Diastereoselective Ammonium-Directed Epoxidation in the Asymmetric Syntheses of Dihydroconduramines (+)-C-2, (-)-C-2, (+)-D-2, (+)-E-2, (+)-F-2, and (-)-F-2.

Epoxidations (40% aq HBF ₄ then m-CPBA) of racemic cis-2-( N-benzylamino)cyclohex-3-en-1-ol and racemic cis-2-( N, N-dibenzylamino)cyclohex-3-en-1-ol proceed with very high levels of diastereoselectivity (>95:5 dr). The latter is in direct contrast to the epoxidation of the corresponding trans-diastereoisomer (which proceeds with essentially no selectivity), showing that the relative configuration of the substrate dramatically influences the diastereoselectivity in these instances. Meanwhile, epoxidations of enantiopure (1 R,2 S,α R)-2-[( N-α-methylbenzyl)amino]cyclohex-3-en-1-ol and (1 S,2 R,α R)-2-[( N-α-methylbenzyl)amino]cyclohex-3-en-1-ol [surrogates for the enantiomers of cis-2-( N-benzylamino)cyclohex-3-en-1-ol] proceed with complete diastereoselectivity (>95:5 dr) under the same conditions, showing that neither the presence of the α-methyl group nor the relative configuration of the α-methylbenzyl stereocenter have an effect upon the established level of diastereoslectivity in these cases. In contrast, epoxidations of enantiopure (1 R,2 S,α R)-2-[ N-benzyl- N-(α-methylbenzyl)amino]cyclohex-3-en-1-ol and (1 S,2 R,α R)-2-[ N-benzyl- N-(α-methylbenzyl)amino]cyclohex-3-en-1-ol [surrogates for the enantiomers of cis-2-( N, N-dibenzylamino)cyclohex-3-en-1-ol] proceed with lower diastereoselectivity (∼70:30 dr). Thus, the presence of the α-methyl group has a detrimental effect on the established level of diastereoselectivity in these cases (although again the relative configuration of the α-methylbenzyl stereocenter is unimportant). The diastereoselective epoxidation pathway is used to enable the asymmetric syntheses of six hitherto unknown, enantiopure dihydroconduramines (+)-C-2, (-)-C-2, (+)-D-2, (+)-E-2, (+)-F-2, and (-)-F-2 (>99% ee in each case).