Practical synthesis of (20S)-10-(3-aminopropyloxy)-7-ethylcamptothecin, a water-soluble analogue of camptothecin

A robust, practical synthesis of (20S)-10-(3-aminopropyloxy)-7-ethylcamptothecin (T-2513, 5), which is a water-soluble analogue of camptothecin, has been developed. The key step in this synthesis is a highly diastereoselective ethylation at the C20 position by using N-arylsulfonyl-(R)-1,2,3,4-tetrahydroisoquinoline-3-carboxylic acid ester as a chiral auxiliary, which affords the key intermediate ethyl-(S)-2-acyloxy-2-(6-cyano-5-oxo-1,2,3,5-tetrahydroindolizin-7-yl)butanoate (8k) in 93% yield and 87% de. Optically pure compound 8k was obtained by a single recrystallization from acetone and its further elaboration through Friedlander condensation afforded compound 5. This synthesis does not require any chromatographic purification steps and can provide compound 5 on a multi-gram scale in 6.3% overall yield (16 steps).