Intramolecular Wittig reaction, chemoselective

A chemoselective synthesis of tetrasubstituted furans has been accomplished using an intramolecular Wittig reaction as the key step. The presence of an additional electron-withdrawing group in the a-position of Michael acceptors controlled the chemoselectivities of the phosphorus ylides presumed to be involved. Mechanistic studies by DFT calculations were carried out to understand the origin of the chemoselectivity observed. 

An efficient synthesis of trisubstituted oxazoles via chemoselective D-acylations and intramolecular Wittig reactions has been also reported. A plausible reaction mechanism was based on the existence of expected and rearranged isomeric oxazoles. 

An intramolecular cycloaddition also occurred with 3-ylidenepiperazine-2,5-diones such as 124 or 125, obtained by Wittig-Horner-Emmons reaction from phosphonate 121 and aldehydes 122 or 123, respectively. The products of the Diels-Alder reaction are the bridged bicyclo[2.2.2]diazaoctane rings 126 and 127 that have been found in biologically active secondary metabolite such as VM55599 and brevianamide A. The different type of structures employed in this case requires a chemoselective reaction in order to produce the expected products as single diastereoisomers after 20 days (Scheme 18) <2001JOC3984>. 

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