Stereoselective dearomatizing addition of nucleophiles to uncomplexed benzene rings: A route to carbocyclic sugar analogues

Article abstract of DOI:10.1002/anie.200801078

(Chemical Equation Presented) Versatile synthetic intermediates were formed by the dearomatization of 2-aryl 4,5-diphenyloxazolines with secondary alkyl lithium reagents in the presence of N,N′-dimethylpropyleneurea (DMPU; see scheme). The resulting cyclo

Full text of DOI:10.1002/anie.200801078

Communications
DOI: 10.1002/anie.200801078
Asymmetric Synthesis
Stereoselective Dearomatizing Addition of Nucleophiles to
Uncomplexed Benzene Rings: A Route to Carbocyclic Sugar
Analogues**
Jonathan Clayden,* Sean Parris, Nuria Cabedo, and Andrew H. Payne
Here we report reaction conditions which enable for the first
time the stereoselective dearomatizing addition of organo-
lithium reagents to simple, uncomplexed benzenoid aromatic
rings. Dearomatizing nucleophilic addition reactions to
arenes provide an efficient way of making complex synthetic
intermediates from simple inexpensive precursors.[1,2] As a
strategy, dearomatization marries the regioselectivity of
aromatic electrophilic substitution with the stereoselectivity
achievable upon the conversion of an arene into a cyclohex-
ane derivative.
The seminal work of the Meyers research group[3] showed
the importance of oxazolines in promoting dearomatizing
addition reactions of organometallic reagents to naphthalene
and pyridine derivatives. However, benzene rings are much
more difficult to dearomatize: the addition of nucleophiles to
Scheme 1. DMPU-promoted dearomatization of a 4,5-diphenyloxazo-
line.
uncomplexed phenyloxazolines has previously led to depro-
[4]
=
tonation or attack at the oxazoline C N bond. Current
solutions to the problem adding nucleophiles stereoselec-
tively addition of nucleophiles to simple substituted phenyl
rings involve stoichiometric coordination to Cr, Mn, or
Os.[2a–e] Racemic dearomatized products may also be obtained
from addition reactions to hindered benzamides[2f,g] or to
carbonyl compounds coordinated to aluminum tris(2,6-diphe-
nylphenoxide) (ATPH).[2h–j]
We have found that the previously unexplored 2-aryl
trans-4,5-diphenyloxazolines promote stereoselective nucleo-
philic attack on simple benzenoid rings without metal
this product was accompanied by a dearomatized adduct 3,
which is formed through the attack of iPrLi on the
p-methoxyphenyl ring. When DMPU[7] is first mixed with
the starting material (in an optimal ratio of 6:1), the cyclo-
hexadiene 3 becomes the major product. Compound 3 was
isolated as a single diastereoisomer (with configuration
assigned by X-ray crystallography[8]) in 70% yield.[9]
The oxazoline group in 1 functions as a chiral auxiliary.[9]
It could be removed from enone 4, the hydrolysis product of 3,
by the alkylation–reduction–hydrolysis–reduction sequence
shown in Scheme 2.[10] The enantiomeric purity (e.r. > 99:1) of
the allylic alcohol 6 was established by HPLC analysis of its
bis-p-bromobenzoate 7.
complexation,
provided
N,N’-dimethylpropyleneurea
(DMPU) is used to activate the organolithium nucleophile
(Scheme 1). Upon lithiation with iPrLi in THF and quenching
with methyl iodide, oxazoline 1[5] was converted principally
into the expected[6] product 2 of ortho lithiation. However,
Carbocyclic sugars and their alkylated, hydroxylated, and
aminated analogues are an important class of natural and
[*] Prof. J. Clayden, S. Parris, Dr. N. Cabedo
School of Chemistry
Universityof Manchester
Oxford Road, Manchester M13 9PL (UK)
Fax: (+44)161-275-4939
E-mail: clayden@man.ac.uk
Homepage: http://clor2.ch.man.ac.uk/home.htm
Dr. A. H. Payne
GlaxoSmithKline
New Frontiers Science Park, Harlow CM19 8AY (UK)
[**] We acknowledge support from the EPSRC and GlaxoSmithKline
(S.P.), and from the Ministerio de Educación of Spain (N.C.). We are
grateful to Tom Baker and Ol’ga Karlubíkovµ for synthetic
assistance.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.200801078.
