Figure 12

1994 a related heptonic amino acid was prepared by nitromethane addition to glucose, reduction of the nitro group, and oxidation of the primary hydroxyl group (Fig.

Fuchs and Lehmann also prepared 7-amino C-glycoside carboxylic acids from the corresponding C-glycoside nitriles, which were hydrolyzed to carboxylic acids. The C6 hydroxyls were selectively converted to azides and subsequently reduced with catalytic hydrogenation [29]. Although Hanessian and Haskell mentioned the possibility that naturally occurring sugar amino acids could be linked via amide bonds rather than glycosides, Fuchs and Lehmann were the first to reduce this to practice in a synthetic arena (see below).

Kim and Hollingsworth prepared a C-glycoside of V-acetylglucosamine by al-kylation of a pyranosyl bromide with malonate anion, giving an V-acetyl y-sugar amino acid after decarboxylation (Fig. 13) [30].

Galantinic acid is a somewhat related compound that can be considered to be a C-glycoside pyranosyl amine. It is an e-amino acid that was thought to be a component of Galantin I, a naturally occurring peptide-based antibiotic. In 1992, however, Sakai and Ohfune showed that galantinic acid is a by-product of the isolation procedure, rather than a component of the natural product [31]. In earlier work, Ohfune and Kurokawa reported a stereocontrolled synthesis of galantinic acid from a serine-derived epoxide. Cuprate opening of the epoxide gave a conjugated ester that underwent Michael addition upon deprotection (Fig. 14) [32]. This example illustrates a subtle relationship between a-amino acids and sugar amino acids, since galantinic acid has structural entities related to both classes of compounds.

Perhaps the interplay between sugar amino acids and a-amino acids is more clearly demonstrated in the synthesis of a-d-glucosyl-(^)-alanine reported by Axon and Beckwith [33]. In the reaction, (2^)-methyleneoxazolidinone was treated with 2,3,4,6-tetra-O-acetyl glucosyl iodide in the presence of sodium cyanoborohydride and tributyltin chloride to give the a-C-glycoside in 88% yield (Fig. 15).

Figure 13

l_l ^NHBoc

0 0

Post a comment