent (high vacuum, up to constant weight). IR spectra were recorded using an attenuated total reflectance
FTIR apparatus and the wavenumbers of maximum absorption peaks are reported in cm–1. 1H NMR spectra were recorded on 400 MHz
spectrometers; chemical shifts are reported in ppm (δ values), in CDCl3, with TMS as internal reference or with the solvent resonance as the
internal standard (CHCl3 impurity in CDCl3, δ 7.26 ppm); data are reported in the following order: chemical shift, multiplicity (s = singlet, d =
doublet, t = triplet, q = quartet, quint = quintuplet, br = broad, m = multiplet), coupling constants in Hz, integration. 13C NMR spectra were recorded
in CDCl3 on the above-mentioned spectrometers (100.6 MHz for 13C) with complete proton decoupling (BB) and DEPT; chemical shifts are
reported in ppm (δ values) with the solvent as the internal standard (CDCl3, δ 77.0 ppm). Where necessary, 2D NMR experiments (HSQC and
NOESY) were carried out to assist in structure elucidation and signal assignments. Optical rotations were measured on a polarimeter at 20 ˚C and
are reported as follows: [α]D (c in g/100 mL, solvent). The high-resolution mass spectra (HRMS, m/z values) were obtained by the electrospray
ionization (ESI, TOF) technique, in the positive or negative mode (as indicated).
(4R,5R)-2,2-Dimethyl-5-(triethylsilyloxymethyl)-1,3-dioxolan-4-yl)methanol, (2R,3R)-2,3-O-isopropylidene-4-(triethylsilyloxy)-1,2,3-
butanetriol (3). Sodium hydride (NaH, 60% dispersion in mineral oil, 492 mg, 0.012 mol) was added to a solution of (2R,3R)-2,3-O-
isopropylidenebutane-1,4-diol (1.90 g, 0.010 mol) in anhydrous THF (58 mL) at 0 ºC. Triethylsilyl chloride (TESCl, 20 mL, 0.010 mmol) was then
added and the reaction mixture was stirred at rt for 3 h, poured into water (150 mL) and extracted with CH2Cl2 (2 × 100 mL). The combined organic
layers were washed with brine (50 mL), dried over Na2SO4, and concentrated under high pressure. Purification by flash column chromatography
(hexanes/EtOAc, 6:4) afforded 3 (2.23 g, 70%) as a colorless oil: Rf (hexanes/EtOAc, 7:3) 0.3; [α]D –15.6 (c 2.07, CHCl3); IR 3465, 2912, 1239, 1079,
1003; 1H NMR 0.62 (q, J = 7.9, 6H), 0.96 (t, J = 8.0, 9H), 1.40 (s, 3H), 1.41 (s, 3H), 3.65 (m, 1H), 3.70–3.78 (m, 2H), 3.85–3.91 (m, 2H), 3.97 (dt, J =
7.6, 4.7, 1H); 13C NMR 4.1, 6.5, 26.8, 26.9, 62.7, 63.3, 78.3, 80.1, 109.0; HRMS (ESI+) calcd for C13H29O4Si(M + H)+ 277.1830, found 277.1827.
(2S,3R)-2,3-O-Isopropylidene-4-(triethylsilyloxy)butanal (4). Sodium hydrogen carbonate (NaHCO3, 1.60 g, 19.0 mmol) and Dess–Martin
periodinane (DMP, 860 mg, 2.02 mmol) were added to a solution of 3 (525 mg, 1.89 mmol) in CH2Cl2 (19 mL). The reaction mixture was stirred
for 1 h at 0 ºC and at rt for 30 min and was then quenched with a saturated aqueous solution of Na2S2O3 (100 mL) and diluted with Et2O (100 mL).
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The layers were separated, the aqueous layer was extracted with Et2O (3 × 70 mL), and the combined organic extracts were dried over MgSO4 and
4
5
6
7
8
9
concentrated. The residue was purified on silica gel (hexanes/EtOAc, 7:3) to give 500 mg (90%) of 4 as a colorless oil: Rf (hexanes/EtOAc, 8:2) 0.45; [α]D –
4.1 (c 1.56, CHCl3); IR 2953, 2912, 2876, 1735, 1456, 1239, 1077; 1H NMR 0.62 (q, J = 8.0, 6H), 0.96 (t, J = 7.9, 9H), 1.42 (s, 3H), 1.48 (s, 3H), 3.80 (m,
2H, AB part of an ABX system), 4.12 (dt, J = 7.3, 4.6, 1H), 4.31 (dd, J = 7.3, 1.6, 1H), 9.77 (d, J = 1.6, 1H); 13C NMR 4.5, 6.8, 26.3, 26.6, 62.8, 77.6, 82.0,
111.5, 200.7; HRMS (ESI+) calcd for C13H27O4Si(M + H)+ 275.1673, found 275.1677.
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(S)-4-Benzyl-3-{(2R,3E)-2-methyl-5-[(1-phenyl-1H-tetrazol-5-yl)thio]-3-pentenoyl}-1,3-oxazolidin-2-one (6). Triphenylphosphine (3.30 g,
12.0 mmol) and 1-phenyl-1H-tetrazole-5-thiol (2.22 g, 12.0 mmol) were added to a solution of alcohol 56c (2.40 g, 8.0 mmol) in THF (80 mL). The
mixture was cooled to 0 ºC and diethyl azodicarboxylate (DEAD, 40% in toluene, 6.6 mL, 14.0 mmol) was added. The solution was stirred for 1 h
at rt and was then quenched by addition of a saturated aqueous solution of NaHCO3 (150 mL). The aqueous phase was extracted with CH2Cl2 (3
× 75 mL), and the combined organic extracts were washed with brine (150 mL), dried over MgSO4, filtered, and concentrated under reduced
pressure. The residue was purified by flash column chromatography (hexanes/EtOAc, 6:4) to yield 3.40 g (91%) of 6 as a colorless oil: Rf
(hexanes/EtOAc, 6:4) 0.55; [α]D +18.5 (c 0.49, CHCl3); 1H NMR 1.27 (d, J = 6.9, 3H), 2.74 (dd, J = 13.4, 9.7), 3.24 (dd, J = 13.4, 3.3), 4.01–4.14
(m, 2H), 4.14–4.22 (m, 2H), 4.44 (quint, J = 7.0, 1H), 4.66 (ddd, J = 13.2, 7.2, 3.5, 1H), 5.84 (dd, J = 15.3, 7.2, 1H), 6.00 (dd, J = 15.3, 7.7, 1H),
7.17–7.21 (m, 2H), 7.27–7.35 (m, 3H), 7.52–7.58 (m, 5H); 13C NMR 16.9, 35.2, 37.7, 40.5, 55.2, 66.1, 123.5, 125.6, 127.3, 128.9, 129.4, 129.8,
130.1, 130.2, 134.6, 135.2, 152.9, 153.6, 174.1; HRMS (ESI+) calcd for C23H24N5O3S+ (M + H)+ 450.1594, found 450.1600.
(2R,3E)-2-Methyl-5-[(1-phenyl-1H-tetrazol-5-yl)thio]-3-penten-1-ol (7). Sodium tetrahydridoborate (sodium borohydride, NaBH4, 67
mg, 1.7 mmol) was added in portions for 1 h to a stirring solution of 6 (619 mg, 1.30 mmol) in 1:1 THF–water (14 mL) at 0 ºC. The reaction
mixture was stirred at rt for 2 h and was then quenched with a 2 M HCl solution (20 mL) and diluted with CH2Cl2 (20 mL). The phases were
separated and the aqueous layer was extracted with CH2Cl2 (3 × 20 mL). The combined organic layers were washed with brine (20 mL) and
dried over MgSO4. After filtration and evaporation of the solvent, the residue was purified on silica gel (hexanes/EtOAc, 1:1) to give 330 mg
(89%) of 7 as a colorless oil: Rf (hexanes/EtOAc, 7:3) 0.3; α]D +22.1 (c 8.19, CHCl3); 1H NMR 0.97 (d, J = 6.8, 3H), 1.82 (br s, 1H) 2.30–2.42
(m, 1H), 3.41 (dd, J = 10.6, 7.4, 1H), 3.49 (dd, J = 10.6, 5.6, 1H), 3.96 (m, 2H), 5.64–5.76 (m, 2H), 7.55 (m, 5H); 13C NMR 16.0, 35.6, 39.4,
67.0, 123.9, 124.1, 129.8, 130.1, 133.6, 139.1, 153.8; HRMS (ESI+) calcd for C13H17N4OS+ (M + H)+ 277.1118, found 277.1124.
(2E,4R)-5-(tert-Butyldiphenylsilyloxy)-4-methyl-1-[(1-phenyl-1H-tetrazol-5-yl)thio]-2-pentene (8). tert-Butyldiphenylsilyl chloride (TBDPSCl,
423 µL, 447 mg, 1.62 mmol) was added dropwise to a stirring solution of alcohol 7 (300 mg, 1.08 mmol) and imidazole (1.49 g, 2.17 mmol) in
THF (5.4 mL) at 0 ºC. After 1 h at rt the reaction was quenched with water (50 mL) and diluted with CH2Cl2 (30 mL). The phases were separated
and the aqueous layer was extracted with CH2Cl2 (3 × 30 mL). The combined organic extracts were dried over MgSO4, filtered, and concentrated.
The residue was purified on silica gel (hexanes/EtOAc, 8:2) to give 489 mg (96%) of 8 as a white solid: mp 46–48 ºC. Rf (hexanes/EtOAc, 8:2)
0.55; [α]D +2.5 (c 0.92, CHCl3); IR 3072, 2955, 2921, 2857, 1606, 1499, 1385, 1110; 1H NMR 0.99 (d, J = 6.8, 3H), 1.03 (s, 9H), 2.33–2.45 (m,
1H), 3.49–3.53 (m, 2H), 4.01 (d, J = 7.1, 2H), 5.66 (dtd, J = 15.2, 7.1, 0.8, 1H), 5.78 (dd, J = 15.4, 7.1, 1H), 7.33–7.44 (m, 6H), 7.51–7.58 (m, 5H),
7.62–7.66 (m, 4H); 13C NMR 16.2, 19.3, 26.8, 35.7, 39.1, 68.1, 122.6, 123.7, 127.6, 129.5, 129.7, 130.0, 133.7, 135.6, 139.6, 154.0; HRMS (ESI+)
calcd for C29H35N4OSSi+ (M + H)+ 515.2295, found 515.2308.
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(2E,4R)-5-(tert-Butyldiphenylsilyloxy)-4-methyl-1-[(1-phenyl-1H-tetrazol-5-yl)sulfonyl]-2-pentene (9). A solution of ammonium hepta-
molybdate—tetrahydrate (687 mg, 0.55 mmol) in 33% w/w H2O2 (7.9 mL) was added to a stirring solution of 8 (714 mg, 1.39 mmol) in EtOH
(27.6 mL) at 0 ºC. After 3 h at rt the reaction was quenched with a saturated aqueous NH4Cl solution (80 mL) and CH2Cl2 (50 mL). The aqueous
layer was extracted with CH2Cl2 (2 × 50 mL) and the combined organic fractions were dried over anhydrous Na2SO4, filtered, and concentrated.
