Sigmatropic 1,2- and 1,3-acyl shifts in excited states: A novel, general protocol for the synthesis of tricyclopentanoids and protoilludanes

Abstract

Abstract A novel and general approach for the synthesis of linearly fused cis:anti:cis tricyclopentanoids and protoilludane framework having fused four, six and five membered rings from common precursors has been delineated. Photochemical sigmatropic 1,2-acyl shift (or oxadi-π-methane rearrangement) and 1,3-acyl shift in annulated bicyclo[2.2.2]octenones are the key features of this approach. An efficient one step synthesis of epoxy ketones 15–17 via π 4 s+ π 2 s cycloaddition of in situ generated cyclohexa-2,4-dienone 11 has been reported. Further transformation of 15–17 and their congeners to a variety of chromophoric systems 22–31 has been described. The structure and stereochemistry of annulated bicyclo[2.2.2]octenones 15–31 has been established through their high field 1 H-NMR (300MHz), 13 C-NMR, decoupling experiments and cosy spectra. Photochemical behaviour of selected chromophoric systems upon triplet ( 3 T) and singlet ( 1 S) excitations has been investigated. While triplet excitation ( 3 T) of 23, 24, 26–28, 30–32 and 35 led to formation of tetracyclic systems 38–47 , the singlet excitation ( 1 S) of 23, 24, 27, 28, 29b, 31b, 36 and 55 furnished the protoilludanoids 56–63 . Interestingly, 29b did not undergo 1,2-acyl shift upon triplet excitation, but it underwent 1,3-acyl shift upon singlet excitation to give 60 . Cleavage of cyclopropane ring in some of the tetracyclic products has also been investigated. Reductive cleavage of 38, 44, 46 and 47 to tricyclic systems 50–53 has also been reported.