An enantioselective organocatalytic oxidative dearomatization strategy

Article abstract of DOI:10.1021/ja077457u

We have developed a catalytic enantioselective strategy for chemical synthesis wherein flat aromatic molecules are directly converted to complex nonracemic molecular architectures. The process involves oxidative dearomatization of substituted phenols foll

Full text of DOI:10.1021/ja077457u

Published on Web 12/15/2007
An Enantioselective Organocatalytic Oxidative Dearomatization Strategy
Ngoc T. Vo, Robert D. M. Pace, Fionn O’Hara, and Matthew J. Gaunt*
Department of Chemistry, UniVersity of Cambridge, Lensfield Road, Cambridge, U.K. CB2 1EW
Received September 27, 2007; E-mail: mjg32@cam.ac.uk
Table 1. Reaction Optimization
The inherent reactivity and functionality stored within aromatic
systems provide numerous possibilities for the synthesis of 3-D
organic structures via dearomatization processes.1,2 Under oxidizing
conditions, the dearomatization of ortho- and para-substituted
phenols forms cyclohexadienones,3 and these products have found
widespread use in the chemical synthesis of natural products.4
Additionally, the groups of Feringa,5a Hayashi,5b and Rovis5c have
developed catalytic desymmetrization methods to convert the cyclo-
hexadienone motif into useful enantioenriched molecules. The ele-
gance of these stepwise tactics leads us to speculate that a catalytic
asymmetric process that can directly transform an aromatic motif
into the nonracemic structure would provide a powerful strategy
for the rapid chemical synthesis of complex molecules (eq 1).6,7
ring from potential oxidation with any residual PhI(OAc)2. More-
over, pyrrolidine 3a also possesses tunable stereocontrolling ele-
ments and has demonstrated a versatile enamine reactivity profile
in a range of organocatalytic enantioselective reactions.10 In testing
this catalytic enantioselective dearomatization strategy we were
delighted to find that treatment of phenol 1a with 1 equiv of PhI-
(OAc)2 and 10 mol% of catalyst (R)-3a in MeCN-H2O at 0 °C
afforded decalin (-)-4a in 45% yield with an enantiomeric excess
(ee) of 87% as a 4:1 mixture of diastereomers (eq 3).
With this promising lead in hand we next sought to improve the
ee of the reaction. By increasing the size of the aryl groups (Ph 3a
to 2-naphthyl 3b) we anticipated that the larger 2-naphthyl moiety
would offer better facial discrimination in the desymmetrization
step. Indeed, using 10 mol % of catalyst (R)-3b under the same
conditions resulted in an increase in ee to 99% although the diaster-
eomeric ratio (dr) and yield of the reaction remained moderate
(Table 1, entries 1,2). To assess the influence of the reaction media
we isolated the meso-cyclohexadienone 2 (R ) H) and subjected
this substrate to the desymmetrization reaction with catalyst (R)-
3b in a range of solvents. To our surprise and delight the reaction
in MeOH proceeded in excellent yield, ee and dr. This result high-
lights an important role of the protic reaction media in controlling
the stereoselectivity in the desymmetrization step.11 A similar out-
come was observed with AcOH as the nucleophile in the oxidation
step followed by cyclization in MeOH to afford (-)-4b with >20:1
dr, 99% ee in 52% yield over the two steps (entry 4).
In this Communication, we report a process that directly converts
a para-substituted phenol to a highly functionalized chiral molecule
via oxidative deraromatization and amine-catalyzed enantioselective
desymmetrizing Michael reaction (eq 2). This one-step transforma-
tion reveals a complex structure formed with exquisite control of
three new stereogenic centers and an array of exploitable orthogonal
functionality directly from a flat molecule that is devoid of
architectural complexity.8
Central to the implementation of this strategy was the realization
of a rapid method to oxidize the phenol ring that would not affect
the sensitive aldehyde function. Hypervalent iodine reagents such
as PhI(OAc)2 facilitate fast oxidation of phenols to meso-cyclo-
hexadieneones in the presence of protic nucleophiles suggesting
that oxidation by this method could be suitable for our proposed
dearomatization process.3,9 Moreover, the fast oxidation of the
phenol would minimize contact between oxidant and the amine
catalyst or enamine intermediate that could lead to deleterious side
reactions. Consequently, in considering the catalyst parameters we
reasoned that a sterically bulky catalyst such as 3a, instead of a
simple proline catalyst, might shield the N-atom of the pyrrolidine
In applying these optimizations and with MeOH as solvent and
nucleophile the catalytic enantioselective dearomatization of 1a
using 1 equiv of PhI(OAc)2 and 10 mol % of (R)-3b delivers (-)-
4c in 75% yield for the one-step process with exquisite control of
stereochemistry (>20:1 dr and 99% ee) (vide supra).
Having demonstrated the viability of this catalytic enantioselec-
tive dearomatization strategy we next investigated the scope of the
transformation. To assess the impact of the structural and functional
motifs on the reaction we tested a range of linking units between
the phenol and aldehyde motifs using MeOH as the oxidation
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10.1021/ja077457u CCC: $40.75 © 2008 American Chemical Society

