Chemoenzymatic synthesis of monocyclic arene oxides and arene hydrates from substituted benzene substrates

Article abstract of DOI:10.1039/c3ob40166a

Enantiopure cis-dihydrodiol bacterial metabolites of substituted benzene substrates were used as precursors, in a chemoenzymatic synthesis of the corresponding benzene oxides and of a substituted oxepine, via dihydrobenzene oxide intermediates. A rapid total racemization of the substituted benzene 2,3-oxides was found to have occurred, via their oxepine valence tautomers, in accord with predictions and theoretical calculations. Reduction of a substituted arene oxide to yield a racemic arene hydrate was observed. Arene hydrates have also been synthesised, in enantiopure form, from the corresponding dihydroarene oxide or trans-bromoacetate precursors. Biotransformation of one arene hydrate enantiomer resulted in a toluene-dioxygenase catalysed cis-dihydroxylation to yield a benzene cis-triol metabolite. The Royal Society of Chemistry 2013.

Full text of DOI:10.1039/c3ob40166a

Organic &
Biomolecular Chemistry
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Chemoenzymatic synthesis of monocyclic arene oxides
and arene hydrates from substituted benzene
substrates†
Cite this: Org. Biomol. Chem., 2013, 11,
3020
Derek R. Boyd,*a Narain D. Sharma,a Vera Ljubez,a Peter K. M. McGeehin,a
Paul J. Stevenson,*a Marine Blaina and Christopher C. R. Allenb
Enantiopure cis-dihydrodiol bacterial metabolites of substituted benzene substrates were used as precur-
sors, in a chemoenzymatic synthesis of the corresponding benzene oxides and of a substituted oxepine,
via dihydrobenzene oxide intermediates. A rapid total racemization of the substituted benzene 2,3-
oxides was found to have occurred, via their oxepine valence tautomers, in accord with predictions and
theoretical calculations. Reduction of a substituted arene oxide to yield a racemic arene hydrate was
observed. Arene hydrates have also been synthesised, in enantiopure form, from the corresponding dihy-
droarene oxide or trans-bromoacetate precursors. Biotransformation of one arene hydrate enantiomer
resulted in a toluene-dioxygenase catalysed cis-dihydroxylation to yield a benzene cis-triol metabolite.
Received 25th January 2013,
Accepted 13th March 2013
DOI: 10.1039/c3ob40166a
www.rsc.org/obc
Introduction
The biotransformation of substituted monocyclic arenes in the
environment can follow different hydroxylation pathways,
according to the type of organism and arene involved.1 For-
mation of phenols and subsequent ring opening of derived
catechols and hydroquinones to yield carboxylic acids are
among the most common early steps followed during minerali-
zation of aromatic substrates. In prokaryotic (e.g. bacterial)
cells, dioxygenase-catalysed dihydroxylation generally yields
2,3-cis-dihydrodiols from monosubstituted benzene substrates,
and is often the initial biodegradation step (Scheme 1).2a–n
While cis-dihydrodiols are intermediates in wild-type strains,
they can be intercepted and isolated using mutant strains, e.g.
Pseudomonas putida UV4, or recombinant strains, e.g. Escheri-
chia coli CL-4t. To date, a wide range of cis-dihydrodiols has
been isolated from toluene dioxygenase (TDO)-catalysed cis
dihydroxylation, mainly at the 2,3-bond, e.g. 1a–h but
occasionally at the 3,4-bond, e.g. 2b, to yield regioisomers but
as very minor metabolites (Fig. 1).2a–n An increasing number of
2,3-cis-dihydrodiols 1 are being used as synthetic precursors
due to their multifunctional nature and chirality (the majority
>98% ee).2a–n cis-Dihydrodiols can however dehydrate either
Scheme 1
spontaneously in vivo, or also under both acidic3a–c and basic
conditions3d to yield phenols.
The metabolism of monocyclic arenes in eukaryotic (e.g.
animal or fungal) cells can also yield phenols, catechols and
ring-opened products, but generally involves monooxygenase
enzymes.1 The migration and retention of a hydrogen atom at
a neighbouring carbon atom, during enzyme-catalysed mono-
hydroxylation of monocyclic arenes, the NIH shift,4a–c provided
the first indirect evidence of possible monooxygenase-cata-
lysed epoxidation of arenes occurring to yield unstable arene
oxide intermediates. It is now established that both aromatic
monohydroxylation and the NIH shift can occur with or
without arene oxide involvement.5a–e However, a number of
relatively unstable monocyclic arene oxides, including 3a in
dynamic equilibrium with the corresponding oxepine valence
tautomer 3′a,6a have been detected as mammalian metabolites
(Fig. 1). Other examples of observed monocyclic arene oxide
and oxepine metabolites include: (i) flavanone 2,3-oxide
aSchool of Chemistry and Chemical Engineering, Queen’s University, Belfast BT9
5AG, UK. E-mail: dr.boyd@qub.ac.uk; Fax: (+44) (0)28-9097 4687;
Tel: (+44) (0)28 9097 4421
bSchool of Biological Sciences, Queen’s University, Belfast BT9 5AG, UK
†Electronic supplementary information (ESI)
available. See DOI:
10.1039/c3ob40166a
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Fig. 1 Structures of arene cis- and trans- dihydrodiols, arene oxides, oxepines and arene hydrates.
diastereoisomers 3i from flavanone, via an unprecedented bac- aromatization of arene hydrate 8a (dehydration), arene oxide
terial dioxygenase-catalysed epoxidation,6b and (ii) the 3,4- 3a (isomerization) and arene cis-dihydrodiol 1a (dehydration)
arene oxides 4c and 4d preferring to exist as the oxepine was found to decrease in the sequence 8a (190) > 3a (32) > 2a
valence tautomers, 4′c and 4′d, following monooxygenase-cata- (0.1).
lysed epoxidation of chlorobenzene6c and bromobenzene6d in
Bacterial metabolism of acetophenone, using P. putida UV4
mammalian cells (Fig. 1). Some substituted 1,2-arene oxides, cells, was reported to yield the unstable arene hydrate 8h
e.g., 5f from methyl (2-trifluoromethyl) benzoate, and oxe- which was intercepted and identified only as a stable iron tri-
pines, e.g. 5′a from methylbenzoate, resulting from monooxy- carbonyl complex (Fig. 1).11a To date no further example of a
genase-catalysed epoxidation in fungi proved to be remarkably similar type of monosubstituted benzene hydrate has been iso-
stable (Fig. 1).6e
lated from biotransformation of this or other monocyclic
Further indirect evidence for the intermediacy of arene arenes using P. putida UV4. However, phenol hydrate 9, stabi-
oxides, was obtained by formation of 2,3- and 3,4-trans-dihy- lized as the preferred keto tautomer 9′, was recently isolated as
drodiols, e.g. compounds 6c and 7c from metabolism of a metabolite of 3-iodophenol using P. putida UV4 (Fig. 1).11b
chlorobenzene following epoxide hydrolase-catalysed ring Preliminary LC-MS studies (unpublished results) have detected
opening of the corresponding 2,3- and 3,4-arene oxide metab- further examples of stable keto tautomers of arene hydrates of
olites 3c and 4c.6c While further examples of arene oxide similar structure to metabolite 9′, from other substituted
metabolites from polycyclic arenes and azaarenes, e.g. phenol substrates. While the isolation as keto tautomers of
naphthalene4b and quinoline,5b have been reported, the total phenol hydrates from phenol biotransformations, could, in
number of isolated arene oxide metabolites is still relatively principle, involve enzyme-catalysed addition of water to a sub-
small. Unlike the arene cis-dihydrodiols, with few excep- stituted benzene ring, it is probable that a multistep redox
tions,7a,b the trans isomers have not yet been made available in sequence is involved; this metabolic pathway is currently
sufficient quantity for synthetic applications.
under investigation.
