Photoinduced molecular transformations. Part 135. New synthesis of taiwanin C and justicidin E based on a radical cascade process involving β-scission of alkoxyl radicals generated from 3- and 8-aryl-1-ethyl-1,2-dihydrocyclobuta[b]naphthalen-1-ols prepared by thermolysis of (Z)-tert-butyl 3-amino-3-(bicyclo[4.2.0]octa-1,3,5-trien-7-yl)propenoates

Abstract

A new general synthesis of naturally occurring phthalide lignans, based on a radical cascade process triggered by a regioselective β-scission of the alkoxyl radicals generated by photolysis of the hypoiodites of 8-aryl-1-ethyl-1,2-dihydrocyclobuta[b]naphthalen-1-ols, is described. Two phases are involved in the present synthesis of phthalide lignans; the first is a new general synthesis of tert-butyl 4-aryl-3- and 4-aryl-8-aminonaphthalene-2-carboxylates by an electrocyclic reaction of o-quinonedimethides thermally generated from (Z)-tert-butyl 3-amino-3-(bicyclo[4.2.0]octa-1,3,5-trien-7-yl)propenoates; the second is a transformation of the protected 4-aryl-3-aminonaphthalene-2-carboxylic acids into the phthalide lignans. This latter phase involves their successive conversions into 3- and 8-arylcyclobuta[b]naphthalen-1 (2H)-ones via the formation of a benzyne intermediate, and then into 3- and 8-aryl-1-ethyl-1,2-dihydrocyclobuta[b]naphthalen-1-ols, followed by β-scission of the alkoxyl radicals generated by photolysis of their hypoiodites, generated in situ with the mercury(II) oxide–iodine reagent in benzene. Simultaneous syntheses of the naturally occurring phthalide lignans taiwanin C and justicidin E were thus achieved.