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Karthikeyan et al.
INTRODUCTION
Cis fused ring systems are prevalent in many natural products that
exhibit biological activity[1] and there are many methods to construct
these ring systems. The Diels-Alder reaction is one such important
synthetic tool in the hands of the organic chemists to build such ring
systems with remarkable stereo-selectivity. On these lines, the Diels-
Alder reactions of 2-cycloalkenones with cyclic dienes assumes
importance, although this reaction has not been utilized to its maximum
potential mainly because of the poor reactivity of these substrates. In this
context, we have developed a simplified route of greater synthetic utility
towards cis fused ring systems.
2-Cycloalkenones are poor dienophiles and take longer periods to
react even with reactive cyclic dienes like cyclopentadiene. It becomes
further difficult with less reactive dienes like cyclohexadiene and these
reactions require high temperature, pressure with the aid of expensive
catalysts and take several days for completion.[1–3]
The novelty of the article lies in the application of the microwave
irradiation technique to overcome difficulties present in the conventional
thermal reactions. This procedure is extremely simple to carry out in an
Erlenmeyer flask under atmospheric conditions. In this article, we would
like to submit the results of the reaction of a series of 2-cycloalkenones
with cyclopentadiene(1) and cyclohexadiene(2) under microwave irradia-
tion conditions. The use of microwave irradiation to accelerate reaction
rates has become almost a standard method to improve the yields and to
reduce the reaction time.[4,5] The poor reactivity of 2-cycloalkenones
makes them ideal to probe these reactions using microwave radiation
in order to make the Diels-Alder intermediates for exploitation as
building blocks for further synthetic transformations.
Thus, the Diels-Alder adducts of cycloalkenones (3a–d) and cyclic
dienes (1 or 2) were synthesized in open Erlenmeyer flasks using a
commercial microwave oven (700 W) within 2 min with good yields,
making these reactions far more simpler to carryout compared to the
previously known hazardous procedures (Sch. 1). Interestingly, it was
also observed that microwave irradiation increased the selectivity of
endo products (Table 1). The synthesis was carried out with the solvents,
toluene and chlorobenzene. It was noted that chlorobenzene provided
better yields than toluene (Table 2). This may be attributed to the
higher dielectric constant of chlorobenzene compared to toluene.[6]
Cyclohexadiene is very unreactive and reactions under thermal
conditions take place very slowly[3] i.e., 3b and 2 on heating neat at
175ꢀC for 24 h yields only 6% of the adduct whereas under microwave
Karthikeyan