The addition of water to alkene CvC bonds is a relatively
The formation of polycyclic arene hydrates, as metabolites
common reaction in metabolism and can be catalysed by of dihydroarenes (e.g. 1,2-dihydronaphthalene) via dioxygen-
hydrolase (hydratase) enzymes, e.g. aconitase, fumarase and ase-catalysed monohydroxylation, was discovered using
crotonase.8 In principle, hydration of arene CvC bonds could P. putida UV4.12a Biotransformations of 1,2-dihydronaphtha-
also occur, but in practice very few examples of arene hydrates lene,12b 1,2-dihydroanthracene,12b 5,6-12c and 7,8-dihydroqui-
have been reported as arene metabolites. The first reports of noline,12c and 9,10-dihydrobenzo[e]pyrene,12d to yield the
arene hydrate intermediate involvement in the metabolism of corresponding arene hydrates have also been reported using
arenes by animal cells appeared in the literature many years mammalian or fungal cells and purified enzymes. In each case
ago.9a–g It was postulated that the formation of metabolites this involved either mono- or di-oxygenase-catalysed benzylic
from naphthalene,9a–e anthracene,9c,f phenanthrene9c and qui- monohydroxylation to yield these more stable polycyclic arene
noline9g was consistent with the corresponding arene hydrate hydrates.12a–d
intermediates being involved. Despite these early proposals,
Although benzene oxides 3a,13a 3c6c and 4c6c and benzene
few reports of arenes being biotransformed into the corres- hydrate 8a13b (Fig. 1, Scheme 2) had been synthesised earlier
ponding arene hydrates have appeared over the intervening from achiral precursors, no generally applicable routes to
period. This may be due to the absence of arene hydratase arene oxides or arene hydrates are available. Our preliminary
enzyme activity on arene substrates and/or due to the instabi- communication13c reported how the enantiopure cis-dihydro-
lity of arene hydrate metabolites. Comparative aromatization diol metabolite 1d of bromobenzene could be used as a pre-
studies of the parent monocyclic arene hydrate 8a with the cor- cursor of the corresponding 2,3-arene oxide 3d (Scheme 2) and
responding arene oxide-oxepine 3a and cis-dihydrodiol 2a have trans-dihydrodiol 6d (Fig. 1). Chemoenzymatic methods
been reported.10a–d Thus, the relative rates (k) of acid-catalysed have since been developed13d,e for the synthesis of the
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Scheme 2
corresponding trans-dihydrodiols, from the available cis-dihy- during attempted purification. trans-Bromoacetate 11f did not
drodiol isomers 1b–f.
yield epoxide 12f when treated with NaOMe but surprisingly
gave an arene hydrate (see later).
The current study demonstrates that:
(i) A relatively short chemoenzymatic route can be applied
Allylic bromination of tetrahydroepoxides 12b–d gave rela-
to the synthesis of four arene 2,3-oxides and to an oxepine, tively unstable diastereoisomeric mixtures of the correspond-
using cis-dihydrodiol metabolites derived from the correspond- ing allylic bromoepoxides 13b–d (Scheme 2). As these
ing halogenated benzene substrates.
bromoepoxides were found to decompose during attempted
(ii) Arene 2,3-oxides formed from monosubstituted benzene purification, they were dehydrobrominated directly, using
substrates can spontaneously racemize and can be used as pre- NaOMe in Et2O, to yield the corresponding arene oxides 3b–d
cursors of racemic arene hydrates.
(iii) cis-Dihydrodiol regioisomers can be utilized in the syn- the more unstable tetrahydroepoxide 12e as a synthetic precur-
thesis of enantiopure arene hydrates. sor to arene oxide 3e was unsuccessful, therefore an alternative
(iv) P. putida UV4 can metabolize a benzene hydrate to yield approach was adopted. Allylic bromination of trans-bromoace-
(ca. 72% yield from epoxides 12b–d, Scheme 2). Application of
the corresponding tetrahydrobenzene cis-triol.
tate 11e yielded a mixture of dibromoacetate isomers 14e.
Treatment of the dibromoacetate mixture 14e, with NaOMe in
Et2O, gave the required arene oxide 3e by a combination of
dehydrobromination and cyclization reactions (Scheme 2).
As expected, the arene oxides 3b–e were rather unstable
oils, and in the presence of traces of acid, or at elevated temp-
eratures, yielded the corresponding phenols. Possibly as a
result of their homoaromaticity,10c,15 the arene oxides were
sufficiently stable to allow purification by distillation under
vacuum at low temperatures,6c but we preferred to use careful
rapid PLC under slightly basic conditions. Of the four halo-
genated arene oxides 3b–e described, only epoxide 3c had
been synthesised earlier.6c However, this literature route gave a
mixture of 2,3-arene oxide 3c and the preferred oxepine tauto-
mer 4′c of the corresponding 3,4-oxide (Fig. 1) which were very
difficult to separate due to their limited stability.6c While com-
pounds 3d and 4′d have been identified as mammalian meta-
bolites of bromobenzene,16a–c neither have previously been
chemically synthesised. The current synthetic approach to
arene oxides 3b–e (Scheme 2), has advantages over earlier
methods in terms of product purity and general applicability.
While arene oxides 3 derived from substituted benzenes
can, in principle, undergo spontaneous valence tautomeriza-
tion with the corresponding oxepines e.g. 3a/3′a (Fig. 1), 2,3-
arene oxides 3b–e were found to be the preferred tautomers at
Results and discussion
Earlier reports of the partial hydrogenation of benzene cis-2,3-
dihydrodiols 1b–f had used both Rh–Al2O3 13e and Rh–graphite
catalysts14a to yield the corresponding cis-tetrahydrodiols 10b–f
(Scheme 2). Use of Pd–C catalyst with cis-diols 1a, 1b and 1f
resulted in disproportionation to give equimolar quantities of
cis-tetrahydrodiols 10a, 10b, 10f and the corresponding
catechols.14b
An earlier study of syn-tetrols derived from the cis-dihydro-
diols 1b–e showed that treatment with acetoxyisobutyl
bromide in MeCN gave the corresponding bis trans-bromoace-
tates in good yield (>80%) as precursors of bis-epoxides.13d
Similar treatment of the stable cis-tetrahydrodiol derivatives
10b–f with acetoxyisobutyl bromide gave the corresponding
trans-bromoacetates 11b–f in similar yields (90–96%,
Scheme 2).
