Threonines protected

For this class of reactions, only a few examples which proceed with reasonable diastereoselectivity are known. Allylation of a-methoxycarbamate 1, easily obtained as a 1 1 mixture of isomers by anodic oxidation of protected threonine, produces an 83 17 mixture of enantiomers on treatment with trimethyl(2-propcnyl)silanel03. Cyanation with trimcthylsilyl cyanide proceeds less stereoselectively (67 33 93 % yield). 

Other N-protected threonine derivatives as well as N-protected serine derivatives can be converted to the corresponding (erf-butyl esters using this method [34]. 

Asymmetric hydrogenation of racemic 2-substituted (3-keto esters to produce 2-substituted (3-hydroxy esters with two new chiral centers is a powerful method, and it is useful in the production of other pharmaceutical intermediates. The methodology can be used in the preparation of protected threonine derivatives 34, where 34d and 34e are key intermediates for the anti-Parkinsonian agent, L-Dops (35). 

Following the building-block approach, artificial glycopeptides were synthesized by Wong et al. [40], as small-molecule mimics of sialyl Lewis . They utilized the fucosylated amino acid (9-allyl carboxylic ester building block (Scheme 14.21). The properly protected, threonine... 

Hepta-2,6-di-0-methyl-p-CD enhanced solubility (by up to 100-fold) such that a-NeuNAc-(2 3)-p-D-Gal-(l- 3)-a-D-GalNAc as a protected threonine glycoside was made enzymically from the corresponding a-D-GalNAc glycoside in 50% yield. ... 

Also carbanions of serine and threonine esters, protected in the form of oxazolines, are capable of addition to nitropyridines to form the corresponding adducts that can be oxidized into a-(nitropyridyl) amino acid derivatives [132]. It should be mentioned that addition of the carbanion of the protected threonine to nitropyridine proceeds with a high diastereoselectivity, which is controlled by the second chiral center present in the oxazoline ring (Scheme 38) [133],... 

More selective examples have used imines derived from amino acids, including phenylalanine and, more generally useful, O-protected threonine... 

SCHEME H.8 Diastereoselective ONSH with carbanion of protected threonine ester [16]. 

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