Enantiopurity and absolute configuration determination of arene cis-dihydrodiol metabolites and derivatives using chiral boronic acids

Article abstract of DOI:10.1002/chir.22764

The relative merits of the methods employed to determine enantiomeric excess (ee) values and absolute configurations of chiral arene and alkene cis-1,2-diol metabolites, including boronate formation, using racemic or enantiopure (+) and (?)-2-(1-methoxyet

Full text of DOI:10.1002/chir.22764

Received: 6 June 2017
Revised: 19 July 2017
Accepted: 24 July 2017
DOI: 10.1002/chir.22764
S P E C I A L I S S U E A R T I C L E
Enantiopurity and absolute configuration determination of
arene cis‐dihydrodiol metabolites and derivatives using
chiral boronic acids
Derek R. Boyd1 | Narain D. Sharma1 | Peter A. Goodrich1 | John F. Malone1 |
Gareth McConville1 | John S. Harrison1 | Paul J. Stevenson1
| Christopher C.R. Allen2
1 School of Chemistry and Chemical
Abstract
Engineering, Queen's University,
The relative merits of the methods employed to determine enantiomeric excess
(ee) values and absolute configurations of chiral arene and alkene cis‐1,2‐diol
metabolites, including boronate formation, using racemic or enantiopure (+)
and (−)‐2‐(1‐methoxyethyl)phenylboronic acid (MEPBA), are discussed.
Further applications of: 1) MEPBA derived boronates of chiral mono‐ and
poly‐cyclic arene cis‐dihydrodiol, cyclohex‐2‐en‐1‐one cis‐diol, heteroarene cis/
trans‐2,3‐diol, and catechol metabolites in estimating their ee values, and 2)
new chiral phenylboronic acids, 2‐[1‐methoxy‐2,2‐dimethylpropyl]phenyl
boronic acid (MDPBA) and 2‐[1‐methoxy‐1‐phenylmethyl]phenyl boronic acid
(MPPBA) and their advantages over MEPBA, as reagents for stereochemical
analysis of arene and alkene cis‐diol metabolites, are presented.
Belfast, UK
2 School of Biological Sciences and
Institute for Global and Food Security,
Queen's University, Belfast, UK
Correspondence
Derek R. Boyd, School of Chemistry and
Chemical Engineering, Queen's
University, Belfast, UK BT9 5AG.
Email: dr.boyd@qub.ac.uk
KEYWORDS
asymmetric synthesis, bacterial metabolism, boronate diastereomers, boronic acid enantiomers,
cis‐1,2‐diol bioproducts, enantiomeric excess values
The first examples of chiral arene cis‐dihydrodiol bacte-
rial metabolites B (X ≠ Y) or B′ (X ≠ Y) (Scheme 1) were
found by Gibson and co‐workers almost 50 years ago.1
Among the early substrates used were toluene A
(X = Me, Y = H),2 p‐chlorotoluene A (X = Me,
Y = Cl)1 and naphthalene.3 The main focus of Gibson's
group was on the structural/stereochemical identifica-
tion of new cis‐dihydrodiol metabolites, from mono‐
and di‐substituted benzene and polycyclic aromatic
hydrocarbon substrates, using different dioxygenase
enzymes.1-10 The instability of arene cis‐dihydrodiol
metabolites, and their unavailability in sufficient quanti-
ties, were early barriers to their utilization in synthetic
chemistry.1-8 In 1983, the development of a proprietary
constituent mutant strain of P. putida (UV4) by ICI,
expressing toluene dioxygenase (TDO), led to the com-
mercial production of arene cis‐dihydrodiols, e.g., B
(X = Y = H and X = Me, Y = H), and the first applica-
tions of this strain in synthesis.11-13
The availability of other mutant (Pseudomonas and
Sphingomonas) and recombinant (Escherichia coli)
strains, expressing TDO, naphthalene dioxygenase
(NDO), biphenyl dioxygenase (BPDO), and benzoate
dioxygenase for biotransformation resulted in a marked
increase in worldwide interest in this type of bacterial
metabolite. To date, more than 400 new cis‐dihydrodiols
have been isolated and are being used in chemoenzymatic
syntheses.1-19
Chirality. 2017;1–14.
wileyonlinelibrary.com/journal/chir
© 2017 Wiley Periodicals, Inc.
1

