CONSTRUCTION OF TRIFLUOROMETHYLATED QUARTERNARY CARBONS VIA DIELS-ALDER REACTIONS OF 2-(TRIFLUOROMETHYL)PROPENOIC ACID-DERIVATIVES - APPLICATION TO THE SYNTHESIS OF 16,16,16-TRIFLUORORETINAL

Diels-Alder reactions of 2-(trifluoromethyl)propenoic acid (1) and its 2,2,2-trifluoroethyl ester 2 with various dienes gave adducts in good yields. In Lewis acid catalyzed Diels-Alder reactions of 2, the combination of the Lewis acid and solvent proved to be crucial. For example, polymerization occurred in the case of TiCl4-CH2Cl2 and adduct formation was observed with TiCl4-toluene and TiCl2(O-i-Pr)2-CH2Cl2. In the TiCl4-catalyzed Diels-Alder reaction of the 2-(trifluoromethyl)propenoate ester 3 of D-pantolactone with butadiene, the formation of the R configurational quaternary carbon bearing the trifluoromethyl group was confirmed by X-ray crystallographic analysis of the adduct 20. No polymerization of ester 3 could be detected in the presence of TiCl4 in CH2Cl2. The reactivity difference between 2 and 3 in TiCl4-catalyzed Diels-Alder reactions may possibly be attributable to the stabilization of the 3-TiCl4 complex or weakening of Lewis acidity by coordination of the bidentate ester group of 3. The synthesis of all-trans-16,16,16-trifluororetinal (4), which is considered to be an important analogue for the study of retinal-binding protein, was conducted on the basis of these results. Comparison of the absorption maximum (362 nm) of 4 with other trifluororetinal 34 (362 nm) and 35 (382 nm) reported previously suggests the possibility of a large torsion of the conjugated system between the ring and the polyenal side chain in 4.