Ortho-quinone methides

The ortho-quinone methides are difficult to isolate due to their high reactivity, which leads to rapid Diels-Alder dimerization or trimerization (Fig. 7.26). At 150°C, a partial retro-Diels-Alder reaction of the trimer can occur to form ortho-quinone methide and bis(2-hydroxy-3,5-dimethylphenyl) ethane (dimer).51... 

Figure 7.26 Dimer and trimer structures of ortho-quinone methides.

Orthopedic fixations, 27 Ortho-quinone methides, 400, 401 Oxalic acid, 379... 

Pyrans and napthopyrans (chromenes) are photochromic compounds that undergo photochemically induced electrocyclic ring opening to give colored ortho-quinone methides.95-98 For example, chromene 153 opens on irradiation to give 154 (Eq. 1.41). 

Chiang, Y. Kresge, J. Zhu, Y. Flash photolytic generation of ortho-quinone methides in aqueous solution and study of its chemistry in that medium. J. Am. Chem. Soc. 2001,123, 8089-8094. 

FIGURE 6.4 Reactions and products of the primary oxidation intermediates of a-tocopherol (1), the tocopheroxyl radical 2, ortho-quinone methide 3, and chromanoxylium cation 4. 

FIGURE 6.9 Confirmed heterolytic formation pathway for 5a-a-tocopheryl benzoate (11) without involvement of 5a-C-centered radicals and its proof by trapping of ortho-quinone methide 3 with ethyl vinyl ether to pyranochroman 13. Shown are the major products of the reaction of a-tocopherol (1) with dihenzoyl peroxide. 

FIGURE 6.15 Detailed mechanism of the oxidation of or t/to-me thy I phenols to ortho-quinone methides by Ag2Q according to DFT computations.38... 

Thus, in complex 17, the stabilized ortho-quinone methide 3 was evidently not present in its traditional quinoid form, but in the form of a zwitterionic, aromatic... 

FIGURE 6.16 ortho-Quinone methide 3 stabilization of the zwitterionic rotamer in a complex with /V-methyImorpholine /V-oxide (17). The zwitterionic, aromatic precursor 3a affords the common quinoid form of the o-QM 3 by in-plane rotation of the exocyclic methylene group. 

The geometry of the zwitterions with its exocylic out-of-plane methylene group was quasi-preserved in the recently reported dibenzodioxocine derivative (18) that was formed in rather small amounts by rapidly degrading the NMMO complex at elevated temperatures.45 Strictly speaking, dibenzodioxocine dimer 18 is actually not a dimer of ortho-quinone methide 3, but of its zwitterionic precursor or rotamer 3a (Fig. 6.17). As soon as the out-of-plane methylene group in this intermediate rotates into the ring plane, the o-QM 3 is formed irreversibly and the spiro dimer 9 results... 

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). 

It was shown that complexes 19 of the zwitterionic precursors of ortho-quinone methides and a bis(sulfonium ylide) derived from 2,5-di hydroxyl 1,4 benzoquinone46 were even more stable than those with amine N-oxides. The bis(sulfonium ylide) complexes were formed in a strict 2 1 ratio (o-QM/ylide) and were unaltered at —78 °C for 10 h and stable at room temperature under inert conditions for as long as 15—30 min (Fig. 6.18).47 The o-QM precursor was produced from a-tocopherol (1), its truncated model compound (la), or a respective ortho-methylphenol in general by Ag20 oxidation in a solution containing 0.50-0.55 equivalents of bis(sulfonium ylide) at —78 °C. Although the species interacting with the ylide was actually the zwitterionic oxidation intermediate 3a and not the o-QM itself, the term stabilized o-QM was introduced for the complexes, since these reacted similar to the o-QMs themselves but in a well defined way without dimerization reactions. 

FIGURE 6.18 Oxidation of ortAo-methylphenols to the corresponding ortho-quinone methide via transient zwitterionic intermediates that are stabilized by forming a complex 19 with the 2,5-dihydroxy[l,4]benzoquinone-derived bis(sulfonium ylide). 

REACTIONS OF THE COMMON TOCOPHEROL-DERIVED ORTHO-QUINONE METHIDE 3... 

Selected Substituent-Stabilized Tocopherols and Conjugatively Stabilized Ortho-Quinone Methides... 

Patel, A. Netscher, T. Rosenau, T. Stabilization of ortho-quinone methides by a bis (sulfonium ylide) derived from 2,5-dihydroxy-[l,4]benzoquinone. Tetrahedron Lett. 2008, 49, 2442-2445. 

SCHEME 9.10 Generation and reversible reaction of an ortho-quinone methide. 

DFT theory at the B3LYP/6-31G(d,p) level was used to investigate the Diels-Alder reactions of o-quinone methides with various ethenes. Calculations show that solvent decreases the activation energy and increase the asynchronicity.116 The Diels-Alder reaction of ortho-quinone methides derived from 3//-1,2-benzoxathiole 2,3-dioxides with maleimides produces chroman 2,3-dicarboxylic acid derivatives.117... 

The silyl group is widely used as an oxygen protecting group, because of the ease of its removal by nucleophilic substitution by fluoride anion. The protected phenols 0-(tert-butyldimethylsilyl)-/ -(bromomethyl)phenol (45) and 0-(tert-butyldimethylsilyl)-2,6-bis(bromomethyl)phenol (46) react rapidly with fluoride anion in water to form the corresponding phenols, which then break down to the ortho-quinone methide 41 (Scheme 21A) and the substituted ortho-quinone... 

The photochromic properties of 2/7-chromene derivatives has generated much interest in recent years. Under UV irradiation these molecules can undergo reversible electrocyclic opening of the pyran ring to afford colored ortho-quinone methides <2005T11730, 2005T1681>. 

The facile electrocyclic ring closure of ortho-quinone methides provides a convenient strategy for the synthesis of 277-chromenes. Oxidation of ortho-allylic phenols using DDQ or potassium dichromate is a popular method for the formation of 277-chromenes, via ortho-quinone methide intermediates, and is discussed in detail in the preceding volumes (Scheme 6) <1984CHEC, 1996CHEC-II>. 

Enolization of the /wtf-quinoncs 28 in the presence of hexamethylphosphoric acid (HMPA) forms the corresponding ortho-quinone methide intermediates, which undergo electrocyclic ring closure to afford 2-acyl-2//-chrornenes (Scheme 7) <20010L3875>. 

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