Strecker other substrates

Preparation of a-monofluoromethyl amino acids is not easy. Indeed, although the amino acids can easily be hydroxymethylated, substitution of the hydroxyl by a fluorine atom is not always easy (SF4, DAST). On the other hand, preparation of monofluoroketones (substrates of the Strecker reaction) involves the use of highly toxic fluoroacetonitrile. ° ... 

Hydroformylations of several other vinyl substrates were carried out (Table 4). Vinyl acetate is a particularly interesting substrate, since, as mentioned earlier, it gives 2-acetoxypropanal, a precursor in the Strecker synthesis of threonine. The product can be converted to 2-hydroxypropanal, a useful intermediate in the synthesis of steroids, pheromones, antibiotics and peptides. 

Shibasaki and co-workers applied (BINOL)Al(III)-derived catalyst 5a, previously developed for the cyanation of aldehydes [28], to the asymmetric Strecker reaction. This catalyst proved to be highly enantioselective for both aromatic and a,p-unsaturated acyclic aldimines (>86% ee for most substrates) (Scheme 8) [63-65]. Aliphatic aldimines underwent cyanide addition with lower levels of enantioselectivity (70-80% ee). A significant distinction of 5 relative to other catalysts is, undoubtedly, its successful application to the hydrocyanation of quinolines and isoquinolines, followed by in situ protection of the sensitive cx-amino nitrile formed (this variant of the Strecker reaction is also known as the Reissert reaction [66]). Thus, Shibasaki has shown that high enantioselectivities (>80% ee for most substrates) and good yields are generally obtainable in the Reissert reaction catalyzed by 5b [67,68]. When applied to 1-substituted... 

Two other types of catalysts have been investigated for the enantioselective Strecker-type reactions. Chiral N-oxide catalyst 24 has been utilized in the trimethylsilyl cyanide promoted addition to aldimines to afford the corresponding aminonitriles with enantioselectivities up to 73% ee [14]. Electron-deficient aldimines were the best substrates, but unfortunately an equimolar amount of catalyst 24 was used in these reactions. The asymmetric Strecker addition of trimethylsilyl cyanide to a ketimine with titanium-based BINOL catalyst 25 gave fast conversions to quarternary aminonitriles with enantiomeric excesses to 59%... 

Several catalysts have been developed for the asymmetric organocatalyzed hydrocyanation of carbonyls and for the Strecker reaction. This chapter divides the catalysts into several subgroups, defined according to important structural motifs responsible for catalytic activity. Each catalyst group will be discussed in detail with regard to substrate scope, hmitations, and other important factors. In addition, mechanistic insights will be provided, if transition states are satisfyingly revealed by experimental work or in silica studies. 

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