Syntheses and Activities of Bioquinone Substances. I. Total Syntheses of Rhodoquinones

The total syntheses of naturally occurring rhodoquinone-9 (I, n=9) and related compounds are described. The key intermediate, 2-halogeno-3-methoxy-6-methyl-1, 4-benzoquinone (VI), was easily obtained by 5 steps starting from p-cresol. i) Reduction of VI and condensation of the resulting hydroquinone (IX) with solanesol in the presence of BF3·ether gave the nonaprenylated hydroquinone, which, on oxidation with Ag2O, was transformed to 2-halogeno-3-methoxy-5-nonaprenyl-6-methyl-1, 4-benzoquinone (X). Treatment of X with NaN3 followed by reduction gave the azido hydroquinone from which rhodoquinone-9 (I, n=9) was prepared through pyrolysis. ii) Reaction of VI with NaN3 followed by reduction gave the azido hydroquinone which was converted to rhodoquinone-0 (I, n=0) by pyrolysis. iii) An attempt to obtain (I, n=0) by direct conversion of the Br atom of VI to amino group failed, but 2-bromo-3-amino-6-methyl-1, 4-benzoquinone (VII) and the cyclohexenedione compound (VIII) were obtained.