Lipase-catalyzed alcoholysis of diol dibenzoates: Selective enzymatic access to the 2-benzoyl ester of 1,2-propanediol and preparation of the enantiomerically pure (R)-1-O-benzoyl-2-methylpropane-1,3-diol

Article abstract of DOI:10.1016/j.tetasy.2005.03.019

Enzymatic debenzoylation of 1,2-propanediol dibenzoate with 1-octanol has been studied in organic solvent using lipases from different sources. In general a slow, highly regioselective alcoholysis in diisopropyl ether affords exclusively a monoester benzoylated at the secondary hydroxy group although the reaction proceeds with low enantioselectivity. In the presence of Pseudomonas cepacia lipase absorbed onto celite, a faster reaction allows the preparation of the 2-benzoyl ester of (R)-1,2-propanediol (82% ee) and the enantiomerically pure (R)-1-O-benzoyl-2-methylpropane-1,3-diol (>98% ee).

Full text of DOI:10.1016/j.tetasy.2005.03.019

Tetrahedron:
Asymmetry
Tetrahedron: Asymmetry 16 (2005) 1705–1708
Lipase-catalyzed alcoholysis of diol dibenzoates:
selective enzymatic access to the 2-benzoyl ester of
1,2-propanediol and preparation of the enantiomerically
pure (R)-1-O-benzoyl-2-methylpropane-1,3-diol
Enzo Santaniello,* Silvana Casati, Pierangela Ciuffreda and Luca Gamberoni
`
Dipartimento di Scienze Precliniche LITA Vialba-Universita degli Studi di Milano Via G. B. Grassi, 74-20157 Milan, Italy
Received 16 February 2005; accepted 16 March 2005
Abstract—Enzymatic debenzoylation of 1,2-propanediol dibenzoate with 1-octanol has been studied in organic solvent using lipases
from different sources. In general a slow, highly regioselective alcoholysis in diisopropyl ether affords exclusively a monoester benzo-
ylated at the secondary hydroxy group although the reaction proceeds with low enantioselectivity. In the presence of Pseudomonas
cepacia lipase absorbed onto celite, a faster reaction allows the preparation of the 2-benzoyl ester of (R)-1,2-propanediol (82% ee)
and the enantiomerically pure (R)-1-O-benzoyl-2-methylpropane-1,3-diol (>98% ee).
ꢀ 2005 Elsevier Ltd. All rights reserved.
1.Introduction
OH
OH
OH
i
OCOPh
R
R
The selective protection of polyhydroxy compounds still
remains a challenge in organic synthesis.1 Considering
an ester as the protecting group of an alcohol, the selec-
tive acylation or deacylation can be chemically achieved
only by a few methods including microwave heating.2
The biocatalytic approach using enzymes has been
adopted for the introduction/removal of acyl groups,
such as acetates,3 but aliphatic esters are not sufficiently
stable for synthetic manipulations. An additional disad-
vantage of the above using protecting groups is consti-
tuted by the fact that migration towards a vicinal
hydroxy group frequently occurs under a variety of
experimental conditions.4–6 This unfavourable side reac-
tion is much slower for moderately reactive monoesters,
such as benzoates that present the additional advantage
to be more resistant to several synthetic transform-
ations.1 We have recently reported (Scheme 1) that the
selective enzymatic benzoylation of the primary hydroxy
group of 1,2-diols can be obtained with lipase from
Muchor miehei (MML) in an organic solvent and vinyl
benzoate (VB) as an acyl transfer agent.7
R = CH3; C4H9; C8H17; Ph; CH2Ph; CH= CH2
Scheme 1. Reagents: (i) MML, VB, tert-butyl methyl ether (tBuOMe).
