Significant lability of guaiacylglycerol β-phenacyl ether under alkaline conditions

Article abstract of DOI:10.1021/jf071147d

It was observed that the β-O-4 bond cleavage of a dimeric phenolic lignin model compound with an α-carbonyl group at the B-ring, 2-(2-ethoxy-4-formylphenoxy)-1-(4-hydroxy-3-methoxyphenyl)propane-1,3-diol (I), is extremely fast in a mild anaerobic alkaline treatment (0.45 mol/L NaOH, 95°C, 0.8 MPa of N₂). This phenomenon significantly contrasts with the case of a common dimeric phenolic lignin model compound without any specific functional group, 1-(4-hydroxy-3-methoxyphenyl)-2-(2-methoxyphenoxy) propane-1,3-diol (II). The most plausible mechanism is the migration of the B-ring from the β- to the α-position following the S_NAr mechanism. Because this migration affords the alkaline labile phenolic α-O-4-type compound (XI), the formation of the quinone methide as well as the cleavage of the originally alkaline very stable alkyl-aryl ether bond is promoted. This promotion of the quinone methide formation explains why a relatively large amount of 4-hydroxy-3-methoxybenzaldehyde (IV) is produced from I in an oxygen-alkali treatment.