Chemoenzymatic approach to enantiomerically pure (R)-3-Hydroxy-3-methyl-4- pentenoic acid ester and its application to a formal total synthesis of taurospongin A

Article abstract of DOI:10.1002/adsc.200800191

(R)-3-Hydroxy-3-methyl-5-hexanoic acid p-methoxybenzyl ester 1b was prepared by carbonchain elongation on both termini of the starting material, (R)-3-benzyloxy-2-methylpropane-1,2-diol 2a, which was prepared by an over-expressed Bacillus subtilis epoxide hydrolase-catalyzed enantioselective hydrolysis of the racemic 1-benzyloxymethyl-1-methyloxirane 3. One of the key steps of the requisite transformation was the Rhodococcus rhodochrous NBRC 15564-mediated hydrolysis of a cyano group to a carboxyl group under neutral conditions, to exclude any racemization of the intermediate and/or product. The enantiomerically pure form of (R)-1b was applied to a new formal total synthesis of taurospongin A.

Full text of DOI:10.1002/adsc.200800191

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DOI: 10.1002/adsc.200800191
Chemoenzymatic Approach to Enantiomerically Pure (R)-3-Hy-
droxy-3-methyl-4-pentenoic Acid Ester and Its Application to a
Formal Total Synthesis of Taurospongin A
Aya Fujinoa and Takeshi Sugaia,*
a
Faculty of Pharmacy, Keio University, Shibakoen 1-5-30, Minato-ku, Tokyo 105-8512, Japan
Fax : (+81)-3-566-1697; e-mail: sugai-tk@pha.keio.ac.jp
Received: March 30, 2008; Published online: July 9, 2008
This manuscript is dedicated to Professor Chi-Huey Wong on the occasion of his 60th birthday.
Supporting information for this article is available on the WWW under http://asc.wiley-vch.de/home/.
Abstract: (R)-3-Hydroxy-3-methyl-5-hexanoic acid NBRC 15564-mediated hydrolysis of a cyano group
p-methoxybenzyl ester 1b was prepared by carbon- to a carboxyl group under neutral conditions, to ex-
chain elongation on both termini of the starting ma- clude any racemization of the intermediate and/or
terial, (R)-3-benzyloxy-2-methylpropane-1,2-diol 2a, product. The enantiomerically pure form of (R)-1b
which was prepared by an over-expressed Bacillus was applied to a new formal total synthesis of tauro-
subtilis epoxide hydrolase-catalyzed enantioselective spongin A.
hydrolysis of the racemic 1-benzyloxymethyl-1-meth-
yloxirane 3. One of the key steps of the requisite Keywords: amidase; bioconversion; epoxide hydro-
transformation was the Rhodococcus rhodochrous lase; hydrolysis; kinetic resolution; nitrile hydratase
Introduction
hydrolysis of readily available racemic epoxide 3,[2]
we also reported the use of an over-expressed Bacillus
3-Hydroxy-3-methyl-5-hexanoic acid (1a), which has a subtilis epoxide hydrolase (Scheme 1).[3] All the trans-
tertiary alcohol on the b-position of the carboxylic formation steps should be carried out under the mild-
acid together with an appropriately functionalized est conditions possible without any loss of the enan-
side chain, seems very attractive as the starting mate- tiomeric excess of the product. Otherwise, as the
rial for many biologically active compounds. Here we target molecule 1 has rather labile tertiary alcoholic
report an approach to a p-methoxybenzyl ester (R)- hydroxy group at the b-position, it would be suscepti-
1b, by the carbon-chain elongation of (R)-3-benzyl- ble for elimination-addition of water or retro-aldol
oxy-2-methylpropane-1,2-diol (2a) on both termini of and aldol condensation of the molecule.
the molecule. While so far (R)-2a in an enantiomeri-
cally pure state has been available[1] by means of mi-
crobial epoxide hydrolase-catalyzed enantioselective Results and Discussion
Carbon-Chain Elongation and the Functional Group
Transformations under Mild Conditions
Towards the synthesis of (R)-1b, the primary hydroxy
group of diol (R)-2a was selectively tosylated, and the
tosylate 2b was converted to the epoxide (S)-3.[3] This
was treated with vinylmagnesium chloride in THF in
the presence of CuI to give (R)-4a in almost a quanti-
tative yield. Next, the benzyl protective group in 4a
was selectively removed, by treatment with lithium-
ethylenediamine-THF,[4] a modified Benkeser reduc-
tion. These reaction conditions and work-up are ad-
Scheme 1.