We have started to study the enzymatic deacylation of
dibenzoates of 1,2-diols by alcoholysis, a reaction that
should preferably occur at the primary benzoate moiety,
thus allowing the preparation of the 2-monobenzoate
that, at present, can be hardly obtained by selective
chemical procedures.8
2.Results and discussion
2.1. Lipase-catalyzed debenzoylation of 1-O,2-O-di-
benzoylpropane-1,2-diol 1
The dibenzoate of 1,2-propanediol 1 [1-O,2-O-di-
benzoylpropane-1,2-diol] was selected as substrate to
set up the experimental conditions of lipase-catalyzed
debenzoylation in diisopropyl ether (DIPE), a solvent
that has been frequently used in alcoholysis proce-
dures.9,10 Other solvents, such as hexane, toluene, chlo-
roform, tetrahydrofuran and tert-butyl methyl ether
*
Corresponding author. Tel.: +39 025 031 9691; fax: +39 025 031
9631; e-mail: enzo.santaniello@unimi.it
0957-4166/$ - see front matter ꢀ 2005 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetasy.2005.03.019

1706
E. Santaniello et al. / Tetrahedron: Asymmetry 16 (2005) 1705–1708
were used but with DIPE, the reaction rates were the
most satisfactory. On the basis of our recent report on
the enzymatic benzoylation of 1,2-diols,7 we selected
MML as the biocatalyst for the alcoholysis process.
After preliminary experiments, an enzyme/substrate
ratio of 1 g/mmol was selected, since higher amount of
lipase produced only marginal beneficial effects and were
not practical from a preparative point of view. Using 1-
butanol as the deacylating agent (4.4 mmol/mmol sub-
strate), the reaction proceeded to 40% conversion
in 7 days. In an attempt to improve yields and lower
reaction times, other nucleophiles (water, ethanol, 1-
butanol, 1-octanol, 1-dodecanol, benzyl alcohol,
2-phenylethanol) were examined within 7 days reaction.
1-Octanol carried out the fastest alcoholysis affording in
7 days the expected monobenzoate 2 (75% yield). Con-
versely from the unstable acetate of secondary hydroxy
group in 1,2-diols11,12 monobenzoate 2 was a stable
product that did not show favour to migrate during
the purification by silica gel chromatography or on
standing at room temperature.
2.2. Pseudomonas species lipase absorbed onto celite
as a biocatalyst for enantioselective debenzoylation
The observed enantioselectivity of the reaction catalyzed
by PSL prompted us to study experimental conditions
that would accelerate the process. It turned out that
PSL absorbed onto celite16 (30% w/w, enzyme/substrate
ratio 1.2g/mmol, corresponding to 0.36 g of PSL/mmol)
catalyzed, at room temperature, the debenzoylation at an
extent of 33% in 2days, although the enantioselectivity
remained substantially unaffected (82% ee). Further-
more, when compared to the MML-catalyzed alcoholy-
sis (92% conversion to monobenzoate 2 in 14 days), the
complete conversion with PSL/celite was obtained in
6 days. Generally, a positive effect on activity, stability
and selectivity of absorbed17 or immobilized18 lipases
has already been demonstrated. We then applied the
PSL/celite-mediated alcoholysis to the asymmetrization
of 1-O,2-O-dibenzoyl-2-methylpropane-1,3-diol 3. In
the presence of PSL/celite, 50% conversion was achieved
in 2days and the enantiomerically pure ( S)-monobenzo-
ate 4 was obtained (>98% ee). Conversely for all other
native lipases, complete conversion could be reached
with PSL/celite in 14 days, but the enantioselectivity of
the reaction was lower (45% ee). We also carried out
the same reaction with MML and found that in 10 days,
30% conversion was reached, obtaining an enantiomeri-
cally pure product (>98% ee). The enantiomeric excess
and (R)-configuration was established by analyzing the
NMR spectra of the MTPA ester of monobenzoate 4
prepared by enzymatic debenzoylation and comparing
the resonances with those described for (S)-4.19 Results
on the debenzoylation of dibenzoates 1 and 3 with
PSL/celite are illustrated in Scheme 3.
