Chemistry of strained polycyclic compounds—IX: The synthesis and homoketonization of 4-substituted homocuneane acetates

The synthesis of three bridgehead homocuneanes acetates viz 1-bromopentacyclo 4.3.0.0^2.4.0^3.8.0^5.7]nonan-9-one ethylene ketal 4-acetate (10), 1-bromopentacyclo4.3.0.0^2.4.0^3.8.0^5.7]nonane 4-acctate (22) and pentacyclo4.3.0.0^2.4.0^3.8.0^5.7]nonane-9-one ethylene ketal 4-acetate (31) is described, starting from the readely available homocubane carboxylic acid (4). The base and acid catalyzed homoketonization reaction of these acetates has been studied. Under basic conditions the acetates (10, 22 and 31) are converted into tettacyclo4.3.0.0^2.4.0^3.8]nonane derivatives by a cyclopropanol ring cleavage. This homoketonization reaction is a stereospecific process proceeding with retention of configuration. The effect of cage strain on the stereochemistry of base induced homoketonization of bridgehead cage alcohols is discussed.The acid catalyzed homoketonization of acetate (10) was also found to occur exclusively with retention of configuration.