Acid-or Base-Promoted Transformations

Ring enlargements or ring contractions are commonly implemented methodologies to build more complex cyclic targets. This is exemplified by the sequence 

Basic conditions and a nucleophilic base such as PPh3 or DABCO were required to perform the sequence SN2/[2 -I- 2]-cycloaddition displayed in Seheme 5.52 [135]. The allenyl ester 149 reacts with allylamine 150, leading to diastereomerically pure bicycles 151. 

Dimethyl acetylenedicarboxylate was involved in a three-component one-pot reaction that furnished functionalized eyelobutenes 153 from 4-aryl-2,4-diketoesters 152 and dimethyl phosphite (Seheme 5.53) [136]. Trimethyl phosphite adds first to dimethyl acetylenedicarboxylate, and then the resulting adduct is attacked by an enolate derived from the diketoester. The last step is the cyclization with concomitant loss of trimethyl phosphate. The whole process is claimed to be diastereoselective. 

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