Unnatural Amino Acid Derivatives through Click Chemistry: Synthesis of Triazolylalanine Analogues

A novel tert-butyl 2-(1-oxoisoindolin-2-yl)acetate derivative is selectively alkylated with propargyl bromide in the presence of lithium hexamethyldisilazide. After removal of the tert-butyl protecting group, the resulting N-isoindolinyl (ethynylalanine) derivative is reacted with a series of azides under ‘click conditions’. The click reactions afford an array of N-isoindolinyl- 1,2,3-triazolylalanine derivatives as the free carboxylic acids. Following esterification, the N-isoindolinone protecting group is then transformed into the more easily removable phthaloyl group by selective oxidation at the benzylic position.