Chaparrin, the bitter principle from Castela nicholsonii may be dehydrated in two stages to give the partially aromatized substance chaparrol. By further stepwise degradation the latter has been converted to the dihydrophenanthrene ( S I I I ) which allows the assignment of structures XVII and I to chaparrol and chaparrin respectively.In 1922 Bosman (2) reported the isolation, from Castela nicholsonii Hook (Chaparro amargosa), of two bitter substances, castelin and castelamarin. In view of the improbable nature of the structure (the lactone of 2-hydroxy-3-methox~~cyclohexaneacetic acid) proposed for one of these, a reinvestigation seemed called for. However, an attempt to isolate either of these substances failed. Instead, a new and apparently unrelated compound was obtained and characterized. While it was under investigation the isolation of the same substance was reported by Geissman and Chandorkar (3). Although there were originally minor differences in the characterization, essentially similar conclusions had been reached. Further degradative work was then continued with exchanges of information between the two groups resulting, by different means, in the same gross structural conclusions (1, 4).I t is the purpose of this report to present our results leading to the selection of formula I a s the representation of the constitution of chaparrin. Our results leading to the elucidation of the stereochemistry and absolute stereochemistry of this substance together with those of chaparrol and its isomers neochaparrol and isochaparrol will be presented in the following paper in this series.Chaparrin, best isolated (see Experimental) as the acetate mixture, forms a tri-and a tetraacetate froin which chaparrin can be recovered on alkaline hydrolysis. Since chaparrin has the forinula C30H28Oi three oxygen atoms remain to be accounted for.Of these, two must be present in a lactone. Thus, although chaparrin will dissolve in aqueous alkali and is precipitated from the alkaline solution on acidification i t shows, itself, 'Part of the ?naterial here described wasjirst presented in preliminary forvz (1).

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Evidence is presented which leads to the absolute stereostructures S S , S, XI, and S I I for chaparrin, chaparrol, isochaparrol, and neochaparrol respectively. The absolute configuration is arrived a t by conversion to an optically active biphenyl (dihydrophcnanthre~~e).In our preceding papers (1, 2) ive showed that the gross structures I and I1 were to be attributed to chaparrin, the bitter principle extracted from Castela nicholsonii (chaparro amargosa) and to chaparrol, its degradation product, respectively. Similar conclusions were siinultaileously reached by Geissman and Ellestad (3). I t is the purpose of this paper to discuss the stereochemistry of these and related compounds.On the assu~nption that the absolute stereochemistry of the nlolecule requires that the priinary hydroxyl be 0, nine asymmetric centers remain in chaparrin giving a total of 512 possible isomers. Although a tenth center a t Cll is introduced by the heini1;etal formation, the stereochemistry of this center is rigidly defined with respect to the pri~nary hydroxyl group and so does not increase the total nuinber of isomers. Chaparrol (11), derived from chaparrin by the acid-catalyzed loss of two ~nolecules of water, has, aside from Cs and C11, five centers of asy~nmetry (32 isomers) and appeared a suitable starting point for investigations.Whereas chaparrin (I) is stable to the action of mild alkali, and may be recovered from its acetate by alkaline hydrolysis, chaparrol (11) is not stable. Merely its solution in aqueous ethanolic alkali a t room temperature for 30 min is sufficient to iso~nerize it to a new compound, isochaparrol. However, the transforination is not instantaneous because after 7 inin soine chaparrol can be recovered froin a solution in which it has been present a s an anion. Sollle slow step, other than lactone opening, is therefore involved.The forination of isochaparrol could in principle involve isomerization a t C9, C12, CI3 (the last, through Iceto1 equilibration), with, in addition, a possible change in the position of lactonization; or a coillbination of these. The substance is still a heiniketal since it shoivs no

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