A New Thiophene Synthesis and its Synthetic Applications

Abstract

Since diketo sulfides (3-thiapentane-1,5-dione derivatives), both symmetrically substituted and unsymmetrically substituted ones, are readily obtainable from α-haloketones and have three functional groups, i.e., carbonyl, active methylene (methine), and divalent sulfur, which are suitably located for intramolecular transformations, they serve as excellent starting materials for organic syntheses. One of our synthetic applications with these compounds is a new thiophene synthesis, which involves intramolecular reductive carbonyl coupling with a low-valent titanium reagent followed by acid-catalyzed dehydration of the resulting thiolane-3,4-diols. The method is very versatile and allows the preparation of a wide variety of thiophenes including overcrowded 3,4-di-tert-butyl- and 3,4-di-1-adamantylthiophenes, angle-strained 1,2,4,5-tetrahydrodicyclobuta[b,d]thiophene and related derivatives, and functionally interesting α,β-type oligothiophenes. The method is also applicable to the preparation of selenophenes. Quite a number of constrained molecules, which have two bulky substituents on the vicinal positions of alkenes, benzenes, and five- and six-membered heteroaromatic rings, can be synthesized from 3,4-di-tert-butyl- and 3,4-di-1-adamantylthiophenes via thiophene 1,1-dioxides and their properties were investigated in some detail.