ChemInform Abstract: Transition-Metal-Diene Complexes in Organic Synthesis. Part 1. Highly Selective Oxidation of Fe(CO)3-Cyclohexadiene Complexes. Synthesis of 4b,8a-Dihydrocarbazol-3-ones and First Total Synthesis of Carbazomycin A.

ChemInform Abstract

Cyclization of the iron complex (IIIa) according to A) leads directly to 4-deoxycarbazomycin B (V). The transformation includes the intermediate iron complex (IV) (space group P2₁/c; Z=4) which can be isolated when (IIIa) is treated with FeCl3/silica gel. Selective oxidation of (III) with commercial MnO₂ gives the iminoquinones (VI) which are cyclized according to A) to yield the products (VIIa) (space group P1; Z=2) and (VIIb). Demethylation of (VIIa) followed by methylation of the resulting alcohol affords (V) with 48% overall yield from (IIIa), thus proving that the route to aromatic carbazoles via the iminoquinone cyclization is the favorable one. The utility of the above methodology is demonstrated by the total synthesis of the antibiotic carbazomycin A (XI). The isolation of the by-product (XII) reveals that the iminoquinone cyclization route is also suitable for the preparation of (XI).