Synthesis and chiroptical properties of enantiopure tricyclo[4.3.0.03,8]nonane-4,5-dione (twistbrendanedione)

The synthesis of chiral tricyclo[4.3.0.0(3.8)]nonane-4.5-dione was accomplished starting from enantiomerically pure (+)-(1S,5S)-bicyclo[3.3.1]nonane-2.6-dione 1. Ring contraction of the latter with thallium(III) nitrate proceeded with high stereoselectivity giving exclusively methyl (+)-(1S,4S)-(+)exo,exo-bicyclo[2.2.1]heptan-2.5-dicarboxylate 2. Inversion of the configuration of this diester to the required di-endo derivative was realized via the following reaction sequence. Bromination of the corresponding acid dichloroanhydride and subsequent reduction with zinc in acetic acid gave a diastereoisomeric mixture of esters, from which the endo,endo-ester 5 was isolated by column chromatography. Acyloin condensation of the latter with trimethylchlorosilane in toluene led to intramolecular ring closure, and subsequent oxidation of the enol silyl ether 7 in situ gave tricyclo[4.3.0.0(3.8)]nonane-4,5-dione 8. The chiroptical properties of this cage molecule were studied by electronic and vibrational circular dichroism spectroscopy. The (1R.3R,6R,8R) absolute configuration of the title structure was also unambiguously proved. (C) 2002 Elsevier Science Ltd. All rights reserved.