Resolution of a racemic 1,2-diol using triphenylmethyl protection of the primary hydroxyl group and Mucor miehei lipase (Lipozyme) for the kinetic resolution

Article abstract of DOI:10.1016/j.tetasy.2011.10.003

The triphenylmethyl group gives very simple access to the 1-protection of 1,2-diol as exemplified by racemic propane-1,2-diol. This group has, however, been shown to be incompatible with lipases commonly used for the resolution of alcohols. This turned out to be the case for Pseudomonas cepacia lipase, which we have used in our earlier work. Lipozyme, a Mucor miehei lipase, best known for 1,3-selectivity with glycerol is, however, shown to catalyze transacetylation onto the secondary hydroxyl group next to a triphenylmethoxy group. The transacetylation is completely enantioselective for the (R)-enantiomer giving a very simple method for the resolution of this type of 1,2-diol enantiomer.

Full text of DOI:10.1016/j.tetasy.2011.10.003

Tetrahedron: Asymmetry 22 (2011) 1809–1812
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Tetrahedron: Asymmetry
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Resolution of a racemic 1,2-diol using triphenylmethyl protection
of the primary hydroxyl group and Mucor miehei lipase (Lipozyme)
for the kinetic resolution
Sigthor Petursson a, , Sigridur Jonsdottir b
⇑
a Faculty of Business and Science, University of Akureyri, Borgum v/Nordurslod, 600 Akureyri, Iceland
b University of Iceland, Institute of Science, Dunhaga 3, IS-107 Reykjavik, Iceland
a r t i c l e i n f o
a b s t r a c t
Article history:
The triphenylmethyl group gives very simple access to the 1-protection of 1,2-diol as exemplified by
racemic propane-1,2-diol. This group has, however, been shown to be incompatible with lipases com-
monly used for the resolution of alcohols. This turned out to be the case for Pseudomonas cepacia lipase,
which we have used in our earlier work. Lipozyme, a Mucor miehei lipase, best known for 1,3-selectivity
with glycerol is, however, shown to catalyze transacetylation onto the secondary hydroxyl group next to
a triphenylmethoxy group. The transacetylation is completely enantioselective for the (R)-enantiomer
giving a very simple method for the resolution of this type of 1,2-diol enantiomer.
Received 3 October 2011
Accepted 14 October 2011
Available online 15 November 2011
Ó 2011 Elsevier Ltd. All rights reserved.
1. Introduction
oxy-2-propanol and to look for other lipases, if necessary.6 There is
considerable interest in propane-1,2-diol as a chiral building block
Methods, which allow the mono-etherification of vicinal diol
systems are synthetically useful.1–4 This is particularly noteworthy
in cases where regioselectivity is pronounced, for instance be-
tween primary and secondary hydroxyl groups. Primary selectivity
in the etherification of propane 1,2-diol, followed by an enantiose-
lective lipase catalyzed esterification of the remaining hydroxyl
group gives a reasonably direct method for the resolution of race-
mic propanediol. An example of this is the use of the tin(II) bro-
mide catalyzed mono-etherification of racemic propane-1,2-diol
with diazo[bis(4-methoxyphenyl)]methane followed by pseudomo-
nas cepacia lipase catalyzed transacetylation of the 2-OH on the
stereogenic carbon as previously reported.5 Even if the primary
selectivity reported in this work was good, additional chemical
steps had to be added in order to isolate the desired pure 1-ether
product. This involved the triphenylmethylation of the minor
1-alcohol isomer in the mixture and a chromatographic separation.
The use of the well known primary selectivity of the triphenyl
methyl group for the derivatization and removal of the minor
1-alcohol immediately draws attention to the possibility of using
the triphenylmethyl group itself for the primary protection of pro-
pane-1,2-diol. Despite reports that 1-phenoxy-3-triphenylmeth-
oxy-2-propanol is not a substrate for Pseudomonas cepacia lipase,
we decided to investigate if this was also true for 1-triphenylmeth-
and the resolved diol is commercially available. Even if the method
reported here is efficient and may be of interest for the resolution
of this simple racemic diol, the wider general applicability is of
more interest.7–10
2. Results and discussion
The triphenylmethylation of propane-1,2-diol using the stan-
dard method of triethylamine as the acid scavenger in dichloro-
methane and 4-dimethylaminopyridine (DMAP) as the catalyst
gave the primary triphenylmethyl ether in close to quantitative
yield. This very simple reaction has, however, its disadvantages.
