Calcium-mediated fulvene couplings. 3. Reductive coupling of guaiazulene with activated calcium to give a mixture of 8,6′ and 8,8′ (diguaiazulenide)calcium isomers. Thermal rearrangement of the 8,6′ isomer to the 8,8′ isomer and X-ray crystal structure of

The reductive coupling of guaiazulene with activated calcium in THF at 30–35 °C affords a 60:40 mixture of two isomers, 8,8′-(1) and 8,6′-(2) (diguaiazulenide)bis(tetrahydrofuran)calcium, respectively, in nearly quantitative yield. The isomers were separated and purified by recrystallization. The X-ray crystal structure of 1 is reported. A modest dependence of the isomer ratio on the reaction temperature was noticed. At lower temperature (9 °C) the 8,6′ isomer, 2, is favored slightly, and at higher temperature (67 °C) selectivity for the 8,8′ isomer, 1, is slightly enhanced. Thermolyses of each isomer revealed that 1 is the thermodynamically preferred isomer.