Lactone Enolates of Isochroman-3-ones and 2-Coumaranones: Quantification of Their Nucleophilicity in DMSO and Conjugate Additions to Chalcones.

Owing to stereoelectronic effects, lactones often deviate in reactivity from their open-chain ester analogues as demonstrated by the CH acidity (in DMSO) of 3-isochromanone (p K a = 18.8) and 2-coumaranone (p K a = 13.5), which is higher than that of ethyl phenylacetate (p K a = 22.6). We have now characterized the reactivity of the lactone enolates derived from 3-isochromanone and 2-coumaranone by following the kinetics of their Michael reactions with p -quinone methides and arylidenemalonates (reference electrophiles) in DMSO at 20 °C. Evaluation of the experimentally determined second-order rate constants k 2 by the Mayr-Patz equation, lg k 2 = s N ( N + E ), furnished the nucleophilicity parameters N (and s N ) of the lactone enolates. By localizing their position on the Mayr nucleophilicity scale, the scope of their electrophilic reaction partners becomes predictable, and we demonstrate a novel catalytic methodology for a series of carbon-carbon bond-forming reactions of lactone enolates with chalcones under phase transfer conditions in toluene.