A reasonable work-up method is to quench the reaction with a
mild aqueous acid and to extract the product into an organic sol-
vent. This was attempted using aqueous ammonium chloride,
which resulted in a negligible yield of the 1-trityl ether. The trityl
ether was also found to be unstable under these conditions with
the product reverting to the starting material. The addition of ex-
cess silica gel directly into the reaction mixture and removal of
the solvent under reduced pressure followed by purification of
the product on a column of silica gel resulted in about 95% yield.
The attempted transacetylation from vinyl acetate in di-isopropyl
ether catalyzed by the Pseudomonas cepacia lipase, as used previ-
ously, did not work and no sign of reaction was observed.5 Mucor
miehei lipase (Lipozyme) is a lipase which has been shown to be
1,3-specific for glycerol.11,12 It is therefore of interest that (R)-1-tri-
phenylmethyl-2-propanol proved to be a substrate for this enzyme
in the transacetylation from vinyl acetate.
⇑
Corresponding author.
E-mail addresses: sigthor@unak.is (S. Petursson), sigga@raunvis.hi.is (S. Jonsdot-
tir).
0957-4166/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetasy.2011.10.003

1810
S. Petursson, S. Jonsdottir / Tetrahedron: Asymmetry 22 (2011) 1809–1812
2.1. Lipase catalyzed transacetylation onto rac-1-
triphenyloxypropan-2-ol
tion on the HPLC, gave e
= 647 L molꢀ1 cmꢀ1 for the acetate product
and 645 L molꢀ1 cmꢀ1 for the alcohol. The 3% excess of the product
integration could be taken to indicate less than 100% enantiomeric
selectivity of the enzyme but it should be noted that the peak inte-
gration accuracy is probably no better than this.13 At the end of the
reaction, the produced (R)-acetate and the unreacted (S)-alcohol
were easily separated on a column of silica gel eluting with
Hex/EtOAc 9:1 and then 4:1.
rac-1-Triphenyloxypropan-2-ol and vinyl acetate were dis-
solved in di-isopropyl ether. A 0.020 mL sample was extracted
for zero time and diluted with 0.230 mL of 80% methanol. A sample
(0.025 mL) of this diluted solution was injected into the HPLC
instrument eluting with 80% methanol. The immobilized Lipozyme
enzyme (200 mg) was then added, the mixture stirred magneti-
cally, and samples extracted periodically in the same way. The
HPLC results are summarized in Table 1 and Figure 1 clearly show
that the reaction comes to a halt after 24 h when 50% of the race-
mic starting material had reacted. Determination of the extinction
coefficient at 254 nm, which was the wavelength used for detec-
The immobilized lipase catalyzed esterification of 1-phenyleth-
anol reported by Suan and Samidi seemed to fit a second order
mechanism.14 Although the transesterification on the immobilized
enzyme studied here probably follows a similar Ping–Pong mech-
anism it is a pseudo first order reaction rather than a second order
reaction, as can be seen by the plots in Figures 1 and 2. Similar
reactions of other racemic secondary alcohols using immobilized
lipases show exactly the same behavior.
Table 1
HPLC monitoring of the Lipozyme catalyzed transacetylation from vinyl acetate to 1-
triphenyloxy propan-2-ol (SM = % area of starting material; REM = % area of reacting
enantiomer)
Time (min)
SM area%
REM%
Product area%
ln (REM)
0
15
40
70
90
110
140
240
300
360
420
735
1440
100.0
94.6
87.0
80.5
77.6
74.8
70.9
63.2
60.4
58.2
56.5
51.4
49.2
50.0
44.6
37.0
30.5
27.6
24.8
20.9
13.2
10.4
8.2
0.00
5.45
3.912
3.797
3.612
3.419
3.317
3.212
3.041
2.579
2.339
2.100
1.867
0.358
—
12.96
19.45
22.43
25.17
29.07
36.81
39.63
41.84
43.53
48.57
50.79
6.5
1.4
ꢀ0.8
Figure 2. A second plot for the transacetylation of the reacting enantiomer of rac-1-
triphenylmethyloxypropane-2-ol from vinyl acetate.
