An efficient enantioselective total synthesis of a trifluoromethyl analog of blastmycinolactol

Article abstract of DOI:10.1016/j.jfluchem.2004.01.003

A novel synthetic strategy for the total synthesis of fluorinated analog of blastmycinolactol has been developed using α-pinene based alkoxyallylboration, chelation controlled diastereoselective reduction, and a single step lactonization of the 1,4-diol to the γ-lactone as key steps.

Full text of DOI:10.1016/j.jfluchem.2004.01.003

Journal of Fluorine Chemistry 125 (2004) 579–583
An efficient enantioselective total synthesis of a trifluoromethyl
analog of blastmycinolactol
*
P. Veeraraghavan Ramachandran , Kamlesh J. Padiya,
M. Venkat Ram Reddy, Herbert C. Brown
Department of Chemistry, Herbert C. Brown Center for Borane Research, Purdue University,
5
60 Oval Drive, West Lafayette, IN 47907-2084, USA
Received 10 October 2003; received in revised form 31 December 2003; accepted 6 January 2004
Abstract
A novel synthetic strategy for the total synthesis of fluorinated analog of blastmycinolactol has been developed using a-pinene based
alkoxyallylboration, chelation controlled diastereoselective reduction, and a single step lactonization of the 1,4-diol to the g-lactone as key
steps.
#
2004 Elsevier B.V. All rights reserved.
Keywords: a-Pinene; Alkoxyallylboration; Chelation controlled diastereoselective reduction; Hydroboration; Dicyclohexylborane; Ley’s oxidation; Anti-
alkylation
1
. Introduction
2. Results and discussion
Synthesis of fluorine containing molecules in optically
The synthesis of 11 was initiated by the reaction of sec-
butyllithium with allyloxymethoxyethoxymethyl ether 12,
followed by the addition of B-methoxydiisopinocampheyl-
pure form is an important area of research [1]. Fluorine
substitution in a molecule often influences its physical and
biological properties and this phenomenon has found wide
applications in medicinal, materials and agrochemistry
borane (Ipc BOMe) 14 to generate the ‘‘ate’’ complex 15.
2
Addition of BF .Et O liberated the alkoxyallylborane 16 [9].
2
3
[
product of antimycin A , an anti-fungal and antibiotic
2]. Blastmycinolactol 1 (Fig. 1) [3] is a degradation
This upon reaction with fluoral 17 resulted in the polymer-
ization of 17. We could only isolate very low yield (<5%) of
the product homoallylic alcohol 19 after alkaline hydrogen
peroxide oxidation. We rationalized that the polymerization
could be initiated by the strong Lewis acidic nature of
BF ꢁEt O. To arrest the polymerization, 17 was reacted
3
isolated from several members of Streptomyces species
(
Fig. 2). There are several other natural products with 2-
alkyl or alkylidene-3-hydroxy or acyl-4-methyl butyrolac-
tone moiety possessing interesting biological properties.
Some of the examples include blastmycinone (2), (ꢀ)–
NFX-2 (3), (ꢀ)–NFX-4 (4) [4], lipid metabolites 5 and 6
3
2
directly with the ‘‘ate’’ complex 15 at ꢀ78 8C and to our
delight, the allyl transfer took place smoothly to provide the
alcohol 19 in 75% yield. The crude NMR analysis indicated
the diastereoselectivity to be >95% syn. The enantioselec-
tivity as determined on HPLC using Chiracel-ODH column
[10] was found to be 96% (Scheme 1).
[
(
5] and (ꢀ)–litsenolides B (7), B (8), C (9), C (10) [6]
1
2
1
2
Fig. 3).
For the past few years, we have been developing methods
for the synthesis of fluoro-organic molecules via organobor-
anes [7]. As a part of this program, we undertook the total
synthesis of trifluoromethyl analog of blastmycinolactol 11
via asymmetric alkoxyallylboration using a-pinene [8] as
chiral auxiliary.
However, the stereochemistry of the target molecule 11 is
trans and hence we needed the anti-homoallylic alcohol.
