Two schemes for the synthesis of plastoquinone derivatives with omega-substituted alkyl side chains have been developed, based on radical alkylation of 2,3-dimethyl-1,4-benzoquinone with omega-substituted carboxylic and/or dicarboxylic acids in the presence of S2O8(2-) and Ag+. In Scheme 1, radical alkylation of 2,3-dimethyl-1,4-benzoquinone with 11-bromoundecanoic acid, in the presence of (NH4)2S2O8 and AgNO3, gave 2,3-dimethyl-5-(10-bromodecyl)-1,4-benzoquinone, which, after hydration, gave 2,3-dimethyl-5-(10-hydroxydecyl)-1,4-benzoquinone. Esterification of 2,3-dimethyl-5-(10-hydroxydecyl)-1,4-benzoquinone with different acyl chlorides gave 2,3-dimethyl-5-(10-acyloxydecyl)-1,4-benzoquinone. In Scheme 2, radical alkylation of 2,3-dimethyl-1,4-benzoquinone with adipic acid, in the presence of K2S2O8 and AgNO3, gave 2,3-dimethyl-5-(4-carboxybutyl)-1,4-benzoquinone, which was converted to 2,3-dimethyl-5-[4-(alkoxycarbonyl)butyl]-1,4-benzoquinone by esterification with different alcohols in the presence of dicyclohexylcarbodiimide. 2,3-Dimethyl-5-(8-carboxyoctyl)-1,4-benzoquinone and 2,3-dimethyl-5-[8-(alkyloxycarbonyl)octyl]-1,4-benzoquinone were similarly synthesized from 2,3-dimethyl-1,4-benzoquinone and sebacic acid. Plastoquinone derivatives having different kinds of double bonds in the side chain with and without a 6-methyl group were synthesized from 2,3,5-trimethyl-1,4-benzoquinone and 2,3-dimethyl-1,4-benzoquinone by successive reactions with (1) NaBH4; (2) KHSO4, different kinds of allylic alcohols; (3) Ag2O. The allylic alcohols were prepared from their corresponding aldehydes by reduction with NaBH4, 2,3,6-Trimethyl-1,4-benzoquinone was prepared from 2,3-dimethyl-1,4-benzoquinone by radical methylation with acetic acids in the presence of K2S2O8 + AgNO3.
