Switching from S- to R-selectivity in the Candida antarctica lipase B-catalyzed ring-opening of ω-methylated lactones: Tuning polymerizations by ring size

Article abstract of DOI:10.1021/ja071241a

Novozym 435-catalyzed ring-opening of a range of ω-methylated lactones demonstrates fascinating differences in rate of reaction and enantioselectivity. A switch from S- to R-selectivity was observed upon going from small (ring sizes ≤7) to large lactones (ring sizes ≥8). This was attributed to the transition from a cisoid to a transoid conformational preference of the ester bond on going from small to large lactones. The S-selectivity of the ring-opening of the small, cisoid lactones was low to moderate, while the R-selectivity of the ring-opening of the large transoid lactones was surprisingly high. The S-selectivity of the ring-opening of the small, cisoid lactones combined with the established R-selectivity of the transesterification of (aliphatic) secondary alcohols prevented polymerization from taking place. Ring-opening of the large, transoid lactones was R-selective with high enantioselectivity. As a result, these lactones could be polymerized, without exception, by straightforward kinetic resolution polymerization, yielding the enantiopure R-polyester with excellent enantiomeric excess (>99%).

Full text of DOI:10.1021/ja071241a

Published on Web 05/18/2007
Switching from S- to R-Selectivity in the Candida antarctica
Lipase B-Catalyzed Ring-Opening of ω-Methylated Lactones:
Tuning Polymerizations by Ring Size
Jeroen van Buijtenen,† Bart A. C. van As,† Marloes Verbruggen,† Luc Roumen,‡
Jef A. J. M. Vekemans,† Koen Pieterse,‡ Peter A. J. Hilbers,‡
Lumbertus A. Hulshof,† Anja R. A. Palmans,*,† and E. W. Meijer*,†
Contribution from the Laboratory of Macromolecular and Organic Chemistry, Department of
Chemical Engineering and the Biomodeling and Bioinformatics Group, Department of
Biomedical Engineering, EindhoVen UniVersity of Technology, P.O. Box 513,
5600 MB EindhoVen, The Netherlands
Received February 21, 2007; E-mail: a.palmans@tue.nl; e.w.meijer@tue.nl
Abstract: Novozym 435-catalyzed ring-opening of a range of ω-methylated lactones demonstrates
fascinating differences in rate of reaction and enantioselectivity. A switch from S- to R-selectivity was
observed upon going from small (ring sizes e7) to large lactones (ring sizes g8). This was attributed to
the transition from a cisoid to a transoid conformational preference of the ester bond on going from small
to large lactones. The S-selectivity of the ring-opening of the small, cisoid lactones was low to moderate,
while the R-selectivity of the ring-opening of the large transoid lactones was surprisingly high. The
S-selectivity of the ring-opening of the small, cisoid lactones combined with the established R-selectivity of
the transesterification of (aliphatic) secondary alcohols prevented polymerization from taking place. Ring-
opening of the large, transoid lactones was R-selective with high enantioselectivity. As a result, these lactones
could be polymerized, without exception, by straightforward kinetic resolution polymerization, yielding the
enantiopure R-polyester with excellent enantiomeric excess (>99%).
Introduction
example, enantioselective metal-based catalysts or organocata-
lysts resulted in isotactic PLA when racemic D,L-lactide was
The availability of (cheap) enantiomerically pure monomers
has played an important role in the development of synthetic
approaches to chiral polymers. A prominent example of an
optically pure monomer available from the chiral pool is
L-lactide, which is industrially synthesized and employed in the
synthesis of poly(L-lactide) (PLA). This polymer is well-studied
as a biocompatible and biodegradable material.1 Readily avail-
able enantiopure monomers such as D- or L-tartaric acid have
also been investigated in the synthesis of chiral polyamides,
poly(ester amide)s, and polyesters, and the polymers show
promise as biodegradable materials with good mechanical
properties.2 Most monomers, however, are not readily available
in enantiopure form. To circumvent the need for enantiopure
monomers, enantioselective catalysts have been explored to
prepare stereoregular polymers from racemic monomers. For
employed as the monomer.3 In addition, racemic substituted
lactones were selectively polymerized with a chiral Al-based
catalyst, albeit with a significantly lower selectivity.3k
Catalysts from natural sources such as lipases are a valuable
extension in the quest to procure enantiomerically pure polymers
from optically inactive monomers. A range of chiral polyesters
has been synthesized by lipase-catalyzed ring-opening polym-
erization of substituted racemic ꢀ-caprolactones.4-8 In most
cases, commercially available Novozym 435sCandida antarc-
(3) (a) Kasperczyk, J.; Bero, M. Polymer 1999, 41, 391. (b) Ovitt, T. M.; Coates,
G. W. J. Polym. Sci., Part A: Polym. Chem. 2000, 38, 4686. (c) Radano,
C. P.; Baker, G. L.; Smith, M. R., III. J. Am. Chem. Soc. 2000, 122, 1552.
(d) Nomura, N.; Ishii, R.; Akakura, M.; Aoi, K. J. Am. Chem. Soc. 2002,
124, 5938. (e) Zhong, Z.; Dijkstra, P. J.; Feijen, J. Angew. Chem., Int. Ed.