We envisioned that oxidation of syn alcohol 19 to ketone
20 followed by chelation-controlled reduction should lead
to anti-alcohol 21. Chelation controlled diastereoselective
nucleophilic additions on a-alkoxy ketones are well
documented in the literature [11]. Oxidation of 19 with
Dess–Martin periodinane [12] to trifluoromethylketone 20
*
Corresponding author. Tel.: þ1-765-494-5303; fax: þ1-765-494-0239.
E-mail address: chandran@purdue.edu (P.V. Ramachandran).
0022-1139/$ – see front matter # 2004 Elsevier B.V. All rights reserved.
doi:10.1016/j.jfluchem.2004.01.003

580
P.V. Ramachandran et al. / Journal of Fluorine Chemistry 125 (2004) 579–583
O
O
O
O
O
sec-BuLi
BOMe
O
O
o
Li
O
THF, -78 C
1
4
13
1
2
o
OH
THF, -78 C
1
, (-)-Blastmycinolactol
O
O
O
O
Li
O
BF . Et O
O
CF CHO
Fig. 1.
B
OMe
3
2
3
Polymer
BIpc2
-78 o
o
THF, -78 C
C
1
6
1
5
O
H
o
N
CF3CHO, 17 -78 C
O
CHO
iBuCOO
BIpc2
O
OH
OH
O
NaOH, H O2
2
O
R
F3C
19
F3C
25 o
R = -(CH ) CH , Antimycin A
C
2
3
3
3
O
O
1
8
O
O
O
R = -(CH ) CH Antimycin A1
O
2
5
3,
75% yld, 96%ee, >95%de
R
i
OCO Bu
Scheme 1.
NaOH
O
O
R = -(CH ) CH , (+)-Antimycinone
Zn+
OMEM
2
3
3
O
OH
R = -(CH ) CH
3
2
5
O
Zn(BH4)2
F3C
F3C
H
Fig. 2.
2
0
OMEM
F3C
H
21 OMEM
Scheme 2.
and subsequent reduction with Zn(BH ) afforded the anti-
4
2
alcohol 21 in 88% de. The high diastereoselectivity can be
explained by the coordination of the metal ion with the
alkoxy oxygen and the carbonyl oxygen resulting in the
formation of five membered transition state followed by
hydride addition from the least hindered side to provide the
anti-homoallylic alcohol (Scheme 2).
Treatment of olefin 21 with dicyclohexylborane and
standard alkaline hydrogen peroxide workup resulted in
the formation of the diol 22. We envisaged that the oxidation
of the primary alcohol in 22 to the aldehyde (lactol),
followed by further oxidation should lead to the b-
alkoxy-g-lactone 23. Indeed, the oxidation of the diol 22
under Ley’s conditions [13] with tetrapropylammonium
perruthenate (TPAP) and N-methylmorpholine N-oxide
amount of trifluoroacetic acid resulted in the lactonization to
provide the hydroxylactone 24. The lactone 24 was also
obtained in a single step by performing the deprotection
reaction in aqueous THF and HCl. Finally, the target mole-
cule 11 was achieved by the hydroxy-directed anti-alkyla-
tion [14] with lithium diisopropylamide (LDA) and butyl
iodide (Scheme 3).
3. Conclusions
In conclusion, we have developed a novel synthetic
strategy for the asymmetric total synthesis of trifluomethyl
analog of blastmycinolactol. The salient features in the
synthesis include asymmetric alkoxyallylboration of fluoral
with the ‘‘ate’’ complex, chelation controlled diastereose-
lective reduction of the a-alkoxy ketone, oxidation of the
diol to the lactone, and hydroxy directed anti-alkylation.
