A highly stereoselective, modular route to (E)-vinylsulfones and to (Z)- and (E)-alkenes

Article abstract of DOI:10.1021/ja207944c

A recently discovered radical fragmentation of 2-fluoro-6-pyridinoxy derivatives allows a new highly stereoselective and convergent route to (E)-vinylsulfones from allylic alcohols. Reductive desulfonylation or nickel-catalyzed couplings furnish di- and t

Full text of DOI:10.1021/ja207944c

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A Highly Stereoselective, Modular Route to (E)-Vinylsulfones and
to (Z)- and (E)-Alkenes
Marie-Gabrielle Braun, Bꢀeatrice Quiclet-Sire, and Samir Z. Zard*
Laboratoire de Synthꢁese Organique, CNRS UMR 7652, Ecole Polytechnique, 91128 Palaiseau, France
S Supporting Information
b
Scheme 1. Formation of Alkenes from Allylic Alcohols
ABSTRACT: A recently discovered radical fragmentation of
2-fluoro-6-pyridinoxy derivatives allows a new highly stereo-
selective and convergent route to (E)-vinylsulfones from
allylic alcohols. Reductive desulfonylation or nickel-catalyzed
couplings furnish di- and trisubstituted (E)- and (Z)-alkenes.
evising flexible routes to di- and trisubstituted (E)- and
D
(Z)-alkenes remains a major challenge in organic synthesis.
Not only are these motifs ubiquitous in natural products, but they
are also starting points for a vast number of ionic, radical, and
carbene-based reactions and for many transition-metal-mediated
transformations.1 While several methods allow selective access to
the thermodynamically more stable (E)-alkenes, the synthesis of
the less stable isomeric (Z)-alkenes is often problematic. The few
classical reactions, such as partial hydrogenation of triple bonds
with poisoned catalysts or the salt-free variation of the Wittig
reaction,2,3 including the Stork and Zhao modification,4 the
StillꢀGennari modifications of the HornerꢀWadsworthꢀEmmons
condensation,5 in addition to the Peterson olefination6 and various
organometallic couplings,7 occasionally exhibit limitations due to
incompatibility with other functional groups or the relative inacces-
sibility of the precursors. Very recently, efforts have been deployed to
access (Z)-alkenes by controlling the stereochemistry of the powerful
metathesis reaction.8
purity can be secured. This translates into a powerful method
for the assembly ofcomplex stereodefined alkeneswhencombined
with the Julia desulfonylation. Radical reactions have had a very
limited impact on the stereoselective synthesis of alkenes, espe-
cially as regards access to the more precious (Z)-isomers.12
The conversion of allylic alcohols 2 into their 2-fluoropyridy-
loxy derivatives 3 allows β-scission of the carbonꢀoxygen bond
by a homolytic mechanism.13 Homolytic cleavage of the nor-
mally strong carbonꢀoxygen bond to give oxygen-centered
radicals is relatively rare, and apart from the particular case of
epoxides,14 it occurs only in special situations of currently limited
synthetic significance.15 Indeed, the general absence of such
fragmentations is a hallmark of radical processes, which have
proved effective for the selective modification of oxygenated
synthetic intermediates and natural products (e.g., carbohydrates).
