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[摘要]：The preparation of (2S,3S)- and (2R,3S)-2-fluoro and of (3S)-2,2-difluoro-3-amino carboxylic acid derivatives, 13, from alanine, valine, leucine, threonine, and beta 3h-alanine (Schemes 1 and 2, Table) is described. The stereochemical course of (diethylamino)sulfur trifluoride (DAST) reactions with N,N-dibenzyl-2-amino-3-hydroxy and 3-amino-2-hydroxy carboxylic acid esters is discussed (Fig. 1). The fluoro-beta-amino acid residues have been incorporated into pyrimidinones (1113; Fig. 2) and into cyclic beta-tri- and beta-tetrapeptides 1719 and 2123 (Scheme 3) with rigid skeletons, so that reliable structural data (bond lengths, bond angles, and Karplus parameters) can be obtained. beta-Hexapeptides Boc[(2S)-beta 3hXaa(alpha F)]6OBn and Boc[beta(3)hXaa(alpha,alpha F(2))]6-OBn, 2426, with the side chains of Ala, VMonMonMonMonal, and Leu, have been synthesized (Scheme 4), and their CD spectra (Fig. 3) are discussed. Most compounds and many intermediates are fully characterized by IR- and 1H-, 13C- and 19F-NMR spectroscopy, by MS spectrometry, and by elemental analyses, [a]D and melting-point values. |
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