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Synthesis of Hypericin via Emodin Anthrone Derived from a Two-fold Diels-Alder Reaction of 1,4-Benzoquinone

The six-step synthesis of hypericin by the regioselective two-fold Diels-Alder reaction of 1,4-benzoquinone first with (1-methoxy-3-methylbuta-1,3-dienyloxy)trimethylsilane leading to 7-methyljuglone, and next with (1,3-dimethoxybuta-1,3-dienyloxy)trimethylsilane, to give emodin and its O-methylated derivative. The reduction of both compounds with tin(II) chloride in acidic media was accompanied by acid hydrolysis that produced emodin anthrone, whose oxidative dimerization with iron (III) chloride hydrate gave the bianthrone in high yield. The oxidation of the bianthrone in the presence of N-ethyldiisopropylamine gave protohypericin, which was converted into hypericin upon irradiation.