Scheme 2. Removal of the oxazolinyl auxiliary. Ar=p-BrC6H4;
DMAP=4-dimethylaminopyridine, Tf=trifluoromethanesulfonyl.
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Angewandte
Chemie
non-natural compounds that possess a range of antibiotic and
antiviral biological activity.[11] The cyclohexadiene 3 and
cyclohexenone 4 presented themselves as readily available
and versatile synthetic intermediates for the synthesis of
functionalized cyclohexanes structurally related to these
compounds.[12] Schemes 3 and 4 illustrate the conversion of
3 into the alkylated carbocyclic analogues 11 and 15 of
l-altrose and l-mannose, respectively, through short,
protecting-group-free sequences.
The dienyl ether 3 was prepared by the addition of iPrLi to
1 on a 2g scale, and was oxidized to yield a single
diastereoisomer of the base-sensitive hydroxyenone
8
(Scheme 3). The oxazoline substituent was removed by the
method used for 4. Concurrent 1,2-reduction of the enone was
fully diastereoselective, and after hydrolysis of the oxazoli-
dine moiety of 9 and further reduction, the triol 10 was
obtained as a single diastereoisomer. Diastereoselective[13]
dihydroxylation of the alkene[14] yielded a single diastereo-
Scheme 4. Synthesis of an l-mannose analogue. mCPBA=m-chloro-
perbenzoic acid.
secondary organolithium reagents generally led to the
desired products in moderate to good yields. In each
case only a single diastereoisomer and regioisomer of
the product was detected, along with remaining starting
material and sometimes the rearomatized by-product
19.[16] The use of tert-butyllithium led to the formation of
18a’’ in low yield (Table 1, entry 4), and n-butyllithium
failed to add to the ring (Table 1, entry 1).
Protonation of the extended azaenolate 17 gave a
1,3-cyclohexadiene 20 or 1,4-cyclohexadiene 21
(depending on the substitution pattern) in around
50% yield, along with the rearomatized by-product 19
and recovered starting material. Treatment of the
extended enolate 17 with allyl bromide or benzyl
bromide also yielded mixtures of regioisomers.
In conclusion, the dearomatizing reaction provides a
new entry into highly functionalized cyclohexene and
cyclohexanone derivatives, yielding carbocyclic sugar
analogues in six to eight steps and 33–43% yield from
simple aromatic oxazoline derivatives.
Scheme 3. Synthesis of an l-altrose analogue. NMO=4-methylmorpholine N-
oxide.
isomer of the alkylated carbocyclic analogue 11 of a-l-altrose.
The X-ray crystal structure of 11[8] confirmed its configura-
tion.
Received: March 5, 2008
Published online: May 30, 2008
Intermediate 8 from this synthesis was also converted into
a carbocyclic analogue of a-l-mannose (Scheme 4). The
enone underwent 1,2-reduction to give 12 as a single
diastereoisomer, the relative configuration of which was
verified by X-ray crystallography.[8] The directed epoxidation
of 12 yielded 13, and removal of the oxazoline moiety by the
standard method then afforded the epoxytriol 14. The treat-
ment of 14 with aqueous acid led to trans-diaxial ring opening
of the epoxide and provided the a-l-mannose analogue 15 in
81% yield.[15]
The scope and limitations of the dearomatizing addition
were investigated by treating a range of aryl oxazolines 16a–e
with organolithium reagents (1.5–3 equiv; Scheme 5 and
Table 1). A deep green or brown solution formed upon the
successful addition of an organolithium reagent to 16;
quenching of the presumed azaenolate intermediate 17 with
methyl iodide gave a cyclohexadiene 18. The addition of
Scheme 5. Dearomatizing functionalization of aryl oxazolines.
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[5] Oxazolines 1 and 16 were synthesized by a previously reported
method: S. Crosignani, A. C. Young, B. Linclau, Tetrahedron
Lett. 2004, 45, 9611.
Table 1: Addition to aryl oxazolines 16.
Entry 16
X[a]
R[a]
Quench Product,
yield [%]
Yield [%]
of 16;[b] 19
[6] M. Reuman, A. I. Meyers, Tetrahedron 1985, 41, 837.
