Hydrolase-mediated resolution of the hemiacetal in 2-chromanols: The impact of remote substitution

Article abstract of DOI:10.1016/j.tetasy.2017.04.001

Hydrolase-catalysed dynamic kinetic resolutions of chroman-2-ol and 3-methyl chroman-2-ol can be effected with up to 88% conversion and 92% ee through the use of organic solvents. Extension to the resolution of the tolterodine precursor 1 proved more challenging. The presence of the remote phenyl substituent had a significant impact on the resolution and it was not possible to achieve high enantioselectivity together with efficient conversion from the focussed panel of enzymes screened.

Full text of DOI:10.1016/j.tetasy.2017.04.001

Tetrahedron: Asymmetry xxx (2017) xxx–xxx
Contents lists available at ScienceDirect
Tetrahedron: Asymmetry
journal homepage: www.elsevier.com/locate/tetasy
Hydrolase-mediated resolution of the hemiacetal in 2-chromanols:
The impact of remote substitution
Declan P. Gavin a, Aoife Foley a, Thomas S. Moody b, U. B. Rao Khandavilli a, Simon E. Lawrence a,
Pat O’Neill c, Anita R. Maguire d,
⇑
a Department of Chemistry, Analytical & Biological Research Facility, Synthesis and Solid State Pharmaceutical Centre, University College Cork, Cork, Ireland
b Almac, Department of Biocatalysis & Isotope Chemistry, 20 Seagoe Industrial Estate, Craigavon, BT63 5QD N. Ireland, UK
c Pfizer Global Process Development Centre, Loughbeg, Co., Cork, Ireland
d Department of Chemistry and School of Pharmacy, Analytical & Biological Research Facility, Synthesis and Solid State Pharmaceutical Centre, University College Cork, Cork, Ireland
a r t i c l e i n f o
a b s t r a c t
Article history:
Hydrolase-catalysed dynamic kinetic resolutions of chroman-2-ol and 3-methyl chroman-2-ol can be
effected with up to 88% conversion and 92% ee through the use of organic solvents. Extension to the res-
olution of the tolterodine precursor 1 proved more challenging. The presence of the remote phenyl sub-
Received 15 March 2017
Revised 27 March 2017
Accepted 28 March 2017
Available online xxxx
stituent had
a significant impact on the resolution and it was not possible to achieve high
enantioselectivity together with efficient conversion from the focussed panel of enzymes screened.
Ó 2017 Published by Elsevier Ltd.
1. Introduction
most notably in the resolution of alcohols, amines and carboxylic
acids.7 Dynamic kinetic resolutions have also been achieved with
The use of enzymes as catalysts in organic synthesis is associ-
ated with several economic and environmental benefits. Biocat-
alytic processes typically operate at ambient temperature,
neutral pH and without the need for functional group protection.
Moreover, enzymatic catalysis can in some cases replace classical
resolutions or more traditional synthetic methods which often
require metal catalysts with associated cost and toxicity implica-
tions. Hydrolases have found particularly widespread use in the
pharmaceutical, fine chemicals and textiles industries, amongst
others.1 This family of enzymes boasts several characteristics,
which make them excellent catalysts for asymmetric synthesis;
they do not require external cofactors and often display broad sub-
strate specificity, stability and excellent enantioselectivity, even in
organic solvents.2
lipases, principally through racemisation of a stereocentre featur-
ing an alcohol/amine functionality.5,8 The racemisation is often
induced via a compatible transition metal catalyst8 or, in some
cases, it may even occur spontaneously.9
The anomeric centre in the hemiacetal functional group has
been previously resolved using lipase-mediated (trans) esterifica-
tion. Feringa et al. have reported the dynamic resolution of 5-acy-
loxy-2(5H)-furanones and pyrrolidinones and 6-acyloxy-2H-
pyran-3(6H)-one.10,11 Deoxysugars bearing a second stereocentre
have also been enzymatically acetylated, furnishing a mixture of
the cis- and trans-stereoisomers and in some cases the ring-opened
aldehyde product.12 Unsurprisingly, due to the dynamic nature of
the hemiacetal functional group, enzymatic enantiodiscrimination
can be difficult to achieve.
Hydrolases are widely used for kinetic resolutions, one of the
most reliable methods for the enzyme-mediated production of
enantiopure compounds,3,4 despite the limitation of a maximum
yield of 50%. A dynamic kinetic resolution is a more attractive
approach since quantitative yields of the desired (enantiopure)
Lactol 1 (Fig. 1) is of interest to our group as it is an intermedi-
ate in the preparation of tolterodine and fesoterodine, antimus-
carinic drugs.13 Currently these drugs are synthesised via
a
classical resolution from a mix of stereoisomers; therefore, a bio-
catalytic approach is attractive to enable a greener, more sustain-
able synthetic route. The lactol exists in dynamic equilibrium
whereby the hemiacetal interconverts with the corresponding
ring-opened hydroxyaldehyde (Fig. 2). Therefore, in approaching
the resolution of lactol 1, there are two distinct processes to be
considered; the first involves the classical resolution of the remote
stereogenic centre, while the second involves the dynamic resolu-
tion of the hemiacetal centre (Figure 2). From a synthetic perspec-
stereoisomer are attainable in compounds bearing
a single
racemisable stereocentre.5,6
Lipases, a sub-group of hydrolases whose natural function is the
hydrolysis of lipids, have been extensively utilised as biocatalysts,
⇑
Corresponding author.
E-mail address: a.maguire@ucc.ie (A.R. Maguire).
http://dx.doi.org/10.1016/j.tetasy.2017.04.001
0957-4166/Ó 2017 Published by Elsevier Ltd.
Please cite this article in press as: Gavin, D. P.; et al. Tetrahedron: Asymmetry (2017), http://dx.doi.org/10.1016/j.tetasy.2017.04.001

2
D. P. Gavin et al. / Tetrahedron: Asymmetry xxx (2017) xxx–xxx
N
O
N
O
OH
OH
O
R = H, OH
R
HO
1. R = H
tolterodine
fesoterodine
Figure 1.
anomeric centre
O
OH
OH
Ph
O
OH
R = H, OH
R
R
H
R
Ph
O
Ph
remote stereocentre
Figure 2.
tive, it is the resolution of the remote stereocentre which is impor-
tant, since the hemiacetal centre is labile and not retained in tolter-
odine and fesoterodine.
was feasible to clarify the reactivity of the hemiacetal functional
group with the hydrolases. Two potential sites of esterification
were possible; the hydroxyl group of the closed hemiacetal func-
tionality and also the phenolic position of the ring-opened tau-
tomer (Fig. 2). Chroman-2-ol 3a and its 6-methyl analogue 3b
were prepared from commercially available chroman-2-ones via
known methods15 and then acetylated to give the racemic products
4a and 4b (Scheme 1).