9
Flash column chromatography (hexanes/EtOAc, 9:1) provided 666 mg (88%) of 9 as a colorless oil: Rf (hexanes/EtOAc, 8:2) 0.6; [α]D +9.6 (c 1.36,
CHCl3); IR 3070, 2930, 1471, 1345, 1112; 1H NMR 0.98 (d, J = 6.8, 3H), 1.03 (s, 9H), 2.36–2.47 (m, 1H), 3.50 (d, J = 6.3, 2H), 4.35 (d, J = 7.4,
2H), 5.52 (dt, J = 15.6, 7.4, 1H), 5.93 (dd, J = 15.5, 7.2, 1H), 7.34–7.45 (m, 8H), 7.54–7.66 (m, 7H, Ph); 13C NMR 16.0, 19.3, 26.8, 39.6, 59.9,
67.8, 112.7, 125.2, 127.7, 129.7, 131.4, 133.0, 133.6, 134.8, 135.6, 147.4, 153.2; HRMS (ESI+) calcd for C29H34N4NaO3SSi+ (M + Na)+ 569.2013,
found 569.2000.
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(2R,3E,5E,7R,8R)-1-tert-Butyldiphenylsilyloxy-7,8-O-isopropylidene-2-methyl-9-(triethylsilyloxy)-3,5-nonadiene, (3E,5E)-10. A solution
of KHMDS (66 mg, 0.33 mmol) and 18-crown-6 (88 mg, 0.33 mmol) in dry DMF (0.7 mL) was added dropwise to a stirred solution of sulfone
9 (73 mg, 0.13 mmol) and aldehyde 4 (50 mg, 0.16 mmol) in anhydrous DMF (0.6 mL) at −65 ºC under Ar. After the addition, the temperature
was raised to –40 ºC and stirring was continued for 6 h. The reaction was quenched with a saturated NH4Cl solution (20 mL) and diluted with
EtOAc (10 mL). The aqueous phase was extracted with EtOAc (3 × 10 mL) and the organic phases were combined and washed with brine (30
mL), dried over Na2SO4, filtered, and concentrated under reduced pressure. The residue was purified by flash column chromatography
(hexanes/EtOAc, 9:1) to afford 63 mg (80%) of a 82:13:5 mixture of (3E,5E)-10, (3E,5Z)-10, and (3Z,5E)-10. A sample of pure (3E,5E)-10 was
isolated after a second flash chromatography: colorless oil; Rf (hexanes/EtOAc, 9:1) 0.7; [α]D +8.4 (c 0.89, CHCl3); IR 1955, 2930, 2875, 1460,
1427, 1251, 1116, 1087; 1H NMR 0.61 (q, J = 7.9, 6H), 0.95 (t, J = 7.9, 9H), 1.05 (s, 12H), 1.41 (s, 3H), 1.43 (s, 3H), 2.49–2.36 (m, 1H), 3.49
(dd, J = 9.7, 6.6, 1H), 3.54 (dd, J = 9.7, 6.4, 1H), 3.63–3.82 (m, 3H), 4.33 (t, J = 7.5, 1H), 5.56 (dd, J = 15.2, 7.4, 1H), 5.66 (dd, J = 15.3, 7.3,
1H), 6.05 (dd, J = 15.3, 10.5, 1H), 6.26 (dd, J = 15.2, 10.4, 1H), 7.33–7.45 (m, 5H), 7.63–7.67 (m, 5H); 13C NMR 4.4, 6.7, 16.4, 19.3, 26.8,
+
26.9, 27.1, 39.3, 62.3, 68.4, 78.7, 81.6, 109.0, 127.6, 127.0, 128.9, 129.5, 133.9, 134.1, 135.6, 138.4; HRMS (ESI+) calcd for C35H58NO4Si2
(M + NH4)+ 612.3899, found 612.3905.
Selective deprotection of 10. Pyridinium p-toluenesulfonate (PPTS, 98 mg, 0.39 mmol) was added to a mixture of diastereoisomers (10, 289
mg, 0.49 mmol) in CH2Cl2 (20 mL) and EtOH (3.3 mL) at 0 ºC. The reaction was stirred overnight at rt. Triethylamine (Et3N, 0.1 mL) was then
added and the solvent was removed under reduced pressure. A flash column chromatography on silica gel (hexanes/EtOAc, 8:2) allowed us to
isolate (3E,5E)-11 (174 mg, 75%, ca. 65% overall yield) and (3E,5Z)-11 (50 mg, 21%, ca. 10% overall yield). Data for (2R,3R,4E,6E,7R)-9-tert-
butyldiphenylsilyloxy-2,3-O-isopropylidene-8-methyl-4,6-nonadien-1-ol, (3E,5E)-11: colorless oil; Rf (hexanes/EtOAc, 9:1) 0.20; [α]D +3.4 (c
1.15, CHCl3) [lit.4a [α]26D +4.4 (c 0.55, CHCl3); lit.6e [α]D: +6.4 (c 3.9, CHCl3)]; IR 2955, 2874, 1590, 1456, 1427, 1250, 1080; 1H NMR 1.03 (d, J
= 6.8, 3H), 1.04 (s, 9H), 1.44 (s, 3H), 1.44 (s, 3H), 2.39–2.48 (m, 1H), 3.47–3.55 (m, 2H), 3.56–3.62 (m, 1H), 3.75–3.80 (m, 1H), 3.80–3.86 (m,
1H), 4.35 (t, J = 8.0, 1H), 5.53 (dd, J = 15.2, 7.9, 1H), 5.68 (dd, J = 15.3, 7.3, 1H), 6.05 (dd, J = 15.3, 10.4, 1H), 6.28 (dd, J = 15.2, 10.4, 1H), 7.34–
7.44 (m, 6H), 7.63–7.67 (m, 4H); 13C NMR 16.3, 19.3, 26.8, 26.9, 27.1, 39.3, 60.7, 68.3, 77.8, 81.1, 109.0, 127.0, 127.4, 128.7, 129.5, 133.8, 135.0,
135.6, 139.0; HRMS (ESI+) calcd for C29H44NO4Si+ (M + NH4)+ 498.3034, found 498.3032. NMR data are consistent with previously reported
values.4a,6e Data for (2R,3R,4Z,6E,7R)-9-tert-butyldiphenylsilyloxy-2,3-O-isopropylidene-8-methylnona-4,6-dien-1-ol, (3E,5Z)-11: colorless oil;
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Rf (hexanes/EtOAc, 9:1) 0.25; [α]D +12.1 (c 4.19, CHCl3); 1H NMR 1.05 (s, 12H), 1.46 (s, 6H), 2.42–2.53 (m, 1H), 3.48–3.59 (m, 3H), 3.72–3.77
(m, 1H), 3.82 (dd, J = 12.1, 2.8, 1H), 4.85 (t, J = 8.7, 1H), 5.27–5.35 (m, 1H), 5.73 (dd, J = 15.0, 7.4, 1H), 6.18 (t, J = 11.0, 1H), 6.35 (dd, J = 15.0,
11.1, 1H), 7.32–7.46 (m, 6H), 7.62–7.68 (m, 4H); 13C NMR 16.4, 19.3, 26.9, 27.0, 27.0, 39.5, 60.4, 68.4, 72.9, 81.4, 109.1, 124.4, 124.8, 127.6,
129.5, 133.9, 134.1, 135.6, 140.9; HRMS (ESI+) calcd for C29H44NO4Si+ (M + NH4)+ 498.3034, found 498.3037.
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(2S,3R,4E,6E,7R)-9-tert-Butyldiphenylsilyloxy-8-methyl-2,3-O-(1,1-dimethylmethylene)-4,6-nonadien-1-al (12). To a solution of (3E,5E)-
11 (48 mg, 0.10 mmol) in CH2Cl2 (1 mL) was added NaHCO3 (8 mg, 0.1 mmol) and DMP (55 mg, 0.12 mmol) under N2 at rt. After stirring for 1 h,
the mixture was quenched with a saturated aqueous solution of Na2S2O3 (10 mL) and diluted with Et2O (10 mL), and the aqueous layer was
extracted with Et2O (3 × 10 mL). The organic extracts were dried over MgSO4, filtered, and concentrated. The residue was purified on silica gel
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(hexanes/EtOAc, 70:30) to give 34 mg (92%) of 12 as a yellowish oil: Rf (hexanes/EtOAc, 70:30) 0.60; [α]D +11.0 (c 0.90, CHCl3) [lit.4a [α]27
D
+11.7 (c 0.60, CHCl3)]; IR 2929, 2856, 1735, 1427, 1214, 1111, 1073; 1H NMR 1.03 (d, J = 6.7, 3H), 1.05 (s, 9H), 1.46 (s, 3H), 1.50 (s, 3H), 2.38–
2.50 (m, 1H), 3.50 (dd, J = 9.7, 6.5, 1H), 3.55 (dd, J = 9.7, 6.3, 1H), 4.06 (dd, J = 7.7, 2.1, 1H), 4.51 (t, J = 7.3, 1H), 5.58 (dd, J = 15.2, 7.3, 1H),
5.71 (dd, J = 15.3, 7.3, 1H), 6.05 (dd, J = 15.4, 10.4, 1H), 6.29 (dd, J = 15.2, 10.4, 1H), 7.34–7.44 (m, 6H), 7.63–7.65 (m, 4H), 9.72 (d, J = 2.1,
1H); 13C NMR 16.3, 19.3, 26.2, 26.8, 26.9, 39.3, 68.3, 77.8, 84.7, 111.3, 125.8, 127.6, 128.4, 129.6, 133.8, 135.1, 135.6, 139.8, 199.7; HRMS
(ESI+) calcd for C29H42NO4Si+ (M + NH4)+ 496.2878, found 496.2884. NMR data are in agreement with those reported in the literature.4a
(S)-5-tert-Butyldiphenylsilyloxy-1,2-epoxypentane, (S)-2-(3-tert-butyldiphenylsilyloxy)propyloxirane (14). TBDPS-protected 4-penten-1-
ol [5-(tert-butyldiphenylsilyloxy)-1-pentene], a known compound27 (13, 25.0 mg, 0.077 mmol), CH2Cl2 (0.4 mL), and 30% aqueous H2O2
(without stabilizers) (0.050 mL, 0.46 mmol, 6 equiv) were added to the Ti–cis-salalen catalyst, prepared independently from Ti(OiPr)4 (6.8 µL,
0.023 mmol) and cis-salalen (11 mg, 0.023 mmol) in anhydrous CH2Cl2 under Ar, following the protocol described by Berkessel et al.17
A
vigorous stirring was maintained in an open atmosphere, at rt. After 24 h, 6 equiv more of H2O2 were added. Again, 24 h later, further 6 equiv
of H2O2 were added, and the vigorous stirring was continued for 24 h at rt. The reaction mixture was diluted with CH2Cl2 (5 mL) and H2O (5
mL), the layers were separated and the aqueous phase was re-extracted with CH2Cl2 (5 mL). The combined organic phases were washed with
brine (2 × 15 mL), dried over Na2SO4, and filtered. The solvent was removed under reduced pressure and the residue was purified by column
chromatography (90:10 hexanes/EtOAc) to afford 14 (23.5 mg, 90%) as yellowish oil: Rf (hexanes/EtOAc, 80:20) 0.58; [α]D –2.4 (c 1.0,
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CHCl3) [lit.28 [α]D –3.41 (c 1.12, CHCl3), lit.27 [α]D –2.7 (c 1.1, CHCl3)]; H NMR 1.05 (s, 9H), 1.57–1.77 (m, 4H), 2.45 (dd, J = 5.0, J = 2.7,
1H), 2.72–2.74 (m, 1H), 2.89–2.93 (m, 1H), 3.66–3.75 (m, 2H), 7.38–7.67 (m, 10H); HPLC (Chiralpack AS-H, hexanes/isopropanol 99.5:0.5,
0.4 mL/min, λ = 254 nm) tR(R) = 10.7 min (minor), tR(S) = 11.2 min (major), 4:96 ratio. NMR data are consistent with reported values.28
The reaction was repeated under identical conditions but using the binaphthyl analog of cis-salalen (see Scheme 7).17c Only two additions of
0.050 mL of 30% H2O2 (without stabilizers) and two days of reaction were required for the complete epoxidation. Workup as above yielded
26.0 mg (98%) of 14, 96:4 e.r. by chiral HPLC.
