Remarkably tuned regioselectivities are obtained in the condensation of 3,5-dihydroxyphthalide (1) with several α,β-unsaturated aldehydes using DBU and neutral conditions. The linear (65-81%) and angular (51-60%) bioactive natural and unnatural benzopyran derivatives are produced exclusively in one step and are described via phenol-keto resonance.
3,5-dihydroxyphthalide - phenol-keto resonance - regioselective condensation - α,β-unsaturated aldehydes - benzopyran synthesis