As a general comment on the MML-catalyzed debenzo-
ylation, it should be pointed out that the alcoholysis
proceeds with high regioselectivity and constitutes as
the first example of an enzymatic preparation of a 1,2-
diol protected as benzoate at the secondary hydroxy
group (Scheme 2). In order to determine the stereochem-
ical outcome of the enzymatic reaction and the enantio-
meric excess of the product, reference samples of (RS)-2
and (R)-2 were prepared by a conventional protection/
deprotection sequence. Starting from commercial (RS)-
2 and (R)-1, selective protection of the primary hydroxy
group as t-butyldimethylsilyl ether,13 benzoylation of
the secondary hydroxy group, and removal of the silyl
group14 afforded required standards. HPLC analysis
using a chiral column (see Experimental section) allowed
us to obtain 20% ee and establish an (R)-configuration
for the enzymatically prepared monobenzoate 2. Inter-
estingly, the same HPLC analysis was unsuccessful in
resolving the enantiomers of (RS)-1-O-benzoyl ester of
propane-1,2-diol. MTPA esters of (RS)-2 and enzymat-
ically prepared monobenzoate 2 were obtained by
reaction with (S)-a-methoxy-a-(trifluoromethyl)phenyl-
acetyl chloride [(S)-MTPACl].15 1H NMR analysis con-
firmed the enantioselectivity and stereochemical outcome
of the enzymatic reaction. Other lipases such as pPL
(from porcine pancreas), CCL (from Candida cylindra-
cea) and CAL (from Candida antarctica) were then
examined and it was concluded that they were neither
more active nor more enantioselective than MML.
Interestingly, only lipase from Pseudomonas sp. (PSL)
showed a good enantioselectivity (78% ee), although
the (R)-monobenzoate 2 was obtained in only 20% yield
after 14 days at room temperature.
3.Conclusions
The preliminary data on the lipase-catalyzed debenzo-
ylation indicate that PSL/celite may allow a biocatalytic
preparation of 2-O-benzoylpropane-1,2-diol 2 and 82%
ee can be reached. The debenzoylation process can be
probably extended to other 1,2-diol dibenzoates and
we are actively exploring other applications of this enzy-
matic reaction. Furthermore, the deacylation of the dib-
enzoate 3, which affords enantiomerically pure (R)-4 is
an example of enantioselective enzymatic desymmetriz-
ation.20 This latest result is complementary to the
MML-catalyzed benzoylation of 2-methylpropane-1,3-
OCOPh
OCOPh
OCOPh
OH
i
i
H3C
H3C
H3C
1
(R)-2
82% ee
OCOPh
OCOPh
OCOPh
OH
OCOPh
OCOPh
OCOPh
OH
i
H3C
H3C
H3C
1
(R)-2
20% ee
(R)-4
3
>98% ee
Scheme 2. Reagents: (i) MML, 1-octanol, DIPE.
Scheme 3. Reagents: (i) PSL/celite, 1-octanol, DIPE.

E. Santaniello et al. / Tetrahedron: Asymmetry 16 (2005) 1705–1708
1707
diol that affords enantiomerically pure (S)-4.19 Both
enantiomers of monobenzoate 4 are important chiral
building blocks derived from meso-2-methylpropane-
1,3-diol21 and are now available as enantiomerically
pure synthons by a biocatalytic approach.
was stirred at room temperature overnight and then
evaporated at reduced pressure to give a crude product
that was purified by flash chromatography on silica
gel. Elution with petroleum ether/AcOEt (9:1) afforded
title compound 2 (40% isolated yield, colourless oil).
Chiral HPLC (hexane/isopropanol 98:2) tR = 20.9 min
1
for (S)-2 and 22.5 min for (R)-2. H NMR (CDCl3) d
4.Experimental
4.1. General
8.04 (2H, d, J = 7.0 Hz, H-ortho), 7.55 (1H, t,
J = 7.0 Hz, H-para), 7.42(2H, dd, J = 7.0 and 7.0 Hz,
H-meta), 5.23 (1H, ddq, J = 3.5, 6.3 and 6.3 Hz, H-1),
3.79 (1H, dd, J = 3.5 and 11.9 Hz, H-2a), 3.74 (1H,
dd, J = 6.3 and 11.9 Hz, H-2b), 1.36 (3H, d,
J = 6.3 Hz, CH3).