2
BOYD ET AL.
Following the earlier isolation of two non‐enantiopure
arene cis‐dihydrodiol metabolites B/B′ (X = Me, Y = Cl;
X = Me, Y = Br),4 further TDO‐catalyzed biotransforma-
tion studies of these and other 1,4‐disubstituted benzene
substrates were conducted24,25; the ee values of isolated
metabolites were determined by the formation of diMTPA
diastereomers of their stable cycloadducts and 1H‐
(or 19F‐)‐NMR analysis of the distinguishable OMe (or
SCHEME 1 Formation of cis‐dihydrodiols (B and B') from
substituted benzene substances (A)
CF3) signals of the diastereomers (CSRR and C'RRR
,
1 | MATERIALS AND METHODS
Scheme 2).21,22 Formation of diMTPA esters (CSRR and
C'RRR), from 1,4‐disubstituted benzene substrates A
(X ≠ Y), confirmed that most cis‐dihydrodiols were not
single enantiomers and that cis‐diols B/B′, metabolites
from P. putida UV4 (X = Me, Y = Cl, 15% ee and
X = Me, Y = Br, 37% ee), were not racemic.
1.1 | Methods for determining the
enantiopurities and absolute
configurations of arene cis‐dihydrodiols
With the marked increase in the numbers and uses of chi-
ral cis‐dihydrodiols, reliable methods were required for
the determination of both enantiomeric excess (ee) values
and absolute configurations (ACs). During early reports of
cis‐dihydrodiol formation from mono‐ and poly‐cyclic
arene substrates,3-10 stereochemical correlation methods,
e.g., catalytic hydrogenation and oxidative ring cleavage
reactions, were commonly used to assign ee values and
ACs. Due to the instability of most cis‐dihydrodiols, early
The partial hydrogenation of unstable cis‐dihydrodiol
metabolites of polycyclic arenes (naphthalene, phen-
anthrene) and azaarenes (quinoline and isoquinoline)
substrates, to yield the corresponding stable tetrahydro‐
cis‐diols, followed by diMTPA ester formation, using both
(R)‐ and (S)‐MTPA chloride, showed the metabolites to be
single enantiomers (>98% ee).26 A combination of
experimental, and Density Functional Theory (DFT)‐
calculated electronic circular dichroism (ECD) spectra,
was later applied successfully for the direct determination
of ACs of cis‐dihydrodiol metabolites derived from
monosubstituted, 1,2‐, 1,3‐, and 1,4‐ disubstituted
benzenes,23,27-29 bicyclic arenes,30 and azaarenes.31
efforts,
to
form
di[2‐methoxy‐2‐(trifluoromethyl)
phenylacetate] (diMTPA) diastereomers for ee determina-
tion20 were unsuccessful and X‐ray crystallographic anal-
ysis was rarely employed for AC assignments. Thus,
alternative methods for the determination of ee values
and ACs were developed.
Enantiomeric mixtures of cis‐dihydrodiols were avail-
able from biotransformations of substituted benzenes,
with TDO,24,25,32,33 chlorobenzene dioxygenase,34,35 and
from both mono‐ and poly‐cyclic arenes, using site‐
directed mutants of NDO.36 Application of chiral station-
ary phase (CSP) high‐performance liquid chromatography
(HPLC) methods (Chiralcel OJ and AD columns) sepa-
rated individual cis‐dihydrodiol enantiomers and also
gave ee values directly.24,36 Separation of the n‐
butylboronate derivatives of cis‐dihydrodiol enantiomers
was reported, using chiral gas chromatography (CGC)
with a cyclodextrin column. The availability of suitable
CSP HPLC (or CGC) columns thus provided a very useful
method for the determination of ee values of alkene and
arene cis‐dihydrodiol metabolites. Since arene cis‐
dihydrodiol metabolites are often found to be
enantiopure, and as the chiral columns are generally
available in only one enantiomeric configuration, the
observation of a single peak by CSP HPLC (or CGC) anal-
ysis could be due to the presence of a single enantiomer or
an unresolved mixture of enantiomers.
DiMTPA derivatives (CSRR and C'RRR) of stable
Diels‐Alder cycloadducts, obtained by cycloaddition of
cis‐dihydrodiols B (Y = H and X = F, Me, CF3) with
4‐phenyl‐1,2,4‐triazoline‐3,5‐dione, were often crystal-
line. X‐ray crystallographic analysis then provided an
unequivocal indirect assignment method for ACs and
ee values of several cis‐dihydrodiols B (Scheme 2).21,22
Later studies, using low‐temperature X‐ray crystallo-
graphic facilities, showed that ACs of unstable cis‐
dihydrodiols B, derived from monosubstituted benzene
A (X = F, Y = H; X = Br, Y = H),23 and 1,4‐disubstituted
benzene substrates A (X = CF3, Y = F; X = CN, Y = F;
24
X = CN, Y = Me), could be determined directly with
the anomalous dispersion method.
While X‐ray crystallography23,24 and ECD spectros-
copy23,27-31 have provided rigorous methods for AC
assignments of arene cis‐dihydrodiols, better methods for
determination of ee values, than reported earlier,21,22,26
SCHEME 2 Formation of diMTPA diastereomers (CSRR and
C'RRR) derived from 4‐phenyl‐1,2,4‐triazoline‐3,5‐dione adducts of
cis‐dihydrodiol enantiomers (B and B', X not H, Y = H)

BOYD ET AL.
3
were required. A major objective of our work was to syn-
thesize and utilize already available and new chiral
boronic acids for the determination of ee values and ACs
of a wide range of cis‐diol metabolites, by H–NMR anal-
ysis of the corresponding boronate derivatives.
method, Scheme 3).37 Recrystallization of the resulting
chiral alcohol 2aS (85% ee) yielded a single enantiomer
(>98% ee), which was methylated (MeI/NaH). Treatment
of methyl ether 3aS, with tert‐butyl lithium and
triisopropylborate, followed by hydrolysis yielded
enantiopure (+)‐(R)‐MEPBA 4aR.37
1
1H–NMR spectra were generally obtained using a
Bruker (Billerica, MA) Avance DRX‐500 instrument and
CDCl3 as solvent. X‐ray diffraction intensities were mea-
sured on a Siemens (Erlangen, Germany) P3 diffractome-
ter. All chiral vicinal diol and catechol metabolites used in
the study, isolated as single enantiomers in most cases
(>98% ee), were available from earlier dioxygenase
enzyme‐catalyzed dihydroxylations of the corresponding
alkene, arene, and heteroarene substrates with mutant
bacterial strains. Optical rotations and ACs of arene cis‐
diol metabolites were reported in earlier reviews14-16 and
later references in Tables 1–6. The synthesis of racemic
and enantiopure [2‐(1‐methoxyethyl)phenyl]boronic acid
(MEPBA) was conducted according to the literature pro-
cedures.37,38 Diastereomeric boronates were obtained by
reacting the cis‐diol with a slight excess of MEPBA
reagent in CDCl3 solution and the reaction was followed
to completion by 1H–NMR spectroscopy. Baseline resolu-
tions of OMe singlets (δOMe), were generally observed
when ΔδOMe values were >7 ppb for boronate diastereo-
mers formed, either using: 1) single enantiomer cis‐diols
and racemic MEPBA or 2) enantiomeric mixtures of cis‐
diols with (1R)‐ or (1S)‐MEPBA.
Asymmetric reduction of ketone 1a, catalyzed by a
carbonyl reductase (CRED) expressed in baker's yeast
cells was later used to produce (−) (S)‐alcohol 2aS
(>98% ee) as the first step in the synthesis of enantiopure
(−)‐(S)‐boronic acid 4aS (Scheme 3).38 This chemoenzymatic
route to alcohol 2aS, followed by a Mitsunobu inversion
step, yielded the (+)‐(R) enantiomer 2aR and finally
boronic acid (+)‐(R)‐MEPBA 4aR.38 During the study a
SelectAZyme screening kit of CREDs was also used with
substrate o‐bromoacetophenone 1a. In common with
bakers' yeast, most of the available CREDs favored
formation of (−)‐(S) alcohol 2aS with many giving ee
values of >90%. One member of the kit (CRED A161)
produced mainly the (+)‐(R) enantiomer 2aR (87% ee),
which can yield a single enantiomer on careful recrystal-
lization. Thus, while both boronic acids (−)‐(S)‐4aS and
(+)‐(R)‐4aR can be produced by a chemoenzymatic
method, the more readily available (−)‐(S) enantiomer
4aS was used preferentially along with ( )‐MEPBA
4aS /4aR in the study.
The first applications of (+)‐(R)‐boronic acid 4aR for
the determination of ee values were reported by Burgess
and Porte. It was applied to six acyclic 1,2‐diols, two
acyclic 1,3‐diols, and an acyclic β‐amino alcohol, all as
racemates.37 Resnick et al. utilized both (−)‐(S)‐4aS and
(+)‐(R)‐4aR boronic acid enantiomers to determine ee
values and ACs of cis‐dihydrodiol metabolites, e.g., B or
B′ (Y = H and X = F, Cl, I, Me, CF3, Ph), derived from
the corresponding monosubstituted benzenes, and several
polycyclic arene substrates.38-40 cis‐Dihydrodiol metabo-
lites B (Y = H and X = Cl, I, Me, CF3, Ph), previously
shown to be single enantiomers (>98% ee),21,22 were
reacted in situ with boronic acids 4aS and 4aR.38 The
resulting corresponding single boronate diastereomers,
2 | RESULTS AND DISCUSSION
The method used for the determination of enantiopurity
of arene cis‐dihydrodiols in the study entailed formation
of chiral boronate derivatives using ( )‐(MEPBA) 4aS/
4aR, and (−)‐(S)‐MEPBA 4aS or (+)‐(R)‐MEPBA 4aR,
1
followed by their H–NMR analysis (Scheme 3).
The initial step, reported in the literature for the syn-
thesis of MEPBA reagent 4aS involved an asymmetric
reduction of o‐bromoacetophenone 1a with diborane in
the presence of a chiral oxazaborolidine catalyst (CBS
1
DaS and DaR, each exhibiting distinguishable H–NMR
signals associated with the exocyclic chiral center
(Scheme 4). The methoxy group (OMe) appeared as a sin-
glet (δOMe), the other methyl group (CMe) as a doublet
(δMe), and the benzylic proton HBn (δBn) as a quartet.
1
Identification of δOMe (or δMe) signals in the H–NMR
spectrum of DaS diastereomer was possible by the reac-
tion of boronic acid enantiomer 4aS with metabolites B
(Y = H and X = Cl, I, Me, CF3, Ph). cis‐Dihydrodiol
metabolite B (Y = H and X = F) was found to be of lower
enantiopurity (80% ee),21,22 as shown by its reaction with
boronic acid 4aS, which yielded a mixture of boronate dia-
stereomers DaS and D'aS.
SCHEME 3 Formation of chiral boronic acid enantiomers 4S and
4R from ketone 1 (OMe changed to OH)

