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C o m m u n i c a t i o n s to t h e E d i t o r
natural compounds were indistinguishable after conversion into the 9-methyl ether as previously described (0.5% tosic acid in methanol a t 25 O C for 20 min).' With the completion of the first total synthesis of an optically active maytansenoid in natural form by a sequence of highly selective and efficient steps, the stage is now set for the synthesis of maytansine and related active anti-tumor agents.i6 References and Notes Corey, E.J.; Weigel, L. 0.;Floyd, D.; Bock, M. G. J. Am. Chem. SOC.1978, 100, 2916. A second synthetic route to (*)-I has been described by Meyers, A. I.; Roland, 0. M.; Comins, 0.L.; Henning, R.; Fleming, M. P.; Shimizu. K. Ibid. 1979, 101,4734. The crystalline methoxy mercuration product 2 possesses the axial ( a ) orientation of methoxy at C-1, in consonance with a previous report. See Inglis. G. R.; Schwarz, J. C. P.; McLaren, L. J. Chem. SOC. 1962, 1014. Methoxy mercuration of tri-C-acetyl-o-glucal under the same conditions affords a mixture of cy- and P-anomeric methoxy triacetates in a ratio of 55:45. Although the mixture of anomeric methoxy triacetates was obtained in high yield >95 % ) and in principle can be used for the synthesis, in practice the anomer was found to be unsatisfactory in a later step of the synthesis (epoxide opening by methylcopper reagent). See also, Manolopoulos, P. T.; Mednick, M.; Lichtin, N. N. J. Am. Cbem. SOC.1962, 84, 2203 for the mercuration step. Satisfactory infrared, proton magnetic resonance and mass spectral data were obtained using chromatographically homogeneous samples of each synthetic intermediate. Ail reactions involving air or moisture sensitive components were performed under an atmosphere of dry argon. This procedure represents a modification of the method described by Hicks, D. R.; Fraser-Reid, B. Synthesis 1974, 203, which uses tosyl imidazole as reagent. The reaction of trityl ether diol 4 with this reagent was found to produce not only the desired cy-oxide 5 but also (in approximately equal amount) the corresponding P-oxide. Interestingly. the reaction of 5 with lithium dimethylcuprate in ether-THF at -50 'C afforded in high yield the allylic alcohol which results from elimination of a proton from C-2 and oxygen from C-3if halide-free CH3Li was used. Corey. E. J.; Gras, J.-L.; Ulrich, P. Tetrahedron Lett. 1978, 809. Corey, E. J.; Bock, M. G.; Kozikowski, A. P.; Rama Rao, A. V.; Floyd, D.; Lipschutz, B. bid. 1978, 1051. Rigby, W.; Hunt, B. J. Chem. Ind. (London) 1967, 1868. For the original use of this reagent see: Corey, E. J.; Kim, S.; Yoo, S.; Nicolaou, K. C.; Melvin, L. S. Jr.; Brunelle, D. J.; Falck, J. R.; Trybulski, E. J.; Lett, R.; Sheldrake, P. W. J. Am. Cbem. Soc. 1978, 100, 4620. The reagent was prepared form equimolar amounts of boranedimethyl sulfide complex in toluene and n-butyilithium in hexane (under Argon). The solvent and dimethyl sulfide were removed in vacuo and dry toluene added to give a 0.25 M solution of reagent. The Rt values found for 10 and the C-10 epimer using silica gel plates with ether as solvent were 0.37 and 0.32, respectively: rotations for 10 and the C-10 epimer were [ L ? ] * ~ D 4-39.6' and -3.3' ( c 0.7 in CHCI3), respectively. Kelley. T. R.; Dali, H. M.; Tsang, W . G . Tetrahedron Left. 1977,3859. Lithium thiooroooxide in DMF or HMPT was found to be less satisfactorv. Corey['E. J.; Enders, D.; Bock, M. G. Tetradedron Lett. 1976, 7.' Circular dichroism data (CD) in ethanol for 16: A 36 mV. If E l - E2 is large enough (-100 mV), two one-electron waves can often be resolved.I2 (3) Superexchange interactions may occur in one or more oxidation states, which can either increase or decrease the magnitude of El - E l . (4) Electronic delocalization can stabilize mixed-valent species which will be reflected as an increased separation, E I - E z . The electrochemical behavior of the CU"CU"L+~species, 1, can be analyzed in this context. The measured separation E l - E2 = 370 mV is corrected for the statistical factor, 36 mV, to give E1 - E2 = 334 mV. The measured superexchange stabilization in the C U I ~ C U ~ Ispecies L + ~ (-3/4 J = 217 cm-I = 27 mV)13 is used to correct El - E2 to 361 mV. No correction need be applied due to the diamagnetic CulCulL species. The separation E l - E2 = 361 mV then reflects electrostatic interactions and covalent stabilization of the mixed-valent Cu'Cu" species relative to the C U ~ ~and CU~~ CuICu' species. Previous attempts to separate these contributions have relied on estimating the electrostatic component.14 In the present case the heterobinuclear complexes, 3-6, permit the covalent factor to be isolated since the electrostatic factor is constant. Copper(II/!) reduction potentials as a function of the divalent metal ion in the second site c u 1 % p 1 ~ + 2 + e- + C,$M*~L+ (2) MI1 = Mn", Fell, Co", Nil1, Cu", Zn" are listed in Table I. Reduction potentials have been corrected for superexchange stabilization in the CU"M"L+~ species as described above, and as estimated from magnetic susceptibility 0 1980 American Chemical Society