(
NMO), furnished 68% yield of the g-lactone 23. Deprotec-
tion of the methoxyethoxymethoxy (MEM) group with HCl
and methanol provided the mixture of the hydroxylactone 24
and the methyl ester 25 in a 3:7 ratio. Heating the reaction
mixture in 1,2-dichloroethane in the presence of a catalytic
O
O
O
R
R
O
O
O
OCOBui
: (+)-Blastmycinone
OH
: R= -(CH ) CH , (-)-NFX-2
OAc
3
4
5: R = (CH ) CH3
2 13
2
3
3
2
: R= -(CH ) CH , (-)-NFX-4
6: R = (CH ) CH3
2
5
3
2 15
O
O
O
R2
7: R = H, R2 = (CH ) CCH, (-)-Litsenolide B
1 2 9 1
8
: R = (CH ) CCH, R = H, (-)-Litsenolide B2
1 2 9 2
O
R1
9: R = H, R2
= (CH ) CH3, (-)-Litsenolide C1
2 12
1
F3C
OH
1
0: R1 = (CH ) CH , R = H, (-)-Litsenolide C
2 12 3 2 2
OH
1
1: Fluoroblastmycinolactol
Fig. 3.

P.V. Ramachandran et al. / Journal of Fluorine Chemistry 125 (2004) 579–583
581
OH
O
Zn(BH4)2
OH
DMP
Et O
2
CH Cl
F3C
2
2
F3C
F3C
7
1% yld.
83% yld.
88% de
OMEM
OMEM
OMEM
1
9
20
21
O
OH
o
(
i) Chx BH, THF, 0 C
TPAP/ NMO
CH Cl
2
OH
O
o
(
ii) NaOH, H O , 25 C
F3C
2
2
2
2
OMEM
68% yld.
82% yld.
F3C
OMEM
2
2
23
HCl, THF
8% yld.
HCl,
MeOH
7
O
O
TFA
ClCH2CH2Cl
9% yld
(for 2 steps)
OH
(
i) LDA, HMPA
OMe
O
O
o
F3C
(
ii)BuI, -78 C
7
OH
O
F3C 11 OH
48% yld. F3C 24 OH
Scheme 3.
25
Further work is in progress to synthesize several fluoro
analogs of biologically important g-lactones and assess their
biological activity.
(91.0 g, 287.7 mmol, 1.0 M solution in THF) was added
to the reaction mixture and stirred for 1 h at ꢀ78 8C.
Trifluoroacetaldehyde 17 (46.9 g, 479.4 mmol) dissolved
in 50 ml THF was cooled to ꢀ78 8C and added dropwise.
It was stirred at ꢀ100 8C for 2 h and then stirred overnight at
ꢀ78 8C. The reaction was oxidized with 115.2 ml of 3.0 M
sodium hydroxide and 115.2 ml of 30% hydrogen peroxide
and stirred for 10 h at room temperature. The product was
extracted with Et O, washed with water, dried over MgSO .
4
. Experimental
4
.1. General experimental procedures
2
4
All operations were carried out under an inert atmosphere.
The solvent was removed under aspirator vacuum and the
crude product was purified by silica gel column chromato-
graphy (hexane:ethyl acetate, 3:2) to obtain 44 g (75%) of
The H, 11B, and 13C NMR spectra were plotted on a Varian
Gemini-300 spectrometer with a Nalorac-Quad probe. Mass
spectra were recorded using a Hewlett-Packard 5989B mass
spectrometer/5890 series II gas chromatograph or a Finnigan
mass spectrometer model 4000. CI gas used was isobutane.
Enantiomeric excess (% ee) was measured using Dynamax
HPLC fitted with HPXL Solvent Delivery System and
Dynamax UV (l 254 nm) detector. Chiral column used
was CHIRALCEL OD-H. All the chemicals were obtained
from Aldrich Chemical Company and used as received.
