A New Synthesis of Thiophenes

Cyclocondensation of Hydroxylamine with 1,3‐Bis(het)arylmonothio 1,3‐Diketones and 1,3‐Bis(het)aryl‐3‐(methylthio)‐2‐prop­enones: Synthesis of 3,5‐Bis(het)arylisoxazoles with Complementary Regioselectivity

“…As part of our ongoing research programme directed towards the design and development of new methods for the synthesis of substituted and condensed five‐ and six‐membered heterocycles using polarized ketene dithioacetals and other organosulfur compoundsa as versatile three‐carbon 1,3‐bielectrophilic building blocks,19 we have previously reported the highly regioselective synthesis of 5‐(or 3‐)methylthio‐3‐(or 5‐)‐substituted pyrazoles20a and 5‐(or 3‐)methylthio‐3‐(or 5‐)arylisoxazoles17b by cyclocondensation of α‐oxoketene dithioacetals (or β‐oxadithioesters) with arylhydrazines and hydroxylamine, respectively, under different conditions. In a continuation of these studies, we became interested in 1,3‐substituted‐monothio 1,3‐diketones of the general structure 1 ,21,22 and the corresponding 1,3‐bis(het)aryl‐3‐(methylthio)‐2‐propenones 2 , a new class of potentially useful three‐carbon 1,3‐bielectrophile building blocks that are virtually unexplored 22e–22g. Based on these intermediates, we have recently reported efficient regioselective synthesis of unsymmetrically substituted 1‐aryl‐3,5‐bis(het)aryl pyrazoles 3 and 4 with complementary regioselectivity by cyclocondensation of either 1 or 2 with arylhydrazines under different conditions (Scheme ) 23a.…”