| Abstract
| - A new photoaldol route to α-amino-β-hydroxy carboxylic acid esters is initiated by the photocycloaddition of aromatic or aliphatic aldehydes to 5-methoxyoxazoles. The 4-unsubstituted 5-methyloxazole 1 gave the cycloadducts 8a−f in high yields and excellent exo-diastereoselectivities.Hydrolysis of 8a−f gives the N-acetyl α-amino-β-hydroxy esters 9a−f, which could be subsequentlyconverted into the corresponding Z-didehydro α-amino acids 10a−f. Quartenary α-amino-β-hydroxyesters 12, 14, 16, 18, and 20, which are stable against dehydration, were obtained from the4-alkylated 5-methoxyoxazoles 2−6, in most cases highly erythro-selective due to the high degreeof stereocontrol (exo) at the photocycloaddition (to give 11, 13, 15, 17, and 19) level. The relativeconfigurations of the N-acetyl α-amino-β-hydroxy esters were determined by NMR spectroscopyand comparison with chiral pool-derived compounds as well as by X-ray structure determinationof the ester 23, formed by hydrolysis of the cycloadduct 22, derived from photocycloaddition ofpropionaldehyde to the isoleucine-derived oxazole 21.
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