Optical rotations were measured on a Perkin–Elmer 241
polarimeter (sodium D line at 25 ꢁC). Melting points
were obtained using a Stuart Scientific SMP3 instrument
1
and are uncorrected. H NMR spectra were recorded
on
4.3. (R)-2-O-Benzoylpropane-1,2-diol (R)-2
a Bruker AM-500 spectrometer operating at
500.13 MHz and are referenced to the residual CHCl3
proton of the solvent CDCl3 at 7.24 ppm; coupling con-
stants (J) are given in hertz. Thin-layer chromatography
(TLC) was performed using Merck silica gel 60 F254 pre-
coated plates with a fluorescent indicator. Flash chro-
matography22 was performed using Merck silica gel 60
(230–400 mesh) using appropriate mixtures of petro-
leum ether and ethyl acetate as eluants. The progress
of all reactions, column chromatography and compound
purity were monitored by TLC, GLC and/or HPLC.
GLC analyses were carried out using a Hewlett Packard
GC System HP6890 with a HP-5 Hewlett Packard col-
umn (30 m · 0.32mm; 0.25 mm ID, film thickness
0.25 lm). HPLC analyses were carried out using a Per-
kin Elmer HPLC instrument with a Series 200 UV/vis
detector operating at 240 nm using a chiral Merck
(R,R) Whelk-O1 column (4 mm · 25 cm). All reagents
were obtained from commercial sources and used with-
out further purification. Porcine pancreas lipase (24 U/
mg solid) was purchased from Fluka. Lipases from
Pseudomonas sp. (Lipase PS ÔAmanoÕ, 30 U/mg solid)
and from C. cylindracea (Lipase AYS ÔAmanoÕ,
31.6 U/mg solid) were purchased from Amano Pharma-
ceutical. Lipases from C. antarctica (Novozym 435ꢂ,
acrylic resin supported lipase, 11.4 U/mg solid) and
Mucor miehei (Chirazymeꢂ L-9,c.-f., C2, lyo, carrier-
fixed lipase, 8 U/mg solid) were purchased from Novo
Nordisk and Roche Diagnostics GmbH, respectively.
(R)-2 was chemically prepared from the commercially
available enantiomerically pure (R)-1,2-propanediol 1
(Fluka, Switzerland) as previously described for (RS)-
25
2; ½aŠ ¼ À18:6 (c 1, CHCl3). Chiral HPLC (hexane/iso-
D
propanol 98:2) tR = 22.5 min. Significative signals corre-
sponding to the Mosher derivative of (R)-2: (CDCl3) d
5.43–5.37 (1H, m, CHOCOPh), 4.52–4.46 (2H, m, part
AB of ABX system, CH2O), 3.48 (3H, s, OCH3), 1.36
(3H, d, J = 7.0 Hz, CH3).
4.4. MML-mediated alcoholysis of 1-O,2-O-dibenzoyl-
propane-1,2-diol 1
The substrate (1.0 mmol), 1-octanol (4.4 mmol) and
lipase (1.0 g) were suspended in DIPE (10 mL). The mix-
ture was allowed to react at room temperature under
magnetic stirring and the progress of the reaction mon-
itored by TLC (petroleum ether/AcOEt 8:2) and GLC
(160 ꢁC for 5 min; 30 ꢁC/min to 280 ꢁC for 15 min;
tR = 4.5 min for monobenzoate, tR = 10.4 min for di-
benzoate). At the end of reaction, the enzyme was fil-
tered off and washed with methanol, the solvents were
distilled under vacuum and the product purified by flash
chromatography. Elution with petroleum ether/AcOEt
(9:1) afforded monobenzoate 2 as a colourless oil
(0.122 g, 68% for the MML catalyzed reaction at 75%
conversion). At 30% conversion, (R)-2 (ee 20%) was ob-
25
D
tained in 28% yield; ½aŠ ¼ À3:6 (c 1, CHCl3).
