Radical additions of xanthates to vinyl epoxides and related derivatives: A powerful tool for the modular creation of quaternary centers

Article abstract of DOI:10.1002/anie.200601567

(Chemical Equation Presented) An open relationship: The triethylborane/O₂-mediated addition of xanthates to vinyl epoxides and derivatives represents an efficient process to form carbon-carbon bonds. Complex structures can be rapidly assembled in a modular, convergent manner, under mild conditions, using readily available reagents (see scheme, Bn = benzyl). The formation of quaternary centers proved especially facile.

Full text of DOI:10.1002/anie.200601567

Communications
Radical Reactions
DOI: 10.1002/anie.200601567
Radical Additions of Xanthates to Vinyl Epoxides
and Related Derivatives: A Powerful Tool for the
Modular Creation of Quaternary Centers**
Scheme 1. Triethylborane-mediated radical addition of a xanthate to a
vinyl epoxide.
Nicolas Charrier, David Gravestock, and
Samir Z. Zard*
controlling agent for quenching the highly reactive alkoxy
À
Over the past ten years, we have developed a new radical
chain process based on the degenerative exchange of a
radical results in a powerful C C bond-forming process that is
especially effective for the creation of quaternary centers.
Preliminary examples of this approach are shown in
Scheme 2. Thus, exposure of the commercially available
butadiene monoepoxide (3) and xanthate 4 to triethylborane
=
thiocarbonyl thio group (Z-C( S)S-), and especially of
xanthates (or dithiocarbonates, 1).[1] This chemistry has
provided a simple, efficient, and reasonably general solution
to a longstanding problem in organic synthesis, namely the
intermolecular creation of carbon–carbon bonds on non-
activated alkenes. This degenerative exchange also consti-
tutes the basis of the extremely powerful reversible addition
fragmentation chain transfer/macromolecular design through
interchange of xanthates (RAFT/MADIX) technology for
the synthesis of block polymers.[2]
We have mainly used peroxides to initiate the radical
addition, although irradiation with a tungsten halogen lamp
was used when lower reaction temperatures were desired. The
autooxidation of boranes, first reported by Brown and
Midland,[3] and ingeniously exploited as a reaction initiating
system by Oshima and co-workers,[4a,b] also proved to be quite
efficient with xanthates, thereby allowing reactions to be
performed at room temperature or below.[5,6] We have now
found that the application of organoboranes to mediate the
xanthate transfer is in fact far more powerful than initially
anticipated. In particular, additions to vinyl epoxides and
related derivatives have proved especially useful.
Scheme 2. Addition of various xanthates to butadiene monoepoxide.
As outlined in Scheme 1, the addition of a radical RC
derived from the xanthate 1 to the vinyl epoxide leads to
opening of the epoxide ring and the formation of an alkoxy
radical. The latter is rapidly intercepted by triethylborane to
give a borinate and an ethyl radical, which propagates the
chain reaction by reacting with the starting xanthate. Xan-
thates are highly radicophilic and a chain process can be
readily sustained. As for the borinate intermediate, it is
hydrolyzed on aqueous workup to afford the allylic alcohol 2.
Overall, combining the efficacious xanthate-transfer process
with the use of triethylborane as both an initiator and
and air gave rise to the corresponding allylic alcohol 5 in 60%
yield (Scheme 2). This transformation parallels the ones
reported by Brown and co-workers,[7] and later by the
research groups of Oshima,[8] Roberts,[9a] and Kim.[9b] How-
ever, these pioneering studies were of rather limited synthetic
scope because of severe constraints in the choice of substrates.
Xanthates, in contrast, do not suffer from such limitations, as
demonstrated by the other examples shown in Scheme 2. For
example, reaction of xanthate 6 directly furnished the
Weinreb amide 7 (in similar yield to that of the reaction
that gave 5), whereas the formation of allylic alcohols 9 and
11, both containing a newly created quaternary center, proved
to be even more efficient.
The vinyl epoxide can be varied substantially, thus
allowing the synthesis of more complex structures. Three
such examples are shown in Scheme 3, in which xanthate 13, a
compound readily available by Michael addition of potassium
O-ethyl xanthate to the cyclobutenone 12[10] under acidic
conditions, is used as the starting material.[11] The radical
addition to epoxides 3, 14, and 15 provided the corresponding
functionalized cyclobutanones 16, 17, and 18 in good yield,
each containing a new quaternary center.
[*] N. Charrier, Dr. D. Gravestock, Prof. S. Z. Zard
Laboratoire de Synth›se Organique associØ au CNRS (UMR 7652)
DØpartement de Chimie
Ecole Polytechnique
91128 Palaiseau Cedex (France)
Fax: (+33)1-6933-3851
E-mail: zard@poly.polytechnique.fr
[**] N.C. thanks the Minist›re de l’Education Nationale, de la Recherche
et de la Technologie (France) for a fellowship.
Supporting information for this article is available on the WWW
under http://www.angewandte.org or from the author.
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synthesis of triol 22 required just two steps from simple
substrates. The second transformation leading to optically
pure 25 incorporates a fragmentation of the cyclobutane ring
of (À)-b-pinene. It was notable that the first addition of the
ketoester xanthate 23 leading to the cyclohexene 24 occurs
smoothly at the carbon atom bearing the least acidic hydrogen
atoms. The ketoester motif in the final product 25 can be used
for further manipulation.
This modular approach is not limited to the formation of
quaternary centers; for example, it could be applied to
alkenes bearing an ester or a carbonate as shown by the two
transformations in Scheme 5. Radical addition of phenacyl
Scheme 3. Addition of a cyclobutanone xanthate to vinyl epoxides.
TFA=trifluoroacetic acid.
The rapid assembly of intricate architectures can also be
