Sterically hindered C-alpha,C-alpha-disubstituted alpha-amino acids: Synthesis from alpha-nitroacetate and incorporation into peptides

The preparation of sterically hindered and polyfunctional C-alpha,C-alpha-d isubstituted alpha -amino acids (alpha alpha AAs) via alkylation of ethyl n itroacetate and transformation into derivatives ready for incorporation int o peptides are described. Treatment of ethyl nitroacetate with NN-diisoprop ylethylamine (DIEA) in the presence of a catalytic amount of tetraalkylammo nium salt, followed by the addition of an activated alkyl halide or Michael acceptor, gives the doubly C-alkylated product in good to excellent yields . Selective nitro reduction with Zn in acetic acid or hydrogen over Raney N i gives the corresponding amino ester that, upon saponification, can be pro tected with the fluorenylmethyl-oxycarbonyl (Fmoc) group. The first synthes is of an orthogonally protected, tetrafunctional C-alpha,C-alpha-disubstitu ted analogue of aspartic acid, 2,2-bis(tert-butylcarboxymethyl)glycine (Bcm g), is described. Also, the sterically demanding C-alpha,C-alpha-dibenzylgl ycine (Dbg) has been incorporated into a peptide using solid-phase synthesi s. It was found that once sterically congested Dbg is at the peptide N-term inus, further chain extension becomes very difficult using uronium or phosp honium salts (PyAOP, PyAOP/ HOAt, HATU). However, preformed amino acid symm etrical anhydride couples to N-terminal Dbg in almost quantitative yield in nonpolar solvent (dichloroethane-DMF, 9:1).