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The title compound, C₃₉H₃₀OS, was inadvertently prepared as a Diels–Alder adduct between 1,3-diphenylisobenzofuran and 3-(1a,9b-dihydro-1H-cyclopropa[l]phenanthren-1-ylidene)tetrahydrothiophene. A combination of fused, bridged, and spirocyclic ring systems are all featured within a single molecular structure of this highly crowded polycyclic compound. 

The reaction of dibenzonorcarynyliden(e/oid) with phencyclone was recently reported to give a congested spiropentane withendostereochemistry. Herein we report that, in sharp contrast, an analogous reaction using tetracyclone, instead of phencyclone, gives the highly crowded title spiropentane but withexostereochemistry as determined by X-ray crystallography. This new tetracyclone adduct (C₄₄H₃₀O) crystallizes upon slow evaporation from hexanes/ethyl acetate in the monoclinic crystal system andP2₁/n(No. 14) space group. It has one molecule in the asymmetric unit and four molecules per unit cell. DLPNO-CCSD(T)/def2-TZVP//B3LYP/def2-SVP calculations indicate that theendospiropentane diastereomers from phencyclone and tetracyclone are both more stable than the correspondingexoforms by 6.68 and 5.35 kcal mol⁻¹, respectively. As noted previously in the phencyclone system, favorable π-stacking interactions between the two flat biphenyl moieties in the product and transition state may lead to the preferential formation of theendodiastereomer. However, the ability of the phenyl rings in the 3,4-position of the tetracyclone component to rotate could introduce destabilizing steric interactions in the transition state that hinder formation of theendodiastereomer in favor of the less thermodynamically stableexoisomer. 

Bicyclo[3.2.1]oct-2-yne was generated from the Fritsch–Buttenberg–Wiechell rearrangement of 2-norbornylidene carbene. The rearrangement preferentially involves migration of a tertiary carbon over a secondary carbon, a trend that contrasts with rearrangements of acyclic carbenes and which may be attributable to hyperconjugative effects promoted by the bridged structure of the carbene. 

The generation of saturated carbenes (A) and unsaturated carbenes (B) from cyclopropanated aromatic systems is reviewed.