Cassamic acid

Cassanes.— -Caesalpin (41), from Caesalpinia pulcherrima, possesses the cassane skeleton, and is thus related to the bitter principles isolated from the seeds of Caesalpinia bonducella. Cassminic acid is a minor constituent of the bark of Erythrophleum guineense. Spectroscopic measurements and an interrelationship with cassane-16,19-dioic acid have established " it as 6-oxo-7-hydroxycass-13-ene-16,19-dioic acid. N.m.r. measurements and bromination-dehydrobromina-tion studies are in accord with the axial 14a-configuration now accepted for the cassamic acid diterpenes. 

Cassamine crystallizes from pentane and then melts at 86-87° with [a]p —56° (ethanol). The perchlorate crystallizes from dilute acetone, the bisulfate from ethanol-ether, and the hydrochloride from acetone, but the salts do not have sharp melting points. The hydrobromide and the picrate were obtained in the amorphous state. When heated with 2N hydrochloric acid cassamine generates dimethylaminoethanol and cassamic acid, CaiHsoOe, (m.p. 217-218°, [a]D —62° in ethanol). This acid yielded a noncrystalline methyl ester which formed a crystalline p-nitrophenyl-hydrazone (m.p. 227-228") and an oxime (m.p. 96-98°). The p-phenyl-phenacyl ester of cassamic acid crystallizes readily and melts at 127-131°. One methoxyl and a double bond conjugated with the carboxyl are present in the acid, so that the function of only one oxygen remains to be determined. 

In a separate series of experiments 7-hydroxycassanic acid was obtained from dihydrocassamic acid by the conversion of the C-4 carbomethoxy function into a methyl group and reduction of the keto group (31). The structure for cassamic acid must be XL-A and for cassamine XL-B. The stereochemistry depicted in XL follows from the... 

Chapman et al. (32) also studied the isomerization of the methyl group in the diketones analogous to XXI and XXII. These authors concluded in variance with the work of Turner that the methyl at C-14 is /3 in cassamic acid and consequently in cassaic acid. This assignment was ultimately based on the isolation of XLI from the ozonolysis of 7-desoxo-cassamic acid obtained from cassamic acid by a Clemmensen reduction. The configuration of rings B and C in XLI was shown by ORD-data. 

Chapman et al. have reported somewhat earlier (32) the isolation of a dehydrocassamic from a hydrolysis of the total alkaloids of Erythro-phleum guineense to which they have more recently (36) ascribed structure XLV. This assignment is based on the isolation of 7-desoxo-cassamic acid on Clemmensen reduction, compound XLVII on NaBH4 reduction, and XLVIII from ozonolysis followed by a metal reduction— all three of which have been obtained from cassamic acid. In addition. 

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