1,2-bis(trimethylsilyloxy)cyclobutene : a study of acylation methodology and synthetic application

Abstract

Kuwajima and co-workers have reported that the reaction of a ketal and 1,2-bis(trimethylsilyloxy)cyclobutene (1) in the presence of a Lewis acid catalyst with subsequent rearrangement of the cyclobutanone intermediate with trifluoroacetic acid affords 2,2-disubstituted 1,3-cyclopentanediones in reasonable yields. We have found that substituents, such as methyl groups, in the α-position to the spiro center significantly reduce the yield of product. In addition, substitution of the 1,2-ethanediol moiety of the ketal with groups such as methyl and phenyl also reduces product yields. -- Synthesis of the precursors of the juvenile hormone 6-ethyl-10-methyldodeca-5,9-dien-2-one of the moth Cecropia in our first approach involved the addition of an ethyl group to 2-ethyl-2-methylcyclopentane-1,3-dione (71) and reduction with sodium borohydride to afford 80 as a precursor for a Grob-type fragmentation. Analysis of 80 using nmr and X-ray crystallography indicated the structure shown in Figure 5. The alternative approach to the precursors or the juvenile hormone involved the reduction of 71 with sodium borohydride with subsequent introduction of the ethyl moiety to generate 89. However, introduction of the ethyl group proved to be irreproducible. Both of our strategies focused on the construction of the correct relative stereochemistry to enable a Grob-type fragmentation to produce the correct double bond isomer of 69. -- Model studies toward the synthesis of fredericamycin A focused on [2+2] photochemical additions of 1 with various enones, in particular with spiro[3-cyclopentene-1,1'-indan]-2,5-dione (99). However, while tests carried out between a simple enone and 1 provided limited results, the reaction between 99 and 1 afforded only starting material