Oxazoline-oxazinone oxidative rearrangement. Divergent syntheses of (2S,3S)-4,4,4-trifluorovaline and (2S,4S)-5,5,5-trifluoroleucine

(Chemical Equation Presented) Stereoselective syntheses of the valuable fluorinated amino acids (2S,3S)-4,4,4-trifluorovaline and (2S,4S)-5,5,5- trifluoroleucine have been achieved starting from 4,4,4-trifluoro-3- methylbutanoic acid by using a conceptually simple transformation: conversion to a chiral oxazoline, SeO₂-promoted oxidative rearrangement to the dihydro-2H-oxazinone, and face-selective hydrogenation of the C=N bond, followed by hydrogenolysis-hydrolysis. The transformation is limited by the tendency of the intermediate β-trifluoromethyldihydrooxazinone to undergo imine-enamine isomerization. Both amino acids were obtained as configurationally pure hydrochloride salts identical in all respects with those in literature reports.