Applications of cobalt stabilised carbonium ions to the synthesis of fused ring carbocycles
This programme of work has been aimed at developing novel annulation reactions in order to furnish a variety of carbocyclic and heterocyclic ring systems: In all of the examples investigated the key cyclisation step was achieved using a Nicholas reaction, that is, the reaction of a dicobalt hexacarbonyl stabilised propargylic carbonium ion with an intramolecular nucleophile. After an initial review of both cyclisation reactions and the chemistry of organocobalt clusters, the results of these investigations are detailed. In the initial investigations a range of 1,2-disubstituted cycloalkanes were synthesised that contained a dicobalt hexacarbonyl complexed propargyl ether at the a-carbon atom and a pendant silyl enol ether at the ß-carbon atom. These molecules were successfully cyclised, using boron trifluoride diethyl etherate followed by decomplexation of the dicobalt cluster, to afford a bicyclo[4:3:0]nonane, two bicyclo[3:3:0]octanes and a bicyclo[5:3:0]decane. The relative stereochemistry at the fused ring junction for all these molecules was determined on the basis of their corresponding =c NMR spectra. An analogous reaction was successfully achieved between a dicobalt hexacarbonyl complexed prop argyl ether and a terminal alkene, using titanium (IV) chloride, to afford a cis fused bicyclo[3:3:0]octane into which a chlorine atom had been incorporated. A subsequent investigation led to the development of a novel benzopyran synthesis. In this approach two benzopyran derivatives were produced from the one key cyclisation precursor. It was observed that they not only contained different functionalities but that the relative stereochemistry at the newly formed carbon-carbon bonds was different. A benzopyran, that possessed the isopropenyl group, exhibited a trans stereochemistry, whereas the stereochemistry for a second fluorinated derivative appeared, from the magnitude of the coupling constant (6 Hz), to exhibit a cis stereochemistry. The concluding investigation focused upon the intramolecular cyclisation of a precursor derived from S-C - )-citronellal. Both a silyl enol ether derivative andan allyl silane derivative were successfully cyclised, using the Nicholas reaction, to affordtrisubstituted cycloalkanes in low but unoptimised yields. An additional investigation using a dicobalt hexacarbonyl complexed derivative possessing a pendant trisubstituted alkene underwent a novel cyclisation reaction to furnish, upon decomplexation, two trisubstituted cycloalkane derivatives. For one of the cycloalkanes produced, the relative stereochemistry of the ring substituents at the newly formed carbon-carbon bond was found to be trans.