Acyclic compound with £-proline

It is assumed that both reactions start with the formation of the imine of the acyclic structure of carbohydrates with proline. After this initial step, the cyanoacetate 86 dictates aKnoevenagel/Michael cascade, whereas by employment of isocyanoacetate 87 a nucleophilic addition of the carbon atom of the isocyano group and subsequent rearrangement reaction are observed. An explanation for this different behavior of the isostructural compounds (ethyl esters 86 and 87) depicted in Scheme 2.18 is given by the different sites of highest nucleophilicity as described by Mayr et al. [41]. These mechanistic considerations are depicted in Scheme 2.19. 

Cheap and readily available L-proline has been used numerous times for the intermediate and reversible generation of chiral iminium ions from a,/ -unsaturated carbonyl compounds. For example, Yamaguchi et al. reported in 1993 that the rubidium salt of L-proline catalyzes the addition of di-iso-propyl malonate to the acyclic Michael acceptors 40a-c (Scheme 4.13), with enantiomeric excesses as high as 77% [22], With 2-cycloheptenone and 2-cyclohexenone as substrates ca 90% yield and ee of 59% and 49% were obtained. Later the enantioselectivity of this process was increased to a maximum of 88% ee in the addition of di-tert-butyl malonate to the E-pentenone 40a in the presence of 20 mol% Rb-L-prolinate and 20 mol% CsF [23], Taguchi and Kawara employed the L-proline-derived ammonium salts 41a and... 

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