A comparison of the first order rate constant using different
substrates and enzymes with greatly different activities, as shown
in Table 2, is not very informative; however comparisons between
different substrates on the same enzyme under similar conditions
is valid. Thus, the reactions of vinyl acetate with 1-[bis(4-methoxy-
phenyl)]methoxypropane-2-ol and fluorenyloxypropane-2-ol cata-
lyzed by the Pseudomonas cepacia lipase shows the latter to be
about twice as good a substrate as the former. The rigid flat struc-
ture of the fluorenyl system seems therefore to fit better into the
active site pocket of the enzyme. The Mucor miehei and the Candida
Antarctica lipases often have similar activities.15 It is therefore of
interest to compare their activities toward triphenylmethoxy-
propan-2-ol and 1-phenyloxy-3-triphenylmethoxypropan-2-ol. As
shown in Table 2, the former is about 26 times better as a substrate
for the Mucor miehei enzyme (with a quoted activity of 30 units/g)
than a phenoxyglycerol derivative is for Novozyme 435 (with a
quoted activity of close to 10,000 units/g) (Scheme 1).16
The crystal structures of many lipases have been determined
and although there is considerable variation within this group of
enzymes, there is sequence similarity around the active site, which
contains a catalytic triad of serine, histidine and aspartic acid.17
The active site dimensions have also been studied by using sub-
strates of various sizes.18 Kinetic studies showed strict adherence
to Kazlauskas’s rule even if both enantiomers were bound to the
active site. This results in the non-reacting enantiomer [(S) in the
case of 1-phenylethanol] being a competitive inhibitor for the reac-
tion. The apparent Km for the alcohol substrate is below 0.1 M,
which is somewhat lower than the substrate concentrations used
here.19,20 The simple kinetic data collected herein does not allow
any detailed analysis of the reaction, but it is clearly pseudo first
order with respect to the alcohol substrate as already stated. The
Figure 1. (a) Percentage of product formed during transesterification. (b) First
order rate plot–ln(% reacting enantiomer, REM) versus time.

S. Petursson, S. Jonsdottir / Tetrahedron: Asymmetry 22 (2011) 1809–1812
1811
Table 2
Comparison of the activity of different lipases during the transacetylation from vinyl acetate onto a secondary hydroxyl group
Lipase
Enzyme activity(U/g)
Substrate structure
Enzyme (g)
Substr. conc (M)
Vinyl acetate (M)
0.170
k0 minꢀ1
0.0053
k0/g enz.
OH
Pseudomonas cepacia5
(Fluka 17261)
15,000
0.040
0.142
0.13
OCH(C6H4OMe(p))2
H3C
H3C
H3C
OH
OH
*
Pseudomonas cepacia
(Fluka 17261)
15,000
30
0.100
0.200
2.00
0.100
0.133
0.164
0.100
0.160
0.300
0.0212
0.0047
0.0018
0.212
OFluorenyl
OCPh3
Mucor miehei Lipozyme
(Fluka 62350)
0.0235
0.000905
OH
Candida Antarctica
(Novozyme) 435)
P10,000
OCPh3
PhOCH2
*
Unpublished results.
H3C
HC CH2
O
C
Lipozyme
OCPh3
(R)
H3C
Mucor miehei
(S)
OCPh3
+
H3C
O
+
OCPh3
O
(i-Pr)2O
OH
C
CH3
OH
O
CH3
(R)-1-Triphenylmethoxy-
(S)-1-triphenylmethyloxy-
(R)-1-triphenylmethyloxy-
vinyl acetate
propan-2-ol
propan-2-ol
2-actoxypropane
Scheme 1. The Lipozyme catalyzed enantiospecific transesterification of an acetate to (R)-1-triphenylmethoxypropan-2-ol.
fact that the rate of the reaction does not depend on the vinyl
acetate as well as the alcohol concentration must mean that its
reaction with the enzyme and the enzyme-acyl formation are not
rate determining.
was distilled from, and stored over, sodium. Ethyl acetate and hex-
ane were of HPLC grade and used as received. The enzyme used
was Fluka Pseudomonas cepacia lipase immobilized on ceramic par-
ticles and immobilized Mucor miehei Lipozyme supplied by Sigma/
Aldrich. The HPLC instrument was Shimadzu Prominence with a re-
versed phase 150 ꢁ 4.6 mm SS Wakosil C18RS 3 mm column. The
elution was isocratic with methanol/water 4:1 and detection was
carried out with a UV detector at 254 nm.
