Studies toward Total Synthesis of Non-aromatic Erythrina Alkaloids. (5). Cyclization of Perhydro-2-epoxymethylene-6-oxo-6H-pyrido[2, 1-i]indole-1-acetic Acid : Synthesis of 8-Oxoerythroidans

Abstract

Treatment of the epoxy-acid (7) with acidic reagents produced a variety of products, hydroxy-γ- and δ-lactones (8 and 9), unsaturated δ-lactones (10, 11, and 18), and seco derivatives (15 and 17), depending on the reagents used. Alkaline hydrolysis of the hydroxy-γ-lactone (8) followed by acidification yielded an isomeric δ-lactone 21, which on further contact with acid regenerated the γ-lactone (8). The hydroxy-δ-lactones, 9 and 21, were stereoselectively dehydrated with thionyl chloride-pyridine to the unsaturated lactones, 18 and 10, respectively. Thus, successive treatments of the epoxy-acid (7) with NaOH, acidification, and dehydration of the product with thionyl chloride-pyridine gave 10 in an overall yield of 86%, and 10 was isomerized to the conjugated lactone (22) on treatment with 1, 8-diazabicyclo[5.4.0]undec-7-ene. For the above unsaturated δ-lactones, 22, 10, 11, and 18, the names α-, β-, γ-, and δ-erythroidans are proposed, respectively.