Control of ditryptophan crosslinking: dihydrotryptophan as a tryptophan precursor in peptide synthesis

In neat trifluoroacetic acid, tryptophan side chains cross-link to form a d iastereomeric mixture of tryptophan dimers. Convergent oxidation with 2,3-d ichloro-5,6-dicyano-1,4-benzoquinone (DDQ) converts tryptophan dimers to di tryptophan. Since cross-link formation is under thermodynamic control, ther e has been no simple way of controlling the regiochemistry of the cross-lin king process when more than one tryptophan side chain is present. Here, we show that dihydrotryptophan (Dht) can be incorporated into peptides as a tr yptophan precursor, which reforms tryptophan upon treatment with DDQ. Dihyd rotryptophan was prepared as a mixture of gamma S and gamma R diastereomers and the indoline nitrogen was protected with a Cbz group. The resulting am ino acid, N-alpha-BOC-Dht(Cbz)-OH, was then incorporated into peptides as a mixture of diastereomers. Dht was resistant to tryptophan cross-linking in neat trifluoroacetic acid and was converted back to tryptophan during conv ergent oxidation of tryptophan dimers. While Dht is useful for control of d itryptophan regiochemistry and as a potential tryptophan analog, it is not a general strategy for Trp protection since DDQ is unlikely to be compatibl e with easily oxidized amino acids such as cysteine.