Controlling the Reaction Mechanism to Construct Dihydrocoumarins

Significance

Enders and colleagues report the synthesis of chromans 4 and dihydrocoumarins 5 by a dienamine-catalyzed Michael reaction- hemiacetalization cascade. Despite using the same substrates 1 and 2 and the same catalyst 3 - used previously to construct isomeric chromans 9 - the authors were able to enforce a different catalytic regime. Thus, through a judicial choice of acidic additives, the formation of dienamine 7 (pathway A) could be favored over iminium activation (pathway B), which leads to an undesired oxa-Michael reaction. The reported reaction offers entry to a different type of chroman scaffolds 4 as well as ­dihydrocoumarin derivatives 5.