When the bromoacetates 11b–d were stirred with NaOMe in
Et2O at ambient temperature, the corresponding benzene
tetrahydroepoxides 12b–d were obtained as oils (93–95% yield,
Scheme 2). Problems were, however, encountered during
attempted formation of the tetrahydroepoxides 12e and 12f.
Epoxide 12e, synthesized from the corresponding trans-bro-
moacetate 11e, was found to be very unstable and decomposed
1
room temperature. This conclusion was derived from H-NMR
spectroscopic analysis and was in accord with predictions
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based on MINDO/3 and ab initio calculations.17a–c Based on
NMR spectroscopic evidence alone, it might be concluded that
compounds 3b–e were present exclusively as the arene oxide
tautomer. However, the lack of any optical rotation or other chir-
optical activity for arene oxides 3b–e, derived from enantiopure
(1c–e) or enantioenriched (1b, ca. 70% ee) cis-dihydrodiol pre-
cursors, is consistent with rapid arene oxide-oxepine valence
tautomerization at ambient temperatures via an undetected pro-
portion of oxepine. On this basis, it was concluded that none of
the substituted benzene oxides 3b–e synthesised in this study
were configurationally stable, i.e. having a barrier to racemiza-
tion (ΔG‡) of >23 kcal mol−1. This observation is in accord with
earlier experimental results from a dynamic NMR spectroscopic
study of a typical monosubstituted benzene 2,3-oxide,17d where
the barrier was found to be ΔG‡ = 7.6 kcal mol−1
.
The CF3 substituted methylbenzoate 1,2-oxide 5f (Fig. 1),
obtained by enzymatic epoxidation of the methylbenzoate sub-
strate in whole cells of the wood rotting fungus Phellinus ribis,
was found to be enantiomerically enriched.6e
Scheme 3
identified products were the corresponding arene hydrate
The initial optical rotation of the chiral arene oxide 5f ([αD] enantiomers 15c and 15d, which were obtained as oils
−14, CHCl3), was found to decrease slowly on storage at 4 °C,6e (50–70% yield). Purification of the unstable arene hydrates 15c
indicative of a racemization barrier (ΔG‡) of >23 kcal mol−1. A and 15d was achieved by careful rapid PLC (30% Et2O : hexane,
range of configurational stability is now evident between trace of Et3N). The steric effects of halogen substituents in
monocyclic arene 2,3-oxides, e.g. 3b–e, and arene 1,2-oxides epoxides 12c and 12d could account for the absence of the cor-
from methylbenzoates e.g. 5f. This trend is similar to that responding substitution products. Similar treatment of epox-
found earlier among arene oxide derivatives of polycyclic aro- ides 12e and 12f with MeLi gave an inseparable mixture of
matic hydrocarbons (PAHs), where barriers to racemization of unidentified products. Reaction of trans-bromoacetate 11f with
polycyclic arene oxides also varied over a wide range.5c Arene NaOMe yielded arene hydrate 15f, without the corresponding
oxides from PAHs, were also found to exist both as configura- epoxide intermediate 12f being detected, but only in relatively
tionally stable or unstable enantiomers, racemizing via their low yield after PLC purification (15%). DIBAL reduction of the
undetected oxepine tautomers, according to the type of arene bromoacetate 11f gave bromohydrin 17f whose reaction with
ring system and position of the epoxide ring. This type of be- NaOMe also produced arene hydrate 15f in slightly better yield
haviour had been predicted using PMO and ab initio calcula- (27%).
tions17a–d and has now been experimentally verified for arene
oxides of both monocyclic and PAH ring systems.5c
Schemes 2 and 3 illustrate how 2,3-cis-dihydrodiols 1b–e,
from the corresponding monosubstituted benzene substrates,
Vogel and Gunther reported that the reduction of benzene can be utilized through an improved route for the synthesis of
oxide 3a using LiAlH4 yielded the parent benzene hydrate 15a arene-2,3-oxides 3b–e and arene hydrates 15c, 15d and 15f.
as a racemate (Scheme 3).13a When the substituted benzene
3,4-cis-Dihydrodiols of fluorobenzene,18a biphenyl,18b 2-phenyl-
oxide 3c was treated under similar reducing conditions, pyridine18c and flavanone18d are among the few to have been
racemic hydrate 15c was obtained as the major product, pre- isolated as minor metabolites from dioxygenase-catalysed cis-
sumably via an SN2′ type mechanism. This route suffers from dihydroxylation. The general unavailability of 3,4-cis-dihydro-
several disadvantages including limited stability of benzene diol metabolites, has restricted their application in chemo-
2,3-oxides and their facile racemization leading to racemic enzymatic synthesis. To circumvent this difficulty a method
arene hydrates. An alternative route to racemic arene hydrate has been developed for the chemoenzymatic synthesis of 3,4-
15a, using the more stable tetrahydroepoxide 12a as precursor, cis-dihydrodiols from the available 2,3-cis-dihydrodiol metab-
has also been reported.13b
olites derived from both mono- and di-substituted benzene
When MeLi was added to a solution of epoxide 12a, it was substrates.18a The use of 2-substituted iodobenzenes e.g. 21, as
reported that both rearrangement to form benzene hydrate 15a substrates for P. putida UV4, to yield the corresponding cis-
(63%), and an SN2 ring-opening reaction, to yield the trans dihydrodiol metabolite 22 (Scheme 4), and its selective hydro-
methyl cyclohexenol 16a (37%), had occurred (Scheme 3).13b
genolysis has provided an indirect approach to the chemoenzy-
It was anticipated that a similar use of the substituted tetra- matic synthesis of a series of 3,4-cis-dihydrodiols. Thus the
hydroepoxides 12c or 12d, would yield arene hydrates 15c and very minor 3,4-cis-dihydrodiol metabolite 2b (Fig. 1) from
15d as mixtures with the corresponding addition products 16c fluorobenzene and other substituted benzene 3,4-cis-dihydro-
and 16d requiring potentially difficult separations.
diols were readily available by this method.18a
However, when the reaction with MeLi was repeated, using
Partial hydrogenation (H2, Rh/Al2O3, THF) of enantiopure
the enantiopure cyclohexene epoxides 12c and 12d, the only cis-dihydrodiol 22,18b yielded cis-tetrahydrodiol 23 (86% yield,
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Scheme 4
Scheme 4). Hydrogenolysis of the iodine atom from diol 23 to
give 3,4-cis-tetrahydrodiol 24 (92% yield) was achieved by using
Pd/C catalyst in MeOH.18b The synthesis of 3,4-arene oxide 4d
from diol 24 was completed in a similar manner to 2,3-arene
oxides 3b–d, i.e. involving trans-bromoacetate 25, epoxide 26,
dibromoepoxide 27 intermediates in similar yields (Scheme 4).