4
BOYD ET AL.
2.1 | MEPBA‐derived boronates of cis‐
dihydrodiols 5–32 formed from the
corresponding substituted benzene
substrates
1
The relevant diagnostic H–NMR signals (δOMe and δMe
)
from MEPBA derivatives of cis‐dihydrodiols, formed by
TDO‐catalyzed cis‐dihydroxylation of the corresponding
monosubstituted (5–23) and disubstituted benzene sub-
strates (24–32), are shown in Table 1.14,16,33,41-43
SCHEME 4 Formation of boronates DaS, DaR, D'aS and D'aR
from the reactions of cis‐dihydrodiol enantiomers B and B' with
(−) ‐(S)‐MEPBA 4aS and (+)‐(R)‐MEPBA 4aR
Only one MeO singlet (δOMe) was observed, for
boronate derivatives DaS (Y = H) of monocyclic arene
cis‐diol metabolites (5–31), formed using (−)‐(S)‐MEPBA
4aS (Scheme 4). With ( )‐MEPBA 4aR/4aS, two MeO sin-
glets (ΔδOMe) of equal peak area were obtained for
boronates DaR and DaS (Y = H), consistent with metabo-
lites (5–31) being single enantiomers (>98% ee). The pres-
ence of a doublet for Me (δMe) in the spectrum of boronate
DaS (Y = H), with (S)‐MEPBA 4aS and two separated dou-
blets (ΔδMe) in the spectrum of boronates DaR and DaS
(Y = H) with ( )‐MEPBA 4aR/4aS) was also employed
in some cases to establish enantiopurity. A wide variation
in the separation of OMe singlets (ΔδOMe ppb) was found
with boronates DaR and DaS (Y = H) formed with ( )‐
MEPBA 4aR/4aS (Diagram 1, Table 1).
The smallest ΔδOMe values for boronate derivatives of
cis‐dihydrodiols (ΔδOMe 0–3) were associated with vinyl
halides,38 e.g., metabolite 5 (ΔδOMe, 0). Those bearing
alkyl (15), substituted alkyl (9, 10, 16, and 17) and vinyl
substituents (11–14) showed slightly larger values (5‐18).
The largest values were associated with electron‐
withdrawing groups (6–8, ΔδOMe 11–27) and particularly
aryl or azaaryl groups (18–23, ΔδOMe 56–89). The ( )‐
MEPBA 4aR/4aS reagent was also effective in forming
boronates from cis‐dihydrodiol metabolites of ortho‐,
meta‐, and para‐disubstituted benzenes (24–30, 32, ΔδOMe
10–24). The 1,4‐disubstituted benzene cis‐dihydrodiol 32
was found to be a mixture of enantiomers (15% ee, using
MEPBA 4aS) and cis‐diol metabolites (6–31) were con-
firmed as single enantiomers (Table 1, >98% ee, using
MEPBA 4aS and ( )‐MEPBA 4aR/4aS).
The diastereomeric boronate method was also applied
to confirm ee values (>98%) for cis‐diol metabolites from
polycyclic
arenes
(naphthalene,
anthracene),38
heteroarenes (dibenzofuran, dibenzothiophene),39 and
dihydroarenes
(1,2‐dihydronaphthalene,
9,10‐
dihydroanthracene and 9,10‐dihydrophenanthrene).38,40
The relative trends observed, for 1H‐NMR directional
chemical shift values of the MeO (δOMe) and Me (δMe
)
groups of boronate diastereomers DaS and D'aS with
boronic acid (−)‐(S)‐4aS, or DaR and D'aR with boronic
acid (+)‐(R)‐4aR, were also used to assign ACs of cis‐
dihydrodiol metabolites B and B′.38-40 Thus, a consistent
upfield shift of δOMe signals of boronates, formed using
(−)‐(S)‐4aS, was observed with monocyclic arene cis‐
dihydrodiols B (Y = H and X = Me, CF3 and Ph), relative
to the δOMe value found using (+)‐(R)‐4aR. While
stereodifferentiation of cis‐diol enantiomers was shown
by the separation of MeO singlets (ΔδOMe) and Me dou-
blets (ΔδMe) of boronate diastereomers in CDCl3 solvent,
in some cases better signal separations were observed
using C6D6 solvent.38-40
Following the earlier reports of the successful applica-
tion of MEPBA reagent in the racemic (4aS/4aR) and
enantiopure (4aS or 4aR) forms, for the determination of
both ee values and ACs of cis‐dihydrodiol metabolites iso-
lated from six monocyclic and four polycyclic arenes,38-40
this valuable reagent has since been used extensively in
our laboratories. The new results presented herein
include: 1) the applicability of the MEPBA reagent for
determining the ee values and ACs of a range of other
arene cis‐diol metabolites (e.g., from carbocyclic and het-
erocyclic arenes and meta phenols) and of cis/trans‐diol
metabolites (from heterocyclic arenes), 2) the synthesis
of two new chiral boronic acids 2‐[1‐methoxy‐2,2‐
dimethylpropyl]phenyl boronic acid (MDPBA) and 2‐[1‐
methoxy‐1‐phenylmethyl]phenyl boronic acid (MPPBA)
and preliminary comparative results showing their great
potential for the stereochemical analysis of cis‐diol metab-
olites of arenes or alkenes and chiral catechol metabolites
derived from benzylic alcohols.
The consistently smaller upfield δOMe chemical shifts
and negative ΔδOMe values recorded earlier for boronate
derivatives of monocyclic cis‐dihydrodiols B (Y = H and
38
X = Me, CF3 and Ph), using MEPBA 4aS (relative to
δOMe values using MEPBA 4aR), were again observed
for boronates of cis‐dihydrodiols (6–32). As most of the
ACs of metabolites (6–32, Table 1) had already been
established by other methods, the advantage of this
approach for the determination of ACs of monocyclic
cis‐dihydrodiols by formation of boronates using
MEPBA 4aS and ( )‐MEPBA 4aR/4aS, is clearly
demonstrated.

