O-Benzoquinone methide

Preparation of 3-Methyl-4H-1,2-Benzoxazine and Its Reactivity as a Precursor of o-Benzoquinone Methide. 

Several types of 1,2-oxazines undergo thermal pericyclic reactions in which the N—O bond is cleaved. Thus (506, R = Me, Ph) undergo a thermal retro Diels-Alder reaction on heating to give the corresponding nitrile and o-benzoquinone methide, which can be intercepted by alkenes (Scheme 56) (90JA5341, 94TL7273). 

The same intermediate occurs in the Lehmann method463 for converting the methiodides of phenolic Mannich bases into 2,3-dihydrobenzo-furans, through reaction with dimethyloxosulfonium methylide. Thus, o-benzoquinone methide (207) leads to 2,3-dihydrobenzofuran according to Scheme 4. Similarly, 2-naphtholgives l,2-dihydronaphtho[2,l-6]furan through 208 and 209, formed and rearranged in situ.i6i The reaction has been applied to the synthesis of polycyclic benzofurans.483,485... 

The simpler substrate o-benzoquinone methide (5) affords (6), but only in 127o yield. 

Bispyrano-l,4-benzoquinones 18 are readily formed regioselectively but as separable diastereomers when a mixture of 2,5-dihydroxy-l,4-benzoquinone, paraformaldehyde and an electron-rich alkene is heated under microwave irradiation. A double domino Knoevenagel -HDA is involved with generation of an o-quinone methide type of intermediate <07T3066>. 

QMs exist in three isomeric forms, namely, o-, m-, and p-quinone methides (also known as o-, m-, and p-QMs) (Figure 7.5). The importance of o- and p-QMs in organic synthesis and their role in biochemistry have been studied in detail. Urdike benzoquinones, o- and p-QM derivatives are highly polarized compounds, usually observed with difficulty or postulated as reactive intermediates because of facile reactions driven by the formation of aromatized phenol derivatives. 

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