In marked contrast to arene oxides 3b–e, where the dynamic
equilibrium favoured the 2,3-arene oxide tautomers, the
initially formed 3,4-arene oxide tautomer 4d was found to exist
preferentially as the corresponding oxepine valence tautomer
4′d, as predicted from theoretical calculations.17a Arene oxide
4d or the preferred oxepine 4′d had been indirectly identified
as mammalian metabolites of bromobenzene through for-
mation of trans-dihydrodiol 7d,16a–c but had not been chemi-
cally synthesised. The regioisomeric arene hydrate 28 was
obtained by treatment of the tetrahydroepoxide precursor 26
with MeLi under similar conditions to those used with tetra-
hydroepoxides 12c and 12d.
The relative rates of dehydration of unsubstituted mono-
cyclic and polycyclic arene hydrates, under acidic conditions,
to yield the corresponding phenols, were investigated using a
UV spectrophotometric procedure.10a–d Comprehensive kinetic
studies of substituted monocyclic arene hydrate dehydration,
under acidic conditions to yield the original arene substrate,
are currently in progress and the results will be presented
separately.
In view of the earlier reports of a monocyclic arene hy-
drate,11a and a keto tautomer of an arene hydrate,11b being
formed during the metabolism of arenes by whole cells of
P. putida UV4, the metabolic fate of an arene hydrate produced
in this study, was also investigated. In this context the most
stable of the chemically synthesised arene hydrates, 15f, was
metabolized under similar biotransformation conditions11b,19a
(Scheme 5). LC-MS analysis of biotransformation products,
present in the crude aqueous culture medium, showed that
substrate 15f had been dihydroxylated and that the major
metabolite had an identical structure to an authentic sample
of the cis,cis-triol 18.
Scheme 5
20′. The cyclohexenone cis-diol 20′ was, in turn, reduced, via
a
carbonyl reductase (CRED)-catalysed process, into
(−)-(1R,2S,4R)-cis,cis-triol 18 (Scheme 5).19a This authentic
sample of (−)-triol metabolite 18 was structurally indistin-
guishable from the new triol metabolite derived from (−)-(1S)-
arene hydrate 15f, which thus had the (+)-(1S,2R,4S)-18 absol-
ute configuration. The opposite absolute configurations found
in cis-diol metabolites obtained from the biotransformation of
a substituted benzene, e.g. phenol 19, and a substituted cyclic
diene, e.g. benzene hydrate 15f, using P. putida UV4, is in
accord with earlier results obtained with a much wider range
of benzene and conjugated cyclic diene substrates.19b Based
on this preliminary observation, it is evident that other arene
hydrate metabolites may also be further metabolised by
P. putida UV4 cells and that the presence of cis,cis-triol meta-
bolites could provide indirect evidence for their involvement
as metabolic intermediates.
Conclusion
cis-Dihydrodiol metabolites 1b–e and 22, formed by dioxygen-
ase-catalysed dihydroxylation of substituted benzene sub-
strates have been converted into the corresponding arene
oxides 3b–e and oxepine 4′d by a chemoenzymatic synthesis
route. The arene oxide enantiomers 3b–e, derived from enan-
tiopure cis-dihydrodiols 1b–e, racemized rapidly via their
oxepine tautomers and a racemic arene hydrate 15c was
obtained from chemical reduction of arene oxide 3c. Enantio-
pure arene hydrates 15c, 15d, 28 and 15f were synthesized
Biotransformation of 3-trifluoromethylphenol 19 under
these conditions had earlier yielded the corresponding cis-
dihydrodiol 20 which preferred to exist as the keto tautomer
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from the corresponding arene tetrahydroepoxides 12c, 12d, 26
(+)-(1S,2R)-1-Acetoxy-2-bromo-3-chlorocyclohex-3-ene
11c.
and bromohydrin 17f respectively. Arene hydrate 15f was Yield (3.24 g, 95%); m. p. 75–77 °C (from Et2O–hexane); Rf 0.11
metabolized, using P. putida UV4, to yield (+)-cis,cis-triol (5% Et2O–hexane); [α]D +103 (c 0.91, CHCl3); (Found: C, 37.8;
metabolite 18 of identical structure but opposite absolute con- H, 3.9. C8H10O2BrCl requires C, 37.9; H, 4.0%); 1H-NMR
figuration to (−)-cis-triol metabolite 18 obtained from 3-tri- (500 MHz, CDCl3) δ 1.93 (1 H, m, 6-H), 2.10 (3 H, s, OCOMe),
fluoromethylphenol 19.19a
2.27 (3 H, m, 5-H, 5′-H, 6′-H), 4.46 (1 H, m, 2-H), 5.36 (1 H, dt,
J1,2 4.5, J1,6 2.3, 1-H), 6.06 (1 H, ddd, J4,5 5.1, J4,5′ 2.7, J4,6 0.8,
4-H); 13C-NMR (100 MHz, CDCl3) δ 20.0, 21.0, 22.0, 48.3, 72.8,
129.4, 154.7, 170.1; LCMS (TOF): m/z 251.9553 (M+, 79Br35Cl,
38%), 253.9530 (M+, 81Br35Cl and 79Br37Cl, 50%), 255.9505
(M+, 81Br37Cl, 5%).
(+)-(1S,2R)-1-Acetoxy-2,3-dibromocyclohex-3-ene 11d. Yield
(2.78 g, 90%); m. p. 57–59 °C (from Et2O–hexane); Rf 0.50 (30%
Et2O–hexane); [α]D +132 (c 1.22, CHCl3); (Found: C, 32.2; H,
3.4. C8H10O2Br2 requires C, 32.3; H, 3.4%); 1H-NMR (500 MHz,
CDCl3) δ 2.02 (1 H, m, 6-H), 2.10 (3 H, s, OCOMe), 2.31 (3 H,
m, 5-H, 5′-H, 6′-H), 4.59 (1 H, m, 2-H), 5.38 (1 H, m, 1-H), 6.55
(1 H, m, 4-H); LRMS (EI): m/z 300 (M+, 81Br81Br, 2%), 298 (M+,
81Br79Br, 7%), 296 (M+, 79Br79Br, 5%), 78 (60), 61 (37), 43 (100).
Experimental
1H and 13C NMR spectra were recorded on Bruker Avance 400,
DPX-300 and DRX-500 instruments. Chemical shifts (δ) are
reported in ppm relative to SiMe4 and coupling constants (J)
are given in Hz. Mass spectra were recorded at 70 eV, on a VG
Autospec Mass Spectrometer, using a heated inlet system.
Accurate molecular weights were determined by the peak
matching method, with perfluorokerosene as the standard.