(R)-1-(tert-Butyldiphenylsilyoxy)-7-octen-4-ol (15). A solution of allylmagnesium bromide (1 M in Et2O, 7.3 mL, 7.3 mmol) was added
dropwise to a stirred suspension of CuCl (Cu2Cl2, 76 mg, 0.76 mmol of CuI) in anhydrous THF (12 mL), at –50 ºC under N2 atmosphere. Then,
a solution of 14 (1.00 g, 2.94 mmol) in anhydrous THF (6 mL) was added via cannula and the reaction mixture was stirred at –50 ºC for 30
min. The reaction was quenched by addition of an aqueous NaHCO3 solution (5%, 40 mL) and Et2O (40 mL). The layers were separated and
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the aqueous phase was extracted with Et2O (3 × 40 mL). The combined organic layers were dried over MgSO4 and filtered. The solvent was
removed under reduced pressure and the residue was purified by column chromatography (CH2Cl2 to 99:1 CH2Cl2/MeOH) to afford 15 (0.99 g,
88%) as a yellowish oil: Rf (CH2Cl2) 0.24; [α]D –0.84 (c 1.8, CHCl3); 1H NMR 1.05 (s, 9H), 1.45–1.62 (m, 6H), 2.08–2.18 (m, 2H), 3.62–3.66
(m, 1H), 3.71 (t, J = 7.8, 2H), 4.98 (ddt, J = 10.2, 2.0, 1.0, 1H), 5.06 (dq, J = 17.1, 1.6, 1H), 5.85 (ddt, J = 17.1, 10.2, 6.7, 1H), 7.36–7.47 (m,
6H), 7.66–7.71 (m, 4H). NMR data agree with those previously reported.28
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(2S,5S)-5-[3-(tert-Butyldiphenylsilyloxy)propyl]tetrahydrofuran-2-methanol (16). Compound 15 (64.0 mg, 0.167 mmol), CH2Cl2 (0.9
mL), and 30% aqueous H2O2 (without stabilizers) (0.110 mL, 1.0 mmol) were added to the complex of TiIV and ent-cis-salalen catalyst (0.05
mmol, prepared as above from Ti(OiPr)4 and the enantiomer of cis-salalen, see Scheme 7). The reaction mixture was vigorously stirred at room
temperature for 3 days, open to the air. More H2O2 was added each day, as above (up to 3 mmol, 18 equiv). The reaction was then diluted with
CH2Cl2 (10 mL) and H2O (10 mL), the phases were separated and the organic phase was washed with brine (2 × 15 mL), dried over MgSO4,
and filtered. The solvent was removed under reduced pressure and the residue was analyzed by chiral HPLC (Chiralpack AS-H,
hexanes/isopropanol 99:1, 0.6 mL/min, λ = 254 nm) tR(SS) = 22.6 min (≥ 85%); tR(RR) = 23.6 min (≤ 1%); tR(RS), tR(SR) = 25.4 min (7%) and
26.1 min (7%). The desired compound (16, major component of the residue) was isolated after flash chromatographic separation
(hexanes/EtOAc, 85:15) as a yellowish oil: 51 mg (76%); Rf (hexanes/EtOAc, 7:3) 0.2; [α]D +4.38 (c 1.00, CHCl3); IR: 3420, 1106, 1006, 822;
1H NMR 1.05 (s, 9H), 1.43–1.50 (m, 1H), 1.53–1.73 (m, 4H), 1.84–2.00 (m, 3H), 3.46 (dd, J = 11.3, 5.6, 1H), 3.67–3.70 (m, 3H), 3.83–3.89
(m, 1H), 3.96–4.01 (m, 1H), 7.35–7.42 (m, 6H), 7.65–7.68 (m, 4H); 13C NMR 19.2, 26.8, 27.0, 29.3, 31.4, 32.1, 63.7, 65.3, 79.2, 79.9, 127.6,
129.5, 133.9, 135.5; HRMS (ESI+) calcd for C24H35O3Si+ (M + H)+ 399.2350, found 399.2345.
(2S,5S)-5-[3-(tert-Butyldiphenylsilyloxy)propyl]tetrahydrofuran-2-carboxaldehyde (17). A solution of 16 (50 mg, 0.13 mmol) in
CH2Cl2 (1.3 mL) was treated with DMP (70 mg, 0.20 mmol) at 0 ºC. After 3 h at rt, the mixture was quenched with a saturated aqueous
solution of Na2S2O3 (10 mL) and diluted with Et2O (10 mL). The aqueous layer was extracted with Et2O (3 × 10 mL) and the combined organic
extracts were washed with brine (20 mL), dried over Na2SO4, and concentrated. The residue was purified on silica gel (hexanes/EtOAc, 7:3) to
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give 45 mg (88%) of 17 as a yellow oil: Rf (hexanes/EtOAc, 7:3) 0.5; [α]D –19.4 (c 10.0, CHCl3); H NMR 1.05 (s, 9H), 1.43 (m, 1H), 1.56–
1.75 (m, 4H), 1.95–2.17 (m, 3H), 3.70 (dd, J = 8.1, 3.9, 2H), 3.99–4.05 (m, 1H), 4.22 (ddd, J = 8.4, 5.4, 1.8, 1H), 7.35–7.44 (m, 6H), 7.65–7.68
(m, 4H), 9.66 (d, J = 1.8, 1H); 13C NMR 19.2, 26.8, 27.8, 29.2, 31.1, 31.9, 63.6, 81.1, 82.9, 127.6, 129.5, 133.9, 135.5, 203.3; HRMS (ESI+)
calcd for C24H33O3Si+ (M + H)+ 397.2193, found 397.2185.
(4S,7S)-8-Benzyloxy-7-(tert-butyldimethylsilyloxy)-1-octen-4-ol (19). To a stirred solution of aldehyde 1820 (420 mg, 1.30 mmol) in
anhydrous Et2O (2.3 mL) at –78 ºC was added (+)-Icp2B(allyl) (1.4 mL of 1 M hexane solution, 1.4 mmol) diluted with Et2O (2 mL). One hour
later, a 3 M solution of NaOH (0.5 mL, 1.6 mmol) was slowly added, followed by the addition of a 33% solution of H2O2 (0.5 mL, 4.9 mmol).
After stirring at rt for 5 h, the aqueous phase was extracted with Et2O (3 × 20 mL). The combined organic phases were washed with brine (30
mL), dried over MgSO4, filtered, and concentrated. Flash column chromatography (hexanes/EtOAc, 7:3) afforded 19 (449 mg, 95%): colorless
oil; Rf (hexanes/EtOAc, 7:3) 0.5; [α]D −11.6 (c 1.55, CHCl3); 1H NMR 0.05 (s, 3H), 0.06 (s, 3H), 0.88 (s, 9H), 1.50–1.67 (m, 4H), 1.78 (s, 1H),
2.12–2.19 (m, 1H), 2.26–2.32 (m, 1H), 3.37 (dd, J = 9.6, 5.5), 3.42 (dd, 1H, J = 9.6, 5.5), 3.61–3.68 (m, 1H), 3.84–3.90 (m, 1H), 4.49–4.55 (m,
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2H), 5.10–5.11 (m, 1H), 5.13–5.15 (m, 1H), 5.77–5.88 (m, 1H), 7.27–7.37 (m, 5H); 13C NMR −4.8, −4.3, 18.1, 25.9, 30.4, 32.0, 41.9, 70.6,
71.4, 73.3, 74.2, 117.8, 127.5, 127.6, 128.3, 134.9, 138.3; HRMS (ESI+) calcd for C21H37O3Si+ (M + H)+ 365.2506, found 365.2512.
(2S,5R)-2-(Phenylmethoxy)methyl-5-(2-propen-1-yl)tetrahydrofuran, (2R,5S)-2-allyl-5-[(benzyloxy)methyl]oxolane (20). Methane-
sulfonyl chloride (MsCl, 129 µL, 1.55 mmol) was added to a stirred solution of 19 (434 mg, 1.19 mmol) and Et3N (248 µL, 1.78 mmol) in
CH2Cl2 (2.4 mL) at 0 ºC. The reaction mixture was stirred for 2 h at 0 ºC and for 2 h at rt and was then quenched with water (10 mL). The
aqueous phase was extracted with Et2O (3 × 10 mL). The combined organic phases were washed with brine (20 mL), dried over MgSO4, and
filtered. The solvent was removed under reduced pressure to obtain 518 mg (98%) of the Ms derivative of 19 (which was used in the next step
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without purification): colorless oil; Rf (hexanes/EtOAc, 7:3) 0.5; H NMR 0.04 (s, 3H), 0.05 (s, 3H), 0.87 (s, 9H), 1.57–1.80 (m, 4H), 2.47–
2.49 (m, 2H), 2.96 (s, 3H), 3.30 (dd, J = 9.6, 5.9, 1H), 3.37 (dd, J = 9.5, 5.4, 1H), 3.81–3.89 (m, 1H), 4.47–4.55 (m, 2H), 4.70–4.78 (m, 1H),
5.12-5.13 (m, 1H), 5.16–5.19 (m, 1H), 5.71–5.85 (m, 1H), 7.27-7.38 (m, 5H); 13C NMR −4.8, −4.3, 18.1, 25.9, 29.5, 29.8, 38.7, 39.1, 70.6,
73.3, 74.3, 82.7, 119.0, 127.5, 127.6, 128.3, 132.3, 138.2. Deprotection and cyclization. Tetrabutylammonium fluoride (TBAF·3H2O, 664 mg,
2.10 mmol) was added to the preceding compound (518 mg, 1.17 mmol) in THF (20 mL). After stirring for 18 h, the reaction was concentrated
under reduced pressure and the residue was purified by flash column chromatography, using hexanes/EtOAc (8:2) as eluent, to afford
tetrahydrofuran derivative 2029 (263 mg, 92% yield): colorless oil; Rf (hexanes/EtOAc, 7:3) 0.6; IR 2854, 1641, 1090, 1028; [α]D −1.4 (c 13.0,
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CHCl3); H NMR 1.52–1.59 (m, 1H), 1.65–1.72 (m, 1H), 1.88–1.98 (m, 2H), 2.21–2.28 (m, 1H), 2.36–2.43 (m, 1H), 3.45 (dd, J = 10.0, 4.9,
1H), 3.50 (dd, J = 10.0, 5.8, 1H), 3.90-3.97 (m, 1H), 4.05-4.12 (m, 1H), 4.54-4.62 (m, 2H), 5.02-5.10 (m, 2H), 5.82 (tdd, J = 17.2, 10.2, 7.0,
1H), 7.26-7.36 (m, 5H); 13C NMR 28.1, 30.1, 40.2, 73.0, 73.3, 78.1, 79.2, 116.7, 127.5, 127.6, 128.3, 135.0, 138.4; HRMS (ESI+) calcd for
C15H21O2 (M+H)+ 233.1536, found 233.1546.