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Enantiomerically Pure (R)-3-Hydroxy-3-methyl-4-pentenoic Acid Ester
Scheme 3.
that neither the nitrile hydratase nor the amidase in
this microorganism worked in an enantioselective
manner on the nitrile 5 or the intermediacy amide 7.
While amidases had in many cases worked enantiose-
lectively on b-alkoxy- and acyloxy-substituted ni-
triles,[16–19] the loss of stereoselection had also been
observed when the substituent was altered from
Scheme 2.
vantageous for water-soluble or highly volatile com- alkoxy and/or acyloxy to a hydroxy group.[16,20–21] In
pounds, as the work-up and purification procedure is this case, as the antipodal substrate, (S)-5 can be pre-
very simple. The desired diol (R)-4b was obtained in pared from the enantiomerically pure form of (R)-ep-
61% yield, under these conditions at À108C for 3 h. oxide 3,[3] this total procedure in Scheme 2 and
Prolonged reaction at higher temperature caused the Scheme 3 would also work well to provide (S)-1b.
unexpected over-reduction on the terminal olefin.
The liberated primary hydroxy group in 4b was tosy-
lated, and the resulting tosylate 4c (83%) was succes- Application to the Formal Total Synthesis of
sively treated with KOH in EtOH and NaCN to give Taurospongin A
nitrile (R)-5 in one pot without the isolation of the in-
termediate, an epoxide 6 (Scheme 2).
After the above-mentioned tertiary hydroxy ester
Next step was the transformation of the cyano (R)-1b, bearing a terminal olefin, in an enantiomeri-
group to a carboxylic acid. In our first attempts for cally pure state became available, it was further trans-
the hydrolysis under strongly basic conditions, the re- formed to a known synthetic intermediate [(R)-11b]
action was very slow. The conversion required harsh of taurospongin A (11c),[22,23] by means of Grubbsꢁ
conditions such as 1208C for 2 days, however, the cross-metathesis coupling reaction. To this end, the
yield of the desired product 1b, after isolation as the counterpart, allylic acetate (1R,3R)-9b (syn-9b) was
corresponding p-methoxybenzyl ester (PMB) was as prepared starting from ethyl (R)-3-hydroxybutanoate,
low as 7%, due to the labile properties of the b-hy- via an ethanolysis of the biodegrable polymer, poly b-
droxynitrile structure of the substrate. Moreover, this hydroxybutyrate (PHB).[24] The known aldehyde 8[25]
key functional group transformation was not satisfac- was treated with vinylmagnesium bromide and the re-
torily possible by any attempted conditions which had sulting allylic alcohol 9a was subsequently acetylated
been known for effective transformations of cyano to to give a stereoisomeric mixture of acetates 9b, in a
carboxyl group under mild conditions,[5–11] such as range of syn:anti=1.3–1.5:1 (Scheme 4). Disappoint-
transition metal complex-catalyzed reactions.
ingly, all attempts towards the more selective forma-
To solve this obstacle, an enzyme-catalyzed hydra- tion of the syn-alcohol were in vain, even by changing
tion of nitrile followed by hydrolysis of the amide[12–15] the Lewis acidity of the vinylmetal species (MgCl,
was effective. In our hands, with the cultured whole MgBr, Ce) and the protective group of the remote hy-
cells of Rhodococcus rhodochrous NBRC 15564, the droxy group from silyl to, e.g., p-methoxybenzyl. As
sequential action of two enzymes, nitrile hydratase either the diastereomeric pair of alcohols 9a or ace-
and amidase in one pot provided the desired product, tates 9b were hardly separable on silica gel column
(R)-1b in a total yield of 60% from tosylate 4c in four chromatography, we decided to use a lipase-catalyzed
steps, after direct treatment with PMB chloride of the kinetic resolution of the diastereomers of 9b. The two
crude hydrolysate (Scheme 3). HPLC analysis with a candidates for enzymes which so far had been report-
chiral stationary phase supported the 100% ee of 1b. ed to satisfy such requrements, the stereoselective hy-
In a separate experiment, incubation of racemic ni- drolysis of allylic acetates, Pseudomonas cepacia
trile 5 provided racemic 1b. These results indicate lipase[26,27] and Candida antarctica lipase[28] were com-
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Conclusions
We were successful in developing a route to the suita-
bly functionalized b-hydroxy ester, (R)-1b by carbon-
chain elongation and functional group transformation
of (R)-2a. As an application of the enantiomerically
pure form of (R)-1b, the formal total synthesis of
taurospongin A was demonstrated. In contrast to all
the attempted chemical transformations, the biocata-
lytic hydrolysis of a cyano to a carboxyl group with
Rhodococcus rhodochrous NBRC 15564 under neu-
tral conditions worked well, to exclude any racemiza-
tion of the intermediate.
