Synthesis of erythrina and related alkaloids. XXII. Intramolecular cyclization approach. 1): New synthetic route to erythrinan and related heterocycles and synthesis of (.+-.)-3-demethoxy-erythratidinone.
Synthesis of erythrina and related alkaloids. XXII. Intramolecular cyclization approach. 1): New synthetic route to erythrinan and related heterocycles and synthesis of (.+-.)-3-demethoxy-erythratidinone.
Synthesis of erythrina and related alkaloids. XXII. Intramolecular cyclization approach. 1): New synthetic route to erythrinan and related heterocycles and synthesis of (.+-.)-3-demethoxy-erythratidinone.
Heating of cycloalkanone-2-carboxylate with β-arylethylamine, and oxalylation of the resulting enamino-ester followed by Lewis acid-catalyzed intramolecular cyclization afforded various erythrinan-type heterocycles in excellent yields. This method is widely applicable not only to the synthesis of erythrinans but also to that of A-nor and A-homo analogs and ring-D variants of erythrinan. The alkoxycarbonyl group on the products was readily removed by a new decarbalkoxylation method (heating with magnesium chloride-dimethyl sulfoxide combination). Thus, starting from 2-ethoxycarbonyl-4, 4-ethylenedioxy-cyclohexanone, the 2, 8-dioxo-erythrinan derivative (35) was synthesized in several steps in high yield, and was readily converted to the natural Erythrina alkaloid, 3-demethoxyerythratidinone.