O-tert-Butyl-L-serine

Condensation of a silylated beta-D-galactopyranosylaldehyde (3) with isolevoglucosenone (4) in the presence of Et(2)AlI provided bicyclic enone 5. Subsequent addition of BnNHOMe gave adduct 6, which was converted into 4-O-acetyl-1,6-anhydro-3-C-[(1 R)-1,3,4,5,7-penta-O-acetyl-2,6-anhydro-D-glycero-L-manno-heptitol-1-C-yl]-2-azido-2,3-dideoxy-beta-D-galacto-hexopyranose after liberation of the 2-amino group, its transformation into a 2-azido moiety, desilylation, and peracetylation. Ring-opening of the 1,6-anhydro galactopyranosyl unit and O-glycosidation with Fmoc-Ser-O-tBu afforded a 5:1 mixture of alpha- and beta-galactosides. Treatment with CH(3)COSH gave pure N-[(9H-fluoren-9-ylmethoxy)carbonyl]-{4,6-di-O-acetyl-3-C-[(1 R)-2,6-anhydro 1,3,4,5,7-penta-O-acetyl-D-glycero-L-manno-heptitol-1-C-yl]-2-[(N-acetyl)amino]-2,3-dideoxy-alpha-D-galactopyranosyl}-l-serine tert-butyl ester (2), a protected form of a C-disaccharide analogue of the Thomsen-Friedenreich (or T) epitope (beta-D-Galp-(1-->3)-alpha-D-GalNAcp) alpha-O-conjugated to L-serine.

We have analyzed free chiral amino acids (aspartate and serine) in the human frontal cortex at different ontogenic stages (from 14 weeks of gestation to 101 years of age) by HPLC with fluorometric detection after derivatization with N-tert-butyl-oxycarbonyl-L-cysteine and o-phthaldialdehyde. Exceptionally high levels of free D-aspartate and D-serine were demonstrated in the fetal cortex at gestational week 14. The ratios of D-aspartate and of D-serine to the total corresponding amino acids were also high, at 0.63 and 0.27, respectively. The concentration of D-aspartate dramatically decreased to a trace level by gestational week 41 and then remained very low during all postnatal stages. In contrast, the frontal tip contained persistently high levels of D-serine throughout embryonic and postnatal life, whereas the D-amino acid content in adolescents and aged individuals was about half of that in the fetuses. Because D-aspartate and D-serine are known to have selective actions at the NMDA-type excitatory amino acid receptor, the present data suggest that these D-amino acids might play a pivotal role in cerebral development and functions that are related to the NMDA receptor.

A synthetic strategy for the preparation of two orthogonally protected methyl esters of the non-proteinogenic amino acid 2,3-l-diaminopropanoic acid (l-Dap) was developed. In these structures, the base-labile protecting group 9-fluorenylmethyloxycarbonyl (Fmoc) was paired to the p-toluensulfonyl (tosyl, Ts) or acid-labile tert-butyloxycarbonyl (Boc) moieties. The synthetic approach to protected l-Dap methyl esters uses appropriately masked 2,3-diaminopropanols, which are obtained via reductive amination of an aldehyde prepared from the commercial amino acid Nα-Fmoc-O-tert-butyl-d-serine, used as the starting material. Reductive amination is carried out with primary amines and sulfonamides, and the process is assisted by the Lewis acid Ti(OiPr)4. The required carboxyl group is installed by oxidizing the alcoholic function of 2,3-diaminopropanols bearing the tosyl or benzyl protecting group on the 3-NH2 site. The procedure can easily be applied using the crude product obtained after each step, minimizing the need for chromatographic purifications. Chirality of the carbon atom of the starting d-serine template is preserved throughout all synthetic steps.