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Chisholm, M. H.; Patmore, N. J.; Zhou, Z. Chem. Commun. 2005, 127. (i)
Russell, S. K.; Gamble, C. L.; Gibbins, K. J.; Juhl, K. C. S.; Mitchell, W.
S., III; Tumas, A. J.; Hofmeister, G. E. Macromolecules 2005, 38, 10336.
(j) Dove, A. P.; Li, H. B.; Pratt, R. C.; Lohmeijer, B. G. G.; Culkin, D. A.;
Waymouth, R. M.; Hedrick, J. L. Chem. Commun. 2006, 2881. (k) Ten
Breteler, M.; Zhong, Z.; Dijkstra, P. J.; Palmans, A. R. A.; Peeters, J.;
Feijen, J. J. Polym. Sci., Part A: Polym. Chem. 2007, 45, 429.
† Laboratory of Macromolecular and Organic Chemistry.
‡ Biomodeling and Bioinformatics Group.
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Y.; Tsuji, H. Macromol. Rapid Commun. 2000, 21, 117.
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1993, 26, 5664. (b) Regan˜o, C.; de Ilarduya, A. M.; Iribarren, I.; Rodr´ıguez-
Gala´n, A.; Galbis, J. A.; Mun˜oz-Guerra, S. Macromolecules 1996, 29, 8404.
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Polymer 1997, 38, 4935. (d) Bou, J. J.; Iribarren, I.; de Ilarduya, A. M.;
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9
10.1021/ja071241a CCC: $37.00 © 2007 American Chemical Society
J. AM. CHEM. SOC. 2007, 129, 7393-7398
7393

A R T I C L E S
van Buijtenen et al.
tica lipase B (CALB) adsorbed on an acrylic resinshas been
employed. Promising results were obtained, although the
enantiomeric excess (ee) of the polymer is generally moderate
at best (<90%), and some lactones appeared to be unreactive.
The moderate ee’s of the polymers obtained by Novozym 435-
catalyzed ring-opening polymerizations are primarily related to
the moderate selectivity of CALB for substituted ꢀ-caprolac-
tones. For example, the enantiomeric ratio E was 17.6 for the
Novozym 435-catalyzed hydrolysis of 4-methyl-ꢀ-caprolactone
at 45 °C, and E was 12 for the transesterification of 6-methyl-
ꢀ-caprolactone with benzyl alcohol in toluene at 60 °C.8,9 In
both cases, the S-enantiomer was the more reactive monomer.
In contrast, it is well-known that CALB is highly enantiose-
lective (typically E > 100) in the transesterification of secondary
alcohols, which is exploited in the kinetic resolution of
secondary alcohols.10 Interestingly, R-secondary alcohols are the
faster reacting enantiomer in this case. This dramatic difference
in the degree and nature of the enantioselectivity of CALB for
substituted ꢀ-caprolactones and secondary alcohols can be
ascribed to different binding sites for the nucleophile (the
alcohol) and the acyl donor (the lactone) in the active site.11
Since the binding site for the alcohol is much more confined,
the discrimination between the two enantiomers is more
pronounced, resulting in high selectivities. In this respect, the
S-enantiomer of ω-methyl-ꢀ-caprolactone is a particularly
fascinating substrate for Novozym 435-catalyzed ring-opening
polymerizations. After ring-opening of this lactone, the S-
secondary alcohol that is formed cannot act as a nucleophile
for propagation, since only R-secondary alcohols are accepted
as a nucleophile and, hence, act as a chain-stopper. This was
indeed observed in the lipase-catalyzed ring-opening polymer-
ization of ω-methylated δ-valerolactone (5-MeVL) and ꢀ-ca-
prolactone (6-MeCL), which did not occur on a realistic time
scale.5,6
Figure 1. Cisoid and transoid conformations of the ester bond.
