Base-promoted P-elimination

Reduction (lithium aluminium hydride/tetrahydrofuran) of the tetraester 34 to the tetraol 35, followed by chlorination (thionyl chloride), afforded 36 in good yield. This tetrachloride was then subjected to base-promoted P-elimination (potassium rerf-butoxide) giving the desired bisdiene 37 in quantitative yield without purification. The sensitivity of 37 toward both thermal and photochemical degradation and its propensity to polymerize necessitated its immediate use following its preparation. 

While 1,2-cleavage of penams by base-promoted P-elimination requires activation with thiophiles, penam sulfones readily react with DBN to give azetidinone sulfinic acids by an ElcB mechanism [98]. Stoodley et al., who discovered this reaction, also developed the sulfinic acid chemistry necessary for application to penem synthesis, namely the conversion of the ring-opened products into either 4-alkylsulfonyl- or 4-acyldithioazetidinone derivatives. 

The reaction proceeds by addition of the nucleophile to the ortho- or/ ara-posi-tion of the nitroarene and subsequent base-promoted P-elimination of XH from the intermediate a-adduct 1. The VNS reaction is very general and the ortho/para re-gioselectivity is controlled by steric factors. Tertiary earbanions replaee hydrogen in flie VNS reaction exclusively in the para-position (Scheme 36.2). 

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