Lipase-catalyzed asymmetric synthesis of oxathiazinanones through dynamic covalent kinetic resolution

Article abstract of DOI:10.1039/c4ob00365a

A domino addition-lactonization pathway has been applied to a dynamic covalent resolution protocol, leading to efficient oxathiazinanone formation as well as chiral discrimination. A new, double biocatalytic pathway has furthermore been proposed and evaluated where the initial product inhibition could be efficiently circumvented. This journal is the Partner Organisations 2014.

Full text of DOI:10.1039/c4ob00365a

Organic &
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Lipase-catalyzed asymmetric synthesis of
oxathiazinanones through dynamic covalent
kinetic resolution†
Cite this: Org. Biomol. Chem., 2014,
12, 3572
Received 17th February 2014,
Accepted 28th March 2014
Lei Hu, Yan Zhang and Olof Ramström*
DOI: 10.1039/c4ob00365a
www.rsc.org/obc
A domino addition–lactonization pathway has been applied to a
dynamic covalent resolution protocol, leading to efficient oxathia-
zinanone formation as well as chiral discrimination. A new, double
biocatalytic pathway has furthermore been proposed and evalu-
ated where the initial product inhibition could be efficiently
circumvented.
Scheme 1 Lipase-catalyzed dynamic covalent resolution to chiral
oxathiazinanones.
Dynamic kinetic resolution (DKR), defined as a combination
of a kinetic resolution (KR) process with in situ racemization,
is an efficient approach to enantioenriched compounds.1–7 In
a DKR process, substrate racemization takes place concurrently
to product formation, thus in principle leading to quantitative
yields while maintaining high enantiomeric purities. When
applied to biocatalysis, this can lead to environmentally
friendly protocols, successfully applied to asymmetric syn-
thesis due to the high stereoselectivities.8–16
Efficient access to diverse, stereochemically pure, hetero-
cyclic structures is a prevailing challenge in chemistry.17
Recently, we reported that Candida antarctica lipase B (CAL B)
catalyzes the lactonization of hemithioacetals from a dynamic
system with both high yields and enantiopurities.18 Herein, we
report the synthesis of new, six-membered N,O,S-containing
heterocycles (oxathiazinanones) using a dynamic domino
nitrone19,20 addition–cyclization pathway coupled to lipase cata-
lysis in a DKR system (Scheme 1). To our knowledge, this is
not only the first report on the synthesis of such new struc-
tures, but also demonstrates the efficiency in using biocatalysts
to easily obtain substituted six-membered lactone-type struc-
tures in one-pot reactions. Moreover, this DKR protocol in
principle enables dynamic systemic resolution (DSR) strat-
egies, where more complicated dynamic systems could be
selectively resolved.21–26
philic addition reaction was initially evaluated. Under basic
conditions, very rapid equilibration of the systems was
observed, with an equilibrium displacement favoring the start-
ing materials (cf. ESI†). Thus, with a nitrone : thiol ratio of
1 : 1, the systems showed a virtual dynamic character and the
intermediates 1 : 2 were not visibly expressed.
Lipase-catalyzed esterifications or lactonizations generally
display better performance when carried out in organic media
of low polarity, and for this reason toluene and tert-butyl
methyl ether (TBME) were primarily evaluated.18,21–24,27,28 It
was shown that similar enantiomeric excesses (ees) could be
obtained with both solvents; however, in toluene, higher con-
versions and reaction rates were observed. Among the enzymes
tested, viz. lipases from Burkholderia (formerly Pseudomonas)
cepacia (PS, PS-C I, PS-C II), CAL B and Candida rugosa (CRL),
only CAL B was able to catalyze the lactonization, which was in
accordance with our previous study.18
The addition of an acyl donor, commonly used in many
enzyme-catalyzed DKR protocols, was initially considered
unnecessary for the intramolecular lactonization. However,
when CAL B was first applied to the DKR process with nitrone
1a in the absence of any acyl donor, no product was observed.
Considering that methanol is released upon reaction of methyl
2-sulfanylacetate 2 with the enzyme in the catalytic cycle, it
was suspected that enzyme inhibition occurred.29,30 For this
reason, either 4 Å molecular sieves or CaCl2 was added to
absorb the produced methanol. However, only trace amount
of product observed in both cases, and the conversion was
not improved by increasing the loading of molecular sieves or
CaCl2 from 50 mg to 200 mg. Interestingly, when isopropenyl
acetate was added, the conversion increased to up to 96%
The racemization rate being of high importance for
efficient resolution schemes, the reversibility of the nucleo-
Royal Institute of Technology, Department of Chemistry, Teknikringen 30, Stockholm,
Sweden. E-mail: ramstrom@kth.se; Fax: +46 8 7912333
†Electronic supplementary information (ESI) available. See DOI: 10.1039/
c4ob00365a
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Table 1 DKR with different acyl donorsa
Entry
Acyl donor
Conversion [%]
eeb [%]
1
2
3
4
5
4-Chlorophenyl acetate
Phenyl acetate
Isopropenyl acetate
Isopropyl acetate
Ethyl acetate
84
87
93
—
—
85
87
90
—
—
a Reaction conditions: 1a (0.1 mmol), 2 (0.3 mmol), TEA (0.1 mmol),
acyl donor (0.36 mmol), CAL B preparation (30 mg), CaCl2 (200 mg),
toluene (0.5 mL), 40 °C. b Determined by HPLC analysis using a
Chiralpak OJ chiral column.
Scheme 2 Proposed process of CAL B-catalyzed oxathiazinanone for-
mation. R’’ = H (2/2E) or C(R)NR’OH (1 : 2/1 : 2E).
within 19 hours, and the formation of methyl acetate was also
1
observed in the H NMR spectra. From this result, it was clear
