Enantiospecific syntheses of alpha-(fluoromethyl)tryptophan analogues: interactions with tryptophan hydroxylase and aromatic L-amino acid decarboxylase.

alpha-Fluoromethyl amino acids are enzyme-activated irreversible inhibitors of amino acid decarboxylases. Aromatic L-amino acid decarboxylase (AADC) is the enzyme responsible for the final step in the biosynthesis of both dopamine and serotonin via decarboxylation of L-dopa and 5-hydroxy-L-tryptophan, respectively. Our goal is to utilize antagonists of the serotonin-producing enzymes (tryptophan hydroxylase and AADC) as the basis for a chemotherapeutic approach to the treatment of carcinoid tumors, a rare tumor type characterized by the overproduction of serotonin. We report here an enantiospecific synthesis of alpha(S)-(fluoromethyl)tryptophan [(S)-11a] and alpha(S)-(fluoromethyl)-5-hydroxytryptophan [(S)-11b], as well as the (R)-enantiomers, based upon recent methodology involving the face-selective alkylation of cyclic tryptophan tautomers. Our synthetic route provided both enantiomers of 11a and 11b with greater than 97% enantiomeric purity based upon evaluation of the NMR spectra of their Mosher's acid derivatives. (S)-11a was evaluated as a substrate for P815 tryptophan hydroxylase and determined to have an apparent Km of 4.31 +/- 1.07 mM, essentially half the value previously reported for the racemic mixture of 11a with rat brain stem tryptophan hydroxylase. (R)-11a was not a substrate for P815 tryptophan hydroxylase. (S)-11b was evaluated as an enzyme-activated irreversible inhibitor of murine liver AADC and determined to have a KI of 24.3 +/- 3.01 microM and a k2 of 2.26 +/- 0.44 min-1. (R)-11b was not an inhibitor of murine liver AADC.