Conformational analysis disubstituted derivatives

In 2,5-disubstituted derivatives (60) and (61), the number of possible conformations amount to 8 for (60) and 10 for (61). Conformational analysis of these derivatives has been carried out employing experimental dipole moment determinations together with semiempirical calculations (MNDO and AMI) <92JST(253)179>. This indicates that compounds (60 R = Me, OMe) and (61 R = Me) exist as an equilibrium mixture of the possible conformers with different populations, while (61 R = OMe) exists in only one conformation (62). 

Hyperflorin, an antibiotic isolated from St. John s wort Hypericum perforatum L.), has been found on the basis of chemical and spectroscopic evidence to possess structure (639). ° A review on the conformational analysis of bicyclo[3,3,l]nonanes has appeared,and the conformations of l,5-disubstituted-3,7-dimethyIenebicyclo-[3,3,l]nonane derivatives have been shown, on the basis of lanthanide-induced n.m.r. shifts, to be predominantly in the double-chair form. In a notable paper that contains a wealth of information pertinent to adamantane synthesis, it has been reported that bicyclo[3,3,l]nonane-3,7-dicarboxylic acid, its dimethyl ester, and various derivatives exist mainly in the chair-boat conformation. The synthetic entry to these systems is illustrated for the above diester (643) in which the starting point is adamantanone (640) thus, ring expansion with diazomethane followed by SeOj oxidation gave the a-diketone (641) which was cleaved with periodate to give the diacid (642). Esterification using diazomethane gave (643) which exists, as does the diacid, as... 

Exciton coupling between the two styryl chromophores in each of the conformers (A)-(C) leads to a negative Cotton effect at around 270 nm, as shown by theoretical analysis using the 71-electron SCF Cl dipole velocity MO method. Therefore, the tram-2,.1-disubstituted spirocyclo-propane-1,9 -fluorene derivative 6, from which the distyryl derivative was obtained, must have accordingly the 2S3S absolute configuration122. 

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