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Thc intramolecular reaction of Na-formyltryptophan to 3,4-dihydro-13-carboline-3-carboxylic acid is realized with high yield either in trifluoroacetic acid or in formic acid -concentrated hydrochloric acid solutions. Optically active products are obtained when fornlylated L-or D-tryptophan are used as starting compounds. The cxpcrimental conditions employed are highly specific and the cyclization of N"-formyltryptophyl rcsiduc is rcal~zcd also in pcptides without secondary lytic reactions. The general behavior of Nu-forn~yltryptophan in formic acid solutions is discussed.Canadian Journal of Chemistry, 46, 3404 (1968) Scheme 1 The compounds containing the P-carboline nucleus are of great interest in connection with their occurrence in some alkaloids. A synthetic procedure available to obtain the members of this class consists in a condensation of tryptophan, or its simple substituted derivatives, with aldehydes to yield a 1,2,3,4-tetrahydro-P-carboline (1). These products can be converted t o the fully aromatic conlpounds by dehydrogenation. Some difficulties arise when the 3,4-dihydro-Pcarboline nucleus is desired. The partial dehydrogenation is generally difficult to realize since the dihydro derivatives are completely aromatized a t a rate that does not permit their accumulation ill the reaction medium. An alte~mative procedure to synthesize the 3,4-dihydro-P-carboline nucleus has been pointed out by Spath and Lederer (2) who directly converted the amides of tryptanline to the corresponding 3,4-dihydro-Pcarboline.The attempts to effect an analogous cyclization on Nu-acyltryptophan to obtain 3,4-dihydro-Pcarboline-3-carboxylic acids showed that secondary reactions such as decarboxylation and dehydrogenation occurred and in the case of Na-formyltryptophan no product was obtained (3,4). 'Contribution No. 15 The present report is concerned with the preparation of optically active 3,4-dihydro-P-carboline-3-carboxylic acid by cyclization of Nuformyltryptophan. The reaction conditions are also used to cyclize some Nu-formyltryptophyl peptides to obtain the corresponding 3,4-diliydro-P-carboline derivatives in good yields as depicted in Scheme I.Experimental Ultraviolet (u.v.) spectra were recorded with a Cary model 15 (I cnl cells) spcctrophotonieter. Anlino acid analyses were performed with a Becknian an~ino acid analyzer model 120 B using ~i~icrocells with a 6.6 rnm light path. Paper chromatography was carried out on Whatman No. 1 paper. The two solvents eniployed were aqueous solutions of ammonia 5 % (v/v) and a~nnionium chloride 3 % (w/v). 3,4-Diliydro-!3-carboline derivatives were easily detected as yellow fluorescent spots in U.V. light (4). Na-forniyltryptoplian was obtained by treatnient of the amino acid with o n e equivalent of acetic anhydride in formic acid (5). Na-for~nyl-L-tryptophyl-L-phenylalan.ine methyl ester, No-formyl-L-tryptophyl-L-tyrosine iiiethyl ester, and No-fonnyl-L-tryptophyl-L-leucine nlethyl ester were syntliesized froni Na-forniyl-L-tryptophan, and the corresponding amino acid est...

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Amino acid esters are racemized by dissolution in a mixture of aliphatic ketones and carboxylic acids. The racemization rate mainly depends on the structure of the amino acid and on the kind of ketone and carboxylic acid used, the best racemizing medium being acetone containing 15% acetic acid. The mechanism of the racemization and the practical consequences of this study in the optical resolution field are discussed.