Enantioselective Syntheses of Multiply Functionalized Six-Membered Carbocycles from Nitroalkenes and ,-Unsaturated -Ketoesters using Chiral Catalysts Derived from Cobalt(III) and 1,2-Diphenylethylenediamine
Academic Article
Overview
Research
Identity
Additional Document Info
Other
View All
Overview
abstract
AbstractCondensations of the title educts R4CH=CH(C=O)CH2CO2Et and R6CH=CHNO2 in the presence of NEt3 and the catalyst [Co((S,S)dpen)3]3+2ClB(C6F5)4 (both 10mol%, 0C; dpen=1,2diphenylethylenediamine) afford R6CHCH(NO2)CH(R4)CH2C(OH)CCO2Et, with three contiguous carbon stereocenters, as >99:1 to 85:15 mixtures of diastereomers (ten examples). The major diastereomers are isolated in 8163% yields and 9776% ee. When R4 and R6 are both aryl groups, additions of 1,1,3,3tetramethylquanidine/CH3CN epimerize the CHNO2 moieties, affording 78:22 to 67:33 mixtures of the new/old diastereomers (2h, RT). Alternatively, epimerization to >99:1 mixtures occurs spontaneously over 4 weeks in hexanes/isopropanol. These transformations entail two consecutive Michael additions, and two crystal structures confirm the relative and absolute product configurations. These assignments are supported by NMR data in solution and literature studies of similar compounds. Overall, catalyst [Co((S,S)dpen)3]3+2ClB(C6F5)4 affords superior yields and enantio or diastereoselectivities compared to those previously reported for the title reaction.
