Dioxocine dimer

FIGURE 6.17 Oxidation of a-tocopherol (1) conventionally leads to its spiro dimer (9) via ortho-quinone methide 3 (path A). The zwitterionic o-QM precursor 3a is stabilized by NMMO in complex 17, which upon rapid heating produces small amounts of new dioxocine dimer 18 (path B). Acid treatment of 18 causes quantitative conversion into spiro dimer 9, via o-QM 3 (path C). 

Acid hydrolysis of the bridged dioxocin 299 led to the cyclohexane 300 by cleavage of the acetal moiety (Equation 31). Similar cleavage was observed in the case of a dimer of 299, in which the two units were bound to each other through two diethyloxy moieties, to give cisoid tetrahydroxy groups < 1999J(P 1)1885 >. 

The intramolecular carbene ligand dimerization of chromium bis-carbene complex 206 furnished the benzo [ /][l,4]dioxocines 207 substituted in the dioxocine ring (Equation 45) <2001JA851>. 

All other reported routes to the 1,3-dioxocin ring system involve condensation of an appropriate one-carbon dielectrophile with 1,5-diols. A low (21%) yield of the 2,2-diphenyl derivative (66) is obtained, together with comparable amounts of a cyclic dimer and an acyclic 2 1 adduct, as well as traces of an apparent cyclic trimer, upon treatment of pentanediol with a reagent prepared in situ from diphenyldiazomethane and DDQ (Scheme 29) <84JOC4060>. 

Dimerization of o-fuchsones, generated from o-hydroxybenzyl halides by treatment with either triethylamine or a diethylamino-derivatized polymer, affords dibenzo[i>/][l,5]dioxocin (115) and dinaphtho analogues (116 Ar = Ph, 4-FQH4, 4-MeOC6H4) in unspecified yield (Scheme 34). These reactions are reported to follow a stepwise, ionic pathway rather than radical or concerted [ttSs + r8a] thermal cycloaddition paths <82CCC838>. Curiously, no [4 + 2] dimers are formed in these reactions, although such products are favored for simpler o-quinone methides. 

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