Intramolecular [4+2] cycloadditions of conjugated ynones and related species ; Studies directed toward the total systhesis of glycinoeclepin A

Abstract

The intramolecular [4 + 2] cycloaddition reactions of c,-alkynyl carbonyl compounds are described. This reaction, the first heterocyclic variant of the enyne cycloaddition reaction, affords a product with a dihydroisobenzofuran ring system. For this reaction, we propose a mechanism in which a highly strained heterocyclic allene intermediate undergoes an unusual rearrangement leading to a 3-furfuryl carbene. A 1,2-C-H insertion then produces the polycyclic furan product. A detailed analysis of the scope and mechanism of this reaction is presented. The synthetic utility of the method for the synthesis of complex organic molecules is illustrated by two sequences demonstraing further transformations of the dihydroisobenzofuran products. A two-step formal benzannulation process generates a tetrahydroanthracene derivative. Ozonolysis of a 7-oxabicycloheptene derivative prepared from a dihydroisobenzofuran affords a product that contains the core oxabicyclo[6.2.1]undecane ring system of eleutherobin and the sarcodictyin family of natural products. Glycinoeclepin A is the natural hatching stimulus agent of the soybean cystnematode. A new strategy for the synthesis of an advanced A-ring intermediate in the total synthesis of this important compound is presented. This strategy provides the key A-ring enyne intermediate in seven steps from 2,2-dimethylcyclohexanedione, utilizing a novel acid-catalyzed cyclization reaction of a hydroxy enedione.