K. Burger et al., SYNTHESIS OF (2S)-4,4-DIFLUOROPROLINE, (2S,4R)-4-FLUOROPROLINE AND THEIR DERIVATIVES FROM (S)-ASPARTIC ACID [1], Journal of fluorine chemistry, 66(1), 1994, pp. 87-90
Syntheses for (2S)-4,4-difluoroproline, (2S,4R)-4-fluoroproline and th
eir derivatives are described starting from (S)-aspartic acid, using h
exafluoroacetone as the protecting reagent and DAST as the fluorinatin
g agent.