A focused panel of diverse commercial hydrolases was tested
for activity against the chosen substrate; the widely used vinyl
acetate was selected as both the acyl source and the solvent for
the initial enzyme screening (Table 1, selected results shown). As
summarised in Table 1, from a panel of over 50 biocatalysts, sev-
eral enzymes were active against substrates 3a and 3b, generating
the acylated products 4a and 4b. Indeed a number of hydrolases
gave high enantioselectivity, albeit with modest efficiency. For
compound 3a, of the 50 enzymes tested, 10 gave a conversion
greater than background acetylation (ꢀ1%). In particular, lipases
from the Alcaligenes family were relatively selective. Immobilised
CAL-B furnished the highest ee (95%) for product 4a of all the
enzymes tested albeit with low conversion in 24 h. Extending the
reaction time to 120 h gave the acetylated product 4a with excel-
lent ee and 20% yield (Table 1, entry 6).
Therefore, theoretically a single enantiomer (from the 4 possi-
ble lactol stereoisomers) can be garnered from a lipase-mediated
acetylation, giving a possible conversion of 50%. Building on previ-
ous work in our group on substrates featuring a remote stereocen-
tre,14 we aimed to achieve a highly selective resolution of the
remote benzylic stereocentre of lactol 1. For the purposes of this
work stereocontrol at the anomeric centre is not critical. A resolu-
tion of this type would therefore require either: (1) an enzyme
which is non-selective at the anomeric centre but selective at the
remote stereocentre or; (2) dynamic kinetic resolution involving
enzymatic acylation of only one of the 4 possible lactol enan-
tiomers, i.e. a kinetic resolution of the remote stereocentre com-
bined with a dynamic resolution of the anomeric centre. A
resolution of either type could enable a maximum possible yield
of 50% with enzymatic discrimination of the remote stereocentre.
2. Results and discussion
2.1. Model substrates
As limited examples of dynamic resolutions of hemiacetals have
been reported only in monocyclic systems,11 model systems 3a and
3b were first explored to ensure the fused aromatic ring system
was compatible with this methodology. Initial enzyme screening
focused on the model compounds 3a and 3b. Firstly, the aim was
to establish if a dynamic kinetic resolution at the anomeric centre
2.2. Use of organic solvents
The use of organic solvents in conjunction with an acyl source is
a well-established approach for improving yield and/or enantiose-
lectivity in resolutions involving hydrolase biocatalysts.16 Accord-
O
O
O
OH
O
O
O
OAc
R
R
Me
R
2
. R = Me
3a
. R = H
4a
. R = H
3b
. R = Me
4b
. R = Me
Scheme 1. Synthesis of model lactols 3a and 3b and their acetylated analogues, 4a and 4b. Reagents and conditions: (i) Pd/C, H2, EtOAc; (ii) DIBAL, MePh, À78 °C; (iii) Ac2O,
DMAP, pyridine, DCM.
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D. P. Gavin et al. / Tetrahedron: Asymmetry xxx (2017) xxx–xxx
3
Table 1
Enzyme screen against substrates 3a and 3b
hydrolase
O
OAc
O
OH
vinyl acetate (100 equiv.)
150 rpm, 30 °C, 24 h
R
R
4a
3a
. R = H
. R = H
4b
3b
. R = Me
. R = Me
Entry
R
Hydrolase
Lipase A from Candida rugosa
Lipase from Pseudomonas cepacia
Lipase B from Alcaligenes sp.
Conversiona (%)
Product
Product (% ee)b
1
2
3
4
5
6
7
8
H
H
H
H
H
H
H
H
Me
Me
Me
Me
Me
Me
Me
25
29
14
28
8
20
47
13
26
28
30
19
12
20
25
4a
4a
4a
4a
4a
4a
4a
4a
4b
4b
4b
4b
4b
4b
4b
73 (R)
58 (R)
80 (R)
78 (R)
94 (R)
95 (R)
77 (R)
33 (R)
64 (+)
65 (+)
70 (+)
33 (+)
97 (+)
59 (+)
8 (+)
Lipase D from Alcaligenes sp.
Candida antarctica lipase B (immob.)
Candida antarctica lipase B (immob.)*
Lipase E from Alcaligenes sp.
Lipase from Pseudomonas stutzeri
Lipase A from Candida rugosa
Lipase from Candida cylindracea
Lipase E from Alcaligenes sp.
Lipase from Pseudomonas stutzeri
Lipase from Thermomyces lanuginosus
Lipase B from Candida rugosa
Candida antarctica lipase A
9
10
11
12
13
14
15
Selected results. Lactol ee not determined due to spontaneous racemisation.
*
Reaction time 120 h.
a
Determined by 1H NMR.
b
Determined by chiral HPLC analysis. 4a [Phenomenex Cellulose 4, hexane/i-PrOH = 99:1, flow rate 1 mL/min, 25 °C, k = 209.8 nm]. 4b [Phenomenex Cellulose 4, hexane/i-
PrOH = 95:5, flow rate 1 mL/min, 25 °C, k = 209.8 nm].
ingly, the lead enzymes from the previous screening were further
screened against the substrates with vinyl acetate as the acyl
source, with variation of the reaction medium. Selected results
are shown in Table 2. As can be seen from this table, the use of
organic solvents was found to have a profound effect on both con-
version and enantioselectivity. Initially, 4.2 equiv of vinyl acetate
were used (entries 1–7; R = H, substrate 3a). The Alcaligenes family
of lipases were especially active and enantioselective with this
Table 2
Solvent screen with active enzymes against substrates 3a and 3b
hydrolase
O
OH
vinyl acetate
O
OAc
solvent (2 mL)
R
150 rpm, 30 °C, 96 h
R
3a. R = H
4a. R = H
3b
. R = Me
4b
. R = Me
Entry
R
Hydrolase
Solvent
Equivalents vinyl acetate
Conversiona (%)
Product
Product (% ee)b
1
2
3
4
5
6
7
8
H
H
H
H
H
H
H
H
Me
Me
Me
Me
Me
Me
Me
Me
Me
Me
Me
Me
Candida Antarctica lipase B (immob.)
Lipase B from Alcaligenes sp.
Lipase B from Alcaligenes sp.
Lipase D from Alcaligenes sp.
Lipase D from Alcaligenes sp.
