AbstractWe report on the synthesis of new and previously describedβ‐peptides (1 – 6), consisting of up to twelveβ2,2‐ orβ3,3‐geminally disubstitutedβ‐amino acids which do not fit into any of the secondary structural patterns ofβ‐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). Fully13C‐ and15N‐labeled 3‐amino‐2,2‐dimethylpropanoic‐acid derivatives were prepared from the corresponding labeled precursors (see asterixed formula numbers andScheme 4). Coupling of these amino acids was achieved by methods which we had previously employed for otherβ‐peptide syntheses (intermediates18 – 23). Crystal structures of Boc‐protected geminally disubstituted amino acids (16a – d) and of the corresponding tripeptide (23a), as well as NMR and IR spectra of an isotopically labeledβ‐hexapeptide (2a*) are presented (Figs. 1 – 4) and discussed. The tripeptide structure contains a