Experimental Section
Characterization data of all of the compounds including
spectroscopic and analytical data and the general remarks
about the equipment used are available as Supporting Infor-
mation.
(R)-1-Benzyloxy-2-methyl-4-pentene-2-ol (4a)
A solution of vinylmagnesium chloride (1.0M, 13.5 mL,
13.5 mmol in THF) was degassed to remove traces of
oxygen by the repeat of evacuation and then introduction of
N2 under atomospheric pressure several times. The mixture
was then cooled to À788C. CuI (0.171 g, 0.898 mmol) was
added under the flow of N2. The reaction temperature was
raised to À508C, 3[3] (0.811 g, 4.55 mmol) in dry THF
(0.4 mL) was added to the reaction mixture and stirred for
3 h. After the mixture had been warmed to room tempera-
ture, a saturated aqueous solution of NH4Cl (10 mL) was
added to the dark-colored mixture reaction. The mixture
was diluted with EtOAc, and then the combined organic
layer was washed with brine, dried over Na2SO4 and concen-
trated in vacuo. The residue (0.97 g) was charged on a silica
gel column (28 g). Elution with hexane-EtOAc (4:1) gave
(R)-4a as a colorless oil; yield: 0.845 g, 4.10 mmol (90%).
[a]2D1: À10.38 (c 1.02, CHCl3) [lit.[30] (R)-4a: [a]2D0: À28 (c 0.5,
CHCl3)]. Its NMR spectrum was identical with that reported
previously.[30]
Scheme 4.
pared. Preliminary experiments revealed that the
former had a higher catalytic activity and enantiose-
lectivity (E value: 48) than the latter (26). Incuba-
tion of the mixture of 9b with P. cepacia lipase
(Amano PS-C) in buffer solution at 408C provided
(1R,3R)-9b in 59% yield and 99.9% de as the unreact-
ed recovery.
Grubbsꢁ second-generation catalyst (10 mol%)-
mediated cross-metathesis coupling reaction[29] be-
tween the two components (R)-1b and (1R,3R)-9b
(two molar excess) worked well to give the desired
ester 10 in 77% yield. Finally, saturation of the olefin
as well as the deprotection of PMB ester via hydroge-
nolysis with Pd(OH)2 in EtOAc, and subsequent ally-
(R)-2-Methyl-4-pentene-1,2-diol (4b)
A solution of 4a (2.00 g, 11.2 mmol) in THF (51 mL) and
ethylenediamine (5.2 mL, 78.6 mmol) was degassed to
remove any traces of oxygen by the repetitive application of
lation of the free carboxyl group in 11a provided (R)- ultrasonic (70–80 W) under evacuation and then introduc-
tion of N2 under atomospheric pressure several times. The
mixture was then cooled to À108C. Lithium shot (0.39 g,
56.2 mmol) was added in several portions under a flow of
N2. After 2 h, the reaction was quenched by addition of 2M
HCl and the mixture was extracted with EtOAc. The organ-
ic layer was washed with brine, dried over Na2SO4, and con-
centrated under vacuum. The residue (1.02 g) was charged
on a silica gel column (40 g). Elution with hexane-EtOAc
(2:1) afforded 4b as a colorless oil; yield: 0.794 g, 6.84 mmol
(61%). [a]1D9: +21.88 (c 1.02, EtOH). Its NMR spectrum was
identical with that reported previously.[31]
11b in 76% through two steps (Scheme 4). In the
former step, the major side reaction was the hydroge-
nolysis of the allylic oxygen functionality, and the use
of conventional Pd-C, other polar solvents, or even
homogeneous Wilkinsonꢁs catalyst promoted this un-
desired path and, moreover, the loss of TBS protec-
tive group. The spectral and optical rotation data of
the present (R)-11b were in good agreement with the
previously reported value, [a]2D3: À1.458 (c 0.97,
CHCl3) {lit.[23] [a]D: À1.178 (c 1.37, CHCl3)}.
