Control of competing N-H insertion and Wolff rearrangement in dirhodium(II)-catalyzed reactions of 3-indolyl diazoketoesters. synthesis of a potential precursor to the marine 5-(3-indolyl)oxazole martefragin A.

Dirhodium(II)-catalyzed reaction of 3-indolyl α-diazo-β-ketoester 25 in the presence of hexanamide results in competing metal carbene N−H insertion and Wolff rearrangement. The corresponding phenyl diazoketoester 32, on the other hand, gives only the product of N−H insertion, suggesting that the indole moiety is more prone to 1,2-rearrangement. The competing processes were investigated in a range of 3-indolyl α-diazo-β-ketoesters (36, 38, 40, 44); these studies established that the Wolff rearrangement could be effectively suppressed by the presence of a strong electron-withdrawing group on the indole nitrogen. Dirhodium(II) catalysts were also more effective than copper or Lewis acid catalysts in favoring the insertion process. The products of N−H insertion, the ketoamides (26, 47, 49, 51, 53), were readily cyclodehydrated to the corresponding 5-(3-indolyl)oxazoles. The N−H insertion/cyclodehydration methodology was used in a formal synthesis of the marine natural product martefragin A. Thus the N-Boc hom...