C O M M U N I C A T I O N S
Table 2. Catalytic Enantioselective Dearomatization
In summary, we have developed a catalytic enantioselective
strategy for chemical synthesis wherein substituted phenols are
directly converted to complex nonracemic molecular architectures.
The process involves oxidative dearomatization of substituted
phenols followed by a desymmetrizing secondary amine catalyzed
asymmetric intramolecular Michael addition and forms a range of
highly functionalized polycyclic molecules with excellent selectiv-
ity. We are currently investigating the application of this process
in the synthesis of natural products, in particular with carbon
nucleophiles, and these results will be reported in due course.
Acknowledgment. We gratefully acknowledge EPSRC (N.T.V.),
Pfizer Global Research & Development, Sandwich, U.K. (to
R.D.M.P), Nuffield Foundation (F.O’H.), the Royal Society (for
University Research Fellowship to M.J.G.), Philip & Patricia Brown
(for Next Generation Fellowship to M.J.G.) and EPSRC Mass
Spectrometry Service (Swansea). We also thank Professor Steven
Ley and Dr. Christelle Lauret (Pfizer, U.K.) and Dr. Andy McNally
for generous support and useful discussion.
nucleophile (Table 2). In addition to the formation of the [4.4.0]-
ring system (-)-4c we also found that a [4.3.0]-bicycloalkanone
(-)-4d could be generated with excellent dr and ee.11,12a Biaryl-
phenols were smoothly converted to polycyclic enones products
(-)-4e,f, again in good yield and very high selectivity from the
direct process.12b Substitution on the phenolic ring was not well
tolerated, and a poor ee was observed for (-)-4g when a
2,6-dimethyl phenol was tested. However, heteroatoms can be
incorporated to the tether unit and they produced highly function-
alized enantiopure products (-)-4h,i in high yield and with excellent
stereocontrol. It was noticeable that these substrates were more
reactive than the carbon analogues, and this was highlighted by
the successful cyclization to the more challenging seven-membered
oxacene ring (-)-4j with excellent ee.
These scope studies demonstrate the flexibility of the catalytic
asymmetric dearomatization process to efficiently generate a range
of useful, architecturally complex, enantio-enriched molecules. In
extending this potentially useful transformation, we also found that
non-oxygen nucleophiles can participate in this reaction. For
example, reaction of 1a in HFIPA-MeCN allowed the -CN function
to act as a nucleophile through a Ritter-type reaction to afford amide
(-)-4k in high ee (eq 4).13a Moreover, oxidation in the presence of
HF‚pyridine complex formed the fluorinated meso-cyclohexadiene
that,13b on catalytic desymmetrization, afforded decalin (-)-4l in
excellent ee (eq 5). This example represents a simple method for
the installation of a tertiary C-F bond within a complex chiral
molecule using enantioselective catalysis.
Supporting Information Available: Experimental data and pro-
cedures for all compounds. This material is available free of charge
via the Internet at http://pubs.acs.org.
References
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(11) The oxidation is the “yield limiting” factor and the cyclization proceeds
almost quantitatively in the stepwise case. See Supporting Information
for further details.
(12) (a) The dr is 3:1 after reaction, but equilibration at room temp gives 1:15
(-)-4d. (b) Absolute stereochemistry was confirmed by X-ray diffraction.
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The stereocontrolling elements of catalysts 3a and 3b have been
well documented for enamine reactions involving intermolecular
C-X bond forming processes;10 however, their use in intramo-
lecular reactions is less common. On the basis of the absolute
configuration obtained from the crystal structure of a derivative of
(-)-4e we suggest that the stereochemistry can be rationalized via
a transition state (vide infra) that involves an endo-like attack onto
the Si face of the meso-cyclohexadienone (eq 6).
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