Lipase E from Alcaligenes sp.
Lipase E from Alcaligenes sp.
Lipase A from Candida rugosa
Lipase E from Alcaligenes sp.
Lipase E from Alcaligenes sp.
Lipase E from Alcaligenes sp.
Lipase A from Candida rugosa
Lipase A from Candida rugosa
Lipase from Candida cylindracea
Lipase from Candida cylindracea
Lipase from Candida cylindracea
Lipase from Thermomyces lanuginosus
Lipase from Thermomyces lanuginosus
Lipase from Thermomyces lanuginosus
Lipase from Thermomyces lanuginosus
Hexane
Hexane
4.2
4.2
4.2
4.2
4.2
4.2
4.2
4.2
100
100
100
100
100
100
100
100
100
100
100
100
13
40
76
35
67
81
26
50
93
58
53
48
26
63
41
30
88
38
55
27
4a
4a
4a
4a
4a
4a
4a
4a
4b
4b
4b
4b
4b
4b
4b
4b
4b
4b
4b
4b
91 (R)
70 (R)
77 (R)
73 (R)
75 (R)
56 (R)
77 (R)
59 (R)
44 (+)
58 (+)
62 (+)
67 (+)
69 (+)
66 (+)
63 (+)
73 (+)
92 (+)
95 (+)
94 (+)
96 (+)
Toluene
Toluene
Diethyl ether
Hexane
Toluene
Heptane
Hexane
Heptane
Toluene
Hexane
Toluene
Hexane
Heptane
Toluene
Hexane
Heptane
Toluene
Diisopropyl ether
9
10
11
12
13
14
15
16
17
18
19
20
Selected results. Lactol ee not determined due to spontaneous racemisation.
a
Determined by 1H NMR.
b
Determined by chiral HPLC analysis. 4a [Phenomenex Cellulose 4, hexane/i-PrOH = 99:1, flow rate 1 mL/min, 25 °C, k = 209.8 nm]. 4b [Phenomenex Cellulose 4, hexane/i-
PrOH = 95:5, flow rate 1 mL/min, 25 °C, k = 209.8 nm].
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D. P. Gavin et al. / Tetrahedron: Asymmetry xxx (2017) xxx–xxx
O
O
O
O
OH
O
OAc
OH
OH
(i)
(iii)
(ii)
+
(
)-5
1
)-
(
(
)-6
O
O
O
O
O
OAc
(ii),(iii)
(iv)
(4R)-5
6
(4R)-
Scheme 2. Synthesis of enantiopure and racemic lactone 5, lactol 1 and acetylated analogue 6. Reagents and conditions: (i) I2, 120–130 °C, neat, 70%; (ii) DIBAL, MePh, À78 °C,
( )-1: 66%; (4R)-1: 65%; (iii) Ac2O, DMAP, Pyridine, CH2Cl2; (iv) Rh(acac)2, (R)-Josiphos, PhB(OH)2, 1,4-dioxane/H2O, 60 °C, 39%.
substrate in a range of organic solvents. For example, Lipase B from
Alcaligenes sp furnished the product with 77% ee with 76% conver-
sion in toluene (Table 2, entry 3), indicating that a resolution
involving a dynamic process was occurring.
Due to the low conversion of 3b at a similar loading of vinyl
acetate, 100 equivalents of the acyl source were used (Table 2,
entries 9–20). There was no evidence of an increase in the rate of
the background non-enzymatic acylation under these conditions.
Figure 3. Stacked HPLC chromatograms which allowed identification of each stereoisomer. Shown:
Mixture of cis-6 and trans-6
cis-6 (slightly impure with trans-6)
trans-6 enantiopure (4R)-6. Experiments performed using Daicel Chiralcel OD-H column, hexane/i-PrOH = 98:2, flow rate 1 mL/min, 25 °C, k = 209.8 nm.
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D. P. Gavin et al. / Tetrahedron: Asymmetry xxx (2017) xxx–xxx
5
Lipase from Thermomyces lanuginosus was particularly selective
for substrate 3b in a range or solvents. In hexane the conversion
was 88% with 92% ee (Table 2, entry 17). Other hydrocarbon media
such as heptane and toluene were also effective media for this
asymmetric transformation, giving enantioselectivities of 95% ee
and 94% ee respectively, albeit at a lower conversion (Table 2,
entries 18 and 19). Despite the structural similarity of substrates
3a and 3b, there was not a large crossover of activity amongst
the enzymes with these substrates, although CAL-B and members
of the Alcaligenes family were active against both compounds.
version, and, most significantly, only the cis-isomer was acetylated
indicating that the efficient enzymatic acetylation of the hemiacetal
is sensitive to the remote stereogenic centre, auguring well for our
overall objective of resolution of the remote stereocentre. Encour-
aged by the high activity exhibited by this enzyme against our cho-
sen substrate, we then screened organic solvents as we had already
seen a large effect in our studies with the model substrate. Indeed,
the effect of organic solvents on this transformation was exhaus-
tively investigated. Disappointingly, the lipase from Pseudomonas
stutzeri did not yield the acylated product with high ee in any of a
wide range of solvents, and indeed addition of organic solvents
resulted in decreased enantioselectivity relative to the transforma-
tions in neat vinyl acetate. The use of hydrocarbons hexane and
heptane dramatically reduced the enantioselectivity of the process
(Table 4, entries 4 and 5), in contrast to their effect on the resolution
of products 4a and 4b. Decreasing the amount of vinyl acetate to
5 equiv in these solvents, with the aim of decreasing the rate of
the reaction and potentially allowing increased enantioselection,
also resulted in poor ee (Table 4, entries 9 and 10).
Another factor which can influence the selectivity and conver-
sion is variation of the acyl source. For example, the use of enol
esters such as vinyl acetate and isopropenyl acetate, help to over-
come the challenge of reversibility in enzyme-mediated (trans)es-
terifications. The choice of acyl source can have a dramatic impact
on the enzymatic resolution and many other acyl sources have
been investigated, including carboxylic acids, methoxyacetates
and anhydrides.19 Herein we found that the less reactive esters
ethyl acetate and isopropyl acetate resulted in little or no conver-
sion to product (Table 4, entries 2 and 3) when the lipase from
Pseudomonas stutzeri was used to promote the reaction, which is
in contrast to the outcome in vinyl acetate with the same biocata-
lyst (reaching 39% conversion in only 4 h). Whilst isopropenyl acet-
ate facilitated conversion to product, it led to less
enantioselectivity and considerably less activity than its vinyl
counterpart, taking 24 h to reach 53% conversion with 26% ee
(Table 4, entry 1). Therefore, in a broad range of reaction condi-
tions, despite the consistent, excellent selectivity for the cis-pro-
duct rather than the trans-, this biocatalyst did not exhibit
sufficient enantiodiscrimination to deliver the acetylated lactol 6
with high ee.
