Trifluoro ethyl hemiacetal

The products from addition of cyanide to aldehydes and ketones are often versatile substrates that lend themselves for further transformations to useful compounds. The early use of the dangerous HCN gas for this cyanohydrin formation could be circumvented by using TMSCN instead, as discovered by Evans etaL [43, 44). Loh et al subsequently reported the first adaptation of the TMSCN addition to aldehydes and ketones in water using Inp3 as the Lewis acid catalyst, with success even for trifluoro ethyl hemiacetal, the commercially available form of trifluoroacetaldehyde [45] (Figure 8.22). However, this procedure is not successful for ketones. 

In the presence of catalytic amounts of anhydrons potassinm carbonate, phenols react with trifluoroacetaldehyde ethyl hemiacetal to give the / ara-alkylated prodncts (C-aUcylation of the phenolate anions). Thns, phenol nnder these conditions gives 4-(2,2,2-trifluoro-l-hydroxyethyl)phenol as the predominant prodnct. The reaction catalyzed by zinc halides predominantly gave the ortto-snbstitnted prodnct (eqnation 4) . ... 

The condensation of l-phenyl-3,3,3-trifluoro-l,2-propanedione monohydrate with various aldehydes in the presence of ammoninm acetate afforded the corresponding 4-phenyl-5-trifluorDmethylimidazoles (Scheme 31) [44]. For the preparation of 2,5-bis(trifluoromethyl)imidazoles, trifluoroacetaldehyde ethyl hemiacetal was used in excess instead of the aldehydes. The prerequisite diketone, that was isolated as its monohydrate, was prepared from the corresponding ketone using SeOa-... 

To a solution of 2-(trimethylsilyl)ethyl CMmethyl 5-acetamido-4,7,8,9-tetra-0-acetyl-3,5-dideoxy-D-g/ycero-a-D-galacto-2-nonulopyranosylonate)-(2->3)-2,4,6-tri-0-benzoyl-P-D-galactopyranose (295 mg, 0.28 mmol) in 2 mL of dichloromethane was added trifluoro-acetic acid (2 mL) at 0°C, and the stirring was continued for 2 h at 0°C. Ethyl acetate (3 mL) and toluene (3 ml) were added and the solvents were evaporated. A second portion of toluene was added and the evaporation was repeated. Purification by flash chromatography on silica gel column with dichloromethane-methanol (20 1) gave the hemiacetal 21 (267 mg, quantitative) mp 85°C. 

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