Pyridinium ylides in syntheses of naphthopyrandiones and in regioselective syntheses of acylated anthraquinones related to fungal and bacterial metabolites

Abstract

Improvements have been made in the use of acylated pyridinium ylides for the transformation of 2-methyl-1,4-naphthoquinone into derivatives (15) and (16) of naphtho[2,3-c]pyran-5,10-dione, containing furan and thiophene groups. The substitution and cyclisation steps can be combined effectively by using 2-phenoxymethyl- instead of 2-methyl-naphthoquinone. The use of better leaving groups than phenoxy (especially 4-nitrophenoxy) allows the quinone to react with two proportions of ylide and leads regiospecifically to 1-aroyl-2-arylanthracene-9,10-diones such as (20a). If the leaving group is nuclear bromine as in 2-bromo-3-methyl-1,4-naphthoquinone, another reaction with 2 mol equiv. of ylide leads to complex red intermediates of type (31) which in contact with alumina are quantitatively converted into the regioisomeric 2-aroyl-3-aryianthracene-9,10-diones such as (22a). The structures have been determined by standard methods but special features of the NMR spectra are reported including a case of extreme line broadening by traces of iron. Mechanisms are suggested for the diverse reactions between the quinones and the ylides.