Table 1. ω-Methylated Lactones of Varying Ring Size
n
lactonea
ring size
ester conformationb
0
1
2
3
4
5
9
3-MePL
4-MeBL
5-MeVL
6-MeCL
7-MeHL
8-MeOL
12-MeDDL
4
5
6
7
8
C
C
C
C
C (major) + T (minor)
T (major) + C (minor)
T
9
13
a PL, propiolactone; BL, butyrolactone; VL, valerolactone; CL, capro-
lactone; HL, heptalactone; OL, octalactone; DDL, dodecalactone. b C
represents the cisoid conformation, while T represents the transoid
conformation; assignments based on data for the non-methylated lactones
obtained from ref 14a, assuming that ω-methylation of the lactone does
not substantially influence the conformational preference of the ester bond.
enantioselectivity of Novozym 435-catalyzed ring-opening of
ω-methylated lactones in general. In addition, we recently
observed fascinating differences in the kinetics of the lipase-
catalyzed ring-opening polymerization of unsubstituted lactones
with ring sizes varying from 6-membered to 16-membered.13
Using Novozym 435 as the catalyst, we found that lactones
possessing a cisoid conformation showed a lower reactivity with
respect to their intrinsic chemical reactivity than large lactones
possessing a transoid conformation (Figure 1). The influence
of the conformation of the ester bond in lactones on the reaction
rates suggests that cisoid and transoid esters represent different
substrates for CALB. It can therefore be expected that the
introduction of a substituent at the ω-position of lactones of
varying ring sizes may lead to differences in the enantioselec-
tivity in a CALB-catalyzed ring-opening polymerization.
In this paper, we describe a comprehensive study of the
Novozym 435-catalyzed ring-opening and ring-opening polym-
erization of ω-methylated lactones of ring sizes 4-13 in view
of their potential for the synthesis of enantiopure polymers
(Table 1). Depending on the enantioselectivity of the ring-
opening of the lactone, completely different behavior in the
lipase-catalyzed ring-opening polymerization can be expected.
S-selective ring-opening requires concurrent racemization for
polymerization to occur. In contrast, R-selective ring-opening
could enable straightforward kinetic resolution polymerization
(KRP) as the nucleophile that is formed upon reaction of the
lactone is accepted by the lipase. To rationalize the results
obtained, we take into account the conformational preference
of the ester bondstransoid or cisoidsin the lactones investi-
gated.14 The observed differences in enantioselectivity prompted
us to investigate the flexible docking of the enantiomers of the
By employing iterative tandem catalysis (ITC), we recently
succeeded in the complete conversion of (S)-6-MeCL and rac-
6-MeCL into enantiopure poly-(R)-6-MeCL of high molecular
weight (25 kDa) and high ee.9 This was achieved by combining
the S-selective ring-opening of the lactone by Novozym 435
with the Ru-catalyzed racemization of the alcohol end-groups.
Since ring-opening results in the formation of an S-secondary
alcohol, which, according to Kazlauskas’s rule is the slower
reacting enantiomer in enzyme-catalyzed transesterification, no
further propagation takes place after initiation.12 Ru-catalyzed
racemization is, therefore, required to yield reactive R-terminal
alcohols and to enable polymerization. As a follow-up to this
work on 6-MeCL, we became interested in the activity and
(6) Peeters, J.; Palmans, A. R. A.; Veld, M.; Scheijen, F.; Heise, A.; Meijer,
E. W. Biomacromolecules 2004, 5, 1862.
(7) van As, B. A. C.; Thomassen, P.; Kalra, B.; Gross, R. A.; Meijer, E. W.;
Palmans, A. R. A.; Heise, A. Macromolecules 2004, 37, 8973.
(8) Peeters, J. W.; van Leeuwen, O.; Palmans, A. R. A.; Meijer, E. W.
Macromolecules 2005, 38, 5587.
(9) (a) van As, B. A. C.; van Buijtenen, J.; Heise, A.; Broxterman, Q. B.;
Verzijl, G. K. M.; Palmans, A. R. A.; Meijer, E. W. J. Am. Chem. Soc.
2005, 127, 9964. (b) Van Buijtenen, J.; van As, B. A. C.; Meuldijk, J.;
Palmans, A. R. A.; Vekemans, J. A. J. M.; Hulshof, L. A.; Meijer, E. W.
Chem. Commun. 2006, 3169.
(10) (a) Schulze, B.; Wubbolts, M. G. Curr. Opin. Biotechnol. 1999, 10, 609.
(b) Liese, A.; Villela Filho, M. Curr. Opin. Biotechnol. 1999, 10, 595.
(11) (a) Rotticci, D.; Haeffner, F.; Orrenius, C.; Norin, T.; Hult, K. J. Mol.
