Syntheses of Enantiomerically Pure P-Amino Acids

During this reaction, two new chiral centres are formed. The formation of one diasteromer with high selectivity shows the potential of the chirality transfer from carbohydrate templates. Conversion of the 2,3-diphenyl alanine into the corresponding 3-amino propanol proved the erythro configuration of the major diaster-eomer. 

Carbohydrate-derived homoallylamines have been demonstrated as being useful chiral synthons for the stereoselective synthesis of P-amino acids. With allyltribu-tylstannane or allyltrimethylsilane as the nucleophiles, glycosyl imines form homoallylamines with high efficiency [29-31]. Promoted by SnCl4, the nucleophile al-lyltributylstannane attacks from the stoically less hindered side. 

The nucleophile attack of the allylstannane again proceeds at the unshielded Si-side of the imine and finally leads to the desired homoallylamines with (S)-config-uration. These homoallylamines (16) are detached from the carbohydrate auxiliary by acid-catalyzed hydrolysis, and can be converted into the enantiomerically pure P-amino acids (17) in a two-step procedure via oxidative degradation of the terminal double bond. 

Reactions of glycosyl imines derived from aliphatic aldehydes must be carried out at low temperature due to their higher sensibility towards anomerization. Yields and diastereoselectivity rank in the same region as those achieved for galactosyl homo-allylamines. The results of different stereoselective allylations, are summarized in Table 4.1. 

Recently, P-amino acids have been reported attractive for the construction of P-peptides [33] which adopt interesting secondary structures [34]. A number of alternative stereoselective syntheses of P-amino acids have been reviewed recently [35]. 

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