RACEMIC, OPTICALLY ACTIVE AND RADIOACTIVE a-LIPOIC ACIDS 6483 the terms ~O-~SO-, ne^-,^^ ana-18 and c y c l o p s e~d o -~~ to be used as prefixes to the trivial name of the parent sapogenin. AS now seems apparent, the use of 20-is0 does not cover all cases. With the structure of the 20p-sapogenins now on firm ground there no longer seems to be a need for the non-specific terms neo, ana, or cyclopseudo. We therefore propose 20-is0 or 20,224~0 as shown above. Experimental (a) Micro Procedure.-To a Pyrex tube (8 mm. X 12.5 cm.) sealed a t one end was added 0.2 g. of sapogenin acetate followed by 0.5 ml. of acetic anhydride containing a trace of acetic acid.l0 The tube was sealed close to the open end, wrapped in a copper spira1,m immersed in a heating bath and removed after the desired time interval. The pseudomerization runs were conducted at temperatures of 100, 170, 180 and 188" using heating-baths of water, butyl Cellosolve, aniline and propylene glycol, respectively. Trial runs quickly indicated that for the acetic anhydride pseudomerization of natural sapogenins, a temperature of 170" was best for determination of pseudomerization rates, the reaction proceeding too slowly at lower and too rapidly at higher temperatures. For 20-iso-or 20,22-isosapogenins, a temperature of 100" was found most suitable. In experiments in which these last named sapogenins were heated without addition of acetic anhydride, a temperature of 188" was convenient with quantities of 0.10 g. Determination of Pseudosap0genins.-After the appropriate heating periods, the tubes were opened, and the contents refluxed one-half hour with 20 ml. of methanol containing 1.0 g. of potassium hydroxide. Three volumes of water was added, and the precipitated product filtere:, thoroughly washed with hot water and dried in vacuo a t 80 .The ultraviolet absorption of a weighed sample (0.025 g. in 100 ml. of methanol, and further diluted if necessary) was determined a t 215 mp.21 The percentage of conversion to pseudosapogenin was determined by dividing the specific absorption coefficient found experimentally by the corresponding value of the pure pseudosapogenin. In the case of the 188" pseudomerization of the 20-isosapogenins with omission of acetic anhydride, the product was, on completion of the heating period, dissolved directly in methanol and the ultraviolet absorption determined as above. The data for natural sapogenins are shown in Table I, for 20-is0 and 20,224sosapogenins in Table 11. The results obtained by ultraviolet measurement were checked by paper chroma-Pseudomerization of Sapogenins. (18) D. H. W. Dickson et ai., Chemistry & rxdustiy, 692 (1954). (19) To obtain consistent results a t 170a i t was necessary to use reagent grade acetic anhydride, 454.5 g., t o which was added 0.5 ml. of glacial acetic acid. (20) This was both a safety precaution and a convenient means of removing the tube from the heating-bath by means of a length of copper wire attached to the spiral. . Soc., 2807 (1055).tography. Whatman No. 4 paper, 12 by 18 inches, wa...
