NOTESFabacein showed relatively low cytotoxicity (EDso = 1 pg/ml) toward human carcinoma of the nasopharynx in tissue culture (KB),'O in contrast with the potent cytotoxicity shown by cucurbitacin B (EDbo E hg/ml).5 Earlier studies in this laboratory have demonstrated the importance of highly electrophilic conjugated systems in relation to the cytotoxicity of several classes of terpenoids.'l Saturation of the conjugated AZ3 double bond in the cucurbitacins is accompanied by a profound lessening in cytotoxicity of the resultant dihydrocucurbitacin derivatives.6p6 Consequently, reactions of the side chain conjugated ketone with biological macromolecules may play an important role in the mechanism by which cucurbitacins exert their cytotoxic effects. The marked diminution in cytotoxicity which accompanies the acetylation of the (3-16 hydroxyl group of cucurbitacin B suggests that the free hydroxyl group may be important for the reactivity of the conjugated ketone. Thus, hydrogenbonding interaction between the C-16 hydroxyl group and the C-22 ketone could activate the a,p-unsaturated ketone toward nucleophilic attack by a biological macromolecule, as shown. The lessened cytotoxicityof fabacein, then, may result from the diminished reactivity of the conjugated ketone in the C-16 acetate ester.
Experimental SectionMelting points were determined on a Mettler FP2 melting point apparatus. Optical rotations were recorded on a Perkin-Elmer 141 polarimeter. Ultraviolet spectra were recorded on a Coleman Hitachi EPS-3T recording spectrophotometer. Nuclear magnetic resonance spectra were recorded on a Varian Associates HA-100 spectrometer using TklS as an internal slandard. Mass spectra were recorded on either Hitachi Perkin-Elmer RMU-63 or AEI MS-902 spectrometers, equipped with direct insertion probes. Elemental analyses were performed by Spang Microanalytical Laboratory, Ann Arbor, Mich.Acetylation of Fabacein to Triacetate 3.-A solution of faba-cein4 (1, 12 mg) in anhydrous pyridine (0.5 ml) was treated with acetic anhydride (0.5 ml). The reaction mixture was stirred overnight at room temperature under nitrogen. The solution was evaporated in vacuo and the residue was dissolved in ethanol and reevaporated. The oily residue (12 mg) was separated by preparative tlc on Brinkmann Bilplates with lYG methanol-chloroform. The major band was eluted with ethyl acetate. Attempts to crystallize the product were unsuccessful. The amorphous product (3, 10 mg) showed Rf 0.68 on Brinkmann Silplates with 274 methanol-chloroform; Rf 0.80 on ChromAr plates with 17~ methanol-ether; Rf 0.70 on polyamide plates with 70y0 methanol-water; uv (CHC1,) 230 nm ( E 10,000); [ c Y ]~~D +2.5" (c 3.60, CHCl,); ir (CHC13) 2.92, 3.36, 3.43, 5.76, 5.92, 6.15, 7.38, 8.09, and 9.70 M ; nmr' (CDC13) T 2.94 (1 H, d, J : = 16 Hz), 3.66 (1 H , d, J = 16 H B ) , 4.32 (1 H, m), 4.62 (1 H, d of d, J = 14, 5 Hz), 4.90 (1 H, b t , J = 8 Hx), 5.80 (1 H, s), 7.92 (3 H, s), after preparative tlc (35 mg) showed the same rotation, ir, uv, nmr, mass spectrum, and Rr values...