made by using a modular approach that depends on the
unique ability of xanthates to add across non-activated
olefins. This strategy is illustrated by the two sequences in
Scheme 4. Thus, addition of xanthate 19 derived from
Scheme 5. Further modular assembly of complex structures. Piv=
pivaloyl.
xanthate 26 to vinyl pivalate furnished adduct 27 in high yield,
and exposure of the latter to vinyl epoxide 3 in the presence of
triethylborane and air resulted in the clean formation of
allylic alcohol 28. Interestingly, no tetralone 29 was observed,
indicating that the intermolecular addition of the intermedi-
ate radical to the vinyl epoxide 3 is faster than the intra-
molecular ring closure onto the aromatic nucleus, despite the
modest radicophilicity of the non-activated vinyl group. We
had previously reported that tetralones can be readily
obtained by treatment of xanthates such as 27 with
peroxide.[12] There is thus a considerable degree of flexibility
in the way xanthates may be employed.
Scheme 4. Creation of quaternary centers by a modular approach.
Bn=benzyl.
A more significant competition between the intermolec-
ular addition process and intramolecular ring closure was
observed with adduct 31, derived from the addition of
xanthate 30 to vinylidene carbonate.[13] The mixture of the
two diastereoisomers of adduct 31 could be separated, but the
subsequent triethylborane-mediated allylation of one of the
two epimers to afford 32 occurred in only moderate yield
chloroacetone to methallyl acetate gave adduct 20 in 60%
yield. Triethylborane-mediated addition to the functionalized
vinyl epoxide 21 gave triol 22, in which two of the alcohol
groups are differentially protected. The xanthate entity has
thus served to create two carbon–carbon bonds and at the
same time to generate a quaternary center. Overall, the
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mixture was diluted with CH2Cl2 and then washed once with water
and once with brine. The organic phase was dried over anhydrous
MgSO4, filtered, and concentrated. The residue was purified by
chromatography on silica gel to give the desired products.
(40%; E/Z 80:20), because of a competing hydrogen abstrac-
tion from the solvent to give 33 as the main side product. The
reaction was repeated on the other epimer, but with the
amount of vinyl epoxide increased to four equivalents and
with the solvent 1,2-dichloroethane replaced by chloroform.
The yield of 32 could thereby be improved to 58%. Interest-
ingly, there was also a slight change in the ratio of the
geometrical isomers (E/Z 90:10, Scheme 5).
Received: April 20, 2006
Revised: August 5, 2006
Published online: September 14, 2006
By analogy with earlier studies by Brown and co-workers
in which the addition of ethyl and cyclopentyl radicals
(generated from the corresponding boranes) to alkynes was
described,[14] we attempted a similar transformation involving
the highly functionalized xanthate 34 derived from levulinic
acid (Scheme 6).[15] The reaction with alkyne epoxide 35 was
Keywords: allylation · allylic alcohols · radical reactions ·
vinyl epoxides · xanthates
.
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[6] The use of triethylborane/O2 worked efficiently for most
xanthates, except a-xanthyl ketones, which proved to be capri-
cious. The problem was traced to an unexpected reduction which
gave the parent ketone. It appears that the spin density on the
oxygen atom in the intermediate a-ketoradical is sufficient to
induce attack on triethylborane before the addition to the olefin
actually occurs. Hydrolysis upon workup of the enol boronate
gave the ketone. Analogous reduction of iodides were previously
observed by Oshima and co-workers and very recently exploited
by Lambert and Danishefsky in the total synthesis of
UCS1025A: a) K. Nozaki, K. Oshima, K. Utimoto, Bull.
Chem. Soc. Jpn. 1991, 64, 403 – 409; b) T. H. Lambert, S. J.
Danishefsky, J. Am. Chem. Soc. 2006, 128, 426 – 427.
Scheme 6. Additions to an alkyne epoxide and to a vinyl aziridine.
somewhat sluggish, although clean and gave rise to the
unusual allene 36, which was isolated as a mixture of
diastereoisomers. We also found it was possible to replace
the epoxide by an aziridine ring (Scheme 6). The reaction of
adduct 20 with aziridine 37 was also slow, but furnished 38 in
useful yield (53%). As far as we are aware, vinylic aziridines
have not hitherto been used in this manner.
It is clear from this preliminary study that the triethyl-
borane-mediated radical reaction of xanthates with vinyl
epoxides and vinyl aziridines represents a very powerful tool
for the formation of carbon–carbon bonds under mild
conditions. The approach is flexible, convergent, very easy
to implement experimentally, and involves readily available
starting materials and reagents. It is also worth noting that
nonracemic products could be prepared by starting with
optically pure vinyl epoxides, obtained for example by the
kinetic resolution procedure developed by Jacobsen and co-
workers.[16]
[7] A. Suzuki, N. Miyaura, M. Itoh, H. C. Brown, G. W. Holland, E.-
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Experimental Section
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[10] For the preparation of cyclobutenone 12, see: A. A. Frimer, H.
Pizem, Tetrahedron 1999, 55, 12175 – 12186.
[11] G. Binot, B. Quiclet-Sire, T. Saleh, S. Z. Zard, Synlett, 2003, 382 –
387.
Typical procedure for the radical addition on vinyl epoxides and
aziridine: Triethylborane (1.0m solution in hexane, 2 equiv) was
added every 30 minutes over two hours to a stirred solution of the
xanthate (1equiv) and vinyl epoxide (2 equiv) in CH 2Cl2
(1mLmmol À1) under nitrogen at room temperature. During the
addition, the syringe needle was lowered into the solution. Further-
more, a small volume of air (about a quarter of the volume of the
borane solution) was introduced by syringe following each addition of
triethylborane. After stirring the reaction mixture overnight, the
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