A synthesis of 2, the chromophore of rubrolone (I), is described. The key constructive step is the regiospecific cycloaddition 10 + lla + 12a.
S~repromyces echimruber produces a red pigment that is known as Nbrolone and possesses structure 1.1 The pentacyclic ring system which constitutes the skeletal framework of 1 is unprecedented and the tricyclic pyrindenotropolone unit (2) that imbues 1 with its color is also unique to 1 and its derivatives. The structure of rubrolone was reported in 197&l but no synthesis of either 1 or 2 has yet been recorded? In conjunction with efforts directed at the construction of 1,3 we now report the synthesis of 2 whtch is HO 0 Thus dialkyl pyrindanone 7, obtained3 by a one-pot assembly of the three components 3-5, was converted to the unsaturated ketal lo4 by ketalization, bromination and dehydrobromination5 A deMayo-type6 strategy for synthesis of the tropolone ring wzas then initiated by photoaddition of 10 to lla. The cycloaddition proceeds regiospecttically to give a single adduct which, as expected,7 was shown to be 12a by 1H NMR. Attempts to cleave 12a directly6 to 2 (or the ethylene ketal thereof) failed, as did equivalent Scheme 1' HsO+, 24 air MEMO a, FkTBDMS b, R=Ac *For experimental and spectral data see notes 10 and 11.