Asymmetric synthesis of podophyllotoxin analogs

Two analogs of podophyllotoxin, with the same absolute stereochemistry as the natural product, have been synthesized from the cycloadduct between α-hydroxy-α′-phenyl-o-quinodimefhane and the fumarate of S-methyl lactate. After initial attempts to produce the cycloadduct from photochemically generated α-hydroxy-α′-phenyl-o-quinodimethane failed, a study of the thermal generation and reaction of α-hydroxy-o-quinodimethane with the fumarate and acrylate of S-methyl lactate was made. A comparison was made of the diastereoselectivity of these cycloaddition reactions to those previously reported, in which the o-quinodimethane was generated photochemically. The α-hydroxy-o-quinodimethane was produced both by the known thermolysis of benzocyclobutenol and by thermolysis of 1-hydroxy-1,3-dihydrobenzo[c]thiophene-2,2-dioxide. The diastereomeric excess for the cycloaddition reactions was found to be greater than 95% with modest (ca. 55%) isolated yields of the major cycloadducts. Following these model studies, it was found that α-hydroxy-α′-phenyl-o-quinodimethane produced thermally from 1-hydroxy-3-phenyl-1,3-dihydrobenzo[c]thiophene-2,2-dioxide could be added to the fumarate of S-methyl lactate with high diastereoselectivity and good yield. The product of this reaction was converted to the podophyllotoxin analogs 7 and 17. Keywords: o-quinodimethanes, asymmetric, Diels–Alder, lactate, podophyllotoxin, lignan.