Enantioenriched selenonium ylides have been generated by addition of benzyl bromide to C2 symmetric (2R,5R)-2,5-dimethylselenolane in the presence of NaOH, and subsequently reacted with a variety of aldehydes to give oxiranes with excellent enantiomeric excesses (a catalytic version has been achieved); also, an aliphatic cyclic hypervalent dibromoselenurane structure has been demonstrated by X-ray analysis
A new chrial organosulfide was synthesized through an unexpected Wagner–Meerwein rearrangement. This organosulfide could catalyze the epoxidation reaction of various aromatic aldehydes smoothly with benzyl bromide to give trans-diaryl epoxides in satisfactory yields (60–84%) with excellent diastereoselectivities (trans:cis=95:5–100:0) and good to excellent enantioselectivities (86–96% ee).
Six ferrocenyl sulfides, exhibiting planar and central chiralities, have been screened as a catalytic source of asymmetric sulfonium ylides. A one-pot reaction has been achieved, involving the addition of an aldehyde, benzyl bromide, 20% molar equivalent of the ferrocenyl sulfide, sodium iodide in a mixture of tert-butanol and water. The best results were observed with enantiopure sulfide 3a, bearing a tert-butyl group. Good yields of stilbene oxides were obtained, with enantiomeric excesses ranging from 74% to 94%. trans/cis-Diastereomeric ratios ranged from 60:40 to 86:14. The chiral sulfide was recovered. An unexpected case of stereoconvergence was observed with diastereoisomers 3a and 3b. A model is proposed to account for the asymmetric induction, based on a conformation locked by the tert-butyl group and the interplay of planar and remote central chiralities. (C) 2004 Elsevier Ltd. All rights reserved.
Pyridyl and Furyl Epoxides of More Than 99% Enantiomeric Purities: The Use of a Phosphazene Base