Traceless Stereoinduction in the One-Pot Assembly of All Three Rings of Hexahydrodibenzopyrans

摘要

The chemistry associated with o-quinone methides is quite extensive and includes a diverse array of cleverly designed tactics for their generation, a host of elegant applications in the total synthesis of natural products, their natural role as key intermediates in biosynthetic pathways, and as reactive intermediates, the focal point of a range of both natural and intelligently designed bio-activity. The Diels-Alder reactions of o-quinone methides with alkenes represents a well-established method for the synthesis of chromanes. For o-quinone methides bearing a substituent on the methide carbon (2, R~1 ≠ H), the stereochemistry at that carbon in the precursor is lost and the cycloadduct is produced in racemic form (eq 1). Asymmetric Diels-Alder reactions of o-quinone methides that produce an asymmetric carbon at the benzylic position are essentially nonexistent, which is undoubtedly a function of the high reactivity of these fleeting intermediates.