3. Conclusion
The triphenylmethyl group is an excellent and long established
group for the selective protection of primary hydroxyl groups.4
Herein it was used in the effective primary protection of pro-
pane-1,2-diol. It has already been established that a secondary
alcohol next to a diarylmethyl ether group is an excellent substrate
for the Pseudomonas cepacia lipase, but the equivalent triphenyl-
methyl ether studied here gives no reaction with vinyl acetate in
the presence of this enzyme. The 1,3-selective Mucor miehei lipase,
Lipozyme did, however, catalyze this transacetylation, albeit with a
much lower reaction rate. In spite of this, the use of this enzyme
gives a much more efficient resolution of the model rac-propane-
1,2-diol than previously reported.
4.2. Triphenylmethylation of rac-propane-1,2-diol
rac-Propane-1,2-diol (761 mg, 10.0 mmol) was dissolved in
dichloromethane (40 mL) and triethylamine (1.52 g, 15.0 mmol,
2.00 mL). 4-Dimethylaminopyridine (DMAP) (50 mg, 0.40 mmol)
was added followed by triphenylmethyl chloride (3.06 g,
11.0 mmol). The reaction was left to proceed overnight after which
TLC (Hex/EtOAc 3:2) showed complete reaction of the starting
material (Rf 0.1) and one product (Rf 0.56). The reaction was
quenched by the addition of excess silica gel and the solvent re-
moved on a rotary evaporator. The product was isolated as a white
solid (3.02 g, 94.0%) from a column of silica gel eluting with Hex/
EtOAc 9:1 to 3:2, which crystallized from EtOAc–hexane, mp
100–101 °C. 1H NMR (250 MHz, CDCl3). dH 1.17 (3H d, JH3,H2
6.37 Hz, CH3), 2.42 (1H s, –OH), 3.06 (1H dd, JAB 9.21, JH1A,H2
7.85 Hz, H1A), 3.20 (1H dd, JBA 9.26, JH1B,H2 3.38 Hz, H1B), 4.04
(1H m, 2-CH, 7.29–7.41 (15H m, aromatic protons); 13C NMR
(100 MHz, CDCl3) dC 18.81, (C-3), 67.06, (C-2), 69.11 (C-1), 86.91
(Ph3C), 127.15, 127.91, 128.72, 143.93, aromatic carbons. Calcd
for C22H22O2.Na, m/z 341.1512. Found 341.1497.
4. Experimental
4.1. General
Thin layer chromatography (TLC) was carried out on aluminum
sheets coated with silica gel 60-F254 and detected under a UV lamp
and using a spray of 0.2% w/v cerium(IV) sulfate and 5% ammonium
molybdate in 2 M sulfuric acid with heating. 1H and 13C NMR spectra
were run on a Bruker Avance 400 (Bruker Biospin GmbH, Rheinstet-
ten, Germany) instrument with Me4Si as the external standard using
chloroform (7.26 ppm for H-1 and 77.0 ppm for C-13) or dichloro-
methane (5.32 ppm for H-1 and 53.8 ppm for C-13) as internal stan-
dards. H-1 spectra were measured at 400.12 and C-13 at
100.61 MHz, both at 298 K. Mass spectra were run on a Bruker
MicroTof-Q mass spectrometer using electrospray ionization (ESI).
