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Total Synthesis of (−)-Fumiquinazolines A, B, C, E, H, and I. Approaches to the Synthesis of Fiscalin A

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journal contribution
posted on 19.12.2002, 00:00 by Barry B. Snider, Hongbo Zeng
The first syntheses of (−)-fumiquinazolines A, B, and I, which proceed in 14 steps from protected tryptophan, anthranilic acid, leucine, and alanine in 7% overall yield, are described. Tricycle 30 was formed by a palladium-catalyzed cyclization. Oxidation of 30a with saccharine-derived oxaziridine 21 for fumiquinazolines A and B and oxidation of 30b with dimethyldioxirane for fumiquinazoline I selectively formed the appropriate imidazoindolone stereoisomers. Application of the Ganesan−Mazurkiewicz cyclization completed the syntheses. Efficient 14-step syntheses of (−)-fumiquinazolines C (7) and E (3) and a 15-step synthesis of (−)-fumiquinazoline H (8) using FmocNHCH(CH2SePh)CO2H as a dehydroalanine precursor that spontaneously eliminated benzeneselenol without oxidation under the cyclization conditions are also reported. Model 86 for fiscalins A with the H and OH anti to each other has been prepared, but the procedure that worked for the model failed with the fully functionalized side chain.