An improved procedure for the conversion of certain coumarins to 2,2-dimethylchromenes: efficient transformation of umbelliferone to precocene 1

Abstract
An improved method for transforming certain coumarins to 2,2-dimethylchromencs has been developed. Umbelliferone (1b) was efficiently converted to precocene 1 (2b) (73%) by etherification to herniarin (1c) with tetramethylorthocarbonate, followed by reaction with methyllithium and cyclization of the resulting phenolic alcohol on silica gel.