Stereoselective Approach to Hydroxyindolizidines: Protection/Deprotection of the Nitrone Functionality via Cycloaddition/Retrocycloaddition
作者:Franca M. Cordero、Martina Gensini、Andrea Goti、Alberto Brandi
DOI:10.1021/ol006125q
日期:2000.8.1
The enantiomericallypure indolizidine (-)-21 has been synthesized starting from L-malic acid. The key intermediate 20 has been assembled through an intramolecular 1,3-dipolar cycloaddition of a nitrone generated in situ by retrocycloaddition from isoxazolidine 17 or 18. The configuration of the new three stereocenters was set up with complete control in the cycloaddition step. The presented synthetic
allowing the selective synthesis of both enantiomers, (−)-24 and (+)-24. This strategy required protection of the nitrone functionality to avoid racemization of the unprotected hydroxy nitrone during the introduction of the dipolarophile moiety. Protection/deprotection were achieved by cycloaddition/retro-cycloaddition reactions. The different propensity of some pyrrolo[1,2-b]isoxazolidines to undergo