1502
K. L. DHUMASKAR, C. BHAT, AND S. G. TILVE
problem associated with this sequence is the handling of the intermediate aldehyde,
particularly in the case of aldehydes, which are unstable, volatile, and toxic. This prob-
lem has been overcome to a great extent by the development of what is called as tan-
[1]
dem oxidative procedure (TOP) or domino reactions. These reactions slightly differ
from the ‘‘one-pot’’ procedures wherein first aldehyde is obtained and then immedi-
[
2]
ately reacted with a stabilized Wittig reagent. In TOP, oxidation is carried out in
the presence of a Wittig reagent so that as soon as aldehyde is formed it can further
condense with the Wittig reagent. In this procedure the crucial part is the choice of
the oxidizing agent, as it should not react with the Wittig reagent to oxidize it or over-
oxidize the initial alcohol further to an acid or ester. There are several reagents avail-
able for the selective oxidation of a primary alcohol to aldehyde but all cannot be used
[
2]
for TOP. For example, the most popular Swern oxidation has to be used in a sequen-
tial manner and not in TOP as intermediate oxalyl chloride readily reacts with phos-
phorane. The first such TOP was reported by Barrett et al. in 1997 wherein primary
alcohol was condensed with stable phosphorane in the presence of Dess–Martin per-
[
3]
iodinane as an oxidizing agent. Benzoic acid was added to the reaction mixture to
expedite the reaction in dimethylsulfoxide (DMSO)–CH Cl (1:6) solvent mixture.
In 1998 Wie and Taylor reported TOP for activated alcohols such as allylic, benzylic,
2
2
[
1]
and propargylic alcohols using excess of MnO2. The procedure is suitable for sensi-
tive aldehydes that are normally difficult to isolate. In the same year Shuto and
[4]
coworkers used BaMnO4 again for activated alcohol in TOP. In 1999 Davies and
[
5]
McKervey extended the MnO TOP for N-protected b-amino alcohols under reflux-
2
ing acetonitrile conditions. In the same year Crich and Mo reported use of iodoxyben-
[6]
zoicacid (IBX) in dimethylsulfoxide (DMSO) for homologation of nucleosides.
[7]
Catalytic aerobic oxidation using Ru catalyst for TOP was reported by Kim et al.
[
8]
in 2001. The popular Corey reagent pyridinium chlorochromate (PCC) in combi-
nation with NaOAc was reported from our laboratory in 2004. Recently Karama
et al. in 2010 reported synthesis of a-bromo-a,b-unsaturated esters using a combi-
[9]
nation N-bromosuccinimide (NBS), stable Wittig reagent, and primary alcohol
under ultrasonic conditions. Because PCC is not a choice of reagent for the oxidations
of allylic and propargylic alcohols, we were interested in using pyridinium dichromate
[
4]
(
PDC) for such TOP. It was reported by Shuto and coworkers that PDC is not suit-
able for such TOP. Hence we thought of investigating this reaction as it would be
complementary to our PCC methodology for synthesizing homologated compounds
of activated alcohols. In this article, we describe PDC-mediated tandem oxidative
Wittig reaction procedure for the synthesis of a,b-unsaturated esters.
Scheme 1. Tandem oxidative Wittig reaction.