Reduction of Tosylamino Acids and Related Compounds with Sodium in Liquid Ammonia; stoichiometry and products

Cleavage at acyl-proline bonds with sodium in liquid ammonia: application with nanomolar amounts of peptides and separation of products by high-performance liquid chromatography for structural analysis

Cleavage of X-Pro bonds with metallic sodium in liquid ammonia is a little-used method due to difficulties with handling of reagents, variable cleavage yields, and separation of peptides from salt byproducts. Construction of a small distillation/reaction apparatus permitted peptide incubations at the nanomole scale. Reverse-phase high-performance liquid chromatography (HPLC) of the residue after removal of NH3 allows separation of the salts and fractionation of the cleaved peptides which may be taken directly for sequence analysis. HPLC also allows rapid assessment of the degree of cleavage before structural analysis. Cleavage of some peptides proceeded in high yield while others were cleaved poorly or not at all, modifying earlier generalizations on factors influencing cleavage.

Reexamination of sodium-liquid ammonia reduction in the peptide chemistry

Sodium-liquid ammonia reduction has been used for over 50 years for removal of benzyl-type protecting groups in peptide chemistry. Up until now a definitely blue end-point has generally been accepted for detection of the completion of reaction. Systematic investigation with model compounds has revealed that this is not only unnecessary for the complete removal of the protecting groups but also that the application of sodium in excess results in many undesired transformations which can simply be suppressed or even eliminated by optimizing the sodium consumption. Cleavage of tert.-butyloxycarbonyl group and N-C alpha bond, reduction of carboxamide groups to carbinol derivatives, transpeptidation and formation of a hydantoin derivative have been observed in model experiments by using sodium in excess.