N-Acyl-α-amino acids were efficiently transformed in a two-step procedure into 1-N-(acylamino)alkyltriphenylphosphonium salts, new powerful α-amidoalkylating agents. The effect of the α-amino acid structure, the base used [MeONa or a silica gel-supported piperidine (SiO(2)-Pip)], and the main electrolysis parameters (current density, charge consumption) on the yield and selectivity of the electrochemical decarboxylative α-methoxylation of N-acyl-α-amino acids (Hofer-Moest reaction) was investigated. For most proteinogenic and all studied unproteinogenic α-amino acids, very good results were obtained using a substoichiometric amount of SiO(2)-Pip as the base. Only in the cases of N-acylated cysteine, methionine, and tryptophan, attempts to carry out the Hofer-Moest reaction in the applied conditions failed, probably because of the susceptibility of these α-amino acids to an electrochemical oxidation on the side chain. The methoxy group of N-(1-methoxyalkyl)amides was effectively displaced with the triphenylphosphonium group by dissolving an equimolar amount of N-(1-methoxyalkyl)amide and triphenylphosphonium tetrafluoroborate in CH(2)Cl(2) at room temperature for 30 min, followed by the precipitation of 1-N-(acylamino)alkyltriphenylphosphonium salt with Et(2)O.
Easily accessible 1-(N-acylamino)alkyltriphenylphosphonium salts react smoothly with nitrogen, sulfur, phosphorus and oxygen nucleophiles in the presence of (i-Pr) 2 EtN to give the expected α-amidoalkylation products, usually in good or very good yields. α-Amidoalkylation of dialkyl malonates or acetylacetates requires the application of a much stronger base (DBU) and gives the best results under the influence of microwave irradiation. α-Amidoalkylation of enamines was carried out successfully in the presence of (i-Pr) 2 EtN in a microwave reactor. 1-(N-Acylamino)alkyltriphenylphosphonium salts can be considered as new, convenient and effective α-amidoalkylation agents.
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