N-Dioxides and Bis-Formamides as Catalysts for the Strecker Reaction

In 2001, axial chiral N,N -dioxides were introduced as promoters for the asymmetric Strecker reaction of aldimines. However, stoichiometric quantities of 

By replacing the linker in previously used N,N -dioxides with a rigid axial chiral (S)-BlNOL-derived backbone, high yields and enantioselectivities were obtained from aryl (entries 1,2), heteroaryl (entry 3), aliphatic (entry 4), and alkenyl (entry 5) methyl keto-imines 64 by use of just 2mol% 65 (Table 30.7) [39]. In addition, the use of cyclic keto-imine 60 (entry 6) as well as ethyl, propyl (entry 7), and phenyl substituted aryl keto-imines (entry 8) as substrates were possible, yielding products 66 in high to excellent enantiopurities. Despite convenient reaction conditions, the reaction times were lengthy ( 72h). 

The concept of connecting two L-prolinamides portions over a chiral backbone structure was applied on the C2-symmetrical bis-formamide catalyst 68 for the 

R = aryl 74-99% yield 73-86%ee ( 99%ee after recrystallization) R = fert-butyl 92% yield 73%ee R = 2-furyl 94% yield 43%ee 

The same group achieved further improvements in the three-component Strecker reaction by applying the trarts-4-hydroxy-L-proUnamide derived N,N -dioxide catalyst 71 (Table 30.8) [41]. In general, high yields and good to high enantioselectivities were obtained by the reaction of aryl (entries 1-3), a,(3-unsaturated (entry 5), a-unbranched (entry 6), and a-branched (entry 7) aldehydes 70 with (l,l-diphenyl)methylamine and TMSCN. Poor stereoinduction was 

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