A general method for the synthesis of indoles bearing a variety of substituents at the β-position, and its application to the synthesis of l-tryptophan
A general synthetic method of β-substituted indoles such as indoleacetic acid, tryptamine and L-tryptophan has been exploited utilizing α-methoxylated amides, lactams, a carbamate, and sulfonamides, easily obtainable by an electrochemical method, as key intermediates.
A variety of N-(1-methoxyalkyl)amides react with benzotriazole in the presence of PPh3·HBF4 and organic bases (Hünig's base, DBU or DABCO) or solid-state-supported bases (SiO2-Pip or IRA-67) in CHCl3 to give N-[1-(benzotriazol-1-yl)alkyl]amides in good yields. The most convenient and efficient procedure for obtaining N-[1-(benzotriazol-1-yl)alkyl]amides consists, however, of the addition of benzotriazole
conditions to give 1-(N-acylamino)alkyl sulfones in good yields. A combination of this reaction with the recently described electrochemical decarboxylative α-methoxylation of N-acyl-α-amino acids to give N-(1-methoxyalkyl)amides in the presence of 3-(1-piperidino)propyl-functionalized silica gel (SiO2–Pip) enables an effective two-pot transformation of N-acyl-α-amino acids to 1-(N-acylamino)alkyl sulfones
Reactions between 2-chloromethyl-3-trimethylsilyl-1-propene 1 and N-acyliminium ions, which were generated in situ from α-methoxy or α-acetoxy amides 2, give allylic chlorides 3 in good yields and high stereoselectivities. The allylic chlorides 3 were further transformed to bicyclic methylenepyrrolidines 4 which are building blocks for pharmacologically interesting compounds.