Diels-Alder Reactions of 4-Triflyloxy-2,6,6-trimethyl-2,4-cyclohexadienone. An Expedient Methodology for the Synthesis of Bicyclo[2.2.2]oct-5-en-2-ones and Bicyclo[2.2.2]octa-5,7-dien-2-ones

Abstract

The synthesis of 4-triflyloxy-2,6,6-trimethyl-2,4-cyclohexadienone (13), bicyclo[2.2.2]octenones 1a-j and 15a-j, and bicyclo[2.2.2]octadienones 2a-f, 6a-d, and 11a-f is described. The 2,4-cyclohexadienones 4 and 13 were used for the first time as nondimerizing and easily accessible alternatives to 2,6,6-trimethyl-2,4-cyclohexadienone 12 in Diels-Alder reactions with acetylene derivatives 5a-d to prepare the adducts 6a-d and 11a-e in excellent yields. Compounds 11a-d were initially prepared by the alcoholysis of 6a-d to afford bicyclo[2.2.2]octene-2,5-diones 7a-d followed by treatment of 7a-d with N-phenyltriflimide in the presence of LHMDS at -78 °C. Diels-Alder reaction of 13 with an acetylene equivalent, phenyl vinyl sulfoxide, was also studied. A detailed study of the Diels-Alder reactions of various olefinic dienophiles 14a-j with 13 has been carried out to furnish cycloadducts 15a-j in high yields. Reductive removal of triflyloxy group of vinyl triflates 11a-f and 15a-j was performed in the presence of [Pd(PPh3)2Cl 2-Bu3N-HCO2H] to obtain the desired bicyclo [2.2.2] octadienones 2a-f and bicyclo [2.2.2] octenones 1a-j, respectively, in good overall yields.