Organofluorine compounds and fluorinating agents Part 17: Sonochemical-forced preparation of perfluoroalkanals and their use for non-conventional acetalations of carbohydrates

The homologous 1-iodo-perfluoroalkanes 1a-1c and α,ω-dibromo-perfluoroalkanes 4a, 4b were carbonylated with DMF in the presence of Al/SnCl2 Al/PbBr2 under sonication in a short reaction time. The hydrated aldehydes 2a-2c and 5a, 5b respectively were obtained in good yields allowing dehydration to 3a-3c and 6a, 6b. Some of the fluorinated aldehydes were selected as substrates in a Wittig-Horner olefination assisted by ultrasound and in non-conventional acetalations of methyl α-l-rhamnopyranoside (9). Thus, (E)-1-perfluorooctyl-2-phenylsulphonyl-ethene (8) was prepared from 3c and the phosphonate 7 by Wittig-Horner synthesis. Acetalations of 9 were carried out with the aldehydes (3a, 3b, 6a), hydrated aldehydes (2a, 2b), and the aldehyde hemiacetal 12 respectively, in the presence of dicyclohexylcarbodiimide (DCC). In all cases, a selective epimerization was observed at the C-atom 3 of the monosaccharide, i.e. polyfluoroalkylidenated 6-deoxy-α-l-altropyranosides 10,11,13, and 14 were obtained.