BOYD ET AL.
5
TABLE 1 [α]D, AC and % ee values of cis‐diols (5‐31), δ and Δδ values of boronates
Lit.f
a
c
d
e
Cis‐ diol
[α]D
AC
δOMe b (−)‐(S)
ΔδOMe (ppb)
δMe (−)‐(S)
ΔδMe (ppb)
% ee
5
+41
1S,2S
3.257
0
1.441
1.435
1.410
1.415
+1
>98
14
6
+201
1S,2R
1S,2R
1S,2R
1S,2R
1S,2R
1S,2R
1S,2R
1S,2R
1S,2R
1S,2R
1S,2R
1S,2R
1S,2R
1S,2R
1S,2R
1S,2R
1S,2R
1S,2R
1S,2S
1S,2S
1S,2S
1S,2S
1S,2R
1S,2R
1S,2R
1S,2R
1R,2R
3.234
3.206
3.209
3.152
3.151
3.212
3.200
3.218
3.218
3.225
3.222
3.349
3.176
3.179
3.123
3.135
3.143
3.151
3.227
3.229
3.228
3.229
3.215
3.238
3.217
3.216
3.219
−11
−25
−27
−6
+74
+31
+24
>98
>98
>98
>98
>98
>98
>98
>98
>98
>98
>98
>98
>98
>98
>98
>98
>98
>98
>98
>98
>98
>98
>98
>98
>98
>98
15
14
14
14
41
41
14
14
14
14
16
14
41
42
42
14
42
42
42
33
33
33
33
14
43
43
43
43
7
+98
8
+59g
+69g
+73g
+115g
+128
+78
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
−7
−18
−10
−8
1.416
1.428
1.418
1.418
1.420
1.410
+55
+42
+26
+29
+21
+34
+87
−14
−11
−18
−5
+80
+70h
+33g
+144h
+140h
+252
+129h
+249h
+181h
+110
+103
+123
+139
+64
−56
−56
−89
−77
−70
−71
−12
−10
−12
−13
−24
−19
−23
−26
−16
1.421
1.415
1.562
1.393
1.400
1.418
+102
+111
+128
+136
+150
+149
1.420
1.426
1.421
1.421
1.431
+18
+37
+43
+38
+41
+28
+56
+60
+5
aMeOH;
bupfield δ value using 4aS;
cdifference in δMeO values using 4aS and 4aR;
ddownfield δ value 4aS;
edifference in δMe values of 4aS and 4aR;
fLiterature reference;
gCHCl3;
hTHF.
cyclohex‐2‐en‐1‐one cis‐diol tautomers ES (Scheme 5).41-44
Reacting MEPBA reagents 4aS and ( ) 4aR /4aS with
tautomers ES, the corresponding boronate diastereomers
FSS and FSR were formed. Their ΔδOMe values indicated
that cis‐diol metabolites (33–41) were enantiopure (>98%
ee, Table 2).41-44 Boronate derivatives of cis‐diol metabolites
having an aryl substituent (18–23, Table 1) resulted in
2.2 | MEPBA derivatives of cyclohex‐
2‐en‐1‐one cis‐diols 33–41 derived from the
corresponding meta‐phenols
TDO‐catalyzed cis‐dihydroxylation of meta phenols
resulted in the initial formation of the corresponding
monocyclic cis‐dihydrodiols, which preferred to exist as

6
BOYD ET AL.
The OMe signals (δOMe) recorded for MEPBA 4aS
derivatives of cis‐diols (6–32) were located upfield in all
cases (Table 1). Conversely, OMe signals for cis‐diols
(33–38 and 41) were found downfield, except for cis‐diols
(39 and 40), which had upfield δOMe signals
(Table 2).44-47 These results demonstrate the value of the
MEPBA reagents for ee determination and also their lim-
itations as a reliable method for the determination of ACs
of cyclohex‐2‐en‐1‐one cis‐diol metabolites of phenol
substrates.
DIAGRAM 1 Structures 5–32
2.3 | MEPBA derivatives of cis‐
dihydrodiols 42–67 derived from the
corresponding polycyclic arenes and
heteroarenes
Relevant MeO singlet values (δOMe) and their separations
(ΔδOMe), for boronate derivatives, of bi‐, tri‐, and tetra‐
cyclic arene cis‐diol metabolites (42–67) (Diagram 2),
formed using (S)‐MEPBA 4aS, and ( ) MEPBA 4aR/4aS,
are presented in Table 3.9,10,14,15,36,42,48-50 As the MeO sig-
nal separation values (ΔδOMe) were generally higher (30–
160) compared to those reported (0–89, Table 1), δMe and
ΔδMe values for Me doublets were not recorded. Polycyclic
cis‐dihydrodiol metabolites from carbocyclic arenes (42–
44, 55, 59, 61, 64) and heterocyclic arenes (45–54, 56–58,
60, 62, 63, 65–67) were single enantiomers (>98% ee),
based on ΔδOMe values of boronates (Table 3).
The enhanced effect of aromatic substituents on
ΔδOMe values of boronates, of monocyclic cis‐dihydrodiols
(18–23, ΔδOMe 56–71, Table 1), and a cyclohex‐2‐en‐1‐one
cis‐diol (39, ΔδOMe 136, Table 2) was also evident in
Table 3, where fused aromatic rings had a similar effect.
SCHEME 5 Boronates FSS and FSR from (S)‐ or (R)‐MEPBA and
cis‐dihydrodiols ES (33–41)
larger ΔδOMe values (56–89). A much larger value was
observed for the boronate of cis‐diol 39 (ΔδOMe 136),
which could be the result of the anisotropic effect of
the phenyl substituent and closer proximity of the OMe
group compared with the corresponding boronate of
metabolite 20 (ΔδOMe 89, Table 1). As the number of
cyclohex‐2‐en‐1‐one cis‐diols studied was relatively small,
other trends in the magnitude of ΔδOMe values were not
apparent.
TABLE 2 [α]D, AC and % ee values of cis‐diols (33‐41), δOMe and ΔδOMe values of boronates
a
c
d
Cis‐diol
[α]D
−52
AC
δOMeb (−)‐(S)
δOMe (+)‐(R)
ΔδOMe (ppb)
% ee
Lit.e
33
4S,5S
3.249
3.228
3.225
3.235
3.202
3.204
3.224
3.153
3.221
3.195
+21
+20
+26
+20
+9
>98
44
34
35
36
37
38
39
40
41
−45
−38
4S,5S
4S,5S
4R,5S
4R,5S
4R,5S
4R,5S
4R,5S
4S,5S
3.245
3.261
3.222
3.215
3.231
3.017
3.214
3.213
>98
>98
>98
>98
>98
>98
>98
>98
44
45,46
45, 46
47
−151
−79
−9
+7
47
−20
−65
−120
−136
−7
45
45
+18
46
aMeOH;
bOMe δ value using 4aS;
cOMe δ value using 4aR;
ddifference (ppb) in δOMe values using4aS and 4aR/4aS;
eLiterature reference.

