Bridgehead carboxylic acids rearrangement

Quasi-Favorskii rearrangement Skeletal rearrangement of bicyclic a-halo ketones in which the halogen is located at the bridgehead position to afford carboxylic acids or carboxylic acid derivatives. 370... 

Reduction of the photo-adduct (446) derived from (+)-isopiperitone and cyclobutene-1-carboxylic acid with NaCNBH3 gives the lactone (447). Thermolysis of this compound affords the 6a-epimer of isoaristolactone (448) and the elemanolide (449).213 A novel approach to the synthesis of germacranes involves the thermal opening of a bridgehead cyclobutene which, in turn, is derived by an oxy-Cope rearrangement (Scheme 57).214... 

Dehydration of compounds (653 R = H or Ph) by toluene-p-sulphonic acid in benzene proceeds by homo-allylic rearrangement to give (654 R = H or Ph) some unrearranged olefin is obtained in the dehydration of (653 R = Ph). The cycliza-tion of ethyl (cyclo-oct-4-enylidene) acetate with BFajOEtj in hot benzene affords (655) and the corresponding acid (656). The bridgehead cation (657) is the presumed reaction intermediate. Aluminium chloride-catalysed cyclization of the lactone (658) resulted in the stereospecific formation of the carboxylic acid (660) the epimeric lactone (659) produced a mixture of a hexahydrofluorene carboxylic acid and the epimeric benzobicyclononene acid (661) mechanisms are discussed. 

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