2.3. Tolterodine intermediate
Having established that a dynamic resolution was possible and
that exclusive acetylation of the hemiacetal functionality in 3a and
3b could furnish an enantioenriched product in high conversion,
we turned our attention to the tolterodine intermediate 1. Conden-
sation of cinnamic acid with p-cresol furnished chroman-2-one 5,17
which was reduced to the corresponding lactol 1 at low tempera-
ture. The hemiacetal group was then acetylated to give compound
6 (Scheme 2) in a 1:1.5 cis:trans mixture. The 4 stereoisomers of
acetylated product 6 were resolved on a single HPLC trace along
with lactol 1 to facilitate analysis of the enzyme-catalysed acetyla-
tions. Compound trans-6 was preferentially crystallised from a
crude mixture of the two diastereomers in ethanol and a crystal
of this diastereomer was subsequently grown. The tolterodine pre-
cursor (4R)-5 (Scheme 2) was also synthesised via a known
method.18 Subsequent reduction gave (4R)-1 and acetylation gave
(4R)-6, which allowed for the identification of each stereoisomer
on the HPLC trace (Fig. 3).
With the analytics for the reaction in hand, we then undertook
the biocatalyst screening (Table 3). Again, vinyl acetate was both
the solvent and acyl source for the initial screen. Although over
50 hydrolases were tested, the vast majority returned starting
material only. Evidently, overall acetylation of hemiacetal 1 is more
challenging than that of the model systems 3a and 3b, with lower
conversions than in the absence of the aryl ring at the 4-position
of the lactol (see Table 1). By far the most active enzyme was the
lipase from Pseudomonas stutzeri, which exclusively furnished the
cis-acetate 6 in 39% conversion in 4 h. This enzyme also displayed
some enantiodiscrimination, giving cis-6 in a 3:1 ratio of enan-
tiomers (Table 3, entry 6). Interestingly, this biocatalyst was consid-
erably less active with model substrates 3a and 3b, with poor
conversion and enantioselection for both (Table 1, entries 8 and
12). Lipase from Pseudomonas stutzeri uniquely provided a high con-
The most selective enzyme tested in the initial screen was acy-
lase from Aspergillus sp. (Table 3, entry 8), however this transfor-
mation suffered from very poor conversion (6%). Despite the low
activity of the enzyme under these conditions, the exclusive con-
version to a single cis-stereoisomer [>98% ee, (2S,4S)] of product
Table 3
Enzyme screen against substrate 1
O
OH
O
OAc
O
OAc
acyl source
hydrolase
+
solvent
150 rpm, 30 °C
rac-1
cis-6
trans-6
Entry
Hydrolase
Time (h)
Solvent
Acyl source (equiv)
cis-6
trans-6
Conversion (%)
Conversion (%)
ee (%)
ee (%)
1
2
3
4
5
6
7
8
Lipase A from Candida rugosa
Lipase B from Candida rugosa
Lipase A from Alcaligenes sp.
Lipase F from Alcaligenes sp.
Lipase from Pseudomonas stutzeri
Lipase from Pseudomonas stutzeri
Lipase from Candida cylindracea
Acylase from Aspergillus sp.
24
24
24
24
12
4
—
—
—
—
—
—
—
—
Vinyl acetate (100)
Vinyl acetate (100)
Vinyl acetate (100)
Vinyl acetate (100)
Vinyl acetate (100)
Vinyl acetate (100)
Vinyl acetate (100)
Vinyl acetate (100)
8
5
7
14
57
39
7
77 (4R)
82 (4R)
11 (4S)
12 (4S)
39 (4S)
50 (4S)
78 (4R)
6
5
>98 (4S)
>98 (4S)
—
—
—
—
<1
<1
<1
<1
5
24
24
>98 (4S)
—
6
>98 (4S)
<1
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6
D. P. Gavin et al. / Tetrahedron: Asymmetry xxx (2017) xxx–xxx
Table 4
Screening of conditions for enzymes active against substrate 1
Entry
Hydrolase
Time (h)
Solvent
Acyl source (equiv)
cis-6
Conversiona
trans-6
Conversiona
eeb (%)
eeb (%)
(%)
(%)
1
2
3
4
5
6
7
8
Lipase from Pseudomonas stutzeri
Lipase from Pseudomonas stutzeri
Lipase from Pseudomonas stutzeri
Lipase from Pseudomonas stutzeri
Lipase from Pseudomonas stutzeri
Lipase from Pseudomonas stutzeri
Lipase from Pseudomonas stutzeri
Lipase from Pseudomonas stutzeri
Lipase from Pseudomonas stutzeri
Lipase from Pseudomonas stutzeri
Acylase from Aspergillus sp.
24
24
24
24
24
24
24
24
16
16
36
36
72
36
36
36
—
—
—
Isopropenyl acetate (100)
Ethyl acetate (100)
Isopropyl acetate (100)
Vinyl acetate (100)
Vinyl acetate (100)
Vinyl acetate (100)
Vinyl acetate (100)
Vinyl acetate (100)
Vinyl acetate (5)
53
—
—
26 (4S)
—
—
<1
—
—
<1
<1
<1
<1
2
<1
2
—
<1
—
—
—
—
—
—
—
—
—
66 (4S)
—
54 (4S)
—
—
—
—
—
—
Hexane
Heptane
Cyclo-hexane
Toluene
MTBE
Toluene
Hexane
—
Hexane
Hexane
Heptane
Toluene
Toluene
80
72
51
50
42
52
74
6
17
20
13
6
14 (4S)
18 (4S)
29 (4S)
24 (4S)
21 (4S)
25 (4S)
2
98 (4S)
97 (4S)
94 (4S)
97 (4S)
97 (4S)
94 (4S)
9
10
11
12
13
14
15
16
Vinyl acetate (5)
Vinyl acetate (100)
Vinyl acetate (100)
Vinyl acetate* (100)
Vinyl acetate (100)
Vinyl acetate (100)
Vinyl acetate* (4)
Acylase from Aspergillus sp.
Acylase from Aspergillus sp.
Acylase from Aspergillus sp.
Acylase from Aspergillus sp.
—
<1
Acylase from Aspergillus sp.
