Two Approaches to the Aromatic Core of the Aminonaphthoquinone Antibiotics

Two complementary approaches are presented for the synthesis of the quinone chromophores of the naphthoquinone ansamycins and related natural products. The first involves the use of an improved protocol for the manganese­(III) acetate mediated cyclization of 5-aryl-1,3-dicarbonyl compounds to β-naphthols, leading to the simple, scalable preparation of building blocks suitable for the synthesis of naturally occurring aminonaphthoquinones. The second approach involves the Diels–Alder reaction of a series of new, ester-containing Danishefsky-type dienes with N-protected aminobenzoquinones to allow more expeditious access to similar intermediates.