Addition to the diphenyldiketone 11 of the reagent prepared from rigorously dried cerium(II1) chloride and phenyllithium yields the tetraphenyldiol endo,endo-13 after extended periods of time. The configuration of this diol, which was previously assigned the (incorrect) configuration exo,exo-13, is established by an X-ray diffraction analysis. Dehydration of endo,endo-13 with sulphuric acid in acetic acid affords a mixture of the oxatwistane 16 and the tetraphenyldiene 17 (3:l). Only the latter is obtained from endo,endo-13 by the action of boron trifluoride-diethyl ether in dichloromethane solution. While attempts at allylic bromination of 17 with Nbromosuccinimide in conventional solvents inevitably lead to complete decomposition, use of cyclohexane as solvent allows to obtain solutions of allylic dibromides that can be cyclised with the zinc-copper couple to afford tetraphenylbarbaralane 9 in 58% yield based on 17. Thus, 9 is now available from the diphenyldiketone 11 in only three steps with an overall yield of 42%. -The autoxidation of 9, studied in various solvents, yields mixtures of products of which the epoxyendoperoxides 20 and 21, the unsaturated ketone 22, and a labile, probably oligomeric or polymeric product C are isolated in pure form. The structures 21 and 22 are elucidated by X-ray diffraction analysis and independent synthesis, respectively. On warming, 20 rearranges into 21. Traces of acid convert the unknown autoxidation product A instantaneously into the unsaturated ketone 22. -Rate studies show that the autoxidation of 9 starts after a short initiation period. 2,6-Ditert-butyl-4-methylphenol exhibits powerful inhibitory effects. These results demonstrate the free radical nature of the autoxidation of 9. -The results are interpreted in terms of a mechanistic scheme involving initiation of the radical chain by addition of triplet oxygen to 9 to generate the diradical 28 followed by a radical chain 1:l copolymerisation of 9 and oxygen to produce diradical29. Eventually, endo addition of oxygen to 29 gives rise to the formation of the epoxyendoperoxides 20 and 21. Intramolecular hydrogen abstraction from the methylene group (C9) is accompanied by cleavage of a bridgehead bond (Cl-C2) and the adjacent 0-0 bond to afford the extremely labile product A (perhaps 32 or 33) which undergoes acid-catalysed elimination to furnish ketone 22.Many olefinic hydrocarbons react with triplet o~y g e n [~-~] but only a limited number react spontaneously [5 -lo], that is, with an appreciable rate at room temperature or below and in the absence of light and sources of neutral[2~1'] or cation radicalsll2I that initiate the generation of chain-carrying radical intermediates. Most of these alkenes are destabilised by strain or resonance, and the degree of destabilisation is reflected by their propensity to undergo autoxidation. The spin-forbidden reactions may afford endoperoxides, 1 : 1 cooligomers or copolymers with oxygen, ketones by cleavage of intermediate 1,2-dioxetanes, and epoxides. Several semibullvalenes re...