1,5‐Dipolare Elektrocyclisierung von Acyl‐substituierten ‘Thiocarbonyl‐yliden’ zu 1,3‐Oxathiolen

1,5‐Dipolar Electrocyclization of Acyl‐Substituted ‘Thiocarbonyl‐ylides’ to 1,3‐Oxathioles The reaction of α‐diazoketones 15a, b with 4,4‐disubstituted 1,3‐thiazole‐5(4 H )‐thiones 6 ( Scheme 3 ), adamantanethione ( 17 ), 2,2,4,4‐tetramethyl‐3‐thioxocyclobutanone ( 19 ; Scheme 4 ), and thiobenzophenone ( 22 ; Scheme 5 ), respectively, at 50–90° gave the corresponding 1,3‐oxathiole derivatives as the sole products in high yields. This reaction opens a convenient access to this type of five‐membered heterocycles. The structures of three of the products, namely 16c, 16f , and 20b , were established by X‐ray crystallography. The key‐step of the proposed reaction mechanism is a 1,5‐dipolar electrocyclization of an acyl‐substituted ‘thiocarbonyl‐ylide’ ( cf. Scheme 6 ). The analogous reaction of 15a, b with 9 H ‐xanthen‐9‐thione ( 24a ) and 9 H ‐thioxanthen‐9‐thione ( 24b ) yielded α,β‐unsaturated ketones of type 25 ( Scheme 5 ). The structures of 25a and 25c were also established by X‐ray crystallography. The formation of 25 proceeds via a 1,3‐dipolar electrocyclization to a thiirane intermediate ( Scheme 6 ) and desulfurization. From the reaction of 15a with 24b in THF at 50°, the intermediate 26 ( Scheme 5 ) was isolated. In the crude mixtures of the reactions of 15a with 17 and 19 , a minor product containing a CHO group was observed by IR and NMR spectroscopy. In the case of 19 , this side product could be isolated and was characterized by X‐ray crystallography to be 21 ( Scheme 4 ). It was shown that 21 is formed – in relatively low yield – from 20a . Formally, the transformation is an oxidative cleavage of the CC bond, but the reaction mechanism is still not known.