Preparation and Structure ofβ-Peptides Consisting of Geminally Disubstitutedβ2,2- andβ3,3-Amino Acids: A Turn Motif forβ-Peptides
作者:Dieter Seebach、Stefan Abele、Thierry Sifferlen、Martin Hänggi、Sibylle Gruner、Paul Seiler
DOI:10.1002/(sici)1522-2675(19981216)81:12<2218::aid-hlca2218>3.0.co;2-0
日期:1998.12.16
We report on the synthesis of new and previously described beta-peptides (1-6), consisting of up to twelve beta(2,2-) or beta(3,3)-geminally disubstituted beta-amino acids which do not fit into any of the secondary structural patterns of beta-peptides, hitherto disclosed. The required 2,2- and 3,3-dimethyl derivatives of 3-aminopropanoic acid are readily obtained from 3-methylbut-2-enoic acid and ammonia (Scheme 1) and from Boc-protected methyl 3-aminopropanoate by enolate methylation (Scheme 2). Protected (Boc for solution-, Fmoc for solid-phase syntheses) 1-(aminomethyl)cycloalkanecarboxylic-acid derivatives (with cyclopropane, cyclobutane, cyclopentane, and cyclohexane rings) are obtained from 1-cyanocycloalkanecarboxylates and the corresponding dihaloalkanes (Scheme 3). Fully C-13- and N-15-labeled 3-amino-2,2-dimethylpropanoic-acid derivatives were prepared from the corresponding labeled precursors (see asterixed formula numbers and Scheme 4). Coupling of these amino acids was achieved by methods which we had previously employed for other beta-peptide syntheses ( intermediates 18 - 23). Crystal structures of Boc-protected geminally disubstituted amine acids (16a-d) and of the corresponding tripeptide (23a), as well as NMR and IR spectra of an isotopically labeled beta-hexapeptide (2a*) are presented (Figs. 1-4) and discussed. The tripeptide structure contains a ten-membered H-bonded ring which is proposed to be a turn-forming motif for beta-peptides (Fig. 2).