BOYD ET AL.
7
DIAGRAM 2 Structures 42–67
TABLE 3 [α]D, AC and % ee values of cis‐diols (42–70) and ΔδOMe values of boronates
a
Cis‐diol
[α]D
Ab. Config.
δOMe b(−)‐(S)
δOMe c(+)‐(R)
ΔδOMe
−44
% ee
Lit.d
42
+236
1R,2S
3.164
3.208
>98%
14
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
+202
−188
+140
+80
1R,2S
1S,2R
5R,6S
5R,6S
5R,6S
5R,6S
8R,7S
1R,2S
5R,6S
5R,6S
8R,7S
8R,7S
4R,3S
10R,9S
10R,9S
10R,9S
1R,2S
7R,8S
4R,3S
4R,3S
10R,9S
1R,2S
11R,10S
11R,10S
1R,2S
3.174
3.139
3.095
3.170
3.102
3.174
3.117
3.115
3.19
3.212
3.207
3.129
3.212
3.132
3.203
3.157
3.179
3.25
−36
−68
−34
−42
−30
−34
−40
−64
−60
−60
−50
−32
−126
−130
−130
−110
−72
−50
−140
−140
−150
−70
−91
−160
−60
>98%
>98%
>98%
>98%
>98%
>98%
>98%
>98%
>98%
>98%
>98%
>98%
>98%
>98%
>98%
>98%
>98%
>98%
>98%
>98%
>98%
>98%
>98%
>98%
>98%
14
14
14
48
48
49
48
48
50
50
50
50
15
42
42
42
36
42
9
+220
+172
+148
+102
+214
−110
+138
+94
3.19
3.25
3.19
3.24
3.198
3.115
3.12
3.230
3.241
3.25
+35
+167
+82
3.09
3.22
−280
+vef
+46
3.08
3.19
3.148
3.16
3.220
3.21
+112
+114
+94
2.97
3.11
3.15
3.29
9
3.08
3.23
9
−222
+21
3.25
3.32
10
10
10
10
3.14
3.23
+42
3.01
3.17
+18
3.15
3.21
aMeOH;
bupfield δ value using 4aS;
cdownfield δ value using 4aR;
dLiterature reference;
eCHCl3;
fonly positive sign of rotation recorded.

8
BOYD ET AL.
Larger values (ΔδOMe 30–68) were similarly observed for
boronates of cis‐dihydrodiol metabolites of bicyclic (42–
49) and linear tricyclic arenes (50–54). The effect was fur-
ther magnified for boronates in which the cis‐diol bonds
were located within the bay regions of tricyclic arenes
(55–58, ΔδOMe 110–130) and tetracylic arenes (61–63,
ΔδOMe 140–150), compared to those in the non‐bay region
positions of tricyclic arenes (59 and 60, ΔδOMe 50–72). Rel-
atively large ΔδOMe values were also observed for
boronates of tetracyclic arene cis‐dihydrodiols located
within the fjord (64, ΔδOMe 70) or pseudo‐fjord regions
(65–67, ΔδOMe 60–160). In common with the results
shown in Table 1, a consistent upfield shift of the OMe
singlet (negative ΔδOMe) was also noted when using (−)‐
(S)‐MEPBA 4aS (Table 3). The results of boronates of
more than 50 cis‐dihydrodiol metabolites, presented in
Tables 1 and 3, demonstrate strong support to the earlier
proposal37; that AC assignments for mono‐ and poly‐
cyclic arene and heteroarene cis‐dihydrodiols can be
made, based on the direction of chemical shifts of the
boronate OMe signals (δOMe), formed using (−)‐(S)‐
MEPBA 4aS and ( )‐MEPBA 4aR/4aS reagents.
D2O) and cis isomers in less polar solvents (60–80% cis
in CDCl3).51-54 Reaction with (−)‐(S)‐MEPBA 4aS simpli-
fied the problem of stereochemical analysis, presented
by cis/trans isomerization of the heterocyclic diols, as
the corresponding boronates (70a–d and 73a–c) were only
formed from cis isomers.
Separation of the isomeric mixtures of diol metabolites
(68acis‐dcis/68atrans‐dtrans and 71ccis /71ctrans
) was
unsuccessful, due to their rapid epimerization; therefore,
individual [α]D values could not be recorded. Recrystalli-
zation of mixtures of diols (71acis /71atrans) and (71bcis
/
71btrans) from CHCl3 led to the isolation of enantiopure
samples of cis isomers (71acis and 71bcis), and single enan-
tiomers of trans isomers (71atrans and 71btrans) were crys-
tallized from MeOH. The maximum [α]D values of
isomers (71acis, 71bcis, 71atrans, and 71btrans) were
recorded immediately after dissolution in order to mini-
mize epimerization.
The enantiomeric mixtures of diols (68acis‐dcis and
71acis‐ccis), on reaction with (−)‐(S)‐MEPBA 4aS, formed
the major boronate diastereomers (70a–d and 73a–c).
1H–NMR analyses of the boronates showed the upfield
OMe signals between (δOMe 3.06–3.27) and provided a
range of ΔδOMe values (15–120); the ee values were found
in the range 43–63% (Table 4).53,54 As recorded earlier for
boronates of carbocyclic cis‐dihydrodiols, from polycyclic
arenes, larger ΔδOMe values were again observed for
metabolites (71acis‐ccis, ΔδOMe 90–120 Diagram 4) com-
pared with the much smaller ΔδOMe values from the
monocyclic cis diols (68acis‐dcis, ΔδOMe 15–25). The
upfield shift of δ values noted using MEPBA 4aS
(Table 5) is consistent with similar ACs for the major
enantiomer of heterocyclic cis‐diols (68acis‐dcis and
71acis‐ccis) and carbocyclic cis‐dihydrodiols (5–32 and
42–67) shown in Tables 1 and 3. The ee values of cis/trans
metabolites (68a–d and 71a–d), found following reaction
with MEPBA 4aS, were confirmed by the formation of cor-
responding diMTPA derivatives, and the ACs were
established in some cases by X‐ray crystallography.
2.4 | Application of MEPBA reagents for
the determination of ee values of chiral cis/
trans dihydrodiols 68a–d and 71a–d derived
from the corresponding heteroarenes
Stereochemical characterization of cis‐diol metabolites,
initially formed by TDO‐catalyzed dihydroxylation of
thiophene (68acis‐dcis, 71acis, and 71bcis) and furan
(71ccis) rings (Scheme 6), proved to be difficult, due to
their equilibration (epimerization) with the correspond-
ing trans isomers (68atrans‐dtrans and 71atrans‐ctrans) at
ambient temperature.51-54
The epimerization involved a spontaneous ring open-
ing process via acyclic aldehyde intermediates (69a–d
and 72a–c), with the equilibrium favoring trans isomers
in hydroxylic solvents (80–100% trans in CD3OD or
2.5 | Synthesis of the new chiral boronic
acids MDPBA and MPPBA
The advantages and limitations of (−)‐(S)‐MEPBA 4aS
and ( )‐MEPBA 4aR/4aS, as reagents for determination
of ee values and ACs of arene cis‐dihydrodiol (5–32 and
46–67), and cyclohex‐2‐en‐1‐one cis‐diol metabolites (33–
41), is clearly evident from the results (Tables 1–3). In
some cases, the use of MEPBA resulted in: 1) very low
ΔδOMe values precluding base‐line separation of diagnos-
tic OMe singlets in the 1H–NMR spectra of boronate
diastereomers, formed from monocyclic cis‐dihydrodiols,
e.g., B or B′ (Y = H and X = F, Cl, I; ΔδOMe 0–3),38
SCHEME 6 Boronates (70a–d and 73a–c) formed from the
reaction of (S)‐MEPBA 4aS with heteroarene cis/trans‐
dihydrodiols (68–d and 71a–c)

