while the 13C spectrum provides additional evidence for two quinonoid (190.0, 184.1) and four ketonic (202.7, 196.3, 189.7, 183.6) carbonyl groups. Olefinic protons are absent but evidently a methylene bridge is present probably derived, as in zeylanone (A), from the methyl group of one of the plumbagin units. Thus plumbazeylanone appears to be formed from three plumbagin molecules, two of which are probably linked as in zeylanone, and an additional methyl group.
It decomposes on sublimation, one product being plumbagin. 0 (3) 0 0 -Me v--a b (4) The arrangement of substituents around the cyclopentane ring can be deduced from the mass spectrum which shows a weak molecular ion at m/z 576(19%), and is dominated by three intense peaks at 388(88%, M'-Cr1Ha03),6 202(94%, M+-C~~H~I+O~), and 188(100%, CllH803) consistent with fragmentations a+H-lla and a (A), respectively. Other major peaks occur at m/z 120(81X) and 92(73X) as expected for juglone derivatives unsubstituted in the benzenoid ring, and at 374(50X, C2zH11+06) and 373(62%, M+-CrrHaOr-Me).
The proposed stereochemistry shown in (4) and the location of the plumbagin "substituent" at C-5b, is suggested by a likely mode of biosynthesis (see Scheme) wherein the trimer is derived from the monomer (L) and a dimer, most probably 3,3'-biplumbagin, the methylene bridge being formed by nucleophilic addition to the quinone methide tautomer (1) (cf. zeylanone3).
Cyclisation of ( 5) to (1) should occur as shown to produce a cis-anti-cis system in which the substituents at C-5a and C-5b are tras, final methylation at C-12a occurring necessarily cis