Catal. B 1998, 5, 267. (b) Orrenius, C.; Haeffner, F.; Rotticci, D.; Ohrner,
N.; Norin, T.; Hult, K. Biocatal. Biotransform. 1998, 16, 1. (c) Orrenius,
C.; Ohrner, N.; Rotticci, D.; Mattson, A.; Hult, K.; Norin, T. Tetrahedron:
Asymmetry 1995, 6, 1217.
(13) van der Mee, L.; Helmich, F.; de Bruijn, R.; Vekemans, J.; Palmans, A. R.
A.; Meijer, E. W. Macromolecules 2006, 39, 5021.
(14) (a) Huisgen, R. Angew. Chem. 1957, 69, 341. (b) Huisgen, R.; Ott, H.
Angew. Chem. 1958, 70, 312. (c) Huisgen, R.; Ott, H. Tetrahedron 1959,
6, 253.
(12) Kazlauskas, R. J.; Weissfloch, A. N. E. J. Mol. Catal. B: Enzymatic 1997,
3, 65.
9
7394 J. AM. CHEM. SOC. VOL. 129, NO. 23, 2007

S-/R-Selectivity in Ring-Opening of
ω
-Methylated Lactones
A R T I C L E S
Table 2. CALB-Catalyzed Ring-Opening of ω-Methylated
Lactones at 70 °Ca
1
k
cat (s-
)
enantioselectivity
ring
size
S-enantio-
mer
R-enantio-
mer
entry
lactone
exptlb
dockingc
1
3-MePL
5-MeVL
6-MeCL
7-MeHL
8-MeOL
12-MeDDL
4
6
7
8
9
45.7
7.9
ndd
7.6
8.5
204.4
10.3
23.3
S
-
S
R
R
R
S
S
S
R
R
R
f
2e
3
49.3
0.01g
ndd
4
5
6h
13
ndd
a Reaction conditions: lactone (4 mmol), BA (1 mmol), Novozym 435
(27 mg), and 1,3,5-tri-tert-butylbenzene (0.3 mmol, internal standard) in
toluene (2 mL); reaction at 70 °C. b Enantioselectivity experimentally
determined by chiral GC. c Enantioselectivity predicted from docking
experiments. d Could not be determined. e Experiment performed with
1-octanol (8 mmol) as initiator; enzyme dried overnight at 50 °C over P2O5.
f No significant enantioselectivity was observed for the reaction. g Deter-
mined in a separate experiment using 2 mmol of isolated (S)-7-MeHL.h 2
mmol of 12-MeDDL.
various lactones in the active site of CALB by computer-aided
molecular modeling. Finally, we describe the polymerization
of the larger racemic lactones (ring sizes g8) to enantiopure
polyesters.
Results and Discussion
CALB-Catalyzed Ring-Opening of ω-Methylated Lactones
with Increasing Ring Sizes. We first evaluated the enantiose-
lectivity of the Novozym 435-catalyzed ring-opening of a range
of ω-methylated lactones (Table 1). The ω-methylated lactones
that were not commercially available were synthesized by
methylation of the suitable cycloketone, followed by Baeyer-
Villiger oxidation employing m-CPBA (see Experimental Sec-
tion in the Supporting Information). The racemic lactones were
obtained in moderate to good purity. The non-methylated lactone
was present as an impurity in the large lactones (ring sizes g8),
but this did not affect the enantioselectivity of the ring-opening
experiments. Experiments were performed with benzyl alcohol
(BA) as the nucleophile and the appropriate lactone as the acyl
donor. We will refer to BA as the initiator (I) and to the lactone
as the monomer (M), even if conditions are selected in which
polymerization cannot take place. For the ring-opening experi-
ments, an M/I ratio of 4 was selected in order to prevent possible
polymerization from taking place. In all cases, Novozym 435
was dried overnight in Vacuo over P2O5 at 50 °C prior to use
(see Experimental Section in the Supporting Information) to
limit the amount of water.15 The results are summarized in Table
2. The turnover frequency (TOF) for the ring-opening of the
enantiomers of the lactones was determined, and under the
applied conditions the TOF equals kcat (see Experimental Section
in the Supporting Information). Table 2 also shows the
experimentally determined enantioselectivities (chiral GC) as
well as the enantioselectivities that were predicted by docking
experiments (Vide infra).