Significative signals corresponding to the Mosher deriv-
ative of the minor enantiomer (40%), (S)-2: (500 MHz,
CDCl3) d 5.43–5.37 (1H, m, CHOCOPh), 4.52(1H,
dd, J = 3.3 and 11.8 Hz, CHHO), 4.46 (1H, dd, J = 6.1
and 11.8 Hz, CHHO), 3.50 (3H, s, OCH3), 1.39 (3H,
d, J = 7.0 Hz, CH3). For the signals corresponding to
the Mosher derivative of the major enantiomer (60%)
(R)-2 see Section 4.3.
4.2. (RS)-2-O-Benzoylpropane-1,2-diol 2
To
a
solution of (RS)-1,2-propanediol (1.0 g;
13.14 mmol) in pyridine (10 mL), tert-butyldimethyl-
chlorosilane (2.9 g; 19.24 mmol) was added. The mixture
was allowed to react under continuous stirring at room
temperature and the progress of the reaction monitored
by TLC (petroleum ether/AcOEt 7:3) and GC. At the
end of reaction, the 1-silyl derivative was purified by
silica gel column chromatography (petroleum ether/
AcOEt 7:3) and reacted with benzoyl chloride (2.7 g;
19.24 mmol) in pyridine (10 mL) at 0–5 ꢁC. After stir-
ring overnight, addition of 1 M HCl and work-up afford-
ed the 1-O-silyl,2-O-benzoyl derivative that was used
for the next step without purification. A solution of
1 M lithium tetrafluoroborate in CH3CN (25.0 mL)
was slowly added to the solution of the above com-
pound in CH2Cl2–CH3CN (1:1, 40 mL). The reaction
4.5. PSL/celite-mediated alcoholysis of 1-O,2-O-di-
benzoylpropane-1,2-diol 1 and 1-O,3-O-dibenzoyl-
2-methylpropane-1,3-diol 3
The substrate (1.0 mmol), 1-octanol (4.4 mmol) and
lipase (1.2g) were suspended in DIPE (10 mL). The mix-
ture was allowed to react at room temperature under
magnetic stirring and the progress of the reaction
monitored by GLC. The enzyme was filtered off and

1708
E. Santaniello et al. / Tetrahedron: Asymmetry 16 (2005) 1705–1708
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25
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4.5.1. (R)-2-O-Benzoylpropane-1,2-diol (R)-2. ½aŠ
¼
1
À15:4 (c 1, CHCl3). H NMR of the Mosher ester of
enzymatically prepared 2 showed two singlets at
3.50 ppm [9%, (S)-2] and 3.48 ppm [91%, (R)-2] corre-
sponding to resonances of the three hydrogen atoms of
the OCH3 group of MTPA [82% ee for (S)-4].
4.5.2. (R)-1-O-Benzoyl-2-methylpropane-1,3-diol (R)-
25
25
4. ½aŠ ¼ À8:0 (c 1, MeOH); lit:19 ½aŠ ¼ þ7:9 (c 1,
D
D
MeOH) for enantiomerically pure (S)-4; 1H NMR
(CDCl3) d 8.02(2H, d, J = 7.0 Hz, H-ortho), 7.54 (1H, t,
J = 7.0 Hz, H-meta), 7.42(2H, dd, J = 7.0 and 7.0 Hz,
H-para), 4.35 (1H, dd, J = 5.4 and 11.2Hz, C HHOBz),
4.28 (1H, dd, J = 6.3 and 11.2Hz, CH HOBz), 3.60 (1H,
dd, J = 5.4 and 11.2Hz, C HHOH), 3.56 (1H, dd,
J = 6.3 and 11.2Hz, CH HOH), 2.23 (1H, br s, OH),
2.14–2.08 (1H, m, CHCH3), 1.04 (3H, d, J = 7.0 Hz,
CH3). For the configuration and evaluation of the ee,
1
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Acknowledgements
`
This work was financially supported by Universita degli
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´
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