1,4-Naphthoquinone methides, formation

The Popik group has recently begun to explore the potential of phenols to photorelease ethers and alcohols attached to an ortho-hem,y c group. For example, they have shown that esters and ethers of (3-hydroxymethyl)naphthalene-2-ol (i.e., 70) efficiently release the corresponding alcohols and acids upon exposure to UV irradiation ( 0.3),50 with formation of naphthoquinone methide 71 occurring in the process (Eq. 1.17). 

Irradiation of 3-hydroxy-2-naphthalenemethanol and 2-hydroxy-l-naphthalene-methanol results in efficient dehydration and the formation of isomeric naphthoquinone methides, 2,3-naphthoquinone-3-methide and 1,2-naphthoquinone-1-methide,... 

Stable o- and p-naphthoquinone methides leading to (316) have been obtained by a two-stage process, starting with 6-methoxy-l,3,8-tri-hydroxynaphthalene, and studies on the stability conferred on ylides by the phenylimino-group (PhN=) have led to the synthesis shown in Scheme 41 for the formation, in high yield, of the ylide (317) from 2-anilino-3-methylthio-1,4-naphthoquinone (318). 

The anion derived from 2,3-dimethyl-l,4-naphthoquinone behaves as a quinone methide and undergoes a [l,4]-cycloaddition with the benzoquinone (193). The product is the xanthene derivative (194) (70JCS(C)722). There is no indication of the formation of the isomeric xanthene. A [l,3]-cycloaddition occurs simultaneously which leads to the fluorene derivative (195). 

It is pertinent at this point to refer briefly to the sources of quinone methides, though these have been reviewed (B-74M122400). The general approach used in chroman syntheses involves the thermal elimination of HX from an -substituted phenol. Commonly the eliminated molecules are water, methanol or dimethylamine (287 X = OH, OMe, NMe2, respectively). However, these methods are not entirely suitable because the eliminated molecules may promote side reactions. In the case of 1,2-naphthoquinone 1-methide, the thermal dissociation of the spirodimer (288) is a better source than the other methods. Its formation represents another example of dimerization by a [4+2]-cycloaddition, since it is prepared by heating l-dimethylaminomethyl-2-naphthol in dodecane or xylene with careful exclusion of moisture (73JCS(P1)120,81CJC2223). 

A quinone methide is a probable intermediate in the formation of dimethylnaphthylgeranine E, which involves a photoannulation of a 2-aryl-3-alkoxy-1,4-naphthoquinone, itself formed by thermal rearrangement of a squaric acid derivative <97JOC5658>. The thermal decomposition of chroman which leads to o-quinone methide has been studied <97JOC4804>. 

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