Figure 2. Conversion-time history for CALB-catalyzed ring-opening of
(a) 6-MeCL and (b) 7-MeHL. Reaction conditions: lactone (4 mmol), BA
(1 mmol), Novozym 435 (25 mg), and 1,3,5-tri-tert-butylbenzene (0.3 mmol,
internal standard) in toluene (2 mL); reaction at 70 °C; conversions were
determined with chiral GC.
opening of the 5-membered ring, 4-methyl-γ-butyrolactone (4-
MeBL), does not occur at all, which is related to its thermo-
dynamic stability. As a result of a highly unfavorable ring-chain
equilibrium for the 6-membered lactone, ring-opening of
5-methyl-δ-valerolactone (5-MeVL) with M/I ) 4 and BA as
the initiator did not produce meaningful results.17 The reaction
was, therefore, repeated with 1-octanol as the initiator and M/I
) 0.5. 1-Octanol is a stronger nucleophile and a weaker leaving
group than benzyl alcohol. As a result, the ring-chain equilibrium
is more favorable with 1-octanol than with benzyl alcohol. In
order to push the equilibrium to a high lactone conversion, an
excess of 1-octanol was employed. Comparable kcat values were
observed for the R- and S-enantiomers, 7.6 and 7.9 s-1
,
respectively, indicative of a virtually aselective reaction (Table
2, entry 2). Conversion of both enantiomers reached equilibrium
at approximately 65%. Apparently, CALB is unable to dis-
criminate between the enantiomers of this particular lactone.
Ring-opening of 6-MeCL was S-selective with a moderate
enantioselectivity, as was previously reported (Figure 2a).9 The
kcat was 49.3 s-1 for the S-enantiomer and 8.5 s-1 for the
R-enantiomer.
Ring-opening of 3-methylpropiolactone (3-MePL) was S-
selective, with kcat ) 45.7 s-1 for the S-enantiomer. No
significant conversion of the R-lactone was observed on the time
scale of the reaction (Table 2, entry 1).5a,16 Only one molar
equivalent of (S)-3-MePL is consumed with respect to BA. Ring-
Ring-opening of an S-lactone furnishes an S-secondary
alcohol, which is not accepted as a nucleophile by CALB (Vide
supra). In the cases of 3-MePL and 6-MeCL, BA reacts with
(15) de Geus, M.; Peeters, J.; Wolffs, M.; Hermans, T.; Palmans, A. R. A.;
Koning, C. E.; Heise, A. Macromolecules 2005, 38, 4220.
(16) Watanabe, S.; Fujiwhara, M.; Sayo, N. U.S. Pat. Appl. US2006046286-
A1, 2006.
9
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A R T I C L E S
van Buijtenen et al.
Scheme 1. Ring-Opening of ω-Methylated Lactones with M/I ) 4a
esterification of the latter substrates represents the reverse
reaction of the well-known R-selective esterification of second-
ary alcohols and, therefore, necessarily exhibits the same (R-)
enantioselectivity.11,18 Surprisingly, the significant population
of the cisoid conformers in 7-MeHL and (to a lesser extent)
8-MeOL does not appear to influence the enantioselectivity of
ring-opening, while it is reasonable to assume thatsanalogous
to the smaller lactonessthe selectivity of ring-opening of the
cisoid conformer of the lactone will be limited and possibly
even S-selective. Presumably, the transoid conformer reacts
much faster than the cisoid conformer, which is also in
agreement with the observation that ring-opening of the small,
cisoid-only lactones is relatively slow, despite their high ring
strain.13 The enantioselectivity of the 8-membered lactone
7-MeHL is unexpectedly high, with kcat for the R-enantiomer
of 7-MeHL several orders of magnitude higher than that of the
S-enantiomer.
a Basic scenarios on short time scales for S- and R-selective ring-opening
of ω-methylated lactones with M/I ) 4 (I represents the initiator):
S-selective ring-opening furnishes a nonreactive S-terminal alcohol, and,
therefore, the reaction essentially stops after 1 equiv of S-lactone is
consumed; (highly selective) R-selective ring-opening results in both
equivalents of R-lactone being consumed, while the S-lactone remains
unreacted.
one molar equivalent of the S-lactone (corresponding to 50%
conversion), and no further consumption is observed. In these
cases, the R-lactone is virtually unreactive (3-MePL) or reacts
only slowly (6-MeCL). For the small lactones, the spatial
orientation of the methyl substituent is highly dependent on the
ring size, which can explain the differences in selectivity for
the lactones with ring sizes e7.
In contrast to the ring-opening of the lactones with ring sizes
e7, ring-opening of the larger lactones with ring sizes between
8 and 13 was R-selective. This resulted in the formation of a
reactive R-secondary alcohol, and both molar equivalents of the
R-lactone (with respect to BA, M/I ) 4) were quickly consumed
(entries 4-6, Figure 2b). No consumption of the S-lactone could
be observed in these cases. For (R)-7-methylheptalactone (7-
MeHL), a kcat value of 204.4 s-1 was calculated (Table 2, entry
4, Figure 2b). The S-enantiomer was isolated after reaction, and
in a separate experiment a kcat value of only 0.01 s-1 was
calculated for (S)-7-MeHL, indicative of a very high enantio-
meric ratio E for the ring-opening of 7-MeHL. Similar behavior
was observed for 8-methyloctalactone (8-MeOL) and 12-
methyldodecalactone (12-MeDDL). The basic scenarios on short
time scales for S- and R-selective ring-opening of ω-methylated
lactones with M/I ) 4 are visualized in Scheme 1.
