4.4.4- trifluorocrotonic acid

The chemical shifts of trifluoromethyl groups at the terminus of a,(3-unsaturated carbonyl compounds are not affected by the presence of the carbonyl group, as is indicated by the examples in Scheme 5.37, and as exemplified by the fluorine NMR of 4,4,4-trifluorocrotonic acid, given in Fig. 5.14. 

Proton and Carbon Spectra of a -Unsaturated Carbonyl Compounds. Characteristic proton and carbon data for a,(3-unsaturated carbonyl compounds are provided in Scheme 5.38. Such spectra are exemplified by the proton and carbon spectra of 4,4,4-trifluorocrotonic acid in Figs. 5.15 and 5.16. 

FIGURE 5.18. HNMRspectrum of 4,4,4-trifluorocrotonic acid (vinylic area)... 

Starting from ethyl 4, 4, 4-trifluorocrotonate, racemic aziridine carboxylic acid 96 was prepared as shown in Scheme 9.22 [46] and was then subjected to lipase-catalyzed esterification. Methyl ester 97 was obtained in 35% yield with excellent enantiomeric purity. Acid-catalyzed ring opening of aziridine 97 proceeded regio- and stereoselectively, affording 2-substituted (2R,3R) or (2/ ,3S)-3-amino-4,4,4-trifluorobutanoates 98 in high yields [47]. 

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