BOYD ET AL.
9
TABLE 4 [α]D, AC, % ee values and ΔδOMe values from boronates of diols (68acis‐dcis / 68atrans‐dtrans) and (71acis‐ccis / 71atrans‐ctrans
)
a
d
Cis‐diol
[α]D
‐83c
ACb,c
ΔδOMe
−20
% ee
Lit.e
68acis /68atrans
2S,3R (60)
2R,3R (40)
43
53
68bcis /68btrans
+19c
2S,3R (60)
2R,3R (40)
−20
−15
−25
−90
−90
−120
48
49
44
53
53
53
54
54
54
68ccis /68ctrans
‐55c
2S,3S (65)
2R,3S (35)
68dcis /68dtrans
‐36c
2S,3S (63)
2R,3S (37)
g
g
71acis /71atrans(80/20)c
71bcis /71btrans (78/22)c
71ccis /71ctrans
+117cisf‐97trans
+136cisf‐80trans
‐14c
2S,3R
2R,3R
63 → >98d
63 → >98e
2S,3R
2R,3R
>98d
>98d
2S,3R (60)
2R,3R (40)
55
aCDCl3;
bmajor isomer;
ccis/trans equilibrium mixture in CDCl3;
ddifference (ppb) in δOMe values using 4aS;
eLiterature reference;
fisomers 71acis and 71bcis recrystallized from CHCl3 (>98% ee);
gisomers 71atrans and 71btrans recrystallized from MeOH (>98% ee).
TABLE 5 [α]D, AC and % ee values of cis‐diols (5, 76–79) and ΔδMeO, ΔδMe, ΔδBn and Δδt‐Bu values of boronates (5'a‐c) and (76′‐79'a‐c) using
racemic MEPBA 4aR /4aS, MDPBA 4bR /4bS and MPPBA 4cR /4cS
Lit.b
a
Cis‐diol
[α]D
AC
ΔδOMe, ΔδMe 4aR /4aS
ΔδOMe, ΔδBn, Δδt‐Bu 4bR /4bS
ΔδOMe, ΔδBn 4cR /4cR
% ee
5
+41
1S,2S
0OMe, 1Me
29OMe, 30Bn, 12t–Bu
5OMe, 60Bn
>98
15, 21
76
77
78
79
+39
+21
+103
−60
1S,2R
1R,2S
1R,2S
1R
115OMe, 61Me
7OMe, 5Me
97OMe, 22Bn, 189t–Bu
3OMe, 5Bn, 5t–Bu
129OMe, 32Bn
6OMe, 76Bn
24OMe, 64Bn
7OMe, 56Bn
>98
59
43
43
43
>98
9
5
OMe, 3Me
OMe, 0Me
24OMe, 13Bn, 58t–Bu
8OMe, 2Bn, 4t–Bu
>98
88→ > 98
aCHCl3;
bLiterature reference.
1
and 2) signal multiplicity, and possible overlap of H–
NMR signals associated with the exocyclic chiral group
of boronates DaS or DaR, including methyl doublets
(Me) and benzylic quartets (Bn). To address these limita-
tions, the syntheses of two new racemic phenylboronic
acids, ( )‐MDPBA 4bS/4bR and ( )‐MPPBA 4cS /4cR,
earlier for MEPBA (Scheme 3).37,38 Ketones 1b, 1c and
racemic alcohols 2bR/2bS and 2cR/2cS were prepared
using literature methods. The alcohols were obtained in
good yield (91–95%) by reduction (NaBH4/MeOH) of the
corresponding ketones 1b and 1c (see Supporting Infor-
mation). The synthesis of enantiopure alcohols 2b and
2c via asymmetric reduction of ketones 1b and 1c and
their AC assignments had been reported.55,56
1
were carried out (Scheme 3). The H–NMR spectra of
their boronate derivatives would have several diagnostic
singlets, e.g., using ( )‐MDPBA 4bS/4bR (OMe, t‐Bu and
Bn) and ( )‐MPPBA 4cS /4cR (OMe and Bn). Thus, reduc-
ing the possibility of signal overlap and the probability of
increasing signal resolution, i.e., larger ΔδOMe, Δδt‐Bu
ΔδBn values.
Asymmetric reduction methods, reported earlier for
ketones (1a–c), were: 1) chemocatalysis with chiral
oxazaborolidines, to give alcohols 2aS or 2aR and 2bS or
2bR,37,55,57 2) chiral Ru‐BINAP/diamine complexes, to
,
56
form alcohols 2cS or 2cR and 3) biocatalysis using
The synthetic sequence used for the new boronic acids
MDPBA and MPPBA, was identical to that reported
CREDs, to give alcohol 2aS.38 Enantiopure (−)‐(1S,2R)‐
cis‐1‐amino‐2‐indanol 741S,2R and its (+)‐(1R,2S)

