Phenylalanine methyl ester, initiation

A/-Methacryloyl-D,L-phenylalanine Methyl Ester. The enantiodiscrim-inating influence of cyclodextrin in the free-radical polymerization of the racemic monomer iV-methacryloyl-D,L-phenylalanine methyl ester (2e) (41) was investigated for the first time. 2e was synthesized in 99% yield by the reaction of phenylalanine methyl ester hydrochlorides with methacryloyl chloride in the presence of triethylamine. The racemic monomer was subsequently added to an aqueous solution of RAMEB yielding a clear colorless solution of complexed monomer. Homopolymerizations were carried out in the presence of redox initiator at various temperatures. It was showed that the L-enantiomer is enriched in the residual monomer mixture. This observation confirms that complexed D-enantiomer polymerizes faster than the complexed L-enantiomer. No polymerization of 2e was observed without cyclodextrin under the same conditions. 

The unusual double melting point behavior of aspartame has been confirmed by differential scanning calorimetry (DSC) [22, 52]. This behavior was interpreted [52] in terms of initial conversion of the methyl ester to I-a-aspartyl-i-phenylalanine, followed by formation of the substituted 2,5-diketopiperazine, although no clear evidence was produced to support this view. However, the alternative explanation that the 2,5-diketopiperazine was produced in a concerted manner from simultaneous self-aminolysis and demethanolation of aspartame seemed more likely to... 

The initial medicinal chemistry route to the azabicyclo[3.3.0]octane-3-carboxylic acid produced the azabicyclo system in a diastereoselective but racemic manner, and required a classical resolution to achieve enantioenriched material (Teetz et al., 1984a, b 1988). Reaction of (R)-methyl 2-acetamido-3-chloropropanoate (43) and 1-cyclopentenylpyrrolidine (44) in DMF followed by an aqueous acidic work-up provided racemic keto ester 45 in 84% yield (Scheme 10.11). Cyclization of 45 in refluxing aqueous hydrochloric acid provided the bicyclic imine, which was immediately reduced under acidic hydrogenation conditions. The desired cis-endo product 46 was obtained upon recrystaUization. The acid was protected as the benzyl ester using thionyl chloride and benzyl alcohol, providing subunit 47 as the racemate. Resolution of 47 was accomplished by crystallization with benzyloxy-carbonyl-L-phenylalanine or L-dibenzoyl-tartaric acid. 

Some of the pathways of animal and bacterial metabolism of aromatic amino acids also are used in plants. However, quantitatively more important are the reactions of the phenylpropanoid pathway,173-1743 which is initiated by phenylalanine ammonia-lyase (Eq. 14-45).175 As is shown at the top of Fig. 25-8, the initial product from phenylalanine is trails-cinnam-ate. After hydroxylation to 4-hydroxycinnamate (p-coumarate) and conversion to a coenzyme A ester,1753 the resulting p-coumaryl-CoA is converted into mono-, di-, and trihydroxy derivatives including anthocyanins (Box 21-E) and other flavonoid compounds.176 The dihydroxy and trihydroxy methylated products are the starting materials for formation of lignins and for a large series of other plant products, many of which impart characteristic fragrances. Some of these are illustrated in Fig. 25-8. 

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