13
Lactol ee not determined due to spontaneous racemisation.
a
Determined by 1H NMR.
b
Determined by chiral HPLC analysis. [Daicel Chiralcel OD-H, hexane/i-PrOH = 98:2, flow rate 1 mL/min, 25 °C, k = 209.8 nm].
Reaction performed at 37 °C, 250 rpm.
*
6 was observed and accordingly this catalyst was pursued as a
constants are expressed in Hertz (Hz). Low resolution mass spectra
were recorded on a Waters Quattro Micro triple quadrupole spec-
trometer in electrospray ionization (ESI) mode using 50% water/
acetonitrile containing 0.1% formic acid as eluent; samples were
made up in acetonitrile. High resolution mass spectra (HRMS) were
recorded on a Waters LCT premier Time of Flight spectrometer in
electrospray ionization (ESI) mode using 50% water/acetonitrile
containing 0.1% formic acid as eluent; samples were made up in
acetonitrile. Elemental analysis was performed by the Microanaly-
sis Laboratory, National University of Ireland, Cork, using Perkin–
Elmer 240 and Exeter Analytical CE440 elemental analysers Melt-
ing points were carried out on a uni-melt Thomas Hoover Capillary
melting point apparatus and are uncorrected. Wet flash chro-
matography was performed using Kieselgel Silica Gel 60, 0.040–
0.063 mm (Merck). Thin layer chromatography (TLC) was carried
out on precoated silica gel plates (Merck 60 PF254). Visualisation
was achieved by UV (254 nm) light detection and KMnO4 staining.
Optical rotations were measured on a Perkin–Elmer 141 polarime-
ter at 589 nm in a 1 cm cell; concentrations (c) are expressed in
g/100 mL. Enzymes were supplied by Almac Sciences/purchased
from Sigma–Aldrich chemical company (see Supporting Informa-
tion). All reagents are analytical grade and purchased from
Sigma–Aldrich chemical company. All enzymatic reactions were
performed on a VWR Incubating Mini Shaker 4450. The enan-
tiomeric purity of products 4a and 4b were determined by chiral
potential candidate for the DKR of product 6. A wide-ranging
screening was conducted with the goal of increasing the conver-
sion of this enzymatic transformation. Although the conversion
improved, particularly with hexane as the solvent (Table 4, entry
12), the overall efficiency of the reaction was poor. The reaction
time and temperature were also increased, but this resulted in a
modest increase in enzyme activity (Table 4, entry 13). All of the
conditions tested furnished almost exclusively the cis-stereoiso-
mer with excellent enantiopurity (up to 98% ee), however, the con-
version to acylated product 6 was never above 20%, even after 72 h.
Other acyl sources, such as isopropenyl acetate and ethyl acetate,
led to complete inactivity for the acylation of 1 with this enzyme.
3. Conclusion
In conclusion, we have demonstrated that dynamic kinetic res-
olution is possible upon treatment of model hemiacetals 3a (up to
76% conversion with 77% ee) and 3b (up to 88% conversion with
92% ee) with hydrolases. Extension to include the resolution of a
remote stereocentre in the tolterodine intermediate 1 proved to
be more challenging in terms of efficiency, while retaining high
enantioselectivity (up to >98% ee).
4. Experimental
4.1. General
HPLC analysis on
a
Phenomenex Cellulose
4
column
(250 Â 4.6 mm); the enantiomeric purity of dihydrocoumarin 5
was determined on a Phenomenex Amylose 2 column, both pur-
chased from Phenomenex Inc., UK. Enantioselectivities for Com-
Dry solvents were distilled prior to use as follows: dichloro-
methane was distilled from phosphorus pentoxide and ethyl acet-
ate was distilled from potassium carbonate; tetrahydrofuran and
toluene were distilled from sodium and benzophenone. Molecular
sieves were activated by heating at 150 °C overnight. Organic
phases were dried using anhydrous magnesium sulphate. Infrared
spectra were measured using a Perkin Elmer FTIR UATR2 spec-
trometer. 1H (300 MHz) and 13C (75.5 MHz) NMR spectra were
recorded on a Bruker Avance 300 MHz NMR spectrometer. 1H
(400 MHz) NMR spectra were recorded on a Bruker Avance
400 MHz NMR spectrometer. All spectra used tetramethylsilane
(TMS) as an internal standard. Chemical shifts (dH and dC) are
reported in parts per million (ppm) relative to TMS and coupling
pound 6 were determined on a Chiralcel OD-H column
(250 Â 4.6 mm), purchased from Daicel Chemical Industries, Japan.
Mobile phase, flow rate, detection wavelength and temperature are
stated in the appropriate Tables 1–4. HPLC analysis was performed
on a Waters alliance 2690 separations module with a PDA detector.
All solvents employed were of HPLC grade.
4.2. Synthesis of model substrates and derivatives
4.2.1. 6-Methylchroman-2-one 220
A solution of 6-methyl coumarin (2.771 g, 16.67 mmol) and Pd/C
(10 wt%, 0.566 g, 0.53 mmol, 5 mol%) in ethyl acetate (17.5 mL) was
stirred under an atmosphere of hydrogen until TLC showed the dis-
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D. P. Gavin et al. / Tetrahedron: Asymmetry xxx (2017) xxx–xxx
7
appearance of the starting material (48 h). The solution was filtered
through a bed of CeliteÒ using ethyl acetate (10 mL) and concen-
trated to give a yellow oil, which solidified on cooling. The solid resi-
due was dissolved in ethyl acetate (10 mL), filtered again through
CeliteÒ, and concentrated to give a pale yellow solid. The product
was dried overnight under high vacuum to give a white waxy solid
(2.704 g, 97%). The product did not require purification mp 77–
2938 (CAH), 1748 (C@O), 1490, 1201, 1199 (CAO). 1H NMR
(300 MHz, CDCl3): d 7.05–7.17 (m, 2H, ArH), 6.84–6.96 (m, 2H,
ArH), 6.52 (app. t, 1H, OCHOCH3), 2.92–3.05 (m, 1H, CH2), 2.65–
2.76 (m, 1H, CH2), 1.97–2.17 (m, 5H, CH2 and CH3); 13C NMR
(75.5 MHz, CDCl3): d 170 (C@O), 151.7, 129.4, 127.8, 121.8, 121.5,
117.2 (6 Â ArC), 90.4 (CHOCH3), 25.2 (CH2), 21.3 (CH3), 19.8
(CH2). HRMS: MH+, found 193.0867. C11H13O3 requires 193.0865.
Enantiomers separated using Phenomenex Cellulose 4 column
(250 Â 4.6 mm), conditions: n-hexane/i-PrOH = 99:1, flow rate
1 mL/min, 25 °C, k = 209.8 nm; t1 = 7.6, t2 = 8.3.