Nevertheless, homolysis of carbonꢀoxygen bonds under mild
conditions through easily accessible derivatives opens numerous
synthetic opportunities not hitherto available, in particular for the
synthesis of alkenes 7 starting from various aldehydes and ketones
1 (Scheme 1).13
The radical addition of xanthate 4 to derivative 3, which is
easily prepared from allylic alcohol 2 by reaction with commer-
cially available 2,6-difluoropyridine in the presence of base,
produces alkene 7 and fluoropyridinoxyl radical (8). The latter
is mostly converted into 2-fluoro-6-hydroxypyridine (9). The
(E)-alkene is the major and, in some favorable cases, the sole
geometrical isomer. This indicates that β-elimination of fluor-
opyridinoxyl radical 8 at the level of adduct radical 5 is sufficiently
slow (in relative terms) with respect to rotation around the various
bonds, allowing the molecule to adopt a propitious transition
Over a quarter of a century ago, Julia and co-workers reported
the highly stereoselective reduction of vinylsulfones using so-
dium dithionite.9 However, this transformation has not met with
a popularity commensurate with its potential, in part because
of a lack of convenient and general access to vinylsulfones with
well-defined geometries. The most generally employed route to
vinylsulfones is through a base-induced ionic elimination of a
leaving group β to the sulfone.10 While (E)- or (Z)-vinylsulfones
may be obtained stereoselectively by treating respectively ery-
thro- or threo-β-tosyloxy sulfones with base, this imposes prior
separation of the diastereoisomeric β-tosyloxy sulfones, with
the resulting serious loss in efficiency and yield.9a In contrast,
exposure ofeithererythro- orthreo-β-acetoxy sulfones topowdered
potassium hydroxide in dry dioxane furnishes (E)-vinylsulfones
stereoselectively;9a however, this latter approach is capricious,
since even slight variations in the experimental conditions cause
the formation of mixtures of (E)- and (Z)-vinylsulfones.9a,b These
practical limitations in the conventional ionic routes have ham-
pered the widespread application of the Julia reduction.11 We have
now found that by exploitation of a recently discovered slow radical
fragmentation of fluoropyridyloxy derivatives of vinylsulfones, a
convenient access to (E)-vinylsulfones in high stereochemical
Received: August 22, 2011
Published: September 19, 2011
r
2011 American Chemical Society
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Scheme 2. Stereoselective Fragmentation Leading to
Scheme 3. Stereoselective Synthesis of (E)- and (Z)-Alkenes
(E)-Vinylsulfones
conformation with minimal steric repulsion. Indeed, in our
preliminary work it even proved possible in some examples to
isolate addition products 6 where no elimination had taken place,
showing clearly that the intermolecular exchange of the xanthate
group can be much faster than the unimolecular elimination step.
While this has no consequence for the final outcome because the
reversibility of the xanthate exchange means that intermediate
adduct radical 5 is continuously regenerated, the relative slow-
ness of the elimination means that control of the stereochemistry
by an appropriate choice of substituents can indeed be possible. A
bulky sulfone group on the starting alkene, as in 10 (Scheme 2),
should exert sufficient steric pressure in transition structure 11 to
force an anti disposition of the sulfone and R0 groups and thus
direct the elimination toward (E)-vinylsulfone 12 in a highly
stereoselective if not exclusive manner. Desulfonylation with
overall retention, according to the Julia procedure, would then
deliver the desired (Z)-alkene 13.
From dihydrocinnamaldehyde as a test starting aldehyde, the
corresponding vinyl sulfide 15a was prepared by addition of the
vinyl anion derived from phenyl vinyl sulfide16 followed by
treatment of the resulting allylic alcohol 14a with 2,6-difluor-
opyridine in the presence of sodium hydride (Scheme 3). Not
unexpectedly, the reaction of xanthate 4a with vinyl sulfide
derivative 15a in the presence of stoichiometric lauroyl peroxide
in refluxing ethyl acetate gave the corresponding radical addi-
tionꢀfragmentation product 16a, but with poor stereoselectivity
in favor of the Z isomer. Oxidation of vinyl sulfide 15a with 2
equiv of m-chloroperoxybenzoic acid (m-CPBA) delivered sul-
fone analogue 10a in good yield. Fortunately, no concomitant
oxidation of the pyridine nitrogen took place. We were indeed
concerned that the known formation of N-oxides from 2-fluor-
opyridines by reaction with peracid would prove to be a serious
complication and an eventual setback.17 In contrast to the case of
sulfide 15a, addition of xanthate 4a to vinylsulfone 10a under
the same conditions resulted in the formation of additionꢀ
elimination product 12a nearly exclusively as the E isomer.18 It is
noteworthy that an acetonyl radical with an electrophilic character
could be successfully added to an electrophilic vinylsulfone. It is
one of the prominent features of the xanthate process that the
intermediate radicals are given a sufficient lifetime to allow them
to undergo relatively slow reactions that would be quite difficult
to accomplish with other radical-based methods.19
The stereoselective formation of vinylsulfones was extended
to more complex derivatives, as shown by the examples compiled
in Table 1 (OFPy = 2-fluoropyridin-6-oxyl). Precursors 10bꢀh
were prepared from various aldehydes and ketones by the same
three-step sequence used for 10a and outlined in Scheme 3 (see
the Supporting Information). Except for 12f and 12g, where a
2:1 E:Z ratio was observed, the selectivity for the E isomer of
vinylsulfones 12 was greater than 20:1; in fact, none of the Z
isomer could be detected by NMR analysis of the crude reaction
mixture in most instances. The low selectivity in the case of
ketone-derived vinylsulfones 12f and 12g reflects the relatively
small difference in the steric bulk of the substituents and the
faster (and therefore less stereroselective) cleavage of the weaker
tertiary CꢀO bond. Stereodefined tetrasubstituted vinylsulfones
such as 12h and 12i may be produced when a really bulky tert-
butyl substituent is present to bias the fragmentation.
