C-15-ALLENIC MODEL COMPOUNDS FOR CAROTENOIDS - SYNTHESIS, COMPARATIVEH-1-NMR DATA AND A NEW INTRAMOLECULAR REACTION

The first synthesis of ethyl-cyclohexylidene]-3-methyl-2,4-pentadien-1 -ol and 2-cis-(4S)-5-[(2' S,4'S)-2',4'- methylcyclohexylidene]-3-methy l-2,4-pentadien-1-ol via the previously undescribed [(1'R,2'R,4'S)-1', 2'-epoxy-4'-acetoxy-2',6',6'-tri ethylcyclohex-1'-enyl]-3-methyl-2-pen ten-4-yn-1-yl acetate and ethylcyclohex-1'-enyl]-3-methyl-2-penten-4-y n-1-yl acetate by a method analogous to that previously reported for t he synthesis of the corresponding 2-trans compounds is described. The two 2-trans compounds were also prepared. The four diastereomeric alle nic triols were employed as H-1 NMR models for allenic carotenoids in a study of the effect on the chemical shift of the allenic proton. It was concluded that the individual effect of geometrical and allenic is omerisation are additive. A new intramolecular dehydration reaction of 2-cis-(4S)-5-[(2' 2',4'-dihydroxy-2',6',6'-trimethylcyclohexylidene] -3-methyl-2,4-pentadien-1-ol involving the allene bond and leading to the formation of rimethylcyclohex-1'-enyl]-3-methyl-2,4-pentadienal wa s observed. A fully concerted reaction mechanism is discussed.