Tetrahydrofuran was distilled from benzophenone ketyl
prior to use. Zn(BH ) was prepared using literature proce-
1
1
the homoallylic alcohol 19. H NMR (300 MHz): d (ppm):
5.72–5.81 (m, 1H), 5.38–5.44 (m, 2H), 5.14 (dd, J ¼ 3:57,
7.20 Hz, 1H), 4.78 (d, J ¼ 7:08 Hz, 1H), 4.69 (d, J ¼
7:08 Hz, 1H), 4.37 (dd, J ¼ 4:68, 7.98 Hz, 1H), 3.82–
3.92 (m, 2H), 3.66–3.72 (m, 1H), 3.57 (t, J ¼ 4:41 Hz,
1
3
2H), 3.40 (s, 3H); C NMR (75.5 MHz) d (ppm): 132.4,
122.31 (q, J ¼ 280:0 Hz), 121.2, 92.1, 74.5, 72.5 (q,
1
9
J ¼ 30:00 Hz, 1C), 71.8, 67.4, 58.9; F NMR (300 MHz)
d (ppm): ꢀ13.4; EI–MS: m/z 196, 59 (100%); CI–MS: m/z
þ
4
2
245 (M þ H) , 139 (100%); HRMS–CI: 245.1002 (actual),
dure [15]. 2,2,2-Trifluoroacetaldehyde hydrate was prepared
from the corresponding carboxylic acid by reduction with
245.1001 (calculated).
0
.5 molar equivalent of lithium aluminum hydride in ethyl
4.1.2. (3R)-2-(Methoxyethoxymethoxy)-1,1,1-trifluoro-
pent-4-en-2-one, 20
ether at ꢀ10 8C for 4 h, followed by quenching with water
and concentrated sulfuric acid. The aldehyde was obtained
by the dehydration of the corresponding hydrate using
phosphorus pentoxide in concentrated sulfuric acid at
90 8C and was distilled twice at room temperature prior
to use [16].
Dess–Martin periodinane (56.2 g, 134 mmol) was sus-
pended in 100 ml CH Cl and alcohol 19 (21.8 g, 90 mmol)
2
2
was added to it at 25 8C. The reaction mixture was stirred for
2 h and filtered over sodium sulfate pad. The crude product
was concentrated under vacuum and purified by column
chromatography (silica gel, hexanes:ethyl acetate (4:1)) to
1
4
1
.1.1. (2S, 3R)-2-(Methoxyethoxymethoxy)-1,1,
-trifluoro-pent-4-en-2-ol, 19
sec-Butyl lithium (141.2 ml, 1.7 M solution, 240.0 mmol)
obtain 15.4 g (71%) of pure ketone 20. H NMR (300 MHz):
d (ppm): 5.76–5.83 (m, 1H), 5.38–5.45 (m, 2H), 4.77
(d, J ¼ 7:20 Hz, 1H), 4.73 (d, J ¼ 7:20 Hz, 1H), 4.32
(d, J ¼ 8:19 Hz, 1H), 3.73–3.87 (m, 2H), 3.57 (t, J ¼
was added dropwise to a well-stirred solution of allylox-
ymethoxyethoxymethane (35.0 g, 239.7 mmol) in 500 ml
THF at ꢀ78 8C and stirred for 0.5 h. (þ) Ipc BOMe
1
9
3:84 Hz, 2H), 3.39 (s, 3H); F NMR (300 MHz) d
(ppm): ꢀ19.3; EI–MS: m/z 167, 59 (100%); CI–MS: 243
2

582
P.V. Ramachandran et al. / Journal of Fluorine Chemistry 125 (2004) 579–583
þ
þ
(
HRMS–CI: 241.0690 (actual), 241.0688 (calculated).
M þ H) , 167 [(M þ H–HOCH CH OCH ) , 100%];
column chromatography (hexane:ethyl acetate, 1:3) to obtain
1
2
2
3
1.8 g (68%) of the lactone 23. H NMR (300 MHz): d (ppm):
4
.89 (q, J ¼ 6:99 Hz, 1H), 4.79 (s, 2H), 4.60 (dt, J ¼ 2:13,
4.1.3. (2R, 3R)-2-(Methoxyethoxymethoxy)-1,1,1-trifluoro-
pent-4-en-2-ol, 21
7.35 Hz, 1H), 3.69–3.75 (m, 2H), 3.54 (t, J ¼ 4:38 Hz, 2H),
3.37 (s, 3H), 2.90 (dd, J ¼ 7:41, 18.66 Hz, 1H), 2.67 (d,
1
9
A solution of 20 (12.8 g, 53 mmol) in anhydrous THF
5 ml) at ꢀ78 8C was treated drop wise with Zn(BH ) (ca
J ¼ 18:63 Hz, 1H); F NMR (300 MHz) d (ppm): ꢀ15.4;
(
0
EI–MS: m/z 183 (M–OCH CH OCH ); 59 (100%); CI–MS:
2
4
2
2
3
þ
.5 M, 120 ml), 2 h after addition, the mixture was allowed
m/z 259 (M þ H) , 89 (100%); HRMS–CI: 259.0790
to warm up to ꢀ25 8C and stirred for 15 h, at which time
(actual), 259.0793 (calculated).