Influence of the Conformation of the Ester Function on
the Ring-Opening of the Lactone. It is reasonable to assume
that ω-methylation does not substantially influence the confor-
mational preference of the ester bond in the lactone. In analogy
to the non-methylated lactones (Vide supra), 6-MeCL will be
almost exclusively in the cisoid conformation, and starting from
7-MeHL the transoid conformation appears (Table 1).13,14
Reactivities that were obtained for the Novozym 435-catalyzed
ring-opening of ω-methylated lactones are roughly in line with
those previously obtained for the unsubstituted lactones (Table
2).13 Notably, ring-opening of 7-MeHL was fastest, while
reactivity of the small lactones (ring size 4-7) was relatively
low. The appearance of the transoid conformer of the ester bond
coincides with the remarkably sharp transition that is observed
in the enantioselectivity of the ring-opening of the lactones:
lactones of ring sizes e7 show limited selectivity for the
S-enantiomer (3-MePL and 6-MeCL) or no selectivity at all (5-
MeVL), while lactones of ring sizes g8 exhibit very high
selectivity for ring-opening of the R-enantiomer.
Computer-Aided Molecular Modeling of Docking of
ω-Methylated Lactones in CALB. In order to verify our
hypothesis regarding the opposite enantioselectivity for the ring-
opening of the small cisoid lactones and the larger transoid
lactones, computer-aided molecular modeling was performed.
The molecular structure of the enantiomers of each lactone was
docked flexibly into the active site cavity of CALB, specifically
mimicking the interactions between the lactones and the catalytic
triad (Ser105, His224, Asp187) before acylation. Furthermore,
several expected stabilizing interactions between protein and
ligand were investigated by additional docking runs. While this
is a very rough approximation, the difference between the
calculated average free energies of binding of the complex for
both enantiomers (∆∆G) can give an indication for the enan-
tioselectivity of the protein.
The predicted enantiopreferences resulting from the docking
study are given in Table 2. The predicted ∆G values for the
lactone enantiomers are listed in the Supporting Information.
Docking of the S-enantiomer of the small cisoid-only lactones
was energetically favored over that of the R-enantiomer,
implying S-selectivity. However, differences in free energy
between the enantiomers are small and in most cases within
the margin of error of the calculation. For 7-MeHL, docking of
the transoid conformation was lower in free energy than that of
the cisoid conformation, and for both conformations the
calculated average free energy of binding suggests R-selectivity
(see Supporting Information). R-enantioselectivity was also
predicted for 8-MeOL and 12-MeDDL. For these larger lactones
the differences in free energy are, in every case, significant.
The lowest energy conformations of (cisoid) (S)-6-MeCL and
both enantiomers of cisoid and transoid 7-MeHL docked in the
active site pocket of CALB are depicted in Figure 3. Clearly,
there is a good agreement between the lowest energy conforma-
tions of (S)-6-MeCLsthe fastest reacting enantiomer of the
7-membered lactonesand transoid (R)-7-MeHL (with respect
to the ring itself, the carbonyl, and the methyl group). For
transoid (S)-7-MeHL, the carbonyl and methyl groups point in
completely different directions (Figure 3a). This is in agreement
with the lower calculated free energy of docking for the
(17) (a) Hall, H. K., Jr.; Schneider, A. K. J. Am. Chem. Soc. 1958, 80, 6409.
(b) Frisch, K. C., Reegen, S. L., Eds. Ring-opening polymerization; M.
Dekker: New York, 1969.
(18) Ohtani, T.; Nakatsukasa, H.; Kamezawa, M.; Tachibana, H.; Naoshima,
Y. J. Mol. Catal. B 1998, 4, 53.
As a substrate for the lipase, the large, transoid lactones are
structurally similar to linear aliphatic (transoid) esters. Trans-
9
7396 J. AM. CHEM. SOC. VOL. 129, NO. 23, 2007

S-/R-Selectivity in Ring-Opening of
ω
-Methylated Lactones
A R T I C L E S
Scheme 2. Polymerization of ω-Methylated Lactones with Ring
Sizes g8a
a Reagents and conditions: (a) Novozym 435, toluene, 70 °C.
significantly different orientation is observed for that of cisoid
(R)-7-MeHL. However, a lower free energy is calculated for
cisoid (R)-7-MeHL than for the S-enantiomer, indicating that
more subtle interactions, not evident from this (2D) figure, play
an important role (∆∆G ) -14 kJ/mol).