Conversion of 20 into 17. A 0.5 M solution of 9-BBN in THF (2 mL, 1 mmol) was added dropwise to a stirring solution of 20 (122 mg,
0.57 mmol) in dry THF (2 mL) at 0 ºC under N2. After 4 h, the ice bath was removed and a 2 M solution of NaOH (1.1 mL, 2.0 mmol) and
H2O2 (33% w/w, 1.1 mL, 8.5 mmol) were added. After 18 h at rt, a saturated solution of NaCl (10 mL) was added to the reaction mixture. The
organic layer was separated and the aqueous phase was extracted with CH2Cl2 (3 × 30 mL). The organic layers were washed with brine, dried
over anhydrous MgSO4, filtered, and concentrated. The residue was purified on silica gel (CH2Cl2/MeOH, 95:5) to give the corresponding
alcohol (128 mg, 91%): colorless oil; Rf (hexanes/EtOAc, 7:3) 0.3; [α]D −9.1 (c 10.8, CHCl3); 1H NMR 1.49–1.75 (m, 6H), 1.89–1.99 (m, 2H),
3.47 (d, J = 5.4, 2H), 3.65–3.68 (m, 2H), 3.88–3.93 (m, 1H), 4.08–4.14 (m, 1H), 4.55 (d, J = 12.2, 1H), 4.59 (d, J = 12.2, 1H), 7.27–7.29 (m,
1H), 7.33–7.35 (m, 4H); 13C NMR 28.0, 29.7, 31.0, 32.6, 62.8, 72.8, 73.3, 78.1, 80.0, 127.5, 127, 128.3, 138.3; HRMS (ESI+) calcd for
C15H23O3 (M+H)+ 251.1642, found 251.1639. Silylation. Imidazole (132 mg, 1.92 mmol) was added to a stirred solution of the preceding
alcohol (240 mg, 0.96 mmol) in THF (4.8 mL). Then TBDPSCl (374 µL, 1.44 mmol) was added dropwise at 0 ºC and the resulting mixture was
stirred at rt for 3 h. The solution was poured into water (20 mL) and the aqueous phase was extracted with CH2Cl2 (3 × 30 mL). The combined
organic extracts were washed with brine (30 mL), dried over Na2SO4, filtered, and concentrated. Flash column chromatography
(hexanes/EtOAc, 9:1) afforded 462 mg (98%) of the TBDPS-protected product: colorless oil; Rf (hexanes/EtOAc, 7:3) 0.6; [α]D −2.0 (c 1.25,
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CHCl3); IR 2855, 1462, 1110, 1087; H NMR 1.04 (s, 9H), 1.04–1.72 (m, 6H), 1.88–1.96 (m, 2H), 3.43 (dd, J = 10.0, 4.9, 1H), 3.47 (dd, J =
9.9, 5.9, 1H), 3.65–3.68 (m, 2H), 3.80–3.86 (m, 1H), 4.02–4.09 (m, 1H), 4.52–4.61 (m, 2H), 7.26–7.42 (m, 10H), 7.65–7.72 (m, 5H); 13C NMR
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19.2, 26.7, 28.2, 29.2, 30.8, 32.1, 63.9, 73.1, 73.3, 77.9, 79.8, 127.4, 127.6, 127.7, 127.8, 128.3, 129.4, 129.6, 134.0, 134.8, 135.5, 138.4;
HRMS (ESI+) calcd for C31H44NO3Si (M+NH4)+ 506.3085, found 506.3096. Cleavage of the benzyl ether. Palladium (10% Pd/C, 60 mg, 0.06
mmol) was added to a solution of the TBDPS derivative (299 mg, 0.60 mmol) in absolute EtOH (4 mL) under a N2 atmosphere. After purging
with hydrogen, the suspension was energetically stirred for 18 h. The heterogeneous mixture was filtered under Celite® and washed with EtOH,
and the solvent was evaporated under vacuum to afford the desired alcohol (215 mg, 88%): yellowish oil; Rf (hexanes/EtOAc 7:3) 0.2; [α]D
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+8.15 (c 1.06, CHCl3); IR 3420, 1106, 1006; H NMR 1.05 (s, 9H), 1.43–1.50 (m, 1H), 1.53–1.71 (m, 4H), 1.84–2.00 (m, 3H), 3.46 (dd, J =
11.3, 5.6, 1H), 3.67–3.70 (m, 3H), 3.83–3.89 (m, 1H), 3.96–4.01 (m, 1H), 7.35–7.42 (m, 6H), 7.65–7.68 (m, 4H); 13C NMR 19.2, 26.8, 27.0,
29.3, 31.4, 32.1, 63.7, 65.3, 79.2, 79.9, 127.6, 129.5, 133.9, 135.5. Oxidation of the hydroxy group with Dess–Martin periodinane. A
solution of the preceding alcohol (50 mg, 0.13 mmol) in CH2Cl2 (1.3 mL) was treated with Dess–Martin periodinane (70 mg, 0.20 mmol) at 0
ºC. After 3 h at rt, the mixture was quenched with a saturated solution of Na2S2O3 (10 mL) and diluted with Et2O (10 mL). The aqueous layer
was extracted with Et2O (3 × 10 mL) and the combined organic extracts were washed with brine (20 mL), dried over Na2SO4, filtered, and
concentrated. The residue was purified on silica gel (hexanes/EtOAc, 7:3) to give 17 (49 mg, 82%). Oxidation of the hydroxy group under
the Swern conditions. Oxalyl dichloride [(COCl)2, 126 µL, 1.2 mmol] was added dropwise to a stirring solution of DMSO (195 µL, 2.4 mmol)
in dry CH2Cl2 (4 mL) at –78 ºC under N2. After 10 min, the alcohol (482 mg, 1.2 mmol) in CH2Cl2 (4 mL) was transferred via cannula, stirring
was continued for 45 min, and Et3N (661 mL, 6.5 mmol) was then added dropwise and the reaction was stirred for 2 h at rt. It was then diluted
with Et2O (50 mL) and the organic extract was washed with saturated NaHCO3 solution (50 mL), dried over MgSO4, filtered, and concentrated.
Flash column chromatography (hexanes/EtOAc 8:2) afforded the desired aldehyde, 17 (460 mg, 96%).
(R)-5-[(4-Bromo-2-methyl-4-penten-1-yl)thio]-1-phenyl-1H-tetrazole (22). Triphenylphosphine (275 mg, 1.05 mmol) and 1-phenyl-1H-
tetrazole-5-thiol (193 mg, 1.05 mmol) were added to a solution of alcohol 2121 (125 mg, 0.70 mmol) in THF (7 mL). The mixture was cooled to
0 ºC and DEAD, 40% in toluene, 564 µL, 1.05 mmol) was added. After stirring for 1 h, the solution was quenched by addition of a saturated
aqueous solution of NaHCO3 (10 mL). The aqueous phase was extracted with CH2Cl2 (3 × 10 mL) and the combined organic extracts were
washed with brine (15 mL), dried over MgSO4, filtered, and concentrated under reduced pressure. The residue was purified by flash column
chromatography (hexanes/EtOAc, 8:2) to obtain 199 mg (84%) of 22 as a colorless oil: Rf (hexanes/EtOAc, 8:2) 0.8; [α]D −8.2 (c 0.9, CHCl3);
IR 1629, 1495, 1384; 1H NMR 1.07 (d, J = 6.5, 3H), 2.32–2.42 (m, 2H), 2.59–2.66 (m, 1H), 3.35 (dd, J = 12.9, 6.8, 1H), 3.49 (dd, J = 12.9, 6.4,
1H), 5.49 (d, J = 1.4, 1H), 5.63 (m, 1H), 7.55–7.61 (m, 5H); 13C NMR 18.3, 31.5, 39.2, 47.3, 118.9, 123.9, 129.8, 130.1, 131.7, 133.7, 154.3;
HRMS (ESI+) calcd for C13H1679BrN4S+ (M + H)+ 339.0274, found 339.0265.
(R)-5-(4-Bromo-2-methyl-4-penten-1-yl)sulfonyl-1-phenyl-1H-tetrazole (23). A solution of 22 (100 mg, 0.29 mmol) in EtOH (3 mL) at 0
ºC was treated with ammonium heptamolybdate—tetrahydrate (73 mg, 0.06 mmol) in H2O2 (33% w/w, 276 µL, 2.93 mmol). After stirring
overnight at rt the resulting suspension, the solvent was evaporated and the residue was partitioned between water (30 mL) and CH2Cl2 (30
mL). The phases were separated and the aqueous layer was extracted with CH2Cl2 (3 × 50 mL). The combined organic extracts were dried over
MgSO4, filtered, and concentrated. Flash chromatography (hexanes/EtOAc, 7:3) provided 101 mg (93%) of 23 as a colorless oil: Rf
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(hexanes/EtOAc, 8:2) 0.5; [α]D +1.5 (c 0.9, CHCl3); IR 1629,1595, 1340, 1151; H NMR 1.21 (d, J = 6.3, 3H), 2.47 (dd, J = 14.0, 7.3, 1H),
2.65–2.78 (m, 2H), 3.65 (dd, J = 14.5, 7.8, 1H,), 3.84 (dd, J = 14.5, 5.6, 1H), 5.54 (d, J = 1.7, 1H), 5.66 (br s, 1H), 7.60–7.64 (m, 2H), 7.68–
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7.70 (m, 3H); 13C NMR 19.1, 27.1, 47.7, 60.4, 120.0, 125.2, 129.7, 130.4, 131.5, 133.1, 154.0; HRMS (ESI+) calcd for C13H1679BrN4O2S+ (M +
H)+ 371.0172, found 371.0185.
