Stereospecific Synthesis of Pyrrolidines with Varied Configurations via 1,3-Dipolar Cycloadditions to Sugar-Derived Enones

Enantiomerically pure pyrrolidines have been obtained by 1,3-dipolar cycloaddition of stabilized azomethine ylides and sugar enones (dihydropyranones) derived from pentoses. Thus, the S-enone (menthyl 3,4-dideoxy-(1S)-pent-3-enopyranosid-2-ulose) was prepared from D-xylose, while the R analogue was obtained from L-arabinose. The dipoles were generated in situ from alpha-arylimino esters of common amino acids (glycine, alanine, or phenylalanine) and aromatic aldehydes (benzaldehyde, 3-formylpyridine and 4-methoxybenzaldehyde). Under optimized conditions, the cycloaddition reactions were highly diastereo- and regioselective to yield, in most of the cases, a very major adduct of the 16 theoretically possible. The diastereoselectivity relies on the strict stereocontrol exerted by the stereogenic center of the pyranone. Thus, the (S)-enone, derived from D-xylose, gave tetrasubstituted pyrrolidines having a defined stereochemistry for the four stereocenters of the ring, while they had the opposite configuration when starting from the (R)-dihydropyranone. Furthermore, some endo-cycloadducts underwent isomerization of the carbons vicinal to the nitrogen atom to afford pyrrolidines with a rather unusual stereochemistry for the direct dipolar cycloadditions.