A Novel Approach to Chiral, Nonracemic Pyrrolidines by 5-exo-trig Diastereoselective Radical Cyclization on Acrylamides Derived from (−)-8-Aminomenthol
摘要:
alpha,beta-Unsaturated amides supported on perhydro-1,3-benzoxazines derived from (-)-8-aminomenthol as chiral auxiliaries undergo regio- and stereoselective 5-exo-trig radical cyclization leading to diastereomeric five-membered lactams. These cyclization products are transformed into enantiopure 3,4-disubstituted pyrrolidines by reduction with aluminum hydride followed by removal of the menthol appendage.
experimental conditions for the preparation of trimethylsilyl enolethers 3 from α-phenylseleno aldehydes 1, compatible with a limited deselenenylation of the substrate, were studied. Starling from α-phenylseleno ketones 2, the enoxysilanes 4, bearing a vinylic PhSe group, are the major products when triethylamine is the base. Trimethylsilyl enolethers 5 with an allylic PhSe group arc also formed beside 4
The stereoselectivity in reactions of 2-oxy-substituted radicals of type B was investigated. As expected, minimization of allylic1,3-strain was the major controllingfactor. Under standard conditions, only a modest level of stereoselectivity was observed. E.g., deuteration of the benzylated radical (R1 = benzyl) gave diastereoisomer ratios ≤ 2:1. Use of a bulky protective group on the O-atom (R1 =