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(2S,5S)-2-[(2E,3R)-5-Bromo-3-methyl-1,5-hexadienyl]-5-[(3-tert-butyldiphenylsilyloxy)propyl]tetrahydrofuran (24).6d A 0.5 M solution of
KHMDS in toluene (2.2 mL, 1.1 mmol) was added dropwise to a stirred solution of sulfone 23 (434 mg, 1.17 mmol) in DMF (4 mL) at −65 ºC
under Ar. After 30 min at –65 ºC, a solution of aldehyde 17 (290 mg, 0.73 mmol) in DMF (3.3 mL) was added dropwise. The reaction mixture
was stirred for 18 h at rt and the reaction was then quenched with H2O (30 mL) and Et2O (20 mL). The aqueous phase was extracted with Et2O
(3 × 20 mL), the organic phases were combined and washed with brine (30 mL), dried over Na2SO4, filtered, and concentrated under reduced
pressure. The residue was purified by flash column chromatography (hexanes/EtOAc, 9:1) to afford 320 mg (80%) of 24 (96:4 E/Z) as colorless
oil, Rf (hexanes/EtOAc, 9:1) 0.6. Spectroscopic data as reported.6d
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Conversion of 276d to 28. Ethyldiisopropylamine (diisopropylethylamine, DIPEA, 450 µL, 2.58 mmol), NaI (64 mg, 0.43 mmol) and
MOMCl (129 µL, 139 mg, 1.72 mmol) were added to a solution of 27 (300 mg, 0.43 mmol) in MeCN (4.3 mL). The mixture was heated to 50
ºC for 3 h. Afterwards the reaction was quenched by addition of a saturated aqueous solution of NH4Cl (20 mL) and diluted with CH2Cl2 (20
mL). The organic layer was separated and the aqueous layer extracted with CH2Cl2 (3 × 20 mL). The combined organic phases were dried over
MgSO4 and concentrated. The residue was purified on silica gel (hexanes/EtOAc, 8:2) to give, apart from 30 mg of the starting material, the
MOM-protected compound (287 mg, 90%), (2S,5S)-2-[(1R,2R,3R)-2-bromo-2-(4-methoxybenzyloxy)-1-methoxymethoxy-3-methyl-5-
hexen-1-yl]-5-[3-(tert-butyldiphenylsilyloxy)propyl]tetrahydrofuran (so-called MOM-protected derivative 27a in the SI): colorless oil; Rf
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(hexanes/EtOAc, 8:2) 0.45; [α]D +3.1 (c 1.46, CHCl3); IR (film) 2996, 2931, 2857, 1613, 1514; H NMR 0.97 (d, J = 6.6, 3H), 1.05 (s, 9H),
1.45–1.51 (m, 1H), 1.58–1.81 (m, 5H), 1.82–1.95 (m, 2H), 2.18 (dd, J = 14.0, 10.6, 1H), 2.24–2.35 (m, 1H), 2.72 (dd, J = 13.7, 2.1, 1H), 3.39–
3.45 (m, 4H), 3.61 (t, J = 5.5, 1H), 3.65–3.72 (m, 2H), 3.76–3.83 (m, 4H), 4.02–4.10 (m, 1H), 4.50 (d, J = 11.0, 1H), 4.60 (d, J = 11.1, 1H),
4.78 (d, J = 6.8, 1H), 4.85 (d, J = 6.8, 1H), 5.42 (s, 1H), 5.57 (s, 1H), 6.84–6.89 (m, 2H), 7.22–7.28 (m, 2H), 7.34–7.44 (m, 6H), 7.64–7.70 (m,
4H); 13C NMR 16.0, 19.4, 26.9, 27.9, 29.4, 31.2, 32.1, 32.6, 43.8, 55.3, 56.2, 63.9, 73.1, 79.3, 79.5, 80.4, 83.2, 98.1, 113.7, 118.0, 127.6, 129.3,
129.5, 130.9, 134.1, 135.5, 159.1; HRMS (ESI+) calcd for C40H5979BrNO6Si+ (M + NH4)+ 756.3290, found 756.3293.
A 1 M solution of TBAF in THF (324 µL, 0.324 mmol) was added to a solution of the previously prepared compound (120 mg, 0.162 mmol) in
THF (1.6 mL) under N2 at 0 ºC. The reaction was stirred at rt for 2 h. A saturated aqueous solution of NH4Cl (10 mL) and CH2Cl2 (10 mL) were
added to the reaction and the layers were separated. The aqueous extract was washed with CH2Cl2 (2 × 10 mL) and the combined organic phases
were dried over Na2SO4 and concentrated. The residue was purified on silica gel (hexanes/EtOAc, 9:1 to 6:4) to yield 76 mg (94%) of a colorless
oil, (2S,5S)-5-[(1R,2R,3R)-2-bromo-2-(4-methoxybenzyloxy)-1-methoxymethoxy-3-methyl-5-hexenyl]-5-(3-hydroxypropyl)tetrahydrofuran
(so-called derivative 27b in the SI): Rf (hexanes:EtOAc, 8:2) 0.15; [α]D +3.2, (c 1.00, CHCl3); IR 3370, 2970, 1453; 1H NMR 0.99 (d, J = 6.3, 3H),
1.50–1.75 (m, 5H), 1.8–2.0 (m, 3H), 2.16–2.30 (m, 2H), 2.67 (d, J = 11.9, 1H), 3.42 (s, 3H), 3.49 (dd, J = 6.0, 4.7), 3.59–3.64 (m, 1H), 3.64–3.77
(m, 2H), 3.80 (s, 3H), 3.84–3.93 (m, 1H), 4.08–4.16 (m, 1H), 4.52 (d, J = 11.0, 1H), 4.63 (d, J = 11.0, 1H), 4.80–4.85 (m, 2H), 5.42 (s, 1H), 5.57 (s,
1H), 6.85–6.89 (m, 2H), 7.24–7.28 (m, 2H); 13C NMR 16.2, 27.6, 30.1, 31.5, 32.4, 32.7, 43.5, 55.2, 56.3, 62.7, 73.6, 79.5, 79.8, 80.3, 83.5, 98.3,
113.7, 118.1, 129.3, 130.8, 134.0, 159.1; HRMS (ESI+) calcd. for C24H4179BrNO6+ (M + NH4)+ 518.2112, found 518.2105.
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A 40% (w/w) solution of DEAD in toluene (146 µL, 0.31 mmol) was added dropwise to a stirring solution of the previously prepared
primary alcohol (84 mg, 0.17 mmol), PPh3 (66 mg, 0.25 mmol) and 1-phenyltetrazole-5-thiol (45 mg, 0.25 mmol) in THF (2 mL) at 0 ºC. The
ice bath was removed and, after stirring the mixture for 1 h, the reaction was quenched with a saturated aqueous NaHCO3 solution (25 mL) and
diluted with CH2Cl2 (25 mL). The organic layer was separated and the aqueous layer extracted with CH2Cl2 (3 × 10 mL). The organic layers
were collected, dried over anhydrous Na2SO4, and concentrated. The residue was purified on silica gel (hexanes/EtOAc, 7:3) to give 108 mg
(99%) of a colorless oil, 5-{[(1R,2R,3R)-5-bromo-2-(4-methoxybenzyloxy)-1-methoxymethoxy-3-methyl-5-hexen-1-yl]-3-[(2R,5S)-tetra-
hydrofuran-2-yl]propylthio}-1-phenyl-1H-tetrazole (so-called thioether derivative 27c in the SI): Rf (hexanes/EtOAc, 5:5) 0.66; [α]D –5.5 (c
0.90, CHCl3); IR (film) 3444, 1775, 1696; 1H NMR 0.97 (d, J = 6.5, 3H), 1.47–2.01 (m, 8H), 2.19 (dd, J = 13.7, 10.5, 1H), 2.24–2.31 (m, 1H),
2.69 (d, J = 12.9, 1H), 3.40 (s, 3H), 3.45–3.50 (m, 3H), 3.60 (t, J = 5.4, 1H), 3.80 (s, 3H), 3.80–3.88 (m, 1H), 4.09 (ddd, J = 14.2, 12.2, 7.1,
1H), 4.50 (d, J = 11.0, 1H), 4.60 (d, J = 11.0, 1H), 4.79 (d, J = 6.8, 1H), 4.82 (d, J = 6.8, 1H), 5.41 (s, 1H), 5.57 (s, 1H), 6.84–6.90 (m, 2H),
7.22–7.27 (m, 2H), 7.49–7.61 (m, 5H); 13C NMR 16.1, 26.1, 27.7, 31.2, 32.6, 33.4, 34.4, 43.6, 55.3, 56.3, 73.3, 78.8, 79.5, 80.3, 83.3, 98.1,
113.7, 118.1, 123.9, 129.3, 129.7, 130.0, 130.8, 133.7, 134.0, 154.4, 159.1; HRMS (ESI+) calcd for C31H4279BrN4O5S+ (M + H)+ 661.2054,
found 661.2051.
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A solution of the sulfide prepared above (30 mg, 0.04 mmol) in EtOH (0.4 mL) at 0 ºC was treated with ammonium heptamolybdate—
tetrahydrate (6 mg, 0.004 mmol) in H2O2 (33% w/w, 0.05 mL, 0.4 mmol). After stirring overnight at rt the resulting suspension, the solvent was
evaporated and the residue was partitioned between water (10 mL) and CH2Cl2 (10 mL). The phases were separated and the aqueous layer was
extracted with CH2Cl2 (3 × 5 mL). The combined organic extracts were dried over MgSO4, filtered, and concentrated. Flash column
chromatography (hexanes/EtOAc, 7:3) provided 27 mg (90%) of a colorless oil, 5-{[(1R,2R,3R)-5-bromo-2-(4-methoxybenzyloxy)-1-
methoxymethoxy-3-methyl-5-hexen-1-yl]-3-[(2R,5S)-tetrahydrofuran-2-yl]propylsulfonyl}-1-phenyl-1H-tetrazole (so-called sulfone derivative
27d in the SI): Rf (hexanes/ EtOAc, 9:1) 0.2; [α]D –8.9 (c 1.60, CHCl3); IR (film) 3444, 1775, 1696; 1H NMR 0.98 (d, J = 6.2, 3H), 1.53–1.62
(m, 2H), 1.73–1.80 (m, 1H), 1.84–1.92 (m, 2H), 1.94–1.98 (m, 1H), 2.07–2.17 (m, 2H), 2.20–2.28 (m, 2H), 2.65 (d, J = 11.3, 1H), 3.41 (s, 3H),
3.50 (dd, J = 6.3, 4.3, 1H), 3.60 (dd, J = 6.3, 4.3, 1H), 3.80 (s, 3H), 3.88–3.92 (m, 3H), 4.07–4.14 (m, 1H), 4.52 (d, J = 11.0, 1H), 4.62 (d, J =
11.0, 1H), 4.78 (d, J = 6.8, 1H), 4.81 (d, J = 6.8, 1H), 5.42 (s, 1H), 5.57 (s, 1H), 6.84–6.88 (m, 2H), 7.24–7.27 (m, 2H), 7.64–7.56 (m, 3H),
7.67–7.71 (m, 2H); 13C NMR 16.1, 19.4, 27.6, 31.0, 32.7, 33.5, 43.4, 55.3, 56.0, 56.3, 73.7, 78.6, 79.4, 80.3, 83.6, 98.3, 113.7, 118.1, 125.0,
129.3, 129.7, 130.9, 131.4, 133.0, 134.0, 153.5, 159.1; HRMS (ESI+) calcd for C31H4279BrN4O7S+ (M + H)+ 693.1952, found 693.1949.
