Fluorinated Amino Acids. Part 3. Synthesis of β-Difluoromethyl-mtyrosine. -A synthesis of the chiral difluoromethyl carboxylic acid (X) has been developed to access the absolute configuration of a oxazole (XI) synthesized in the preceding paper. (XI) as well as its diastereomer are transferred into the corresponding β-difluoromethyl-m-tyrosine (cf. (XIII)). -(SABOL, J. S.; BRAKE, N. W.; MCDONALD, I. A.; Tetrahedron Lett. 35 (1994) 12, 1821-1824 Marion Merrell Dow Res. Inst., Cincinnati, OH 45215, USA; EN)
A series of compounds were studied which had increasing alkyl chain length between an aromatic group and a tertiary amine. In this homologous series, a maximum in retention versus alkyl chain length was found for a wide range of mobile phases using either a silica gel column or a cyanopropyl-bonded silica gel column. One nonaqueous mobile phase using an octadecyl-bonded silica gel column also showed this maximum. The selectivity between E and Z geometric isomers found with the shorter chain analogues decreased with increasing chain length greater than about pentyl, with eventual complete loss of resolution. A few normal-phase solvents produced a reversal of the usual E-Z elution order. 1 H NMR assignments are described for the individual geometric isomers. NMR studies and ab initio calculations indicate that a folded conformation in solution is possible for the longer chain analogues. The unusual retention patterns are most likely due to a combination of localized adsorption and analyte solution conformation effects.
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