Elcb Eliminations cleavage

The Fmoc group is a lineal descendent of the p-nitroethyl acetates whose base-catalysed elimination by an Elcb mechanism (see section 1.2.6), first propounded by Chattaway in 1936, laid the foundation for the numerous progeny which have been tailored to amino and carboxyl protection. Fmoc groups undergo rapid non-hydrolytic cleavage on treatment with simple bases (ammonia, piperidine, morpholine), usually in polar solvents such as dimethylforma-mide or acetonitrile, to liberate the yV-terminus of a peptide in the free base form. The conditions are mild enough to preclude -elimination in the sensitive O-glycosylserine derivatives [Scheme 8.84].2 Other bases include DBU and TBAF.2 ... 

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. 

Nitrile-forming eliminations from (E)-2,4-dinitrobenzaldehyde 0-aryloximes promoted by tertiary amines in acetonittile have been studied kinetically. The results have been interpreted in terms of a highly ElcB-like TS with extensive C -H bond cleavage... 

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