78 °C. m
max/cmÀ1 (ATR): 3476 (CAH, Ar), 2924 (CAH), 1740 (C@O),
1500 (CAC), 1209 (CAO). 1H NMR (300 MHz, CDCl3): d 7.10–6.90
(m, 3H, ArH), 3.0–2.90 (m, 2H, CH2), 2.81–2.72 (m, 2H, CH2) 2.31
(s, 3H, CH3); 13C NMR (75.5 MHz, CDCl3): d 168.8 (C@O), 150.0,
134.0, 128.7, 128.5, 122.3, 116.7 (6 Â ArC), 29.4 (CH2), 23.8 (CH3),
20.7 (CH2). MS (ES+): found 163.3. C9H9O2 requires 163.2.
4.2.5. ( )-6-Methylchroman-2-yl acetate ( )-4b
This was prepared using the procedure for 4a from compound
3a to give a colourless oil (0.093 g, 70%) which used without fur-
4.2.2. ( )-Chroman-2-ol ( )-3a18
ther purification. m
max/cmÀ1 (ATR): 2933, 1748 (C@O), 1498,
To a three-neck round-bottom flask equipped with nitrogen
bubbler and dropping funnel were added dihydrocoumarin (5 g,
33.7 mmol) and dry toluene (100 mL). The solution was cooled to
À78 °C and DIBAL [1 M solution in hexanes, (37 mL, 37.0 mmol,
1.1 equiv)] was added dropwise over a period of 30 min. The reac-
tion was stirred for 2 h, allowed to warm to room temperature and
then quenched with H2O (30 mL). The resulting white suspension
was filtered over CeliteÒ and washed through with diethyl ether
(100 mL). The phases were separated and the aqueous phase was
extracted with diethyl ether (200 mL). The combined organic
phases were washed with H2O (200 mL) and brine (200 mL). The
combined organic layers were dried, filtered and the solvent was
removed under vacuum to give the product (4.83 g, 95%) as a
1199. 1H NMR (300 MHz, CDCl3): d 6.84–6.98 (m, 2H, ArH), 6.76
(d, J = 8.2, 1H, ArH), 6.50 (t, J = 2.6, 1H, OCHOCH3), 2.86–3.03 (m,
1H CH2), 2.57–2.74 (m, 1H, CH2), 2.26 (s, 3H, CH3), 1.90–2.18 (m,
5H, CH2 & CH3); 13C NMR (75.5 MHz, CDCl3): d 169.9, 149.3,
130.7, 129.6, 128.2, 121.3, 116.8, 90.3, 25.2, 21.2, 20.5, 19. HRMS+:
found: 147.0816 [MÀOAc]+. C10H11O requires 147.0816. Enan-
tiomers separated using Phenomenex Cellulose
4 column
(250 Â 4.6 mm), conditions: n–hexane/i-PrOH = 95:5, flow rate
1 mL/min, 25 °C, k = 209.8 nm; t1 = 6.0, t2 = 6.5.
4.3. Synthesis of tolterodine lactol and derivatives
4.3.1. ( )-6-Methyl-4-phenylchromanone ( )-517
colourless oil which was used without further purification. mmax
/
To a solution of trans-cinnamic acid (10.22 g, 69 mmol) in p-cre-
sol (7.2 mL, 69 mmol, 1 equiv) was added I2 (3.5 g, 13.8 mmol,
20 mol %). The solution was stirred at 130 °C for 3 h. It was then
allowed to cool to room temperature, dissolved in ethyl acetate
(300 mL) and washed with saturated aqueous sodium thiosulfate
solution (2 Â 100 mL), H2O (100 mL) and brine (200 mL). The
organic layer was then passed through a silica plug and the solvent
was removed under reduced pressure. The residue was purified by
column chromatography on silica gel with hexane/diethyl ether
(5:1) to afford the pure product ( )-5 (11.06 g, 70%) as a white solid
cmÀ1 (ATR): 3401 (OH), 3040 (CAH, Ar), 2939 (CAH), 1220
(CAO). 1H NMR (300 MHz, CDCl3): d 7.14–7.04 (m, 2H, ArH),
6.92–6.80 (m, 2H, ArH), 5.62 (dd, J = 4 Hz, J = 2.6 Hz, 1H, OCHOH),
3.05–2.99 (m, 2H, CH2 and OH), 2.76–2.66 (m, 1H, CH2), 2.11–
1.93 (m, 2H, CH2); 13C NMR (75.5 MHz, CDCl3): d 152.1, 129.4,
127.6, 122.2, 121.0, 117.0 (6 Â ArC), 92.3 (OCHOH), 27.2, 20.4
(2 Â CH2). HRMS (ESÀ): found 149.0608. C9H9O2 requires 149.0603.
4.2.3. ( )-6-Methylchroman-2-ol ( )-3b21
This was prepared using the procedure for 3a from 6-
methylchroman-2-one 2 (1.557 g, 9.6 mmol) and DIBAL solution
(1 M in hexanes, 11.2 mL, 11.2 mmol, 1.16 equiv) in dry toluene
(30 mL). The crude product was purified by column chromatogra-
phy on silica gel using hexane/ethyl acetate (85:15) to give the
(mp 76–78 °C). m
max/cmÀ1 (ATR): 3028 (CAH), 1763 (C@O), 1493,
1126 (CAO). 1H NMR (300 MHz, CDCl3): d 7.38–7.27 (m, 3H,
ArH), 7.18–7.13 (m 3H, ArH), 7.09 (dd, J = 8.3 Hz, J = 1.8 Hz, 1H,
ArH), 6.78 (s, 1H, ArH), 4.29 (app t, J = 6.9 Hz, CH), 3.10–2.93 (m,
2H, CH2), 2.25 (s, 3H, CH3); 13C NMR (75.5 MHz, CDCl3): d 168.0
(C@O), 149.8, 140.7, 134.5, 129.5, 129.3, 128.8, 127.8, 127.7,
125.5, 117.0 (10 Â ArC), 40.9 (CH), 37.3 (CH2), 20.9 (CH3).
pure product as a colourless oil (0.93 g, 59%).
