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Two novel approaches to N,N-diprotected chiral α-amino acid esters, based on selected imidodi-carbonates as amine synthons, have been explored. Thus, N-alkylation of these substrates was smoothly accomplished by the Gabriel or Mitsunobu methods as well as by the use of triflates. Ethyl (R,S)-2-bromopropionate underwent nucleophilic substitution when treated with the potassium salt of selected imidodicarbonates in dry dimethylformamide to furnish the corresponding fully blocked (R,S)-alanines in high yield. The chirality of ethyl (S)-lactate was largely conserved when it was condensed with free imidodicarbonates and tosylcarbamates under conventional Mitsunobu conditions. The yield of the corresponding N,N-di-protected ethyl (R)-alaninate was strongly dependent on the electron-withdrawing properties of the imidodicarbonate alkyl groups. Thus, Boc2NH gave < 5% of the product whereas Troc-NH-Z afforded the corresponding analogue in 83% yield under comparable conditions. On the other hand, triflates of various lactic acid esters reacted smoothly with the lithium salt of Boc2NH, also with clean inversion, as a result of which, after selective removal of two blocking groups, the N-protected alanine of opposite configuration could be isolated in high yield and excellent stereochemical purity. Both methods have been used for the synthesis of 15N-labelled N-protected (R)- and (S)-alanines, suitable for direct application to peptide synthesis.

An efficient synthesis of 15N-labelled chiral Boc-amino acids by triflate alkylation of di-tert-butyl [15N]imidodicarbonate is reported. Both enantiomers of Boc-leucine and -phenylalanine were synthesized from commercial α-amino acids of opposite configuration viaα-hydroxy carboxylic acids provided by diazotization, thus extending the scope of an earlier exploratory study. The high chiral purity of the final products was confirmed by HPLC. These labelled amino acid derivatives are suitable for direct application to the synthesis of labelled peptides.

Starting from the corresponding precursors, 1 and 2, the two well known procedures for asymmetric synthesis of amino acids, those of Schollkopf and Oppolzer, were compared for the preparation of [1,2-13C2,15N]-labelled (substituted) Boc-leucine enantiomers. Although both gave the final products in comparable overall yields, analysis of optical purity by two independent chromatographic methods revealed that the two procedures differed considerably in this respect. The enantiomeric excess found was 97.2–97.4 and 99.7%, respectively. As a consequence, the latter method was preferred for the synthesis of a number of additional backbone-labelled Boc-derivatives of the three proteinogenic amino acids alanine, phenylalanine and tyrosine, including such deuteriated in the amino acid side-chain. These derivatives exemplify precursors suitable for chemical synthesis of specifically backbone-labelled peptides and should allow greater exploitation of the properties of the 13C and, especially, the 15N nuclei in structural studies.

The synthesis of protected hexa- to nona-peptide precursors of substrates of cyclic AMP-dependent protein kinase, based on a partial amino acid sequence from rat liver pyruvate kinase, as well as of related phosphoserine peptides has been explored. A convenient scheme has been developed which furnishes both N-terminally elongated peptides of variable lengths and intermediates suitable for chemical phosphorylation. The use of adamantyloxycarbonyl as a protecting group for the two important guanidine functions involved, gave rise to the highly lipophilic intermediates (9), (21), (22), (28), and (31), which could easily be purified. Treatment of these with anhydrous hydrogen fluoride [or trifluoroacetic acid in the case of compound (31)] afforded the pure substrate peptides (10), (23), (24), (29), and (32) in high overall yield. All of the free peptides obtained could be phosphorylated by cyclic AMP-dependent protein kinase at a significant rate. The chemical synthesis of two phosphoserine peptides (12) and (14) and their purification by preparative reversed-phase ion-pair chromatography are also reported.