Quinone epoxides. Part III. Stereospecific reductions with metal hydrides

Abstract

Reduction of 1,4-naphthoquinone epoxide with sodium borohydride (1 equiv.) is shown to give the isomeric r-2,3-epoxy-3,4-dihydro-c-4-hydroxynaphthalen-1(2H)-one (IIIa) and r-2,3-epoxy-3,4-dihydro-t-4-hydroxy-naphthalen-1(2H)-one (IIIb) in a ratio of ca. 11 : 1. With the same reagent juglone epoxide (5-hydroxy-1,4-naphthoquinone epoxide) is selectively and stereoxpecifically reduced at the chelated carbonyl to give r-2,3-epoxy-3,4-dihydro-c-4,5-dihydroxynaphthalen-1(2H)-one (IX) as the major product. The role of the chelating hydroxy group is discussed. The configurations of the products are established on the basis of the n.m.r. spectra of the corresponding 2,3-epoxy-1,2,3,4-tetrahydronaphthalene-1,4-diols. N.m.r. evidence also establishes the preferred conformations.