General reagents and catalysts were from chemical suppliers and
usually used without further purification. 1,2-Dimethoxyethane
4.3. Lipozyme (Mucor miehei) catalyzed acetylation of rac-1-tri-
phenylmethoxypropane-2-ol: the formation and separation of
(R)-2-acetoxy-1-triphenylmethoxypropane and (S)-1-triphenyl-
methoxypropan-2-ol
The triphenymethyl ether (636 mg, 2.00 mmol) was dissolved
in di-isopropyl ether (15 mL) and vinyl acetate (206 mg,

1812
S. Petursson, S. Jonsdottir / Tetrahedron: Asymmetry 22 (2011) 1809–1812
0.220 mL, 2.40 mmol) was added. A sample of this solution
(0.020 mL) was diluted with 0.230 mL of 80% methanol for a zero
time HPLC run after which the immobilized Lipozyme enzyme
(200 mg) was added. The reaction was monitored by HPLC eluting
with 80% MeOH. After 24 h, HPLC showed that the reaction had
come to a halt with 50.8% peak integration for the product and
49.2% for the starting material. The acetate produced from the
(R)-enantiomer and the unreacted (S)-alcohol, which were sepa-
rated on a column of silica gel eluting with Hex/EtOAc 9:1 and then
4:1 to give (R)-2-acetoxy-1-triphenylmethoxypropane and (S)-1-
triphenylmethoxypropan-2-ol. The (R)-acetate was isolated from
fractions 4–8 (285 mg, 79.0% based on the reacting enantiomer),
which was crystallized from hexane/ethyl acetate, mp 81–82,
reaction left at room temperature overnight. The resulting (R)-
1,2-propanediol was purified on a column of silica gel (Et2O/MeOH
9:1) (50 mg, 64%). ½a D20
ꢂ
¼ ꢀ25:1 (c 0.45, CHCl3). 1H NMR (400 MHz,
CDCl3). dH 1.09 (3H d, JH3,H2 6.45 Hz, CH3), 2.65 (2H s, 2 ꢁ OH), 3.32
(1H dd, JAB 11.28, JA,H2 7.79 Hz, H1A), 3.55 (1H dd, JBA 11.28, JB,H2
2.96 Hz, H1B), 3.84 (1H m, H2). 13C NMR (100 MHz, CDCl3) dC
18.81, (C-3), 68.040, (C-2), 68.34 (C-1). Identical to the authentic
racemic starting material.
(S)-1-Triphenylmethyloxypropan-2-ol (425 mg, 1.30 mmol)
was dissolved in dichloromethane/methanol 9:1 (15 mL). Sodium
hydrogen sulfate on silica gel (100 mg) was added and the reaction
left at room temperature overnight. The resulting (S)-1,2-propane-
diol was purified on a column of silica gel (Et2O/MeOH 9:1) (80 mg,
½
a 2D0
ꢂ
¼ þ18:1 (c 1.01, CHCl3). 1H NMR (400 MHz, CDCl3). dH 1.25
81%).½a 2D0
ꢂ
¼ þ29:3 (c 1.95, CHCl3). 1H and 13C NMR spectra were
(3H d, JH3,H2 6.32 Hz, CH3), 2.21 (3H s, CH3C@O), 3.10 (1H dd, JAB
9.60, JHA1,H2 4.00 Hz, H1A), 3.18 (1H dd, JBA 9.85, JHB1,H2 6.32 Hz,
H1B), 5.17(1H m, H 2), 7.20–7.50 (15 H m, Haromatic); 13C NMR
(100 MHz, CDCl3) dC 17.0, (CH3), 21.4, (CH3C@O), 66.2 (C-2), 69.9
(C-1), 86.4 (Ph3C), 127,0, 127.8, 128,7, 144.0 (aromatic Cs), 170.6
(CH3C@O). Calcd for C24H24O3.Na m/z 383.1618. Found 383.1633.
Fractions 14–16 gave (S)-1-triphenylmethoxypropan-2-ol
(227 mg, 71.3%), which was crystallized from hexane/ethyl acetate,
identical to the R-enantiomer and the authentic racemic starting
material.
Acknowledgments
The University of Akureyri and Rannis are thanked for the finan-
cial support. Prof. Guðmundur G. Haraldsson and his students at
the Science Institute, University of Iceland, are thanked for helpful
discussions and optical rotation analysis. Sean M. Scully is thanked
for the technical assistance.
mp 82–83, ½a 2D0
ꢂ
¼ þ4:7 (c 1.02, CHCl3). 1H NMR (400 MHz, CDCl3).
dH 1.14 (3H d, JH3,H2 6.32 Hz, CH3), 2.39 (1H d, JOH, H2 3.03 Hz, OH),
3.03 (1H dd, JAB 9.10, JHA1,H2 7.83 Hz, H1A), 3.17 (1H dd, JBA 9.35,
JHB1,H2 3.54 Hz, H1B), 4.02 (1H m, H 2), 7.21–7.51 (15H m, Haromatic);
13C NMR (100 MHz, CDCl3) dC 19.0, (CH3), 67.1 (C-2), 69.0 (C-1),
86.7 (Ph3C), 127,1, 127.9, 128,7, 144.0 (aromatic Cs). Calcd for
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methanolic sodium methoxide (1 mL) was added. The reaction
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