A RING EXPANSION PROCEDURE BASED ON THE TANDEM DEALKOXYCARBONYLATION MICHAEL ADDITION-REACTION

A three-step ring expansion procedure has been developed to convert me thyl alpha,alpha-dialkyl-1-cyclopenteneacetates to highly functionaliz ed cyclohexaneacetic esters. The procedure involves (1) ozonolytic dou ble bond cleavage, (2) chemoselective Wittig olefination of the aldehy de carbonyl of the resulting keto aldehyde, and (3) tandem dealkoxycar bonylation-Michael addition to close the ring. The transformation proc eeds best when the a-carbon of the starting acetate is quaternary, and thus, the method is unique in yielding hindered 2,2-dialkyl-3-oxocycl ohexaneacetic esters. The reaction sequence is easily performed, and o verall yields of 40-50% are readily achieved. The synthetic and mechan istic details of this process as well as optimization studies are pres ented. An analogous five-membered cyclization process was also investi gated. Tandem dealkoxycarbonylation-Michael addition of 1-ethyl methyl (E)-7,7-dimethyl-6-oxo-2-octenedioate was found to afford a 24% isola ted yield of ethyl 2,2-dimethyl-3-oxocyclopentaneacetate by a disfavor ed 5-[enolendo]-exo-trig process.