C. R.NOLLER
1933 [CONTRIBUTION FROM THE
Yo]. 60
DEPARTMENT OF CHEMISTRY,
STANFORD UNIVERSITY ]
Saponins and Sapogenins. VIII. Surface Films of Echinocystic Acid and Derivatives BY
c. I?.. NOI,I,ER
Previous work' has indicated that echinocystic ic triterpenoid of the formula C2sH46(0H)&OOH. n'othing definite is known concerning the position of the functional groups. Since the same trimethylnaphthol is obtained by selenium dehydrogenation of echinocystic acid as has been obtained from hederagenin and aleanolic acid, i t seems likely that all have one hydroxyl group in the same position. Moreover, the non-reactivity of the carboxyl group and of the double bond of these compounds indicates a similar location of these groups.
it was thought that similar measurements on echinocystic acid and its derivatives might give valuable information concerning its structure. Echinocystic acid gives a solid incompressible film occupying an area of about 58 sq. k. per molecule (Curve I) as compared with an area of 54 sq. A. for hederagenin. Methyl echinocystate (Curve 11) and echinocystic acid diacetate (Curve 111) give films very similar to that of echinocystic acid with areas of 56 and 54 sq. 8.,respectively. All films show the phenomenon of contraction even a t low pressures but it is much more pronounced with the methyl ester and the diacetate than with echinocystic acid. Curve I V represents the data on films of echinocystadienol obtained by decarboxylating and dehydrating echinocystic acid. The area occupied is only 45 sq. 8. per molecule or about 20% less than that for the other three compounds. Examination of Askew's curves shows that hederabetulin, which is obtained by the decarboxylation of hederagenin, occupies an area of about 46 sq. in close agreement with that for decarboxylated and dehydrated echinocystic acid. It is difficult to see how formula I proposed by
x.,
CHs CHs
CH? I
..
Ho
(Ju so 100 Sq. A.per molecule. Fig. 1.-Pressure-area curves for surface films of: I, echinocystic acid; 11, methyl echinocystate; 111, echinocystic acid diacetate, 1lr, echinocystadienol 4u
In view of the extensive investigation of Askew2 on surface films of a large number of triterpenoids, ( I ) Bergsteinsson and N d l t r , ?HIS Jutm\Ar., Noller, rbid , 66, 1582 (1934) (2) Askew, J Chem SOL., 1585 il93Ri
56, 1403 (1934)
/z,'
,'\
CH, CH2OH I
I1
Ruzicka4 for hederagenin could account for this decrease in area. With the carboxyl group near the middle of the molecule the free acid should lie flat a t low pressures and occupy a large area. Askew assumes that the molecules in the films of hederagenin have been forced to stand on end with (3) Winterstein and Meyer, Z. ghysrol. Cbem., 199, 43 (1931), Winterstein and Stein, z b d , 190,16 (1931) The formulas given by Askew for hederabetulin and dihydrohederabetulin diacetate are incorrect. Group B should be CHzOH and CHzOAc, respectively while group C should be hydrogen in both cases. (4) Kuzicka and Giacomello, Hclv. Cbzm. Acta, 'do, 301 (1937).
Aug., 1938
SURFACE FILMSOF ECHINOCYSTIC ACIDAND DERIVATIVES
1939
the carboxyl group out of the water but if this of the methyl ester of hederagenin and echinocyswere the case, hederabetulin and echinocys- tic acid and of the diacetate of the latter. If tadienol should occupy the same area as hedera- formula I is correct, the free acids should lie flat genin and echinocystic acid. If, however, the and esterification should decrease the attraction carboxyl group in these compounds occupies the of the carboxyl group for water and permit the position of that in formula I1 proposed by Kita- molecules to stand on end and occupy a smaller ~ a t o one , ~ could assume that in the acids the area; or if it is assumed that the carboxyl group molecules are tilted so that the carboxyl group is out of the water in the case of the free acids, also may touch the water but that in the decar- acetylation of the alcoholic hydroxyl groups should boxylated compounds the molecules are per- decrease their attraction for water and permit mitted to stand on end and occupy a smaller the molecule to lie flat and occupy a larger area. Actually esterification and acetylation have little area. The chief difficulty with this explanation is the effect on the areas occupied which is what one behavior of films of oleanolic acid which has a would expect if the groups are all close together methyl group in place of the hydroxymethyl near the end of the molecule. The procedure used in making the measuregroup. According to Askew’s measurements this acid occupies an area of 48.5 sq. A. How- ments of surface pressure was identical with that ever, Askew’s dotted curve XIIIa, which pre- previously described’ except that the solvent sumably is for the initial film of oleanolic acid, consisted of 80 parts by volume of cyclohexane approaches the curve for hederagenin indicating and 20 parts of isopropyl alcohol. The prepathat the initial film, if i t were stable, would ration of echinocystic acid and of its methyl ester occupy an area near that for hederagenin. It is and diacetate has been reported’ and that of conceivable that a second hydroxyl group nearer echinocystadienol will be described in a later the B ring is necessary to help the carboxyl publication. group to remain in the water.e The fact that Summary oleanolic acid formed gaseous films on sodium Measurements of the area occupied by surface hydroxide solutions does not appear to us to be films of echinocystic acid and its derivatives indisignificant since the methyl ester behaved like- cate a close relationship in structure to hederagenin. wise. It is postulated that the smaller areas occupied by It seems t o us that assigning the carboxyl hederabetulin and echinocystadienol have a beargroup to the position indicated in formula I1 also ing on the position of the carboxyl group in provides a better explanation for the behavior hederagenin and echinocystic acid and support (5) Kitasato, Acfo Phylochim. (Japan), 10, 199 (1937). the formula of Kitasato in which the carboxyl (6) Unpublished work indicates that one of the hydroxyl groups group is placed a t the fusion of the A and B rings. of echinocystic acid is in a postion i3 t o the carboxyl group. We have assumed that the fusion of the A and B rings is t i a m for both STANFORDUNIVERSITY, CALIF. RECEIVED MAY24, 1938 hederagenin and echinocystic acid and that the groups containing oxygen are cis with respect t o each other.
(7) Noller, THIS JOURNAL, 60, 1629 (1938).