The combination of (1) the good agreement between the
lowest energy conformations of transoid (R)-7-MeHL and the
reactive (S)-6-MeCL in contrast to the (visually) significantly
different orientation of cisoid (R)-7-MeHL with (2) the very
high enantioselectivity that was observed experimentally for the
ring-opening (while a limited enantioselectivity can be expected
for ring-opening of the cisoid lactone, Vide supra) suggests that
the transoid conformer of 7-MeHL is the reactive one in CALB-
catalyzed ring-opening. More detailed (quantum mechanical)
molecular modeling studies are in progress to support this
tentative conclusion.
Polymerization of ω-Methylated Lactones. Novozym 435-
catalyzed ring-opening of the small cisoid-only lactones is
S-selective (3-MePL and 6-MeCL) or virtually aselective (5-
MeVL). As a result, S-secondary alcohols are formed as end-
groups which, according to Kazlauskas’s rule, are the slower
reacting enantiomers in the highly enantioselective enzyme-
catalyzed transesterification.12 Therefore, no further propagation
takes place on a realistic time scale, and polymerization is
impossible.
The highly R-selective ring-opening of ω-methylated lactones
with ring sizes g8 enables straightforward KRP of these
substrates (Scheme 2). 7-MeHL, 8-MeOL, and 12-MeDDL were
successfully polymerized with M/I ) 100 (Table 3). The highest
activity was observed for the polymerization of 7-MeHL (kcat
) 270 s-1), followed closely by that of 12-MeDDL (kcat ) 223
s-1) (see Supporting Information for the conversion plots). The
Figure 3. (a) Lowest energy conformations of (cisoid) (S)-6-MeCL
(yellow), transoid (R)-7-MeHL (green), and transoid (S)-7-MeHL (blue)
docked in the active-site pocket of CALB (oxygen atoms are colored red).
The methyl groups of (S)-6-MeCL and transoid (R)-7-MeHL point ap-
proximately in the same direction (indicated with A, right side of the figure),
while their rings nearly overlap; the carbonyl groups point deeper into the
active site. For transoid (S)-7-MeHL, both the carbonyl and the methyl
groups (both indicated with B) are directed into a different part of the active-
site pocket than in the case of (S)-6-MeCL. (b) Lowest energy conformations
of (cisoid) (S)-6-MeCL (yellow), cisoid (R)-7-MeHL (green), and cisoid
(S)-7-MeHL (blue) docked in the active-site pocket of CALB (oxygen atoms
are colored red). The methyl groups of (S)-6-MeCL and cisoid (S)-7-MeHL
point approximately in the same direction (indicated with C, right side of
the figure), while their rings nearly overlap; the carbonyl groups point deeper
into the active site. For cisoid (R)-7-MeHL, the methyl group points in a
different direction (downward, indicated with D) and there is almost no
overlap of the ring with that of (S)-6-MeCL.
polymerization of 8-MeOL was around 3 times slower (kcat
)
44 s-1) than the polymerization of 7-MeHL and 12-MeDDL.
The small amount of non-methylated lactone that is present as
an impurity in the monomer (Vide supra) is also polymerized.
The amount of non-methylated monomeric units in the polymer
was determined by 1H NMR and varied from 11 to 22% (Table
3). In all cases, good molecular weights were obtained for the
chiral polymers (14-16 kDa) after precipitation. These are
higher than the calculated values, which can be rationalized by
the loss of low-molecular-weight species during precipitation.
The polydispersities vary between 1.23 and 2.25 (after precipita-
tion). As an example, the gel permeation chromatography (GPC)
traces of poly-(R)-7-MeHL obtained by KRP of rac-7-MeHL
(entry 1) before and after precipitation in methanol are presented
in Figure 4.
R-enantiomer compared to the S-enantiomer (∆∆G ) -7.5 kJ/
mol), implying R-selectivity; moreover, this matches the
experimentally observed selectivity. For the cisoid conformation
of 7-MeHL, a different picture is obtained (Figure 3b): the
lowest energy conformation of cisoid (S)-7-MeHL appears to
be in good agreement with that of (S)-6-MeCL, while a
To enable the isolation of the unreacted lactones and
unambiguously determine the enantioselectivity of CALB for
9
J. AM. CHEM. SOC. VOL. 129, NO. 23, 2007 7397

A R T I C L E S
van Buijtenen et al.
