Quinonemethide structures

Dithiazolidines containing o-quinonemethide structural units in the 3,5-positions (e.g. (99)) are converted by reagents with an activated CHj group into a range of condensed heterocyclic compounds not easily accessible otherwise (Scheme 14) <85S535,86PHA283>. 

Typically, substitution reactions occur by attack of a nucleophilic reagent on a benzyl carbon present in the form of a carbonium ion or a methine group in a quinonemethide structure. Several representative substitution reactions are illustrated in Fig. 1.5. At moderate temperatures ( 100°C) and under mildly alkaline conditions, benzylic hydroxyl groups in phenolic units are converted to thiols by reaction with bisulfide (Q, Fig. 1.5). At higher temperatures and alkalinities, e.g., under kraft pulping conditions, the mercaptide group undergoes a series of transformations in which the sulfur is ultimately eliminated. 

The quinonemethide content of chemically modified lignins may be much higher. If quinonemethide precursors (—e.g. structures of type III) are present in high yield pulps in residues of condensed lignin attached to the fibers, attempts to bleach the pulps with oxidizing agents may lead... 

The most likely lignin condensation reaction during the kraft cook is the formation of diphenylmethane structures 29, 30). These structures are also proposed 14) as being already present in native lignin. Phenolic diphenylmethanes do not exhibit any color, but they can easily be dehydrogenated to quinonemethides 20, 23) or quinonemethide radicals (P). 

The native Bjorkman milled wood lignin has a very slight yellow color and a low absorption in the visible part of the spectrum (Figure 2). If quinonemethides are present in the native lignin, as suggested by Harkin (20), the amount of such structures must be exceedingly small. 

Pearl and Dickey (20) isolated 3,3 -dimethoxy-4,4 -dihydroxybenzo-phenone, an analog of the diphenylmethanes apparently present in our product mixture. They suggest that the ketone is a rearrangement product of vanillil. In our system the corresponding diphenylmethanes could also be produced by a coupling reaction of phenolic methylols or quinonemethides, and at present they cannot be considered proved structural elements native to lignin. 

Fig. 4-6. Endwise polymerization (Adler, 1977). The monomeric radical b is attached by/3-0-4 coupling to an end group radical a. A guaiacylglyserol-0-aryl ether structure (2) is obtained from the intermediate (quinone methide) (1) by addition of water. As an alternative a phenolic hydroxyl group can be added to quinonemethide (1) resulting in a guaiacylglycerol-a,/3-diaryl ether structure (3).

Since the isolation of celastrol in 1936 by Chinese workers a variety of quinonemethides have been encountered in Celastraceae species and their structures and a wide range of biological activities have been reported thus far, resulting quite interesting activities against microorganisms and tumor cells [9,11]. 

A number of quinonemethides with structures related to pristimerin, tingenone (54) iguesterine (55) and isoiguesterine (56) had been described in these years. All of them are characterized by a conjugated system involving rings A and B, which is named non-extended conjugation, Fig. (30). 

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