Synthesis of the Mammea coumarins. Part 4. Stereochemical and regiochemical studies, and synthesis of (–)-mammea B/BB

Abstract

Acylation of phloroglucinol with (S)-2-methylbutyryl chloride followed by Pechmann condensation with ethyl 3-oxohexanoate, or acylation of 5,7-dihydroxy-4-propylcoumarin with (S)-2-methylbutyryl chloride, gave an 8-(S)-acylcoumarin that was C-alkylated to afford natural (–)-mammea B/BB; the configuration of the 2-methylbutyryl moiety in the natural coumarins was thus demonstrated to be (S). Surangin B was likewise prepared as a mixture of (1′S,2″S)- and (1′R,2″S)-stereoisomers. The crystal structure and conformation of a 6-acylcoumarin that establishes the orientation of other synthetic coumarins is reported.