Practical Asymmetric Synthesis of (S)-4-Ethyl-7,8-dihydro-4-hydroxy-1H-pyrano[3,4-f]indolizine- 3,6,10(4H)-trione, a Key Intermediate for the Synthesis of Irinotecan and Other Camptothecin Analogs

A practical asymmetric synthesis of (S) 4-ethyl-7,8-dihydro-4-hydroxy-1H-pyrano[3,4-f]indolizine-3,6,10(4H)-trione (1), a versatile intermediate for the synthesis of camptothecin analogs, was developed. Commercially available citrazinic acid is converted in four steps into the 2-chloro-6-methoxypyridine 5. An ortho-directed metalation followed by reaction with a formamide produces an aldehyde with the required 2,3,4,6-substituted pyridine (6) with high regioselectivity. After refunctionalization of the aldehyde, the chloropyridine is converted into an ester by a facile palladium-mediated carbonylation reaction. Wittig reaction and racemic osmylation produce the diol 16 which is resolved by an efficient lipase resolution to an ee > 99%, and a one-pot recycle of the unwanted diol enantiomer was developed. A series of high-yielding oxidation and deprotection steps convert (S)-16 into the pyridone 25, which is then converted into 1 with an ee > 99.6%.