10
BOYD ET AL.
The monomeric structures of the ( )‐boronic acids
4bR/4bS and (+)‐4cR, shown in Figure 1, contained dihe-
dral angles of 26o and 60o, respectively, between the
disubstituted aryl rings and the attached planar B(OH)2
groups. Only intermolecular hydrogen bonding was
found in crystalline (+)‐boronic acid 4cR enantiomer,
the molecules being linked in infinite chains via hydro-
gen bonding engaging both hydroxyl groups. Surpris-
ingly, racemic 4bR/S showed a strong intramolecular
hydrogen bond between an OH group and a proximate
OMe group (B‐O‐H ‐ ‐ ‐ O‐Me), thus rendering the
OMe oxygen atom chiral in the solid state. There is also
intermolecular hydrogen bonding between OH groups,
and the molecules existing as hydrogen‐bonded dimers
in the solid state. While the OMe oxygen atom is pyrami-
dal, with respect to the attached covalently‐bonded and
hydrogen‐bonded atoms, the OH oxygen atoms in both
( )‐4bR /4bS and (+)‐4cR are trigonal planar. This
implies sp2 hybridization, with the filled vertical p
orbitals on oxygen interacting with the empty p orbital
on boron.
DIAGRAM 3 Structures 4aR−cR, 4aS−cS, 741R2S, 741S2R, 751R2S
751S2R
,
enantiomer 741R,2S, available commercially and by
chemoenzymatic synthesis,58 were reacted (BH3.SMe2/
THF), to form the corresponding oxazaborolidines
(751S,2R and 751R,2S, Diagram 3). These chiral hydride
transfer reagents were used in the asymmetric reduction
of a range of ketones, to give the corresponding alcohols
(79–95%
ee).58
This
literature
method
and
oxazaborolidines (751S,2R and 751R,2S, Diagram 3), were
applied to the asymmetric reduction of ketones 1b and
1c, to give alcohols 2b (77% yield, 94% ee) and 2c (84%
yield, 98% ee). Reduction of ketone 1a, under similar con-
ditions, yielded alcohol 2a (83% yield, 81% ee); Ru‐com-
plex with aminoalcohol 741S,2R (or 741R,2S) gave better
results (89% yield, 98% ee).
Methylation of racemic and enantioenriched alcohols
(2a–c, 94–98% ee), conducted according to the literature
method (NaH/MeI/THF),37,38 yielded the corresponding
methyl ethers 3a (95% yield), 3b (92% yield), and 3c
(98% yield). Transmetallation, followed by crystallization
from hexane,38,39 yielded both racemic and enantiopure
forms of the chiral boronic acids 4a (59% yield), 4b (64%
yield) and 4c (59% yield) (Scheme 3). Recrystallization
of the new boronic acids (4b and 4c), derived from the
alcohols 2b (94% ee) and 2c (98% ee), provided single
enantiomers, whose structures, ACs and ee values
(>98%) were confirmed by X‐ray crystallography.
Boronic acid monomers are known to dehydrate, to
form the corresponding cyclotrimeric anhydrides
(boroxines), and both readily interconvert in solution at
ambient temperature. Although the racemic boronic
acid 4bR /4bS was found to be a monomer in the crystal-
line state, during the removal of solvent (CH2Cl2) under
vacuum, the (+)‐(R) enantiomer 4bR crystallized out
preferentially as boroxine 4b'R; the molecule lies on a
crystallographic three‐fold axis of symmetry. The
boroxine ring, being isoelectronic with benzene, is pla-
nar with the aryl substituents adopting a propeller con-
formation and an interplanar angle of 35.6o between
the boroxine and aryl rings. The AC and ee value
(>98%) of (+)‐boronic acid monomer 4bR was
established from the corresponding boroxine X‐ray crys-
tal structure 4b'R (Figure 1).
A solution of the
enantiopure crystalline (+)‐boroxine 4b'R was found to
react spontaneously with cis‐diols in a similar manner
to ( )‐monomer 4bR /4bS.
FIGURE 1 X‐ray crystal structures of ( )‐boronic acids 4bR/4bS, (+)‐4cR and (+)‐boroxine 4b'R

BOYD ET AL.
11
DIAGRAM 4 Structures 5, 76−79, 5′a
−c,76′a−c, 77′a−c, 78′a−c, 79′a−c
TABLE 6 [α]D, AC and % ee values of catechols (82a–e) and
ΔδOMe values of boronates (83a–e and 84a–e)
2.6 | Determination of % ee values of cis‐
diols (5, 76–79) using MEPBA 4aR /4aS,
MDPBA 4bR /4bS, and MPPBA 4cR /4cS
a
Catechol
82a
[α]D
AC
ΔδOMeb(−)‐(S)
% ee
Lit.c
+18c
R
20
>98
61
61
61
63
63
The cyclic cis‐diols metabolites (5, 76–78) were isolated as
single enantiomers (>98% ee), but an enantiopure sample
of acyclic diol metabolite 79 was only obtained by recrys-
tallization of the enantioenriched metabolite (88% ee).
1H–NMR separations of diagnostic signals from MeO
(ΔδOMe), benzylic H (ΔδBn) and tert‐butyl (Δδt‐Bu) groups
of boronate diastereomers formed from vicinal cis‐diol
82b
82c
82d
82e
+20c
+16c
+23
+13
R
R
R
S
30
>98
>98
>98
>98
30
120
90
aCDCl3;
bdifference in δOMe values of boronates (83a–e and 84a–e) formed using
MEPBA 4aS;
cLiterature reference.
metabolites (5, 76–79, Diagram 4), using MEPBA 4aR
/
4aS, MDPBA 4bR /4bS, and MPPBA 4cR /4cR are pre-
sented in Table 5.15,21,43,59
A comparison of ΔδMeO values of boronates of cis‐diol
76, having a fused aromatic ring, formed with ( )‐boronic
acids 4aR /4aS, 4bR /4bS, and 4cR /4cR, showed that they
were consistently larger (97–129) compared to cis‐diols 5
(0–29), 77 (3‐7), 78 (9‐24), and 79 (5‐8). The ΔδMe values
for boronates found, using these cis‐diols and boronic acid
4aR /4aS, were smaller than their ΔδOMe values. Boronates
formed using MDPBA 4bR /4bS and diols (5, 76–79)
showed a wide range of values, e.g., Δδt‐Bu (4–189) and
ΔδBn (2‐30), while boronates of MPPBA 4cR /4cR, that
recorded larger ΔδBn values (32–76), proved to be the most
consistently reliable chiral boronic acid.
dehydrogenase enzyme, which catalyzed their biotrans-
formations, to yield individual (+)‐ and (−)‐catechol
enantiomers (82a–e).61-63 As separations (ΔδOMe) of diag-
1
nostic H–NMR signals of the boronates, formed using
MEPBA 4aS with (+) and (−)‐catechols (82a–e), were rel-
atively small (δOMe 3.15–3.28), it was considered appropri-
ate to test the new boronic acid (+)‐MPPBA 4cR
(Table 6).61,63 The magnitude of the observed values
(ΔδOMe 20–120), for boronates from enantiomers of cate-
chols (82a–e) with (+)‐MPPBA 4cR, confirmed that the
reagent can be successfully applied, to determine the ee
values of chiral catechol metabolites derived from ben-
zylic alcohols, and possibly also to catechols derived from
alkylaryl sulfoxides.54
2.7 | Application of (+)‐MPPBA reagent
4cR for the determination of ee values of
chiral catechols
The TDO‐catalyzed cis‐dihydroxylation of substituted
benzyl alcohol enantiomers yielded triol diastereomers
(80a–e and 81a–e, Diagram 5).60 These triols, in turn,
proved to be good substrates for a naphthalene diol
3 | CONCLUSION
The versatility of racemic MEPBA 4aR /4aS and
enantiopure MEPBA (4aR and 4aS) for the determination
of ee values was demonstrated by the formation of
boronate diastereoisomers of chiral cis‐diol metabolites
of alkenes, mono‐ and poly‐cyclic arenes, and
heteroarenes, phenols, and catechol metabolites. The
MEPBA reagents (4aR /4aS, 4aR, and 4aS) were also
utilized in stereochemical analysis of 1) cis β‐amino
alcohols,64 synthesized in five steps from polycyclic cis‐
dihydrodiol metabolites, 2) 1,3‐diols,37 and 3) 2‐
hydroxyacids.37 Preliminary results on the synthesis and
applications of racemic and enantiopure forms of the
new chiral boronic acids MDPBA 4b and MPPBA 4c
DIAGRAM 5 Structures 80a–e, 81a–e, (+)–82a–e, (−)–82a–e,
83a–e, 84a–e

