Multi-step, or Non-concerted Pericyclic Processes

Dipolar intermediates appear to be involved in the conversion of hexamethyl-Dewar-benzene on reaction with tetracyanoethylene into (276 77 %) and (277 8 %). Although the rates of reaction were solvent-dependent, the product yields were not. The structure of (276) was confirmed by X-ray analysis. The isomerization of 3-substituted tropolone prop-2-ynyl ethers in p-cymene at reflux is complete in 2 h, giving the products (278) and/or (279). The furan derivative (278) was the sole product when the substituent R was Cl or Br, and can arise by way of sequential 

Nakamura, R. Hollenstein, J. Zsindely, H. Schmidt, and W. E. Oberhaensli, Helv. Chim. Acta, 1975,58, 1949. 

Reaction of cycloheptatriene with p-benzoquinone in toluene at 110°C gave the [4 + 2] adducts (282), the uic-ditropyl product (283), a small quantity of 7,7 -bitropyl, and hydroquinone. More complex behaviour was observed for the reaction conducted in the absence of solvent in addition to the above compounds, the wic-ditropyl derivative (284) and the cage compound (285), formed by intramolecular Diels-Alder reaction of (283), were also isolated, all in very low yield. The formation of (283) appears to involve a biradical process.  

---

Complex, Multi-step, or Non-concerted Pericyclic Processes.—Pyrolysis of labelled tricyclo[8,2,2,0 ]tetradeca-3(8),ll,13-triene (264 -n = D) at 180°C led to the formation of unlabelled 1,2-dimethylenecyclohexane, [ H jbenzene, 2,3-dideuterio-butadiene, and tetralin labelled at the four aromatic ring positions. The labelling pattern suggests that the 1,2-dimethylenecyclohexane and benzene derive from a common precursor, as do the butadiene and tetralin. The suggested reaction pathway is shown in Scheme 3 and involves preliminary [1,3] sigmatropic or biradical rearrangement of (264) into (265), which in turn is transformed into (266) by intramolecular [4 +2] cycloaddition retro-[4 +2] addition is considered to convert... 

---