DDQ (34 mg, 0.15 mmol) was added to a solution of the sulfone prepared above (78 mg, 0.11 mmol) in a mixture of CH2Cl2 (1.8 mL) and pH 7
buffer (0.2 mL). After 30 min the reaction was quenched by addition of a saturated NaHCO3 solution (10 mL) and diluted with CH2Cl2 (10 mL).
The organic layer was separated and the aqueous layer extracted with CH2Cl2 (3 × 10 mL), dried over MgSO4, concentrated, and purified on silica
gel (hexanes/EtOAc, 8:2 to 7:3) to afford 26 (63 mg, 95%) as a yellowish oil, 5-{[(1R,2R,3R)-5-bromo-2-hydroxy-1-methoxymethoxy-3-methyl-
5-hexen-1-yl]-3-[(2R,5S)-tetrahydrofuran-2-yl]propylsulfonyl}-1-phenyl-1H-tetrazole (28): Rf (hexanes/EtOAc, 5:5) 0.30; [α]D –25.1 (c 0.68,
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CHCl3); H NMR 0.91 (d, J = 6.3, 3H), 1.05–1.67 (m, 2H), 1.67–1.79 (m, 2H), 1.92–2.03 (m, 2H), 2.02–2.20 (m, 5H), 2.62 (d, J = 8.2, 1H),
2.89 (d, J = 12.5, 1H), 3.28 (td, J = 7.9, 2.6, 1H), 3.43 (s, 3H), 3.57 (dd, J = 6.3, 2.7, 1H), 3.81–3.86 (m, 2H), 3.87–3.93 (m, 1H), 4.05–4.17 (m,
1H), 4.73 (d, J = 6.8, 1H), 4.92 (d, J = 6.8, 1H), 5.44 (s, 1H), 5.60 (s, 1H), 7.56–7.64 (m, 3H), 7.67–7.71 (m, 2H); 13C NMR 15.2, 19.3, 27.8,
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30.7, 33.7, 34.2, 40.0, 55.8, 56.3, 75.7, 78.7, 80.1, 80.6, 97.6, 118.1, 125.0, 129.7, 131.4, 133.0, 134.0, 153.4; HRMS (ESI+) calcd for
C23H3779BrN5O6S+ (M + NH4)+ 590.1642, found 590.1646.
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propylsulfonyl}-1-phenyl-1H-tetrazole (29). tert-Butyldiphenylsilyl trifluorosulfonate (TBDPSOTf, 122 µL, 0.36 mmol) was added dropwise
to a stirring solution of alcohol 28 (70 mg, 0.12 mmol) and 2,6-lutidine (52 µL, 0.49 mmol) in CH2Cl2 (0.9 mL) under N2. After stirring
overnight the reaction at rt, it was quenched with a saturated aqueous solution of NaHCO3 (15 mL) and diluted with CH2Cl2 (15 mL). The
layers were separated and the aqueous phase was extracted with CH2Cl2 (3 × 20 mL). The combined organic extracts were dried over Na2SO4
and concentrated. The residue was purified on silica gel (hexanes/EtOAc, 8:2) to recover 28 (5 mg, 7%) and to isolate 29 (82 mg, 83%, 89%
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brsm): yellowish oil; Rf (hexanes/EtOAc, 7:3) 0.5; [α]D +0.77 (c 0.52, CHCl3); IR 2930, 1637, 1340, 1151, 1104, 1073, 1037, 703; H NMR
0.84 (d, J = 6.8, 3H), 1.08 (s, 9H), 1.37–1.45 (m, 1H), 1.57–1.70 (m, 2H), 1.74 (q, J = 7.5, 2H), 1.84–1.90 (m, 1H), 1.90–1.99 (m, 1H), 1.99–
2.11 (m, 2H), 2.17–2.24 (m, 1H), 2.79–2.86 (m, 1H), 3.22 (s, 3H), 3.47 (dd, J = 5.7, 3.6, 1H), 3.75 (dd, J = 5.9, 3.6, 1H), 3.78–3.85 (m, 3H),
4.06–4.13 (m, 1H), 4.41–4.49 (m, 2H), 5.34 (s, 1H), 5.43 (s, 1H), 7.35–7.45 (m, 6H), 7.58–7.63 (m, 3H), 7.67–7.72 (m, 6H); 13C NMR 16.0,
19.3, 19.7, 27.3, 28.0, 31.0, 33.8, 34.3, 44.8, 55.7, 56.0, 77.2, 78.2, 78.6, 81.8, 97.3, 117.7, 125.1, 127.5, 127.6, 129.7, 129.8, 131.4, 133.1,
133.6, 133.6, 134.2, 136.2, 136.3, 153.5. HRMS (ESI+) calcd for C39H55BrN5O6SSi+ (M + NH4)+ 828.2820, found 828.2811.
Compound 30. A 0.50 M solution of KHMDS in toluene (400 µL, 0.20 mmol) was added dropwise to a stirred solution of sulfone 29 (190
mg, 0.23 mmol) and 18-crown-6 (30 mg, 0.20 mmol) in anhydrous DMF (0.9 mL) at –65 ºC under Ar. Aldehyde 12 (57 mg, 0.12 mmol) in
anhydrous DMF (0.5 mL) was slowly added via syringe. The reaction mixture was allowed to warm to rt and stirred for 12 h. It was then
quenched with pH 7 buffer (20 mL) and diluted with Et2O (20 mL). The aqueous phase was extracted with Et2O (3 × 10 mL), the combined
organic phases were washed with brine (30 mL), dried over Na2SO4, filtered, and concentrated under reduced pressure. The residue was
purified by flash column chromatography (hexanes/EtOAc, 9:1 to 7:3) to afford 30 (72 mg, 68%) with recovery of 29 (90–100 mg). Data for
30: colorless oil; Rf (hexanes/EtOAc, 9:1) 0.45; [α]D –2.3 (c 0.59, CHCl3); IR 2957, 2930, 2856, 1624, 1472, 1427, 1383, 1241, 1150, 1111,
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1050; H NMR 0.84 (d, J = 6.8, 3H), 1.03 (d, J = 6.6, 3H), 1.04 (s, 9H), 1.09 (s, 9H), 1.29–1.39 (m, 1H), 1.43–1.49 (m, 7H), 1.54–1.60 (m,
1H), 1.72–1.77 (m, 2H), 1.80–1.86 (m, 1H), 1.90 (dd, J = 14.1, 10.7, 1H), 2.05–2.10 (m, 2H), 2.18–2.24 (m, 1H), 2.37–2.47 (m, 1H), 2.77–2.81
(m, 1H), 3.23 (s, 3H), 3.47–3.56 (m, 3H), 3.72–3.76 (m, 2H), 4.02–4.11 (m, 3H), 4.49, 4.52 (ABq, J = 6.6, 2H), 5.34 (s, 1H), 5.38–5.45 (m,
2H), 5.50 (dd, J = 15.2, 6.8, 1H), 5.64 (dd, J = 15.3, 7.3, 1H), 5.78 (dt, J = 15.0, 6.6, 1H), 6.05 (dd, J = 15.4, 10.4, 1H), 6.25 (dd, J = 15.2, 10.4,
1H), 7.34–7.45 (m, 12H), 7.63–7.67 (m, 4H), 7.67–7.72 (m, 4H); 13C NMR 15.7, 16.3, 19.3, 19.7, 26.9, 27.0, 27.1, 27.3, 28.0, 29.0, 31.1, 34.5,
35.1, 39.3, 44.9, 55.6, 68.4, 77.1, 78.4, 78.7, 81.7, 82.0, 82.3, 97.3, 108.6, 117.6, 125.8, 126.6, 127.5, 127.6, 128.9, 129.5, 129.7, 129.7, 133.6,
133.7, 133.8, 133.9, 134.2, 134.2, 135.6, 135.6, 136.2, 136.2, 136.3, 138.4; HRMS (ESI+) calcd for C61H87BrNO7Si2+ (M + NH4)+ 1080.5199,
found 1080.5212.
Compound 32. A mixture of 30 (68 mg, 0.06 mmol), organotrifluoroborate 316d (30 mg, 0.15 mmol), Pd(OAc)2 (4.3 mg, 0.02 mmol), PPh3
(10 mg, 0.03 mmol) and Cs2CO3 (62 mg, 0.2 mmol) in degassed THF/H2O (10:1, 1.3 mL) was heated for 4 h at 70 ºC under Ar. The reaction
was then quenched with water (20 mL) and diluted with Et2O (10 mL). The organic layer was separated and the aqueous layer extracted with
Et2O (3 × 10 mL). The combined organic extracts were dried over Na2SO4 and concentrated. The residue was purified on silica gel
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(hexanes/EtOAc, 9:1) to give 64 mg (95%) of 32 as a yellowish oil: Rf (hexanes/EtOAc, 9:1) 0.45; [α]D –3.5 (c 1.5, CHCl3); IR 3071, 2957,
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2929, 2856, 1653, 1590, 1471, 1461, 1427, 1377, 1110; H NMR 0.83 (d, J = 6.7, 3H), 1.00–1.05 (m, 12H), 1.08 (s, 9H), 1.28–1.36 (m, 1H),
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1.43 (br s, 6H), 1.47–1.86 (m, 6H), 1.69 (s, 3H), 1.97–2.12 (m, 3H), 2.38–2.48 (m, 1H), 2.69 (br d, J = 14.0, 1H), 2.74 (d, J = 7.3, 2H), 3.23 (s,
3H), 3.45–3.56 (m, 3H), 3.68–3.73 (m, 2H), 3.96–4.12 (m, 3H), 4.50–4.54 (m, 2H), 4.70 (s, 1H), 4.73 (s, 1H), 4.74 (s, 1H), 4.89 (s, 1H), 5.40
(dd, J = 15.3, 7.2, 1H), 5.50 (dd, J = 15.1, 6.8, 1H), 5.60–5.70 (m, 2H), 5.77 (dt, J = 15.3, 6.7, 1H), 5.99 (d, J = 15.9, 1H), 6.05 (dd, J = 15.3,
10.4, 1H), 6.25 (dd, J = 15.1, 10.4, 1H), 7.33–7.44 (m, 12H), 7.61–7.67 (m, 4H), 7.67–7.77 (m, 4H); 13C NMR 16.1, 16.3, 19.3, 19.7, 22.5,
26.9, 27.1, 27.2, 27.6, 27.9, 29.1, 31.1, 35.0, 35.2, 36.5, 39.3, 41.4, 55.6, 68.4, 78.0, 78.3, 79.2, 81.8, 82.0, 82.3, 97.3, 108.6, 110.8, 115.1,
125.7, 126.6, 127.6, 127.6, 127.7, 128.9, 129.5, 133.7, 133.8, 133.9, 134.9, 135.6, 136.2, 136.4, 138.3, 144.6, 144.7; HRMS (ESI+) calcd for
C67H96NO7Si2+ (M + NH4)+ 1082.6720, found 1082.6710.
