Maytansine 1 is a potent antileukemic ansa macrolide isolated in 1971 from Maytenus serrata (Celastraceae) which is currently undergoing Phase II clinical trials under the auspices of the National Cancer Institute (1, 2). A large number of homologous antileukemic ansa macrolides have been isolated from several Maytenus and Putterlickia species (Celastraceae), from Colubrina texensis (Rhamnaceae) and, more recently, from fermentation broths of Actinomyces species No. C-15003(S-1) (1, 3, 4). Approximately 15,000 kg of Maytenus buchananii were collected in Kenya in 1976 and sent to Monsanto Research Corporation for isolation of maytansine. During the course of the isolation, several fractions were obtained which did not correspond to any known maytansinoids. When two of these fractions were combined and subjected to preparative thin layer chromatography (ptlc) on silica gel G6 (EM Labs) two separate times a white solid was obtained which was crystallized from dichloromethane-hexanes to yield 9 mg of normaytansine 2, mp 166-167°. The ultraviolet spectrum of 2 was indicative of a maytansinoid with absorptions at 240, 250, 281, and 288 nm, and the infrared spectrum had bands at 1740, 1710, and 1660 cm⁻¹ which suggested that 2 was a maytanside ester. The mass spectrum was typical of a maytanside ester with peaks at m/e 616 corresponding to M⁺-H₂O-HNCO, m/e 471 resulting from subsequent loss of the sidechain ester at C-3, m/e 456 due to loss of a methyl group from the m/e 471 ion, and m/e 436 due to loss of a chlorine from the m/e 471 ion. The pattern of ions at m/e 471, 456, and 436 was typical of a maytansinoid with a demethyl C-1 amide such as normaysine, and indicated that 2 had the same macrocyclic ring system as normaysine 3 (1). The loss of 145 mass units from m/e 616 to m/e 471 suggested that the C-3 side chain ester was the same as in maytansine 1, an ester of S-acetyl-S-methyl-L-alanine (1). Peaks at m/e 128 and m/e 100 corresponded to loss of -OH and -COOH, respectively, from S-acetyl-S-methyl-L-alanine. Comparison of the proton magnetic resonance (pmr) spectrum of 2 with the pmr spectrum of maytansine 1 confirmed the structure of 2. The major difference between the two spectra was the absence of the three proton singlet assigned to the C-1 amide methyl in the spectrum of 2. Identification of the C-3 side-chain ester as S-acetyl-S-methyl-L-alanine was confirmed by the characteristic signals at δ 1.33 (3H, d, J=7 Hz, C2'-CH₃), δ 1.98 (3H, s, C2'-NCOCH₃), δ 2.85 (3H, s, C2'-SCH₃), and δ 5.29 (1H, q, J=7 Hz, C2'-H). In addition, all the peaks expected for a maytanside ring identical to maytansine were present, and normaytansine was assigned structure 2. In order to confirm the maytanside ring structure, 2 was treated with sodium carbonate in methanol to eliminate the C-3 ester (1). The product obtained was identical by mixed tlc in several systems with normaysine 3. Normaytansine 2 was found to have significant in vivo activity against the P388 lymphocytic leukemia in mice at doses comparable to maytansine (e.g., T/C 181 at 100 μg/kg) and in vitro activity against the KB cell culture (ED₅₀ 10⁻³ μg/ml) (5). Thus the amide methyl at C-1 did not seem to affect the antileukemic activity of the maytansinoids.