The process is tolerant of numerous useful functional groups,
such as Weinreb amides (12b and 12j), masked α-ketoaldehydes
(12d), phthalimides (12f, Phth = phthalimido), epoxides (12j), and
complex scaffolds such as carbohydrates and steroids (12kꢀm).
The steroid examples are particularly interesting: they illustrate the
modification of the side chain by having either the vinylsulfone or
the xanthate on the steroid partner. Developing new strategies for
the construction of steroid side chains remains a worthwhile
endeavor, especially in the context of vitamin D3 derivatives.20
Finally, complexity may be introduced in a modular fashion.
For instance, xanthate 4e is made by radical addition of a simpler
mesityl oxide-derived xanthate [MeC(dO)CH2CMe2SCSOEt,
4j] to vinyl pivalate,21 so each of the resulting vinylsulfones 12e
and 12i is in fact constructed from four different components: an
aldehyde, phenyl vinyl sulfide, vinyl pivalate, and xanthate 4j.
A simple, flexible, and powerful process for the synthesis of
alkenes with defined geometry is now in hand. A few illustrative
examples are displayed in Scheme 4. Thus, reductive desulfonyla-
tion of 12b gives (Z)-alkene 13b in 76% yield, whereas additionꢀ
fragmentation of xanthate 4k to glucose-derived vinylsulfone 10g
furnishes carbohydrate (Z)-alkene 13c via intermediate (E)-
vinylsulfone 12m in good overall yield. It is interesting to note
that radical additionꢀfragmentation on the naked vinyl alcohol
derivative 18 cleanly and directly gives (E)-alkene 17b.13 Both
(Z)- and (E)-alkenes may therefore be prepared through the
inclusion or omission of the phenylsulfonyl group. The stereo-
selective obtention of trisubstituted alkenes combines the radical
additionꢀfragmentation with organometallic coupling reactions
of vinylsulfones developed by Julia and others,22 as shown by the
conversion of vinylsulfones 12a and 12d into (Z)-alkenes 20a
and 20b, respectively, through Ni(acac)2-mediated couplings.
Exposure of vinylsulfone 12a to a refluxing ethanolic solution of
sodium dithionite and sodium bicarbonate resulted in a smooth
desulfonylation to give the desired (Z)-alkene 13a in good yield
with high stereochemical purity (Scheme 3). Interestingly, desul-
fonylation using sodium amalgam furnished the isomeric (E)-
alkene 17a. These conditions are known to deliver the more stable
(E)-alkene irrespective of the stereochemistry of the starting
vinylsulfone.8 It is thus possible to prepare at will either geome-
trical isomer of the alkene from the same vinylsulfone precursor.
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Table 1. Stereoselective Synthesis of (E)-Vinylsulfones
Scheme 4. Stereoselective Synthesis of Di- and Trisubsti-
tuted (Z)-Alkenes
ultimately hinges on the slow homolytic rupture of a fluoropyr-
idinoxy group, which allows control of the stereochemistry of the
key intermediate vinylsulfones. Not only are numerous func-
tional groups tolerated, but the vinylsulfone motif itself has an
exceptionally rich chemistry that may be exploited for the
synthesis of myriad other products.23
’ ASSOCIATED CONTENT
S
Supporting Information. Experimental procedures as
b
well as a compilation of spectral and analytical data of all new
compounds. This material is available free of charge via the
Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION
Corresponding Author
zard@poly.polytechnique.fr
’ ACKNOWLEDGMENT
We thank Ecole Polytechnique for a Monge Fellowship to
M.-G.B. This paper is dedicated with respect to the memory of
Professors Marc and Sylvestre Julia.
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