Flash column chromatography and preparative layer chromato-
graphy (PLC) were performed on Merck Kieselgel type 60
(250–400 mesh) and PF254/366 plates respectively. Merck Kiesel-
gel type 60F254 analytical plates were employed for TLC. A
Perkin Elmer 341 polarimeter was used for optical rotation
([α]D) measurements. The conditions used for LC-MS analysis
of metabolites were as reported earlier.19a
(+)-(1S,2R)-1-Acetoxy-2-bromo-3-iodocyclohex-3-ene
11e.
Yield (3.40 g, 94%); m. p. 64–66 °C (from Et2O–hexane); Rf 0.52
(30% Et2O–hexane); [α]D +130 (c 0.92, CHCl3); (Found: C, 28.0;
H, 2.9. C8H10O2IBr requires C, 27.9; H, 2.9%); 1H-NMR
(500 MHz, CDCl3) δ 1.94 (1 H, m, 6-H), 2.09 (3 H, s, OCOMe),
2.29 (2 H, m, 5-H, 5′-H) 2.40 (1 H, m, 6′-H), 4.54 (1 H, m, 2-H),
5.36 (1 H, m, 1-H), 6.27 (1 H, dd, J4,5 5.0, J4,5′ 2.6, 4-H); LRMS
(EI): m/z 346 (M+, 81Br, 1%), 344 (M+, 79Br, 1%), 286 (11), 284
(12), 265 (10), 223 (9), 205 (47), 78 (63), 61 (27), 51 (13), 43
(100).
cis-Dihydrodiols 1b–f and 22 and cis-tetrahydrodiols 10b–f,
23 and 24 were available from earlier studies.14a,b,19b All cis-
diols were enantiopure (>98% ee) with the exception of com-
pounds 1b and 10b (ca. 70% ee) and had the absolute con-
figurations shown in Schemes 2 and 4.
(+)-(1S,2S)-1-Acetoxy-2-bromo-3-(trifluoromethyl)cyclohex-
3-ene 11f. Yield (1.2 g, 76%); m. p. 30–33 °C; Rf 0.43 (10%
Et2O–hexane); [α]D +248 (c 0.82, CHCl3); (Found: M+ 226.9688.
C7H7F3Br-OAc requires 226.9683); 1H-NMR (500 MHz, CDCl3)
(i) Typical conditions used for the synthesis of trans-
bromoacetates 11b–f and 25 from the corresponding
cis-tetrahydrodiols 10b–f and 24
cis-Tetrahydrodiol 10 (15 mmol) was dissolved in dry MeCN δ 1.98 (1 H, m, 6-H), 2.06 (3 H, s, OCOMe), 2.35–2.50 (3 H, m,
(20 and 1-bromocarbonyl-1-methylethylacetate 6′-H, 5-H, 5′-H), 4.58 (1 H, m, 2-H), 5.35 (1 H, m, 1-H), 6.34
cm3)
(16.5 mmol) was added drop-wise to the solution at 0 °C. The (1 H, t, J4,5 3.4, 4-H); 13C-NMR (125 MHz, CDCl3) δ 19.7, 20.6,
reaction mixture was stirred at 0 °C for 0.5 h, and then for 2 h 21.0, 37.5, 70.8, 122.9, 126.8, 136.0, 169.90; LRMS (EI): m/z 229
at room temperature. Acetonitrile was removed under reduced (M+, 81Br-OAc, 7%), 227 (M+, 79Br-OAc, 8%), 207 (34), 148 (8),
pressure, the residue taken up in diethyl ether (100 cm3) and 147 (45), 128 (7), 127 (58), 95 (6), 79 (22), 61 (55), 43 (100), 39
the ether extract washed thoroughly with 3% aqueous NaHCO3 (8), 28 (14).
(2 × 25 cm3). The aqueous phase was again extracted with
(+)-(1S,2S)-1-Acetoxy-2,4-dibromocyclohex-3-ene
25. Yield
ether (2 × 25 cm3). The combined ether extract was dried (1.47 g, 92%); m. p. 91–92 °C (from Et2O–hexane); Rf 0.55 (30%
(Na2SO4), the solvent evaporated, and the crude product Et2O–hexane); [α]D +144 (c 1.55, CHCl3); (Found: C, 32.1; H,
obtained was purified by crystallization to give the bromoace- 3.3. C8H10O2Br2 requires C, 32.3; H, 3.4%); 1H-NMR (500 MHz,
tate as white crystals or powder.
CDCl3) δ 1.94 (1 H, m, 6-H), 2.08 (3 H, s, OCOMe), 2.37 (1 H,
(+)-(1S,2R)-1-Acetoxy-2-bromo-3-fluorocyclohex-3-ene
11b. m, 6′-H), 2.65 (1 H, m, 5-H), 2.72 (1 H, m, 5′-H), 4.53 (1 H, m,
Yield (1.14 g, 96%); m. p. 31–32 °C (from Et2O–hexane); Rf 0.59 2-H), 5.24 (1 H, m, 1-H), 6.19 (1 H, d J3,2 5.0, 4-H); LRMS (EI):
(30% Et2O–hexane); [α]D +97 (c 1.43, CHCl3); (Found: C, 40.4; m/z 300 (M+, 81Br81Br, 8%), 298 (M+, 81Br79Br, 6%),
H, 4.2. C8H10O2BrF requires C, 40.5; H, 4.3%); 1H-NMR (M+79Br79Br, 7%), 78 (70), 61 (34), 51 (16), 43 (100).
(500 MHz, CDCl3) δ 1.90 (1 H, m, 6-H), 2.10 (3 H, s, OCOMe),
trans-(1S,2S)-2-Bromo-3-(trifluoromethyl)-3-cyclohexen-1-ol
2.30 (2 H, m, 5-H, 5′-H), 2.41 (1 H, m, 6′-H), 4.47 (1 H, m, 2-H), 17f. To a stirred solution of trans-(1S,2R)-1-acetoxy-2-bromo-3-
5.36 (1 H, m, 1-H), 5.52 (1 H, ddd, J4,F 15.0, J4,5 5.1, J4,5′ 3.1, (trifluoromethyl)cyclohex-3-ene 11f (1.02 g, 3.55 mmol), in dry
4-H); 13C-NMR (100 MHz, CDCl3) δ 18.8, 21.0, 21.1, 41.7, 73.3, Et2O (5 cm3) maintained at °C under nitrogen atmosphere, di-
107.0, 154.7, 170.0; LRMS (EI): m/z 238 (M+, 81Br, 2%), 236 (M+, iso-butyl aluminium hydride solution in THF (1 M, 8 cm3) was
79Br, l%), 178 (16), 176 (16), 157 (21), 114 (19), 98 (54), 78 (69), added dropwise. The reaction mixture was allowed to warm
61 (23), 51 (13), 43 (100).
to room temperature and stirring continued overnight.