TLC showed complete disappearance of starting materials.
Fifty milliliter of saturated aqueous NH Cl solution was
4.1.6. (4R, 5R)-4-Hydroxy-5-trifluoromethyl-dihydrofuran-
2-one, 24
4
added and the aqueous layer was extracted with ether
(
3 ml ꢂ 100 ml); the organic layers were combined, dried
Pyrone 23 (0.4 g, 1.5 mmol) was dissolved in 2 ml of
THF, followed by the addition of a solution of THF: water:
12 M HCl (8:1:1) (3 ml). The mixture was stirred at room
temperature for 8 h, extracted with ethyl acetate, and washed
with saturated sodium bicarbonate. The solvent was
removed under aspirator vacuum and the crude product
was purified by column chromatography (silica gel, hexane:
ethyl acetate (1:4)) to obtain 0.20 g (78%) 24 as colorless
(
Na SO ) and concentrated in vacuo. Chromatography
4
2
afforded 21 (10.7 g, 83%) as colorless oil. 1H NMR
(
2
(
300 MHz): d (ppm): 5.80–5.90 (m, 1H); 5.37–5.43 (m,
H), 4.76 (s, 2H), 4.34 (dd, J ¼ 3:30, 7.20 Hz, 1H), 4.20
bs, 1H), 3.82–3.92 (m, 2H), 3.64–3.71 (m, 1H), 3.57 (t,
13
J ¼ 3:96 Hz, 2H), 3.40 (s, 3H); C NMR (75.5 MHz) d
(
7
ppm): 132.1, 121.1 (q, J ¼ 282:1 Hz), 120.4, 93.9, 78.1,
19
1
2.5 (q, J ¼ 28:61 Hz, 1C), 71.7, 67.7, 58.9; F NMR
liquid. H NMR (300 MHz): d (ppm): 4.75 (d, J ¼ 7:08 Hz,
(
300 MHz) d (ppm): ꢀ11.8; EI–MS: m/z 196, 59 (100%);
1H), 4.69 (q, J ¼ 7:02 Hz, 1H), 3.34 (bs, 1H), 2.93 (dd,
þ
13
CI–MS: m/z 245 (M þ H) , 139 (100%).
J ¼ 7:14, 18.66 Hz, 1H), 2.60 (d, J ¼ 18:57 Hz, 1H);
C
NMR (75.5 MHz) d (ppm): 174.16, 122.5 (q, J ¼ 286:9 Hz,
1
9
4
1
.1.4. (2R, 3R)-2-(Methoxyethoxymethoxy)-1,1,
-trifluoro-2,5-pentanediol, 22
Alcohol 21 (4.0 g, 16.4 mmol) was dissolved in 10 ml
1C), 83.0 (q, J ¼ 33:0 Hz, 1C), 66.9, 36.6; F NMR
þ
(300 MHz): d (ppm): ꢀ15.4; EI–MS: m/z 170 (M ), 71
þ
(100%); CI–MS: m/z 171 [(M þ H) , 100%]; HRMS–CI:
THF and added to a suspension of dicyclohexylborane
6.5 g, 36.5 mmol) in 30 ml THF at 0 8C and stirred for
5 h at 25 8C. After the completion of the reaction, the
170.0186 (actual), 170.0191 (calculated).
