Abstract
Selected hetaryl and aryl thioketones react with acetylenecarboxylates under thermal conditions in the presence of LiClO4 or, alternatively, under high-pressure conditions (5 kbar) at room temperature yielding thiopyran derivatives. The hetero-Diels–Alder reaction occurs in a chemo- and regioselective manner. The initially formed [4 + 2] cycloadducts rearrange via a 1,3-hydrogen shift sequence to give the final products. The latter were smoothly oxidized by treatment with mCPBA to the corresponding sulfones.
Highlights
A series of recent publications evidence that, in contrast to earlier opinions, thioketones are useful building blocks for the preparation of diverse sulfur heterocycles [1,2,3]
The goal of the present study is to examine the reactions of diverse hetaryl thioketones with both 2a and methyl propiolate (2b)
We modified the procedure by using THF as a solvent and LiClO4 as a known, efficient catalyst applied frequently in diverse Diels–Alder reactions [17]
Summary
A series of recent publications evidence that, in contrast to earlier opinions, thioketones are useful building blocks for the preparation of diverse sulfur heterocycles [1,2,3]. Studies performed by Huisgen and coworkers are of special importance and they resulted in the formulation of the name ‘superdipolarophiles’ for aromatic thioketones [4,5,6]. Sauer and coworkers called them ‘superdienophiles’ based on kinetic studies [7,8]. Thiobenzophenone (1a) was reported to react as a heterodiene smoothly with cyclooctyne, dicyanoacetylene, and dimethyl acetylenedicarboxylate (2a) to give [4 + 2] cycloadducts of type 3a, which spontaneously rearrange via a 1,3-hydrogen shift yielding rearomatized products of type 4a (Scheme 1) [9,10,11]. The hetero-Diels–Alder reaction of 4-substituted analogues of 1a with in situ generated benzyne is known [13]. Heteroaromatic thioketones are reported to undergo a hetero-Diels–Alder reaction with dienophiles such as maleic anhydride, acrylonitrile, styrene, and α-chloroacryloni-
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