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The reaction mixture was quenched with a mixture of MeOH– 6.33 (1 H, m, 5-H); LRMS (EI): m/z 176 (M+, 81Br, 16%), 174
hexane (1 : 5, 6 cm3) whilst stirring it in an ice-bath. The preci- (M+, 79Br, 16%), 148 (53), 146 (51), 95 (60), 77 (22), 67 (58), 51
pitated inorganic salts were removed by filtration through a (34), 39 (100).
pad of hyflo-supercel and the filtrate concentrated under
reduced pressure. The crude residue obtained was purified by
flash column chromatography (30% Et2O–hexane) to yield bro-
mohydrin 17f as a white crystalline solid (0.427 g, 47%); Rf
0.38 (20% Et2O–hexane); m. p. 53 °C; [α]D +318 (c 0.36, CHCl3);
(Found: M+ 226.9688. C7H8BrF3O–HO requires 226.9683);
1H-NMR (500 MHz, CDCl3) δ 1.88 (1 H, m, 6-H), 2.99–2.44 (2
H, m, 6′-H, 5-H), 2.54 (1 H, m, 5′-H), 4.40 (1 H, m, 1-H), 4.49 (1
H, m, 2-H), 6.63 (1 H, m, 4-H); 13C-NMR (125 MHz, CDCl3) δ
20.2, 22.2, 41.7, 69.8, 126.6, 136.2; LRMS (EI): m/z 229 (M+,
81Br-HO, 3%), 227 (M+, 79Br-HO, 4), 200 (12), 166 (24), 165 (41),
147 (75), 145 (38), 128 (17), 127 (46), 121 (43), 117 (65), 101
(18), 97 (100), 77 (65), 69 (18), 51 (34), 44 (57), 39 (52), 31 (21),
29 (21).
(iii) Typical conditions used for the synthesis of
bromotetrahydroepoxides 13b–d and 27 from the
corresponding tetrahydroepoxides 12b–d and 26
To a solution of tetrahydroepoxides 12 (2.0 mmol) in a mixture
of chloroform and carbon tetrachloride (1 : 1, 10 cm3) was
added freshly recrystallized N-bromosuccinimide (2.4 mmol)
and catalytic amount of AIBN. The reaction mixture was stirred
and refluxed under nitrogen. When the reaction was complete
(1.5 h, monitored by 1H-NMR) most of the solvent was
removed under reduced pressure, the concentrate diluted with
carbon tetrachloride (10 cm3) and the precipitated succinimide
filtered off. The filtrate was concentrated under reduced
pressure and the crude mixture of bromotetrahydroepoxide
diastereoisomers 13 obtained was used for the next step
without further purification due to its instability.
(ii) Typical conditions used for the synthesis of
tetrahydroepoxides 12b–d and 26 from the corresponding
trans-bromoacetates 11b–d and 25
(1aS,5aS)-3-Bromo-5-fluoro-1a,2,3,5a-tetrahydro-1-benzoxirene
13b. Crude yield (0.40 g, 92%); 1H-NMR (500 MHz, CDCl3) δ
1.72 (1 H, m, 2-H), 2.49 (1 H, m, 2′-H), 3.94 (1 H, m, 1a-H),
4.02 (1 H, m, 5a-H), 4.79 (1 H, m, 3-H), 5.56 (1 H, m, 4-H).
(1aS,5aS)-3-Bromo-5-chloro-1a,2,3,5a-tetrahydro-1-benzoxirene
To a solution of trans-bromoacetate 11 (4.0 mmol) in dry Et2O
(25 cm3) was added NaOMe (8.0 mmol) and the mixture
stirred overnight at ambient temperature. The reaction mixture
was filtered and the residual oil obtained after removal of the
solvent from the filtrate was purified by rapid PLC (silica gel,
solvent containing traces of Et3N) to yield the corresponding
tetrahydroepoxide 12.
1
13c. Crude yield (0.378 g, 90%); H-NMR (500 MHz, CDCl3) δ
1.70 (1 H, m, 2-H), 2.40 (1 H, m, 2′-H), 3.45 (1 H, m, 1a-H),
3.75 (1 H, m, 5a-H), 4.75 (1 H, m, 3-H), 6.20 (1 H, m, 4-H).
(1aS,5aS)-3,5-Dibromo-1a,2,3,5a-tetrahydro-1-benzoxirene
(1aS,5aS)-5-Fluoro-1a,2,3,5a-tetrahydro-1-benzoxirene 12b.
Yield (0.424 g, 93%); Rf 0.36 (30% Et2O–hexane); [α]D +70
(c 1.35, CHCl3); (Found: C, 62.9; H, 6.0. C6H7OF requires C,
63.2; H, 6.1%); 1H-NMR (500 MHz, CDCl3) δ 1.65 (1 H, m, 2-H),
2.04 (2 H, m, 3-H, 3′-H), 2.23 (1 H, m, 2′-H), 3.38 (1 H, m,
1a-H), 3.63 (1 H, m, 5a-H), 5.31 (1 H, m, 4-H); LRMS (EI): m/z
114 (M+, 15%), 95 (49), 86 (64), 77 (18), 67 (60), 51 (31), 39 (100).
(1aS,5aS)-5-Chloro-1a,2,3,5a-tetrahydro-1-benzoxirene 12c.
Yield (0.49 g, 92%); Rf 0.53 (30% Et2O–hexane); [α]D +128
(c 1.35 CHCl3); (Found: M+ 130.0183. C6H7O35Cl requires
130.0185); 1H-NMR (500 MHz, CDCl3) δ 1.67 (1 H, m, 2-H),
2.08 (2 H, m, 3-H, 3′-H), 2.25 (1 H, m, 2′-H), 3.39 (1 H, m, 1a-
H), 3.57 (1 H, m, 5a-H), 5.95 (1 H, m, 4-H); LRMS (EI): m/z 132
(M+, 37Cl, 16%), 130 (M+, 35Cl, 46), 104 (45), 102 (100), 95 (71),
77 (35), 67 (56), 65 (72), 51 (30), 43 (45), 41 (47), 39 (100).
(1aS,5aS)-5-Bromo-1a,2,3,5a-tetrahydro-1-benzoxirene 12d.
Yield (0.546 g, 90%); b. p. 78–80 °C; [α]D +55 (c 1.70, CHCl3);
1
13d. Crude yield (0.41 g, 96%); H-NMR (500 MHz, CDCl3) δ
1.68 (1 H, m, 2-H), 2.39 (1 H, m, 2′-H), 3.42 (1 H, m, 1a-H),
3.80 (1 H, m, 5a-H), 4.77 (1 H, m, 3-H), 6.33 (1 H, m, 4-H).
