N,N-Diprotected dehydroamino acid derivatives: versatile substrates for the synthesis of novel amino acids

Abstract

Non-proteinogenic amino acids are an important class of organic compounds that can have intrinsic biological activity or can be found in peptides with antiviral, antitumor, anti-inflammatory or immunosuppressive activities. This type of compounds is also important in drug development, in the elucidation of biochemical pathways and in conformational studies. Therefore, research towards efficient methods that allow the synthesis of these compounds constitutes an important area of peptide chemistry. In our laboratories we have developed a new and high yielding method for the synthesis of N,N-diprotected dehydroamino acid derivatives using tert-butyl pyrocarbonate and 4-dimethylaminopyridine. These compounds were used as substrates in several types of reactions, allowing the synthesis of a variety of new amino acid derivatives. Some of these new compounds are heterocyclic systems or contain heterocyclic moieties such as pyrazole, indole, or imidazole. Thus, several nitrogen heterocycles were reacted with N,N-diprotected dehydroalanine to give new beta-substituted alanines and dehydroalanines. Furanic amino acids were obtained treating the methyl ester of N-(4-toluenesulfonyl), N-(tert-butoxycarbonyl) dehydroalanine with carbon nucleophiles of the beta-dicarbonyl type having at least one methyl group attached to one of the carbonyl groups. Treatment of these furanic amino acids with trifluoracetic acid afforded pyrrole derivatives in good to high yields. A N,N-diprotected 1,4-dihydropyrazine was obtained reacting the methyl ester of N-(4-toluenesulfonyl), N-(tert-butoxycarbonyl)dehydroalanine with 4-dimethylaminopyridine and an excess of potassium carbonate. Tetrahydropyrazines were synthesized by reaction of this 1,4-dihydropyrazine derivative with nucleophiles or by electrochemical reduction. Cleavage of the N-protecting groups from the 1,4-dihydropyrazine gave a disubstituted pyrazine. This review covers the synthesis of N,N-diprotected dehydroamino acids and their application as precursors for the synthesis of new