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Alcohol 33. A 1 M solution of TBAF in THF (31 µL, 0.03 mmol) was added to a solution of 32 (30 mg, 0.28 mmol) in THF (2.8 mL) under
Ar at 0 ºC. After 10 h at rt (or after 18 h at 4 ºC), SiO2 was added to the reaction and the solvent was removed under reduced pressure. The
residue was purified on silica gel (hexanes/EtOAc, 8:2) to yield 22 mg (97%) of 33 as a yellowish oil: Rf (hexanes/EtOAc, 7:3) 0.45; [α]D +3.7
(c 0.78, CHCl3); IR 3481, 2928, 2856, 1458, 1427, 1376, 1217, 1105, 1038; 1H NMR 0.83 (d, J = 6.7, 3H), 1.01 (d, J = 6.8, 3H), 1.08 (s, 9H),
1.30–1.37 (m, 1H), 1.39–1.45 (m, 1H), 1.43 (s, 6H), 1.52–1.66 (m, 3H), 1.70 (s, 3H), 1.70–1.82 (m, 2H), 1.95–2.13 (m, 3H), 2.37–2.47 (m,
1H), 2.68 (m, 1H) 2.74 (d, J = 7.1, 2H), 3.24 (s, 3H), 3.43 (dd, J = 10.4, 7.4, 1H), 3.48–3.54 (m, 2H), 3.69–3.75 (m, 2H), 3.98–4.10 (m, 3H),
4.52 (m, 2H), 3.98–4.10 (m, J = 11.7, 2H), 4.52 (m, 2H), 4.70 (s, 1H), 4.73 (s, 1H), 4.74 (s, 1H), 4.89 (s, 1H), 5.40 (dd, J = 15.3, 7.2, 1H),
5.51–5.68 (m, 3H), 5.76 (dt, J = 15.4, 6.7, 1H), 5.99 (d, J = 15.8, 1H), 6.14 (dd, J = 15.3, 10.3, 1H), 6.28 (dd, J = 15.2, 10.5, 1H), 7.34–7.43 (m,
6H), 7.66–7.76 (m, 4H); 13C NMR 16.1, 16.3, 19.7, 22.5, 27.0, 27.1, 27.3, 27.9, 29.0, 31.0, 35.0, 35.2, 36.5, 39.7, 41.4, 55.6, 67.2, 78.0, 78.3,
79.2, 81.7, 82.0, 82.3, 97.3, 108.6, 110.8, 115.1, 125.6, 127.4, 127.4, 127.5, 127.7, 129.6, 129.7, 130.2, 133.6, 133.7, 133.9, 133.9, 136.2,
136.4, 136.4, 137.7, 144.6, 144.7; HRMS (ESI+) calcd for C51H78NO7Si+ (M + NH4)+ 844.5542, found 844.5539.
Acid 34. A mixture of 33 (22 mg, 0.02 mmol), dry NaHCO3 (20 mg, 0.10 mmol), and CH2Cl2 (0.6 mL) was treated with DMP (stored over
P4O10 under vacuum, 15.0 mg, 0.035 mmol) under Ar at rt. After stirring for 1 h, the mixture was quenched with a saturated aqueous solution of
Na2S2O3 (20 mL) and diluted with Et2O (20 mL), and the aqueous layer was extracted with Et2O (3 × 10 mL). The organic extracts were dried
over MgSO4 and concentrated. The crude aldehyde obtained was dissolved in tBuOH (1.4 mL). 2-Methyl-2-butene (130 µL, 86 mg, 1.2 mmol),
isoprene (25 µL, 17 mg, 0.25 mmol), and a solution of NaClO2 (12 mg, 0.12 mmol) and NaH2PO4 (32 mg, 0.6 mmol) in water (1.4 mL) were
added. After 1 h at 0 ºC, the reaction was quenched with water (20 mL) and diluted with EtOAc (20 mL). The layers were separated and the
aqueous phase was extracted with EtOAc (3 × 10 mL). The combined organic layers were dried over MgSO4 and concentrated. The residue was
purified on silica gel (CH2Cl2/MeOH, 95:5) to afford 20 mg (91%) of acid 34 as a yellowish oil: Rf (hexanes/EtOAc, 7:3) 0.15; [α]D –10.5 (c
0.96, CHCl3); IR 3448, 2927, 2855, 1734, 1711, 1462, 1424, 1377, 1238, 1109, 1038, 704; 1H NMR 0.83 (d, J = 6.6, 3H), 1.08 (s, 9H), 1.22 (d,
J = 7.0, 3H), 1.23–1.33 (m, 2H), 1.43 (s, 6H), 1.42–1.91 (m, 5H), 1.70 (s, 3H), 1.87–2.04 (m, 3H), 2.68 (dd, J = 13.8, 3.6, 1H), 2.74 (d, J = 7.1,
2H), 3.17–3.28 (m, 1H), 3.25 (s, 3H), 3.40–3.53 (m, 1H), 3.68–3.78 (m, 2H), 3.98–4.10 (m, 3H), 4.50–4.57 (m, 2H), 4.69 (s, 1H), 4.74 (s, 2H),
4.89 (s, 1H), 5.38 (dd, J = 15.3, 7.4 1H), 5.51–5.68 (m, 2H), 5.70–5.79 (m, 2H), 5.98 (d, J = 15.8, 1H), 6.16 (dd, J = 14.7, 10.4, 1H), 6.26 (dd, J
= 15.0, 10.4, 1H), 7.33–7.43 (m, 6H), 7.66–7.76 (m, 4H); 13C NMR 16.1, 17.0, 19.7, 22.5, 27.0, 27.1, 27.3, 27.8, 29.1, 31.1, 35.0, 35.4, 36.4,
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41.4, 42.5, 55.6, 77.9, 78.2, 79.3, 81.7, 82.2, 82.4, 97.2, 108.8, 110.8, 115.1, 125.5, 127.5, 127.6, 127.7, 128.5, 129.7, 129.7, 130.7, 133.1,
133.4, 133.7, 133.8, 133.9, 136.2, 136.4, 136.7, 144.6, 144.7, 176.9; HRMS (ESI–) calcd for C51H71O8Si– (M – H)– 839.4924, found 839.4934.
Hydroxy acid 35 (seco-acid 35). A 1 M solution of TBAF in THF (42 µL, 0.042 mmol) was added to a solution of acid 34 (12.0 mg, 0.014
mmol) in THF (0.5 mL) under Ar at 0 ºC. The reaction was stirred at 50 ºC for 2 h. Again, TBAF (1 M in THF, 42 µL, 0.042 mmol) was
injected and the mixture was stirred for 2 h. Finally, the last portion of TBAF (28 µL, 0.028 mmol) was added. Two hours later the crude
reaction mixture was filtered through a short path of silica (hexanes/EtOAc/AcOH, 1:1:0.01). After careful evaporation, the residue was
purified on silica gel (CHCl3/MeOH 95:5) to recover starting material (5.5 mg of 34, which was subjected to deprotection, again) and to isolate
seco-acid 35 (4.3 mg, 50%) as a colorless oil: Rf (CHCl3/MeOH, 95:5) 0.3; 1H NMR 0.85 (d, J = 6.5, 3H), 1.27 (d, J = 7.0, 3H), 1.32–1.37 (m,
1H), 1.43 (s, 6H), 1.46–1.54 (m, 2H), 1.71 (s, 3H), 1.74–1.84 (m, 2H), 1.86–1.95 (m, 3H), 2.00–2.10 (m 1H), 2.17–2.25 (m, 1H), 2.76 (d, J =
7.0, 2H), 2.90–2.98 (m, 1H), 3.10–3.19 (m, 1H), 3.24 (d, J = 10.2, 1H), 3.37 (s, 3H), 3.62 (d, J = 7.6, 1H), 3.70–3.76 (m, 1H), 3.95–4.06 (m,
3H), 4.69 (d, J = 6.7, 1H), 4.69 (s, 1H), 4.73 (s, 1H), 4.88 (s, 1H), 4.99 (s, 1H), 5.03 (d, J = 6.8, 1H), 5.38 (dd, J = 14.9, 7.4, 1H), 5.55 (dd, J =
14.7, 7.8, 1H), 5.61–5.79 (m, 3H), 6.06 (d, J = 15.5, 1H), 6.13–6.23 (m, 2H); 13C NMR 15.7, 17.3, 22.6, 27.2, 27.3, 28.3, 29.1, 30.1, 34.5, 34.9,
36.1, 41.5, 43.3, 56.4, 76.9, 78.7, 79.0, 80.5, 82.4, 82.4, 97.2, 109.1, 110.9, 115.6, 126.1, 128.0, 128.4, 130.9, 133.7, 134.0, 134.1, 137.1, 144.8,
145.0, 182.8 (see Supporting Information; almost all the 13C signals of our sample are 0.2 ppm at higher field than those reported,4 but we used
CDCl3 as the internal reference, δ 77.0 ppm, whereas TMS was employed in ref 4); HRMS (ESI–) calcd for C35H53O8– (M–H)– 601.3746, found
601.3739. The assignments of the NMR signals were confirmed by a standard HSQC experiment.
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ASSOCIATED CONTENT
Supporting Information
1
Copies of H NMR and 13C NMR spectra of the new compounds and of 35. The Supporting Information is available free of charge on the ACS
Publications website at DOI: 10.1021/acs.joc........
AUTHOR INFORMATION
Corresponding Authors
*E-mail: amcosta@ub.edu (A.M.C.)
*E-mail: jvilarrasa@ub.edu (J.V.)
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
Grants CTQ2006-15393, CTQ2009-13590, CTQ2012-39230, and CTQ2015-71506-R (Spanish Government, FEDER), and
2009SGR825 (AGAUR, Barcelona) are acknowledged. L.B., a postdoc in our group (CTQ2015) during 2016, carried out the
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optimization of the allyl sulfide to allyl sulfone reaction and the synthesis of additional amounts of the northern fragment, as well as
the repetition of the last six steps of the sequence in collaboration with E.P. (fellow of the Fundació Cellex during 2017). L.M. was
an UB PhD student (her Doctorate Thesis was defended on 28 June 2016). M.S. was a Master student (2015–2016) who performed
the asymmetric epoxidation studies. J.E., an UB PhD student (2004–2008) and later a fellow of the Fundació Cellex (April 2009–
March 2010), began the project in the fall of 2005. The participation of undergraduate students Guillem Vázquez (2013) and
Cristian Marco (2014) in an approach to 17 (fragment C10–C17) via hydrolytic kinetic resolution of the corresponding intermediate
epoxides also deserves to be mentioned. Useful information from Prof. Albrecht Berkessel (Universität zu Köln) and a gift of the
new binaphthyl analog of salalen (Scheme 7) are deeply acknowledged. This work is dedicated to the late President of the Fundació
Cellex of Barcelona, chemist and entrepreneur, Dr. Pere Mir (deceased 10 March 2017).
