| Abstrakt: |
The molecular structures of isolaurallene and neolaurallene, diastereomeric nonterpenoids (C15H20O2Br2) from the red algae Laurencia, have been determined by X-ray crystallographic analysis. The crystal data are as follows: isolaurallene, orthorhombic, space group P212121, a=10.393(2), b=27.824(4), c=5.397(1) Å, Z=4, Dc=1.669 g cm−3; neolaurallene, monoclinic, space group P21, a=13.338(7), b=4.929(1), c=12.435(5) Å, β=98.22(4)°, Z=2, Dc=1.609 g cm−3. The structures were solved by the heavy-atom method, and refined by the block-diagonal least-squares method; the final R-values were 0.076 and 0.069 respectively. The (Z)-2,3,4,7,8,9-hexahydrooxonin rings in the two substances possess essentially the same conformation. The MM2 force–field calculations have shown that this conformation is almost identical to the most stable one of the 11 minimum-energy conformations found for (Z)-2,3,4,7,8,9-hexahydrooxonin. For comparison, the conformations of (Z)-cyclononene have also been examined by the force–field calculations. Of its 10 minimum-energy conformations obtained, the asymmetric one which Favini et al.already studied has the lowest energy. |
