Two New Syntheses of the Pyranojuglone Pigment α‐Caryopterone

Abstract
By a simple process, 3‐methoxyjuglone (= 8‐hydroxy‐2‐methoxy‐1,4‐naphthoquinone; 9) has been synthesized from 1,2,4‐trimethoxybenzene (5) and converted, after prenylation, to α‐caryopterone (1; Scheme 1), a pyranojuglone pigment from Caryopteris clandonensis. On the other hand, juglone (= 5‐hydroxy‐1,4‐naphthoquinone; 12) was regioselectively prenylated at C(2) via its 1‐methoxy‐cyclohexa‐1,3‐diene adduct 15 (Scheme 2). The 2‐prenyljuglone (4) thus formed led to 1 after oxidation and other reactions.