12
BOYD ET AL.
5. Akhtar MN, Boyd DR, Thompson NJ, et al. Absolute stereo-
chemistry of the dihydroanthracene‐cis‐ and ‐trans‐1,2‐diols
produced from anthracene by mammals and bacteria. J Chem
Soc Perkin Trans l. 1975;2506‐2511.
suggest that they may offer significant advantages over
the MEPBA reagents 4a. Although the boronates of chiral
boronic acids (4b and 4c) in this study were only used to
determine ee values of catechols and cis‐diols of alkenes
and arenes, they should equally be applicable to cis β‐
amino alcohols,1,3‐diols, and 2‐hydroxyacids.
6. Gibson DT, Mahadevan V, Jerina DM, Yagi H, Yeh HJ. Oxidation
of the carcinogens benzo[a]pyrene and benzo[a]anthracene to
dihydrodiols by a bacterium. Science. 1975;189:295‐297.
The consistent upfield shift (CDCl3) in the direction of
OMe signals (δOMe), associated with (−)‐(S)‐MEPBA
boronate derivatives, originally observed for a relatively
small number of arene cis‐dihydrodiols,38-40 was also
recorded in the large number of examples shown in
Tables 1, 3, and 5. These examples lend further support to
the boronate method for the assignment of absolute config-
urations to this type of metabolite. It should be noted, how-
ever, that an inconsistent trend in the OMe signal direction
of shift (δOMe) was observed with boronate derivatives of
cyclohex‐2‐en‐1‐one cis‐diol metabolites with (−)‐MEPBA
(Table 2). Thus, where possible, more rigorous alternative
methods, e.g., X‐ray crystallography and ECD spectros-
copy, should also be used for AC determination.
7. Koreeda M, Akhtar MN, Boyd DR, Neill JD, Gibson DT,
Jerina DM. Absolute stereochemistry of cis‐1,2‐, trans‐1,2‐ and
cis‐3,4‐dihydrodiol metabolites of phenanthrene. J Org Chem.
1978;43:1023‐1027.
8. Jerina DM, van Bladeren PJ, Yagi H, et al. Synthesis and abso-
lute configuration of the bacterial cis‐1,2‐, cis‐8,9‐, and cis‐
10,11‐dihydrodiol metabolites of benz[a]anthracene formed by
a strain of Beijerinckia. J Org Chem. 1984;49:3621‐3628.
9. Boyd DR, Sharma ND, Hempenstall F, et al. bis‐cis‐Dihydrodiols:
A new class of metabolites resulting from biphenyl dioxygenase‐
catalyzed sequential asymmetric cis‐dihydroxylation of polycy-
clic arenes and heteroarenes. J Org Chem. 1999;64:4005‐4011.
10. Boyd DR, Sharma ND, Harrison JS, Kennedy MA, Allen CCR,
Gibson DT. Regio‐ and stereo‐selective dioxygenase‐catalyzed
cis‐dihydroxylation of fjord‐region polycyclic arenes. J Chem
Soc Perkin Trans l. 2001;1264‐1269.
ACKNOWLEDGMENTS
11. Ballard DGH, Courtis A, Shirley IM, Taylor SC. A biotech route
to polyphenylene. J Chem Soc Chem Commun. 1983;954‐955.
We thank Dr. C‐T Yeung (Hong Kong Polytechnic
University) for providing helpful chiroptical information
about compound 2bR and Prof. T. Moody (Almac Sciences
Ltd) for preliminary results obtained using SelectAZyme
CREDs. Funding was provided for CAST Awards by the
Department of Education, Northern Ireland (GMMcC,
PAG) and for a Postgraduate Studentship by the Euro-
pean Social Fund (JSH).
12. Howard PW, Stephenson GR, Taylor SC. Convenient access to
homochiral tricarbonyl complexes. J Chem Soc Chem Commun.
1988;1603‐1604.
13. Ley SV, Sternfeld F. Microbial oxidation in synthesis: Prep-
aration of (+)‐ and (−)‐pinitol from benzene. Tetrahedron.
1989;45:3463‐3476.
14. Boyd DR, Sheldrake GN. The dioxygenase‐catalyzed formation
of vicinal cis‐diols. Nat Prod Rep. 1998;15:309‐325.
15. Hudlicky T, Gonzalez D, Gibson DT. Enzymatic dihydroxylation
of aromatics in enantioselective synthesis: Expanding asymmet-
ric methodology. Aldrichimica Acta. 1999;32:35‐63.
ORCID
Paul J. Stevenson http://orcid.org/0000-0002-1211-983X
16. Johnson RA. Microbial arene oxidations. Org React. 2004;63:
117‐264.
17. Hudlicky T, Reed JW. Applications of biotransformations and
biocatalysis to complexity generation in organic synthesis. Chem
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SUPPORTING INFORMATION
atropselective resolution involving supramolecular interactions.
Additional Supporting Information may be found online
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How to cite this article: Boyd DR, Sharma ND,
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