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Enantiomerically Pure (R)-3-Hydroxy-3-methyl-4-pentenoic Acid Ester
charged on a silica gel column (6.5 g). Elution with hexane-
EtOAc (4:1) gave 9a as a diastereomeric mixture; yield:
0.181 g (0.786 mmol, 82%). The ratio between (3R,5R)-9a
[syn-9a], (3S,5R)-9a [anti-9a] was estimated as 1.44:1 by
(R)-2-Hydroxy-2-methyl-4-pentenyl p-Toluenesulfon-
ate (4c)
To a solution of 4b (0.297 g, 2.56 mmol) in pyridine (5.6 mL)
was added p-toluenesulfonyl chloride (0.740 g, 3.87 mmol) at
08C. After stirring for 24 h at room temperature, the reac-
tion was quenched by the addition of water (10 mL), and
the mixture was extracted with EtOAc. The organic layer
was washed with 1M HCl, saturated aqueous NaHCO3 solu-
tion, and brine. The solution was dried over Na2SO4, and
concentrated under vacuum. The residue (0.601 g) was
charged on a silica gel column (12 g). Elution with hexane-
EtOAc (3:1) afforded 4c as a colorless oil; yield: 0.574 g,
2.12 mmol (83%). [a]1D9: À0.528 (c 0.96, EtOH). This was em-
ployed for the next step without further purification.
1
comparing the H NMR signal of the each component.
(1RS,3R)-3-tert-Butyldimethylsilyloxy-1-vinylbutyl
Acetate (9b)
A solution of 9a (0.178 g, 0.773 mmol) and Ac2O (1.0 mL) in
pyridine (1.0 mL) was stirred at room temperature for 24 h.
The mixture was quenched by the addition of ice and stirred
for 1 h. Then, the mixture was concentrated under vacuum,
and extracted with EtOAc. The organic layer was washed
with 1M HCl, saturated aqueous NaHCO3 solution and
brine, dried over Na2SO4 and concentrated under vacuum.
The residue (0.224 g) was charged on a silica gel column
(6 g). Elution with hexane-EtOAc (10:1) afforded (1RS,3R)-
9b as a colorless oil; yield: 0.215 g (quantitative). This was
employed for the next step without further purification.
p-Methoxybenzyl (R)-3-Hydroxy-3-methyl-5-hexeno-
ate (1b)
A solution of 4c (0.197 g, 0.729 mmol) in EtOH (0.67 mL)
was added KOH (50.0 mg, 0.891 mmol) in EtOH (0.67 mL)
at 08C. After stirring for 10 min, water (2 mL) and NaCN
(0.11 g, 2.24 mmol) was added. The mixture was warmed to
1208C and stirred for an additional 1 h. The mixture was di-
luted with Et2O, the organic layer was washed with brine,
dried over Na2SO4 and concentrated under vacuum to give
crude 5; yield: 96.3 mg.
Pseudomonas cepacia Lipase-Catalyzed Hydrolysis of
(1RS,3R)-9b
A mixture of lipase PS-C (0.2 g) and (1RS,3R)-9b (30.7 mg,
0.113 mmol) in phosphate buffer (0.1M, 0.6 mL, pH 7.0) was
stirred for 18 h at 408C. The reaction mixture was diluted
with brine and extracted with EtOAc. The organic layer was
dried over Na2SO4 and concentrated under vacuum. The res-
idue (29.6 mg) was charged on a silica gel column (1 g). Elu-
tion with hexane-EtOAc (10:1) gave (1R,3R)-9b (syn-9b:
yield: 18.2 mg, 0.0668 mmol, 59%, 99.9% de) and (3S,5R)-9a
(anti-9a: yield: 7.7 mg, 0.0334 mmol, 30%, 87.3% de) as a
colorless oil. (1R,3R)-9b: [a]2D1: À7.968 (c 0.97, EtOH).
A mixture of 5 in phosphate buffer (0.1M, 0.6 mL,
pH 7.0) and cultured wet cells of R. rhodochrous
NBRC15564 prepared according to the previous paper[32]
was stirred for 4 days at 358C. The supernatant was saturat-
ed with NaCl and mixed with EtOAc. The mixture was
stirred for 1 h and filtered through a pad of Celite. The or-
ganic layer of the filtrate was separated and the aqueous
layer was acidified with HCl (2M) to pH 2 and extracted
with EtOAc. The organic layer was washed with brine, dried
over Na2SO4 and concentrated under vacuum to give crude
1a (0.112 g). This was dissolved in DMF (3 mL) and to this
was added KOH (0.10 g, 1.78 mmol) and p-methoxybenzyl
chloride (0.3 mL, 2.21 mmol). After stirring for 14 h, the re-
action was quenched by the addition of water (10 mL), and
the mixture was extracted with Et2O. The organic layer was
washed with brine, dried over Na2SO4, and concentrated
under vacuum. The residue (0.517 g) was charged on a silica
gel column (15 g). Elution with hexane-EtOAc (4:1) afford-
ed (R)-1b as a colorless oil; yield: 0.116 g (0.438 mmol,
60%). [a]2D2: À1.688 (c 0.85, EtOH); HPLC:>99.9% ee
[Chiralcel OJ-H, 0.46 cm”25 cm; hexane-i-PrOH (15:1),
0.5 mLminÀ1], tR =49.9 min [(R), single peak]. No peak as-
cribable to (S)-isomer [tR =52.2 min] was detected.
