Chiral Pyrrolines via Asymmetric 1,3-Dipolar Cycloaddition Reactions

Significance

A Ru-catalyzed three-component diastereoselective route to chiral pyrrolines is reported. It features an in situ formation of a chiral 8-phenylmenthol-tethered Ru-carbene followed by the generation of a dipolarophile which undergoes 1,3-dipolar cycloaddition with acetylenes yielding pyrrolines in good yields and with good ­diastereoinduction. Studies using various N-benzylidene imines allowed the conclusion that imines bearing electron-donating substituents give higher yields compared to those bearing electron-withdrawing substituents. Unfortunately, the cleavage of the auxiliary was not demonstrated in the study. Furthermore, the isolation of a Ru-carbene complex was achieved, thereby providing some mechanistic insight.