Mn
Table 3. Kinetic Resolution Polymerization of ω-Methylated Lactones Using Novozym 435 at 70 °Ca
time
(h)
conv
(%)b
kcat
eem
(%)c
eep
(%)d
% non-methylated
(mol/mol)e
Mn,th
(kDa)f
1
b
entry
lactone
M/I
(s-
)
(kDa)g
PDI
1
2
3
7-MeHL
8-MeOL
12-MeDDL
102
101
99
4
24
7
>99
>99
>99
270
44
223
99
99
>99
>99
>99
>99
16
11
22
7.2
7.9
10.7
16.7
14.4
14.2
1.23
1.29
2.25
a Reaction conditions: lactone (5 mmol), BA (0.05 mmol), Novozym 435 (50 mg), and 1,3,5-tri-tert-butylbenzene (0.25 mmol, internal standard) in
toluene (2.5 mL); reaction at 70 °C under an argon atmosphere. Conversions were determined with chiral GC. b Results for the R-lactone; the S-lactone is
1
practically unreactive. c Determined by chiral GC. d Determined by chiral GC after acid-catalyzed methanolysis of the polymer. e Determined by H NMR
after precipitation of the polymer in methanol. f Calculated molecular weight: Mn,th ) MBA + 100 × Mlactone × 0.50. g Determined by GPC (relative to
polystyrene standards) after precipitation of the polymer in methanol.
oselectivity. Ring-opening of the small, cisoid lactones is
S-selective (3-MePL and 6-MeCL) or aselective (5-MeVL).
Ring-opening of the larger lactones, for which the ester bond
can adopt a transoid conformation, is R-selective with a very
high enantioselectivity. For the intermediate ring sizess7-MeHL
(8-membered) and 8-MeOL (9-membered)sthe significant
presence of cisoid conformers does not appear to affect the
enantioselectivity of the ring-opening. Molecular modeling
studies supported the reversal of selectivity observed upon going
from the small, cisoid-only lactones to the larger lactones which
can adopt a transoid conformation. The R-selectivity for the
transoid lactones was related to the R-selectivity of the trans-
esterification of (transoid) linear, aliphatic esters. Importantly,
this selectivity enables the Novozym 435-catalyzed kinetic
resolution polymerization of these lactones. Poly-(R)-7-MeHL,
poly-(R)-8-MeOL, and poly-(R)-12-MeDDL were all obtained
with good molecular weight and excellent ee (>99%). To the
best of our knowledge, this ee represents by far the highest value
obtained for CALB-catalyzed ring-opening polymerization of
racemic lactones.
Figure 4. GPC traces of polymer obtained by KRP of rac-7-MeHL (Table
3, entry 1) before (A) and after (B) precipitation in methanol (Mn ) 16.7
kDa, PDI ) 1.23).
the larger ω-methylated lactones, KRP of 7-MeHL and 8-MeOL
was performed at a larger scale and the unreacted lactones were
isolated by (vacuum) distillation. Isolated (S)-7-MeHL (ee
>99%) displayed an optical rotation of +58°, while (S)-8-MeOL
(ee >99%) displayed a value of +50°. The latter value is in
agreement with the optical rotation of -41° previously reported
for (R)-8-MeOL of 91% ee.19 These data further confirm the
enantioselectivity of the present polymerizations. Finally, the
polyesters resulting from the experiments described in Table 3
were degraded by acid-catalyzed methanolysis.9a Subsequent
chiral GC analysis enabled determination of the eep (Table 3).
In all cases, an excellent eep >99% was obtained.
Acknowledgment. We thank Gerard Verzijl and Rinus
Broxterman of DSM as well as Emmo Meijer of Unilever for
useful discussions. DSM Pharma Chemicals, DSM Research,
NRSC-C, and NWO are gratefully acknowledged for financial
support.
Supporting Information Available: Experimental details for
the synthesis of the methylated lactones, their ring-opening and
ring-opening polymerizations, and the molecular modeling
studies; conversion-time history of CALB-catalyzed ring-
opening of 3-MePL, 5-MeVL, and isolated (S)-7-MeHL;
calculated average free energy of binding of the protein-ligand
complex for the different lactones; and conversion-time history
of kinetic resolution polymerization of 7-MeHL, 8-MeOL, and
12-MeDDL. This material is available free of charge via the
Internet at http://pubs.acs.org.
Conclusions
Novozym 435-catalyzed ring-opening of ω-methylated lac-
tones exhibits fascinating variations in reactivity and enanti-
(19) Carling, R. W.; Curtis, N. R.; Holmes, A. B. Tetrahedron Lett. 1989, 30,
6081.
JA071241A
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7398 J. AM. CHEM. SOC. VOL. 129, NO. 23, 2007