(1aS,5aR)-3,4-Dibromo-1a,2,3,5a-tetrahydro-1-benzoxirene
27. Crude yield (0.33 g, 90%); 1H-NMR (500 MHz, CDCl3) δ
2.60 (1 H, m, 2-H), 2.71 (1 H, m, 2′-H), 3.41 (1 H, m, 1a-H),
3.63 (1 H, m, 5a-H), 4.82 (1 H, m, 3-H), 6.19 (1 H, m, 5-H).
trans-(1S,2R)-1-Acetoxy-2,5-dibromo-3-iodocyclohex-3-ene
14e. The method used for the allylic bromination of tetrahy-
droepoxides 12b–d and 26 was also applied to trans-bromoace-
tate 11e. Dibromo compound 14e was found to be unstable
and was used directly without purification for the next step.
1
Crude yield (0.32 g, 91%); H-NMR (500 MHz, CDCl3) δ 2.02 (1
H, m, 6-H), 2.12 (3 H, s, OCOMe), 2.41 (1 H, m, 6-H), 2.61 (1 H,
m, 6′-H), 4.80 (2 H, m, 2-H, 5-H), 5.38 (1 H, m, 1-H), 6.58 (1 H,
m, 4-H).
1
(Found: M+ 173.9690. C6H7O79Br requires 173.9680); H-NMR
(iv) Typical conditions used for the synthesis of benzene
oxides 3b–e and oxepine 4′d from the corresponding
bromotetrahydroepoxides 13b–d, 27 and dibromoacetate 14e
(500 MHz, CDCl3) δ 1.68 (1 H, m, 2-H), 2.06 (2 H, m, 3-H, 3′-H),
2.28 (1 H, m, 2′-H), 3.50 (1 H, m, 1a-H), 3.56 (1 H, m, 5a-H),
6.20 (1 H, m, 4-H); LRMS (EI): m/z 176 (M+, 81Br, 18%), 174
(M+, 79Br, 18), 148 (61), 146 (55), 95 (55), 77 (22), 67 (59), 51 Bromotetrahydroepoxide (13b–d, 27) (2.5 mmol) was dissolved
(33), 39 (100).
in dry Et2O (20 cm3) and NaOMe (5.0 mmol) was added to the
(1aS,5aR)-4-Bromo-1a,2,3,5a-tetrahydro-1-benzoxirene
26. solution. The mixture was stirred at ambient temperature for
Yield (0.56 g, 95%); b. p. 77–78 °C; [α]D +113 (c 1.00, CHCl3); 1 h, the salts filtered off, the filtrate concentrated and the
1
(Found: M+ 173.9682. C6H7O79Br requires 173.9680); H-NMR crude benzene oxides (3b–d) or oxepine 4′d were each purified
(500 MHz, CDCl3) δ 1.88 (1 H, m, 2-H), 2.35 (2 H, m, 3-H, 3′-H), by PLC. Similar treatment of dibromoacetate 14e yielded
2.53 (1 H, m, 2′-H), 3.26 (1 H, m, 1a-H), 3.48 (1 H, m, 5a-H), benzene oxide 3e. Due to their instability in the presence of
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traces of acid or at room temperature, benzene oxide and (2 H, m, 6-H, 6′-H), 4.32 (1 H, m, 1-H), 5.97 (2 H, m, 5-H), 6.05
oxepine products were stored below −20 °C.
(1 H, d, J2,1 4.6, 2-H); 13C-NMR (125 MHz, CDCl3) δ 31.8, 63.8,
2-Fluoro-1a,5a-dihydro-1-benzoxirene
(fluorobenzene-2,3- 122.7, 125.8, 128.2, 132.2; LRMS (EI): m/z 132 (M+, 37Cl, 16%),
oxide) 3b. Yield (0.216 g, 77% from 12b); [α]D 0.0 (c 1.11, 130 (M+, 35Cl, 46), 114 (8), 112 (28), 95 (32), 77 (24), 65 (100);
CHCl3); (Found: C, 64.2; H, 4.4. C6H5OF requires C, 64.3; H, ECD: λ 254 (Δε −8.46), λ 210 (Δε +5.22).
1
4.5%); H-NMR (500 MHz, CDCl3) δ 4.22 (2 H, m, 5a-H, 1a-H),
(−)-(1S)-1-Hydroxy-3-bromocyclohexa-2,4-diene
15d. Yield
5.89 (1 H, m, 4-H), 6.19 (1 H, dd, J5,4 9.2, J5,5a 3.6, 5-H), 6.30 (1 (0.179 g, 68%); [α]D −45 (c 0.76, CHCl3); (Found: C, 40.0; H,
H, dd, J3,4 7.3, J3,1a 2.0, 3-H); LRMS (EI): m/z 112 (M+, 12%), 94 3.0. C6H7OBr requires C, 40.1; H, 3.1%); 1H-NMR (500 MHz,
(50), 93 (19), 75 (43), 43 (100).
CDCl3) δ 2.53 (2 H, m, 6-H,6′-H), 4.26 (1 H, m, 1-H), 5.89 (1 H,
2-Chloro-1a,5a-dihydro-1-benzoxirene
(chlorobenzene-2,3- m, 5-H), 6.10 (1 H, d, J4,5 9.9, 4-H), 6.29 (1 H, d, J2,1 5.4, 2-H);
oxide) 3c. Yield (0.265 g, 82% from 12c); [α]D 0.0 (c 1.38, 13C-NMR (125 MHz, CDCl3) δ 31.6, 64.6, 127.0, 127.7, 127.9,
CHCl3); (Found: C, 55.9; H, 3.7. C6H5OCl requires C, 56.1; H, 131.6; LRMS (EI): m/z 176 (M+, 81Br, 16%), 174 (M+, 79Br, 46%),
1
3.9%); H-NMR (500 MHz, CDCl3) δ 4.16 (1 H, m, 1a-H), 4.19 158 (15), 156 (15), 95 (38), 77 (16), 65 (100); ECD: λ 256 (Δε
(1 H, m, 5a-H), 6.32 (1 H, ddd, J4,5 9.1, J4,3 6.8, J4,5a 2.0, 4-H), −5.13), λ 214 (Δε +3.25).
6.36 (1 H, dd, J5,4 9.1, J5,5a 3.6, 5-H), 6.41 (1 H, dd, J3,4 6.8, J3,1a
2.4, 3-H); LRMS (EI): m/z 130 (M+, 37Cl, 12%), 128 (M+, 35Cl, (0.120 g, 61%); [α]D −11 (c 1.75, CHCl3); (Found: M+, 173.9680.
36%), 112 (13), 110 (40), 93 (45), 75 (47), 43 (100).
C6H7O79Br requires 173.9680); 1H-NMR (500 MHz, CDCl3) δ
(−)-(1S)-1-Hydroxy-4-bromocyclohexa-2,4-diene
28. Yield
2-Bromo-1a,5a-dihydro-1-benzoxirene (bromobenzene-2,3- 2.57 (2 H, m, 6-H, 6′-H), 4.26 (1 H, m, 1-H), 5.98 (1 H, dd, J2,1
oxide) 3d. Yield (0.278 g, 76% from 12d); [α]D 0.0 (c 1.53, 4.7, J2,3 9.8, 2-H), 6.10 (2 H, m, 3-H, 5-H); LRMS (EI): m/z 176
CHCl3); (Found: C, 41.4; H, 2.9. C6H5OBr requires C, 41.7; H, (M+, 81Br, 12%), 174 (M+, 79Br, 12%), 158 (19), 156 (19), 95 (46),
2.9%); 1H-NMR (500 MHz, CDCl3) δ 3.69 (1 H, m, 5a-H), 4.07 (1 77 (24), 65 (100); ECD: λ 258 (Δε −1.52), λ 194 (Δε +1.95).