p-Methoxybenzyl (3R,7S,9R)-7-Acetoxy-9-(tert-butyl-
dimethylsilyloxy)-3-hydroxy-3-methyl-5-decenoate
(10)
To a mixture of (R)-1b (63.5 mg, 0.240 mmol) and (1R,3R)-
9b (0.133 g, 0.488 mmol) in CH2Cl2 (1.2 mL) was added
second-generation Grubbsꢁ catalyst (20 mg, 0.0236 mmol).
The reaction was heated to 458C for 15 h and then cooled
to room temperature. The reaction mixture was concentrat-
ed under vacuum. The residue (0.25 g) was charged on a
silica gel column (6 g). Elution with hexane-EtOAc (5:1)
gave 10 as a pale yellow oil; yield: 91.9 mg (0.181 mmol,
77%). [a]2D4: +7.668 (c 1.61, EtOH).
Allyl (3R,7S,9R)-7-Acetoxy-9-(tert-butyldimethyl-
silyloxy)-3-hydroxy-3-methyldecanoate (11b)
A
A solution of 10 (60.9 mg, 0.120 mmol) in EtOAc (1.6 mL)
was stirred with Pd(OH)2 (30 mg, Aldrich, Wet, Degussa
type) at room temperature for 1 h under H2. The mixture
was filtered through No. 5C paper and the filtrate was con-
centrated under vacuum to give crude 11a; yield: 50.4 mg.
To a solution of 11a in CH2Cl2 (1.2 mL) was added N,N-
diisopropylethylamine (0.2 mL, 1.18 mmol), followed by
allyl bromide (0.154 mL, 1.78 mmol). The resulting mixture
was stirred for 14 h, then concentrated. The residue
(59.4 mg) was purified by silica gel column (4.0 g). Elution
with hexane-EtOAc (10:1) afforded 11b as a colorless oil;
(9a)
To a solution of vinylmagnesium bromide (0.94M, 3.3 mL,
3.10 mmol in THF) was added (R)-8 (0.195 g, 0.964 mmol)
in dry THF (0.5 mL) at À788C under an argon atmosphere.
After 1 h, the reaction mixture was warmed to room tem-
perature, stirred for an additional 6 h and quenched by
dropwise addition of saturated aqueous NH4Cl solution
(5 mL) at 08C. The mixture was diluted with EtOAc, the or-
ganic layer was washed with brine, dried over Na2SO4 and
concentrated under vacuum. The residue (0.212 g) was
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yield: 39.2 mg (0.091 mmol, 76% isolated yield; 50% based
on the recovery). [a]23: À1.458 (c 0.97, CHCl3), [lit.[23]: [a]D:
À1.178 (c 1.37, CHCl3D)]. Its IR and NMR spectra were iden-
tical with those reported previously.[23]
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[17] H. Kakeya, N. Sakai, A. Sano, M. Yokoyama, T. Sugai,
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Acknowledgements
The authors thank Drs. Masaya Ikunaka and Hitomi Yama-
guchi of Research & Development Center, Nagase & Co., for
their support on Bacillus subtilis epoxide hydrolase, and Dr.
Yoshihiko Hirose of Amano Enzyme Inc. for generous gift
of lipase PS-C, Dr. Shigeo Yamanoi of Daiichi-Sankyo Ltd.
for his valuable information on taurospongin A synthesis.
Professor Shigeru Nishiyamaꢀs encouragement on this study
was acknowledged with thanks. This work was supported
[18] M. Yokoyama, N. Imai, T. Sugai, H. Ohta, J. Mol.
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both by
a Grant-in-Aid for Scientific Research (No.
18580106) and “High-Tech Research Center” Project for Pri-
vate Universities: matching fund subsidy 2006–2011 from the
Ministry of Education, Culture, Sports, Science and Technol-
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