3401 (OH), 2939 (CAH), 1498, 1206 (CAO). 1H NMR (300 MHz,
CDCl3): 7.00–6.78 (m, 2H, ArH), 6.71 (d, J = 8.1 Hz, 1H,
m
max/cmÀ1 (ATR):
d
ArH),5.58 (br s, 1H, CHOH), 3.25 (s, 1H, OH), 3.04–2.82 (m, 1H,
CH2), 2.66 (dt, J = 16.4 Hz, J = 5.3 Hz, 1H, CH2), 2.25 (s, 3H, CH3),
2.11–1.85 (m, 2H, CH2); 13C NMR (75.5 MHz, CDCl3): d 149.7,
130.1, 129.6, 128.0, 121.7, 116.6 (6 Â ArC), 92.1 (CHOH), 27.1
(CH2), 20.5 (CH3), 20.3 (CH2). HRMS+: found: 147.0808 [MÀH2O]+.
4.3.2. (4R)-6-Methyl-4-phenylchromanone (4R)-518
To a solution of Rh(acac)(C2H4)2 (4.6 mg, 18 mmol), (R)-Josiphos
(12.6 mg, 9.9 mmol) and 6-methylcoumarin (96 mg, 0.60 mmol) in
a deoxygenated mixture of 1,4-dioxane (2 mL) and H2O (0.20 mL)
was added PhB(OH)2 (0.732 g, 6 mmol). The mixture was stirred
at 65 °C for 8 h and then passed through a short pad of silica gel
with Et2O as eluent, and the solvent was removed under reduced
pressure. The residue was purified by column chromatography
on silica gel with hexane/diethyl ether (5/1) as eluent to afford
the pure product (4R)-5 (28 mg, 39%) as a white solid (with identi-
cal spectroscopic properties as those above) in >99% ee.%. The ee
was determined on a Phenomenex Amylose 2 column with 90:10
hexane/isopropanol, flow = 1 mL/min, wavelength = 209.8 nm.
Retention time: 14 min [(S)-enantiomer], 16 min [(R)-enantiomer].
C10H11O requires 147.0810.
4.2.4. ( ) Chroman-2-yl acetate ( )-4a22
To a solution of lactol ( )-3a (0.097 g, 0.648 mmol) in dichloro-
methane (10 mL) were added acetic anhydride (0.48 mL, 5 mmol,
7.7 equiv), DMAP (5 mg) and pyridine (1 mL). The resulting solu-
tion was stirred for 4 h. A saturated solution of NaHCO3 (10 mL)
was added and the reaction was stirred vigorously until efferves-
cence ceased (approximately 30 min). The layers were separated
and the aqueous phase was washed with dichloromethane
(2 Â 10 mL). The organic layers were combined and washed with
saturated CuSO4 (30 mL), saturated NaHCO3 solution (30 mL),
water (20 mL) and brine (30 mL). The organic layer was dried, fil-
tered and the solvent was removed under vacuum to give 0.12 g
[a
]
D
20 = À9 (c 0.2, CHCl3). Absolute configuration confirmed by com-
parison with literature value.18
4.3.3. ( )-6-Methyl-4-phenylchroman-2-ol 118
This was prepared using the procedure for 3a from ( )-6-
methyl-4-phenylchromanone 5 (5.81 g, 24.38 mmol) and DIBAL
(97%) of the pure product as a colourless oil.
m
max/cmÀ1 (ATR):
Please cite this article in press as: Gavin, D. P.; et al. Tetrahedron: Asymmetry (2017), http://dx.doi.org/10.1016/j.tetasy.2017.04.001

8
D. P. Gavin et al. / Tetrahedron: Asymmetry xxx (2017) xxx–xxx
solution (1 M in hexanes, 26.8 mL, 26.8 mmol, 1.1 equiv) in dry
toluene (80 mL). The crude product was purified by column chro-
matography on silica gel using 100% DCM to give the pure product
as a colourless oil which solidified overnight to give a white waxy
solid (3.85 g, 66%) in a 4:1 mix of diastereomers. Mp 86–88 °C.
4.4. Enzymatic resolutions
General procedure for enzymatic acylation screens:
The substrate (10 mg) was added to a small test tube. The acyl
source and solvent (2 mL, if applicable) were then added along
with a spatula tip of enzyme. The reaction was incubated in a
mini-shaker at 30 °C. When the stipulated time period had elapsed,
the solution was passed through a Pasteur pipette containing a
layer each of CeliteÒ and MgSO4, using diethyl ether as eluent.
The solvent was removed under reduced pressure and the resulting
crude mixture was analysed using 1H NMR spectroscopy for con-
version data and chiral HPLC for enantioselectivity.
General procedure for preparative scale enzymatic resolutions:
The substrate (40 mg) was added to a small test tube. Vinyl
acetate (50 equiv) and solvent (4 mL, if applicable) were added
along with a spatula tip of enzyme. The small test tube was sealed
and the reaction was incubated in a mini-shaker at 30 °C. When the
stipulated time period had elapsed, the solution was passed
through a Pasteur pipette containing a layer each of CeliteÒ and
MgSO4, using diethyl ether as eluent. The solvent was removed
under reduced pressure and the resulting crude mixture was puri-
fied using column chromatography. The purified product was ana-
lyzed using 1H NMR spectroscopy for conversion data and chiral
HPLC for enantioselectivity.
m
max/cmÀ1 (ATR): 3433 (OAH), 3031 (Ar CAH), 2968 (CAH), 1493,
1202, 1012 (CAO). 1H NMR (300 MHz, CDCl3): d 7.38–7.16 (m,5H,
ArH), 6.94 (m, 1H, ArH), 6.81–6.76 (m, 1H, ArH), 6.58 (s, 0.79H,
ArH), 6.54 (s, 0.21H, ArH), 5.63 (br s, 0.79H, CHOH), 5.48 (m,
0.21H, CHOH), 4.30 (dd, J = 10.8 Hz, J = 5.9 Hz, 0.79H, CHPh), 4.19
(dd, J = 10.9 Hz, J = 6.0 Hz, 0.21H, CHPh), 3.17–3.04 (m, 1H, OH),
2.49–2.40 (m, 2H, CH2), 2.30–2.22 (m, 0.79H, CH2), 2.19–2.06 (m,
4H, CH2 and CH3). 13C NMR (75.5 MHz, CDCl3): d 151.1, 149.6,
144.0, 130.4, 130.3, 129.9, 129.7, 128.8, 128.7, 128.7, 128.6,
128.5, 129.6, 128.5, 126.9, 126.7, 124.9, 124.8, 116.7, 116.7
(20 Â ArC), 94.3, 91.3 (2 Â CHOH), 41.3 (CH), 38.9 (CH2), 37.0
(CH), 36.4 (CH2), 20.6 (CH3). Anal. Calcd for C8H8O: C, 79.97; H,
6.71. Found: C, 80.00; H, 6.71.
4.3.4. (4R)-6-methyl-4-phenylchroman-2-ol (4R)-1
This was prepared using the procedure for 3a above from (4R)-
6-methyl-4-phenylchromane 5 (0.020 g, 0.084 mmol) and DIBAL
solution (1 M in hexanes, 0.1 mL, 0.1 mmol, 1.19 equiv) in dry
toluene (1 mL). The crude product was purified by column chro-
matography on silica gel using hexane/ethyl acetate (85:15) to give
the pure product as a colourless oil (0.013 g, 65%) with identical
spectroscopic characteristics to ( )-1 above.