H, dd, J1a,5a 5.1, J1a,3 2.1, 1a-H), 5.64 (1 H, m, 4-H), 5.77 (1 H,
(−)-(1S)-1-Hydroxy-3-trifluoromethylcyclohexa-2,4-diene 15f.
dd, J5,4 8.8, J5,5a 3.9, 5-H), 6.15 (1 H, dd, J3,4 7.3, J3,1a 2.1, 3-H); Sodium methoxide (0.12 g, 2.23 mmol) was added to a stirred
LRMS (EI): m/z 174 (M+, 81Br, 32%), 172 (M+, 79Br, 32%), 156 solution of bromohydrin 17f (0.55 g, 2.23 mmol) in Et2O
(33), 154 (19), 93 (47), 75 (55), 43 (100).
(10 cm3). After stirring the reaction mixture, overnight at room
2-Iodo-1a,5a-dihydro-1-benzoxirene (iodobenzene-2,3-oxide) temperature, the salts were filtered off, the filtrate concen-
3e. Yield (0.252 g, 46% from 11e); [α]D 0.0 (c 1.5, CHCl3); trated under reduced pressure and the crude product purified
(Found: C, 32.6; H, 2.4. C6H5OI requires C, 32.8; H, 2.3%); by PLC (30% Et2O–hexane) to yield benzene hydrate 15f as a
1H-NMR (500 MHz, CDCl3) δ 4.42 (1 H, m, 5a-H), 4.69 (1 H, m, viscous oil (0.098 g, 27%); Rf 0.2 (30% Et2O–hexane); [α]D −193
1a-H), 6.09 (1 H, m, 4-H), 6.36 (1 H, dd, J5,4 8.8, J5,5a 4.0, 5-H), (c 1.2, CHCl3); (Found: M+, 164.0441·C7H7OF3 requires
6.86 (1 H, dd, J3,4 7.3, J3,1a 2.0, 3-H); LRMS (EI): m/z 220 164.0449); 1H-NMR (500 MHz, CDCl3) δ 2.57 (2 H, m, 6-H,6′-
(M+, 29%), 212 (34), 93 (41), 75 (35), 43 (100).
H), 4.44 (1 H, m, 1-H), 6.09 (2 H, m, 4-H, 5-H), 6.42 (1 H, m,
3-Bromo-1a,5a-dihydro-1-benzoxirene (4-bromooxepine) 4′ 2-H); 13C-NMR (125 MHz, CDCl3) δ 31.8, 62.4, 118.4, 123.0,
d. Yield (0.281 g, 77% from 26); [α]D 0.0 (c 1.26, CHCl3); 127.4, 128.4, 128.6; LRMS (EI): m/z 164 (M+, 38%), 163 (7), 146
(Found: C, 41.5; H, 2.8. C6H5OBr requires C, 41.7; H, 2.9%); (4), 145 (8), 143 (13), 134 (5), 127 (21), 115 (38), 114 (12), 97
1H-NMR (500 MHz, CDCl3) δ 5.30 (1 H, d, J7,6 5.0, 7-H,), 5.35 (1 (11), 95 (100), 83 (20), 77 (22), 67 (12), 57 (15), 43 (15); ECD:
H, d, J2,3 5.1, 2-H), 5.74 (1 H, m, 6-H), 5.99 (1 H, d, J3,2 5.1, λ 253 (Δε −8.80).
3-H), 6.50 (1 H, m, 5-H); LRMS (EI): m/z 174 (M+, 81Br, 32%),
Similar treatment of trans-1-acetoxy-2-bromo-3-(trifluoro-
methyl)cyclohex-3-ene 11f with NaOMe gave benzene hydrate
15f (17%).
172 (M+, 79Br, 32%).
(v) Typical conditions for the synthesis of benzene hydrates
15c, 15d and 28 from the corresponding tetrahydroepoxides
12c, 12d and 26
(vi) Biotransformation of (−)-1-hydroxy-3-trifluoromethyl-
cyclohexa-2,4-diene 15f using Pseudomonas putida UV4 to
yield (1S,2R,4S)-cis-triol 18
A stirred and cold solution (−78 °C) of tetrahydroepoxide 12
(1.5 mmol) in dry Et2O (10 cm3) was treated, under nitrogen, The biotransformation of (−)-(1S)-trifluoromethylbenzene
with a solution of MeLi in hexane (1.6 M, 3 cm3). After keeping hydrate 15f (0.015 g) was carried out in a shake flask culture of
the mixture for ten minutes at −78 °C, it was gradually allowed P. putida UV4, employing similar conditions to those reported
to come to ambient temperature and the stirring continued for for the toluene-dioxygenase catalysed cis-dihydroxylation of
another 0.5 h. The reaction mixture was then quenched by 3-trifluoromethylphenol 19.19a LC-TOF/MS analysis of the
dropwise addition of water (2 cm3), the ether layer was sepa- aqueous biotransformed material under the previously
rated, dried (MgSO4) and concentrated. The unstable benzene reported conditions,19a showed the presence of only one
hydrate 15 obtained was purified by PLC (30% Et2O–hexane, metabolite: retention time 5.1 min; m/z obs. 199.0571. Calcd.
trace of Et3N; Rf 0.2).
199.0577 for [M + H]+; m/z obs. 216.0845. Calcd. 216.0842 for
(−)-(1S)-1-Hydroxy-3-chlorocyclohexa-2,4-diene
15c. Yield [M + NH4]+; m/z obs. 221.0385. Calcd. 221.0390 for [M + Na]+.
(0.137 g, 70%); [α]D −42 (c 1.45, CHCl3); (Found: M+ 130.0183. The identical LC-MS data for this metabolite and for an auth-
1
C6H735Cl requires 130.0185); H-NMR (500 MHz, CDCl3) δ 2.54 entic sample of (−)-(1R,2S,4R)-cis-triol 18, obtained from
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Organic & Biomolecular Chemistry
phenol 19, allowed the new metabolite to be identified as the
opposite enantiomer, i.e. (+)-(1S,2R,4S)-cis-triol 18f.18
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(e) F. P. Guengerich, Arch. Biochem. Biophys., 2003, 409, 59.
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Acknowledgements
We thank Dr Colin McRoberts for LC-MS analysis of meta-
bolites. Postgraduate funding was provided by Genencor Inter-
national and Donegal County Council (P. McG) and the
Queen’s University of Belfast (V.L.).
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