Synthesis and NMR characterization of cis and trans decahydro-2a,4a,6a,8a-tetraazacyclopent[fg]acenaphthylene. Solid state structure of the trans stereoisomer. Modelling studies

The reduction with Vitride^® of the cis and trans isomers of octahydro-2a,4a,6a,8a-tetraazacyclopentenfg]acenaphthylene 1,2-diones (5 and 6) and octahydro-2a,4a,6a,8a-tetraazacyclopentenfg]acenaphthylene 3,4-diones (7 and 8) led to the cis9 and the so far unknown trans10 isomers of decahydro-2a,4a,6a,8a-tetraazacyclopentfg]acenaphthylene. The stereochemistry of 9 and 10 was determined by comparison of the NMR coupling constants of the proton signals of the ethinic bridge. The results were confirmed by the solid state structure of the trans isomer (10) determined by single crystal X-ray diffraction. Given that the trans bis-aminal species 2 and 4 can undergo rearrangement, i.e. stereoisomerization into the corresponding cis ones (1 and 3, respectively), the conformational behaviour of species 1-4 was investigated by means of both molecular mechanics (MM) and quantum chemical (QC) approaches. The theoretical study was completed by including diamides 5-8 and the final products 9 and 10.