4.4.1. (+)-(R)-Chroman-2-yl acetate 4a
This was synthesised from chromanol 3a. The sample was incu-
bated for 120 h in toluene with immobilised CAL-B as the biocata-
lyst. 1H NMR analysis of the crude mixture indicated a 54%
conversion. The mixture was purified with hexane/dichloro-
methane (1/1) as eluent to give pure 4a in 41% yield and 94% ee.
4.3.5. ( )-6-Methyl-4-phenylchroman-2-yl acetate 6
This was prepared using the procedure for 4a from compound
( )-1 to give ( )-6 as a 1:1.5 cis:trans-mix of diastereomers.
trans-6 was preferentially crystallised from the mixture in ethanol.
[a]
20 = +64.5 (c 0.2, CHCl3).
D
trans-6: White solid. Mp 110–112 °C. m
max/cmÀ1 (ATR) 3029 (Ar
CAH), 2928 (CAH), 1748 (C@O), 1494, 1203, 1184 (CAO). 1H NMR
(300 MHz, CDCl3): d 7.39–7.21 (m, 5H, ArH), 6.95 (dd, J = 2.1 Hz,
J = 8.3 Hz, 1H, ArH), 6.82 (d, J = 8.3 Hz, 1H, ArH), 6.55–6.52 (m,
2H, ArH and CHOAc), 4.23 (dd, J = 10.8 Hz, J = 7 Hz, 1H, CHPh),
2.28–2.16 (m, 2H, CH2), 2.14 (s, 3H, CH3), 2.10 (s, 3H, CH3). 13C
NMR (75.5 MHz, CDCl3): d 169.9 (C@O), 149.2, 143.5, 130.5,
129.5, 128.8, 128.8, 128.7, 127.0, 124.9, 116.8 (10 Â ArC), 90.0
(CHOAc), 36.8 (CHPh), 34.6 (CH2), 21.3, 20.6 (2 Â CH3). Anal. Calcd
for C6H6O: C, 76.57; H, 6.43. Found: C, 76.61; H, 6.44. Enantiomers
separated using Daicel Chiralcel OD-H column (250 Â 4.6 mm),
conditions: n-hexane/i-PrOH = 98:2, flow rate 0.5 mL/min, 25 °C,
k = 209.8 nm; t1 = 14.0 min, t2 = 18.6 min.
4.4.2. (+)-6-Methylchroman-2-yl acetate 4b
This was synthesised from 6-methylchromanol 3b. The sample
was incubated for 96 h with Thermomyces lanuginosus as the bio-
catalyst. 1H NMR analysis of the crude mixture indicated a 65%
conversion. The mixture was purified with hexane/ethyl acetate
(5/1) as eluent to give pure 4b in 32% yield and 94% ee. [
20
a
]
=
D
+26.7 (c 0.2, CHCl3).
4.4.3. (2S,4S)-6-Methyl-4-phenylchroman-2-yl acetate 6
This was synthesised from ( )-1. The sample was incubated for
6 h in hexane with lipase from Pseudomonas stutzeri as the biocat-
alyst. 1H NMR analysis of the crude mixture indicated 51% conver-
sion. The mixture was purified with hexane/dichloromethane (1:1)
cis-6: Characterised from a mixture containing 15% trans-6.
Colourless liquid.
m
max/cmÀ1 (ATR) 3027 (Ar CAH), 2926 (CAH),
as eluent. Yield: 19 mg (47%). ee: 50%. [
a
]
D
20 = À53 (c 0.2, CHCl3).
1752 (C@O), 1493, 1199, 1799 (CAO). 1H NMR (300 MHz, CDCl3):
d 7.39–7.15 (m, 5H, ArH), 6.99 (dd, J = 8.4 Hz, J = 2.1 Hz, 1H, ArH),
6.86 (d, J = 8.2 Hz, 1H, ArH), 6.68 (br s, 1H, ArH), 6.44 (dd,
J = 5.9 Hz, J = 3 Hz, 1H, CHOAc), 4.27–4.19 (m, 1H, CHPh), 2.51–
2.20 (m, 2H, CH2), 2.20 (s, 3H, CH3), 1.80 (s, 3H, CH3). 13C NMR
(75.5 MHz, CDCl3): d 169.7 (C@O), 150.3, 144.5, 131.1, 130.4,
129.1, 128.6, 128.5, 126.6, 123.6, 117.1 (10 Â ArC), 91.9 (CHOAc),
38.8, (CHPh), 34.7 (CH2), 20.9, 20.7 (2 Â CH3). HRMS (ES+): MH+,
found 283.1343. C18H19O3 requires 283.1334. Enantiomers sepa-
rated using Daicel Chiralcel OD-H column (250 Â 4.6 mm), condi-
tions: n–hexane/i-PrOH = 98:2, flow rate 0.5 mL/min, 25 °C,
k = 209.8 nm; t1 = 10.7 min, t2 = 12.3 min.
Acknowledgements
This publication has emanated from research conducted with
the financial support of the Synthesis and Solid State Pharmaceuti-
cal Centre, funded by SFI under Grant Number 12/RC/2275, as well
as academic support from UCC. X-ray crystallography was made
possible through SFI funding (05/PICA/B802/EC07).
A. Supplementary data
Supplementary data associated with this article can be found, in
the online version, at http://dx.doi.org/10.1016/j.tetasy.2017.04.
001.
4.3.6. (4R)-6-Methyl-4-phenylchroman-2-yl acetate 6
This was prepared using the procedure for compound 4a
(0.013 g, 0.054 mmol), acetic anhydride (0.04 mL), DMAP (1 mg)
and pyridine (0.02 mL) to give product (4R)-6 in quantitative yield
(15 mg). Product was isolated in a 45:55 cis:trans ratio, corrobo-
rated via a HPLC chromatogram (5% ee).
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Please cite this article in press as: Gavin, D. P.; et al. Tetrahedron: Asymmetry (2017), http://dx.doi.org/10.1016/j.tetasy.2017.04.001