(
1
4.1.7. (3R, 4R, 5R)-3-Methyl-4-hydroxy-5-trifluoromethyl-
dihydrofuran-2-one, 11
reaction was oxidized with 14.4 ml of 3.0 M sodium hydro-
xide and 14.4 ml of 30% hydrogen peroxide and stirred for
Lithium diisopropylamide in hexanes (0.8 ml, 1.7 M
solution, 1.6 mmol) was cooled to ꢀ60 8and a solution of
the hydroxylactone 24 (0.08 g, 0.48 mmol) and hexamethyl
phosphoramide (0.5 ml) in 2 ml THF was added drop wise
over 40 min and stirred at ꢀ60 8C for 2 h. Iodobutane
(0.42 ml, 3.7 mmol) was added to it in one portion and stirred
for 16 h. The reaction mixture was quenched with water and
worked up with ether. The organic layer was dried over
3
with Et O, washed with water, dried over MgSO . The
h at room temperature. The reaction mixture was extracted
2
4
solvent was removed under aspirator vacuum and the crude
product was purified by silica gel column chromatography
(
hexane:ethyl acetate, 1:5) to obtain 3.5 g (82%) of the diol
1
2
1
4
3
1
2. H NMR (300 MHz): d (ppm): 4.83 (d, J ¼ 7:62 Hz,
H), 4.81 (d, J ¼ 7:62 Hz, 1H), 4.30–4.40 (m, 1H), 3.99–
.04 (m, 1H), 3.68–3.92 (m, 4H), 3.55–3.58 (m, 2H); 3.40 (s,
H), 1.83–2.03 (m, 2H); 13C NMR (75.5 MHz) d (ppm):
24.6 (q, 282.9 Hz, 1C), 96.0, 77.0, 71.8, 71.1 (q,
MgSO , concentrated under aspirator vacuum and purified
4
by column chromatography (silica gel, 3:2, hexane: ethyl
acetate) to obtain 0.04 g (48%) of the a-methyl-b-hydroxy-
J ¼ 29:1 Hz, 1C), 67.5, 59.0, 58.6, 32.1; 19F NMR
lactone 11. H NMR (300 MHz): d (ppm): 4.40–4.51 (m, 2H),
1
(
(
300 MHz) d (ppm): ꢀ19.31; EI–MS: m/z 187, 59
2.94 (bs, 1H), 2.68 (dt, J ¼ 5:55, 7.47 Hz, 1H), 1.25–1.90 (m,
þ
13
100%); CI–MS: m/z 263 (M þ H) , 157 (100%);
6H), 0.92 (t, J ¼ 7:53 Hz, 3H); C NMR (75.5 MHz) d
HRMS–CI: 263.1109 (actual), 263.1106 (calculated).
(ppm): 174.1, 122.9 (q, J ¼ 279:5 Hz, 1C), 79.6 (q, J ¼
3
3:4 Hz, 1C), 72.1, 47.4, 28.6, 28.1, 22.4, 13.8; EI–MS: m/z
þ
4.1.5. (4R, 5R)-4-(2-Methoxyethoxymethoxy)-5-
trifluoromethyl-dihydrofuran-2-one, 23
209, 170, 153 (100%); CI–MS: m/z 227 [(M þ H) , 100%];
HRMS–CI: 227.0891 (actual); 227.0895 (calculated).
Diol 22 (2.6 g, 10.0 mmol) was dissolved in 10 ml
CH Cl and NMO (3.5 g, 30.0 mmol) was added to it at
2
2
˚
0
8C. 4 A molecular sieves (2.6 g) and tetrapropylammo-
Acknowledgements
nium perruthenate (0.35 g, 1.0 mmol) were added to it and
stirred at 25 8C for 15 h. After the completion of the reaction
as indicated by TLC, solvent was removed under aspirator
vacuum and the crude product was purified by silica gel
Financial assistance from Herbert C. Brown Center for
Borane Research [17] and Aldrich Chemical Company are
gratefully acknowledged.

P.V. Ramachandran et al. / Journal of Fluorine Chemistry 125 (2004) 579–583
583
(
d) H.C. Brown, G.M. Chen, M.P. Jennings, P.V. Ramachandran,
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