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C. H.; An, H. J.; Shin, W. K.; Yu, W.; Woo, S. K.; Jung, S. K.; Lee, E. Chem. Asian J. 2008, 3, 1523–1534.
(5) (a) Va, P.; Roush, W. R. J. Am. Chem. Soc. 2006, 128, 15960–15961. For syntheses of stereoisomers of 1, see: (b) Va, P.; Roush, W. R.
Org. Lett. 2007, 9, 307–310. (c) Va, P.; Roush, W. R. Tetrahedron 2007, 63, 5768–5796.
(6) (a) Gurjar, M. K.; Mohapatra, S.; Phalgune, U. D.; Puranik, V. G.; Mohapatra, D. K. Tetrahedron Lett. 2004, 45, 7899–7902 (C12–
C29). (b) Marshall, J. A.; Schaaf, G.; Nolting, A. Org. Lett. 2005, 7, 5331–5333 (C6–C21). (c) Esteban, J.; Costa, A. M.; Gómez, A.; Vilarrasa,
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S.; Nayak, S.; K. Mishra, S.; Pattanaik, P. Lett. Org. Chem. 2013, 10, 65–69 (C1–C9, C10–C18).
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Aïssa, C. Eur. J. Org. Chem. 2009, 1831–1844. Also see: (c) Alonso, D. A.; Fuensanta, M.; Gómez-Bengoa, E.; Nájera, C. Eur. J. Org. Chem.
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2008, 2915–2922, and refs cited therein.
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(8) Billard, F.; Robiette, R.; Pospíšil, J. J. Org. Chem. 2012, 77, 6358–6364.
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(9) For the construction of the C5–C6 double bond we initially examined the J–K reaction between the phenyl tetrazolyl sulfone derived from
TBDPS-monoprotected (4R,5R)-2,3-O-isopropylidenethreitol (prepared from reduction of diethyl (S,S)-tartrate, monoprotection as a TBDPS ether,
Mitsunobu reaction with 1-phenyl-1H-tetrazole-5-thiol (PT–SH), and oxidation of the thioether to the corresponding phenyltetrazole-sulfone (PT–
SO2R) and the aldehyde obtained by Dess–Martin oxidation of 5 (see the main text and reference 6c). Although the J–K reaction of aliphatic sulfones
with α,β-unsaturated aldehydes is well-documented,7 the E isomer being consistently obtained, initial control experiments determined that our aliphatic
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sulfone decomposed when treated with KHMDS (probably via β-elimination), but remained unchanged when NaHMDS or LiHMDS were used (THF,
–78 ºC, 2 h). Moreover, while the J–K reactions of the sodium salt of this sulfone with cinnamaldehyde or acrolein gave high yields and complete
stereoselectivity of the expected products, the reaction with the aldehyde derived from 5 did not afford the expected diene. Analysis by 1H NMR of the
crude product mixtures revealed the presence of substantial amounts of unreacted sulfone. Probably, the aldehyde (with a relatively acidic hydrogen at
C2) is too unstable under the reaction conditions.
(10) McDougal, P. G.; Rico, J. G.; Oh, Y.-I.; Condon, B. D. J. Org. Chem. 1986, 51, 3388–3390.
(11) (a) Blakemore, P. R.; Cole, W. J.; Kocienski, P. J.; Morley, A. Synlett 1998, 26–28. For reviews, see ref 7 and (b) Plesniak, K.;
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(12) (a) Evans, D. A.; Andrews, G. C. Acc. Chem. Res. 1974, 7, 147–155. (b) Hilpert, H.; Wirz, B. Tetrahedron 2001, 57, 681–694. (c) Bastin,
R.; Dale, J. W.; Edwards, M. G.; Papillon, J. P. N.; Webb, M. R.; Taylor, R. J. K. Tetrahedron 2011, 67, 10026–10044, and refs cited therein.
(13) The epimerization (at C7) of the aldehyde under the reaction conditions was also considered, but the practically identical chemical shifts of
the isopropylidene methyl groups suggested that the trans-dioxolane substructure was maintained. See: Chuche, J.; Dana, G.; Monot, M. R. Bull.
Soc. Chim. Fr. 1967, 3300–3307. Migrations of the double bonds are also feasible, but they were not observed.
(14) (a) Thomas, E. J.; Willis, M. Org. Biomol. Chem. 2014, 12, 7537–7550. (b) Sasaki, K.; Urabe, D.; Arai, H.; Arita, M.; Inoue, M.
Chem. Asian J. 2011, 6, 534–543. (c) Domínguez, M.; Álvarez, S.; Álvarez, R.; Rodríguez de Lera, A. Tetrahedron 2012, 68, 1756–1761.
(15) Pospíšil, J. Tetrahedron Lett. 2011, 52, 2348–2352.
(16) Pilli, R. A.; Riatto, V. B. Tetrahedron: Asymmetry 2000, 11, 3675–3686.
(17) (a) Berkessel, A.; Günther, T.; Wang, Q.; Neudörfl, J.-M. Angew. Chem. Int. Ed. 2013, 52, 8467–8471. (b) Berkessel, A.; Ong, M.-C.;
Nachi, M.; Neudörfl, J.-M. ChemCatChem 2010, 2, 1215–1218; (c) Wang, Q.; Neudörfl, J.-M.; Berkessel, A. Chem. Eur. J. 2015, 21, 247–254.
(18) Mancuso, A. J.; Huang, S.-L.; Swern, D. J. Org. Chem. 1978, 43, 2480–2482.
(19) The Jacobsen hydrolytic kinetic resolution of epoxide (±)-14 and of a diastereomeric mixture of epoxides obtained in 90% yield by
treatment of the 4-O-TBS derivative of 15 with 3-ClC6H4CO3H gave practically enantiopure epoxides (in 42–45% yields). (a) The (R,R)-
(salen)CoIIIOTs catalyst was prepared according to: (a) Stevenson, C. P.; Nielsen, L. P. C; Jacobsen, E. N.; McKinley, J. D.; White, T. D.;
Couturier, M. A.; Ragan, J. Org. Synth. 2006, 83, 162−169. (b) For the mechanism, see: Nielsen, L. P. C.; Zuend, S. J.; Ford, D. D.; Jacobsen,
E. N. J. Org. Chem. 2012, 77, 2486–2495.
(20) (a) Fuwa, H.; Nakajima, M.; Shi, J.; Takeda, Y.; Saito, T.; Sasaki, M. Org. Lett. 2011, 13, 1106–1109. (b) We prepared 18 in five
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simple steps from (R)-glycidol, namely, O-benzylation, addition of vinylmagnesium bromide (catalyzed by CuI), protection with TBSCl,
hydroboration/oxidation, and Swern oxidation (all yields > 90%). These synthetic intermediates were known compounds.
(21) (a) Evans, D. A.; Bender, S. L.; Morris, J. J. Am. Chem. Soc. 1988, 110, 2506–2526. (b) Sidera, M.; Costa, A. M.; Vilarrasa, J. Org.
Lett. 2011, 13, 4934–4937.
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(22) (a) Chatterjee, B.; Bera, S.; Mondal, D. Tetrahedron: Asymmetry 2014, 25, 1–55. (b) Legnani, L.; Porta, A.; Caramella, P.; Toma, L.;
Zanoni, G.; Vidari, G. J. Org. Chem. 2015, 80, 3092–3100.
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(23) For a general entry to all these standard protection/deprotection schemes, see: Wuts, P. G. M.; Greene, T. W. Protective Groups in
Organic Synthesis, 4th ed.; Wiley: Hoboken, 2007; pp 123−130.
(24) (a) Our initial retrosynthetic plan considered esterification of the northern and southern fragments followed by macrocycle formation by either
ring-closing metathesis or a J–K reaction. Preliminary experiments showed that Shiina esterification of (2R,3E,5E)-6-[(4R,5R)-5-(tert-butyldiphenyl-
silyloxymethyl)-2,2-dimethyl-1,3-dioxolan-4-yl]-2-methyl-3,5-hexadienoic acid (that is, of a fragment C1–C9, see Scheme 1, with C1 in the form of
COOH and C9 in the form of the CH2OTBDPS group) with isopropanol afforded the desired ester but as a 70:30 mixture of epimers at C2. For the
activation of the carboxyl group as its mixed 2-methyl-5-nitrobenzoyl anhydride and ester formation in the presence of DMAP, see: Shiina, I. Chem. Rev.
2007, 107, 239–273. (b) In contrast, the Kita–Trost procedure, which uses a Lewis acid (a RuII-based catalyst for the activation of the carboxyl group)
followed by a Brönsted acid (CSA), gave the expected ester in quantitative yield. For the Kita–Trost procedure, see: Ohba, Y.; Takatsuji, M.; Nakahara,
K.; Fujioka, H.; Kita, Y. Chem. Eur. J. 2009, 15, 3526–3537, and refs cited therein. (c) However, when this last procedure was applied to the esterification
of this acid with the alcohol derived from the PMB deprotection of 27a (see Experimental Section and Supporting Information), no reaction was observed
at rt. When the reaction mixture was heated, the acid was recovered unaltered but the alcohol was transformed into the corresponding methylene acetal
(derived from an intramolecular transacetalization, with the MOM group). Other esterification methods (Shiina, EDC/HOBt) were also tested, without
success.
(25) Lee et al. (ref 4) used LiHMDS in THF at –78 ºC for the J–K reaction of a similar sulfone (which already contained the side chain),
with an excess (1.5 equiv) of aldehyde 12 in DMF/DMPU. They obtained a 74% yield of a 10:1 E/Z mixture. To avoid the appearance of the Z
isomer (often a difficult-to-separate byproduct), we insisted on using KHMDS, 18-crown-6, and DMF.
(26) Molander, G. A.; Felix, L. A. J. Org. Chem. 2005, 70, 3950–3956.
(27) Brimble, M. A.; Park, J. H.; Taylor, C. M. Tetrahedron 2003, 59, 5861–5868, and refs cited therein.
(28) Yuasa, Y.; Ando, J.; Shibuya, S. J. Chem. Soc. Perkin Trans. 1 1996, 793–802.
(29) This compound has been reported as a mixture of cis and trans isomers (ca. 1:1): (a) Finch, O. C.; Furkert, D. P.; Brimble, M. A. Tetrahedron
2014, 70, 590–596. (b) Larsen, C. H.; Ridgway, B. H.; Shaw, J. T.; Smith, D. M.; Woerpel, K. A. J. Am. Chem. Soc. 2005, 127, 10879–10884.
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