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

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)2Cl2−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.