SolutionsAddition-cyclization reactions
Exercise 1
Which carbonyl compound is obtained, after hydrolysis and decarboxylation, from the glycidic ester formed in the Darzens condensation of cyclohexanone with ethyl chloroacetate?
Cyclohexanecarbaldehyde. Hydrolysis gives the glycidic acid; on decarboxylation the C–C bond to the carboxyl breaks and the epoxide opens to the enol of the aldehyde, which tautomerizes. The net result is the conversion of C=O into CH–CHO: a one-carbon homologation.
Exercise 2
Carvone (an enone) is treated separately with dimethylsulfonium methylide and with dimethylsulfoxonium methylide. What type of product does each reagent give, and why are they different?
The more reactive, less stabilized sulfonium ylide adds rapidly and irreversibly to the carbonyl (1,2-addition) and gives a spiro epoxide, leaving the conjugated double bond untouched. The sulfoxonium ylide, stabilized by S=O, adds reversibly to the carbonyl; conjugate (1,4) addition is slower but leads to a stable enolate, which closes the cyclopropane by displacing DMSO. The cyclopropyl ketone is obtained (the C=O is retained).