Asymmetric Synthesis of Bi(thio)xanthylidene Overcrowded Alkenes

Overcrowded alkenes are a fascinating class of inherent dissymmetric molecules that attract considerable interest for instance as chiroptical molecular switches and unidirectionally rotary motors. A practical synthesis route towards enantiomerically pure overcrowded alkenes is an important goal. We report here the development of an asymmetric synthesis of bi(thio)xanthylidenes. The upper and lower part of the desired alkenes were first coupled to a chiral template. Intramolecular coupling with partial stereocontrol, separation of diastereoisomers and subsequent removal of the templates gave stable enantiomers of bithioxanthylidene. The absolute configuration was determined by X‐ray analysis. In the case of bixanthylidene, stereocontrol during the intramolecular coupling reaction was complete, and it was found that the presence of a binaphthol bridging unit prevents the fast racemization process of the bixanthylidene, which it normally exhibited at ambient temperature. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2006)