Bisdehydrooxa annulenes

Bisdehydrothia-annulene (94) and the annulated bisdehydrooxa- and thia-annulenes (99 and 100) were synthesized73 in two steps as shown in Scheme 4, the key initial step entailing cross-condensation of 45 with properly designed dialdehydes. Acyclic models (93, 97, and 98) were reported to be quite unstable, and this appears to have prevented their... 

The H-n.m.r. spectra of237 and 238 do not indicate any conformational mobility, in contrast to bisdehydrothia[13]- (227) and [17]annulenes (223 and 224) . It was found that the sulphides 237 and 238 are clearly paratropic 16tc electron systems. The H-n.m.r. spectrum of mono-cw-bisdehydrooxa[15]annulene (239) shows that it is atropic molecular models suggest that 239, unlike a -trans analogues (237 and 238), cannot be planar. 

The methylene-bridged bisdehydrooxa- (240) and thia- (241a,b) [17]annulenes, which were found to be diatropic, have been prepared according to the following reaction sequence ... 

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