2,3-Dichloro-5,6-dicyano-1,4-benzoquinone (DDQ)

(A) o-Quinone methides are generated in situ by DDQ oxidation from o-alkyl phenols, and further undergo Diels-Alder cycloadditions with a variety of dienes to yield chromans and xanthenes. Similarly, p-quinomethanes and heterocyclic quinone methides are also generated by DDQ oxidation. (B) Treatment of an electron-rich benzyl ether with DDQ at ambient temperature followed by addition of a silyl enol ether results in a C-C bond-forming reaction to afford 3-alkoxy-3-phenylpropionyl compounds. This is a general reaction and works well with a variety of silyl enol ethers to give carbonyl products. (C) Oxidation of highly hindered electron-rich benzhydrols has been achieved using catalytic amount of DDQ. This method is especially appealing for sterically crowded substrates and furnishes the corresponding benzophenones, which are useful synthons and are difficult to obtain by conventional methods. (D) DDQ in aqueous media was found to be a useful reagent for oxidation of arylalkanes to yield aryl ketones and aldehydes. Greater regioselectivity and higher yields were obtained in aqueous acetic acid. (E) DDQ acts as a Lewis acid to promote the formation of C-O and C-N bonds under very mild conditions. Such methodology enables the preparation of benzimidazoles. (F) Acid-catalyzed condensation of dipyrromethanes and aldehydes, followed by oxidation with DDQ, has a wide applicability in the synthesis of many complex natural products such as porphyrins (e.g. heme) and corrins (e.g. vitamin B12). (G) The Schiff base derived from the condensation of o-aminophenol with benzaldehydes was induced to undergo oxidative cyclization in the presence of DDQ, leading to formation of 2-arylbenzoxazoles. (H) DDQ has been used for the oxidative cleavage of linker molecule from Wang resin in the solid phase synthesis of 1,3-oxazolidines. Deprotection of prenyl ethers as well as a number of other protecting groups was accomplished with the use of DDQ to give the corresponding alcohol. © Georg Thieme Verlag Stuttgart.