[7] DMPU and hexamethylphosphoramide (HMPA) have been
shown to promote other reactions involving dearomatizing
addition steps, probably by facilitating the formation of ion
pairs; see: J. Clayden, F. E. Knowles, C. J. Menet, Synlett 2003,
1701; J. Clayden, J. Dufour, D. Grainger, M. Helliwell, J. Am.
Chem. Soc. 2007, 129, 7488; M. Shimano, A. I. Meyers, J. Org.
Chem. 1996, 61, 5714. By contrast, N,N,N’,N’-tetramethylethyle-
nediamine (TMEDA) does not promote dearomatizing addition
to 1. DMPU also promoted the formation of some dearomatized
products from more conventional substituted oxazolines, such as
those employed by Meyers and co-workers,[3] but none per-
formed as well as 1.
[8] CCDC 670194 (3), CCDC 670195 (11), CCDC 670196 (12),
CCDC 670197 (18a), and CCDC 670198 (18c) contain the
supplementary crystallographic data for this paper. These data
can be obtained free of charge from The Cambridge Crystallo-
graphic Data Centre via www.ccdc.cam.ac.uk/data_request/cif.
[9] The stereoselectivity of both the dearomatizing addition and the
alkylation appears to be greater than 10:1, as no stereoisomeric
dearomatized products were observable in the NMR spectrum
of the crude reaction mixture. We assume that the configuration
of the product arises from coordination of isopropyllithium to
the basic nitrogen atom of the oxazoline, followed by 1,4-
addition to the 2-position of the p-methoxyphenyl ring from the
face anti to the 4-phenyl substituent of the oxazoline ring.
Details of the mechanism are still under investigation; however,
preliminary attempted trapping experiments suggest that radical
intermediates are not involved.
[10] B. A. Barner, A. I. Meyers, J. Am. Chem. Soc. 1984, 106, 1865.
The oxazolidine intermediates 5 and 9 were formed as single
stereoisomers, the configuration of which was not assigned.
[11] O. Arjona, A. M. Gómez, J. C. López, J. Plumet, Chem. Rev.
2007, 107, 1919; S. V. Ley, L. L. Yeung, Synlett 1992, 291; L.
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Ramesh, Eur. J. Org. Chem. 2005, 2225; N. Gathergood, K. R.
Knudsen, K. A. Jørgensen, J. Org. Chem. 2001, 66, 1014.
[12] A number of known carbasugar analogues contain quaternary
centers and/or alkyl branches: A. Berecibar, C. Grandjean, A.
Siriwardena, Chem. Rev. 1999, 99, 779; M. Ferrero, V. Gotor,
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1
2
3
4
5
6
7
8
9
16a
H
H
H
H
nBu MeI
–
95; 0
12; 0
1; 0
16a
16a
16a
16b
16c
iPr
MeI
18a, 70[c]
18a’, 81[d]
18a’’, 17
18b, 32
18c, 54[c]
3, 70[c]
sBu
tBu
iPr
MeI
MeI
MeI
12; 0[e]
12; 30
20; 0[f]
6; 0
4-Ph
3-OMe iPr
MeI
MeI
MeI
16d (1) 4-OMe iPr
16d (1) 4-OMe sBu
16a
16a
18d’, 78
20a, 47
20a’, 56
20d, 30;
21d, 15
21e, 53
5; 7
H
H
iPr
sBu
NH4Cl
NH4Cl
MeOH
32; 5
29; 9
29; 6
10
11
16d (1) 4-OMe iPr
12[g]
16e 4-F iPr
NH4Cl
37; 7
[a] See Scheme 5. [b] Recovered starting material. [c] The configuration
of the product was confirmed byX-raycrystallography. [8] [d] The product
was formed as a 3:1 mixture of diastereoisomers with respect to the
exocyclic stereogenic center. [e] Alkylation of the oxazoline ring occurred
to provide a further by-product in 17% yield. [f] A further by-product was
formed in 9% yield by ortho methylation. [g] The reaction was carried out
in toluene with racemic 16e.
Keywords: carbohydrates · dearomatization ·
.
diastereoselectivity· oxazolines · pseudosugars
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[16] The configuration of the adducts was assigned by analogy with 3,
18a, and 18c.
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