Stereoselective Synthesis of Cyclic Ethers Using Vinylogous Sulfonates as Radical Acceptors:  Effect of <i>E</i>/<i>Z</i> Geometry and Temperature on Diastereoselectivity.

Abstract

Resins were first washed with NMP (10 × 1 mL) and dichloromethane (10 × 1 mL), and then dipeptides were cleaved from the resin using a mixture of trifluoroacetic acid (TFA, 1.80 mL) and H2O (200 µl) (1h), taken to dryness, and analyzed by HPLC (linear gradient from 10% B to 90% B over 20 min).Retention times of diastereomeric peaks as determined using dipeptide 14 prepared from racemic D,L-leucine indicated diastereomers eluting at 18.6 min and 19.1 min.Enantiomeric contamination of azido acid 4 was then determined by similar analysis of dipeptide 15, where diastereomeric contamination accounted for an area less than 3% of that observed for the major diastereomer.These results indicated greater than 94% enantiomeric purity.Acknowledgment.Appreciation is expressed to Dr.