that isopropenyl acetate could promote the conversion of
methanol to methyl acetate through enzyme transesterifica-
tion, resulting in considerably more efficient methanol scaven-
ging than either 4 Å molecular sieves or CaCl2. To further
verify this, one equivalent of methanol was added to the reac-
tion mixture, and the conversion decreased dramatically
from 96% to 31%. When methanol was increased to three
equivalents, the product formation was completely blocked.
Thus, a double biocatalytic pathway of CAL B can be proposed
(Scheme 2), taking advantage of the enzyme’s ability to cata-
lyze both reactions. Methyl 2-sulfanylacetate 2 reacts with the
enzyme, generating acylated enzyme 2E and methanol (cycle I).
This subsequently leads to the formation of chiral product 3
via the domino addition–lactonization process between
enzyme 2E and nitrone 1. In parallel, the enzyme reaction with
acyl donor 5 generates acylated enzyme 5E, which upon reac-
tion with the formed methanol from cycle I generates methyl
acetate (cycle II).
The base, an important parameter in the dynamic system,
was also studied in the process using nitrone 1a and an acyl
donor. When triethylamine (TEA) was applied to the process
with isopropenyl acetate or phenyl acetate, both the highest
conversions and ees were achieved. In comparison, 1,1,3,3-tetra-
methylguanidine (TMG) and morpholine exhibited similar
conversions but considerably lower ees. 1,8-Diazabicyclo[5.4.0]-
undec-7-ene (DBU) and 1,5-diazabicyclo-[4.3.0]non-5-ene
(DBN) proven to be less efficient due to formation of undesired
side products.
○
■
Fig. 1 DKR temperature dependence: ( ) conversion; ( ) ee. Reaction
conditions: 1a (0.1 mmol), 2 (0.3 mmol), TEA (0.1 mmol), isopropenyl
acetate (0.36 mmol), CAL B preparation (30 mg), CaCl2 (200 mg),
toluene (0.5 mL), 40 °C.
aliphatic alcohols, which in turn supported the proposed bio-
catalytic mechanism. The other three acyl donors, isopropenyl
acetate, phenyl acetate and 4-chlorophenyl acetate, generating
acetone and aromatic alcohols instead of aliphatic alcohols
upon reacting with the enzyme displayed good performance as
CAL B inhibitor scavengers. Good conversions and ees were
thus achieved when using these acyl donors, where the best
conversion was achieved with isopropenyl acetate.
The effect of the temperature on the DKR process was next
addressed (Fig. 1). At 60 °C, no product was formed due to
decomposition of the product. Lowering the temperature from
40 °C to −18 °C resulted in lower ees, an unusual result that
contradicts the previous study.18 The reason for this reversed
behavior is unclear, although potentially related to the concept
of the racemic temperature/isoinversion relationship,31–34
which describes the critical temperature that leads to no iso-
meric preference. In this case, the racemic temperature should
then be lower than −18 °C. In order to follow the decreasing
trend of the ees further, the temperature was decreased to
−48 °C, however, not resulting in product formation due to
low enzyme activity at very low temperatures. The optimal
Five acyl donors were subsequently screened to evaluate the
inhibitor-scavenging abilities during the CAL B-catalyzed
process (Table 1). A nitrone–thiol ratio of 1 : 3 was chosen to
increase the formation of intermediates 1 : 2, while keeping
the amount of thiol sufficiently low, and CaCl2 was added to
further increase the conversion. As expected, ethyl acetate and
isopropyl acetate were inefficient, likely due to release of
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Table 2 DKR with different substratesa
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Nitrone
R
Conversion [%]
eeb [%]
1a
1b
1c
1d
1e
Isopropyl
Propyl
Heptyl
Phenyl
2-Pyridyl
93
95
88
32
—
90
83
31
3
—
a Reaction conditions: 1 (0.1 mmol), 2 (0.3 mmol), TEA (0.1 mmol),
isopropenyl acetate (0.36 mmol), CAL B preparation (30 mg), CaCl2
(200 mg), toluene (0.5 mL), 40 °C. b Determined by HPLC analysis
using a Chiralpak OJ chiral column.
temperature for the DKR process under the present conditions
was 40 °C, where the best ee (90%) was achieved together with
high conversions (93%).
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To expand the scope of the reaction, a series of substrates
were applied to the system under the optimized conditions
established above (Table 2). Compound 3a exhibited the best
enantiospecificity with up to 90% ee. Similar conversion, but
slightly lower ee, was recorded for compound 3b, suggesting
the preference for a branched alkyl chain in the active site.
Nitrone 1c led to good conversion but relatively low ee, indicat-
ing unfavorable effects from the longer alkyl chain. Compound
3d was obtained with both low conversion and low ee, and no
product was obtained from nitrone 1e, which indicated that
nitrones with aromatic substituents were much less favored
by CAL B.
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In summary, we have successfully synthesized a series of novel,
chiral oxathiazinanones by a combined dynamic domino
nitrone addition–lactonization process, catalyzed by CAL B in a
one-pot reaction. A double catalytic pathway could be proposed
and verified, leading to optimized reaction conditions for high
conversions and stereoselectivities. The reverse temperature
dependence could be recorded, and the substrate catalog of
CAL B has further expanded. Moreover, this study offers a
method for the efficient one-pot synthesis of enantioenriched
complex heterocycles.
This work was in part supported by the Swedish Research
Council and the Royal Institute of Technology. LH and YZ
thank the China Scholarship Council for special scholarship
awards.
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