Ene reactions of allenes. Part I. Reactions of alkylallenes with hexafluoroacetone

Abstract

The reaction of 2,4-dimethylpenta-2,3-diene with hexafluoroacetone proceeds by rearrangement of the allene to 2,4-dimethylpenta-1,3-diene followed by a Diels–Alder reaction to give 3,6-dihydro-4,6,6-trimethyl-2,2-bis-(trifluoromethyl)-2H-pyran under thermal or photochemical conditions. Treatment of 3-methylbuta-1,2-diene with hexafluoroacetone at ambient temperature gave a 4:1 mixture of 1,1,1-trifluoro-4-methyl-3-methylene-2-trifluoromethylpent-4-en-2-ol and 3,6-dihydro-4-methyl-2,2-bis(trifluoromethyl)-2H-pyran. At 175 °C, an additional product is 3,6-dihydro-5-methyl-4-(2,2,2-trifluoro-1-hydroxy-1-trifluoromethylethyl)-2,2-bis(trifluoromethyl)-2H-pyran.