2706
ing 5a-H and ~ O P - C H I . ’ ~These results therefore add a further variant to the family of tetracyclic diterpene skeletons found in higher plants. Acknowledgments. We are indebted to Professor C. D. Adams, University of the West Indies, for botanical identification of S. maritima. Financial support was provided by the National Science Foundation (J. D. W.). (12) W. Moffitt, R. B. Woodward, A . Moscowitz, W. Klyne, and C. Djerassi,J. Amer. Chem. Soc., 83,4013 (1961). (13) Camille and Henry Dreyfus Foundation Awardee, 1972-1977.
Figure 1. A computer generated perspective view of stemodinone
P. S. Manchand*
(5).
Chemical Research Department, Hoffmann-La Roche Inc. Nutley, New Jersey 07110
system was effected through a single-crystal X-ray analysis of stemodinone (5). Crystals of stemodinone were grown from a solvent system of chloroform and ethyl acetate. The crystals belong to the orthorhombic crystal clast with a = 12.96 (l), b = 6.78 (2) and c = 20.16 (1) A. Systematic extinctions uniquely indicated space group P212121, and a calculated density of 1.15 g/cm3 for 2 = 4 implied one molecule of stemodin per asymmetric unit. A total of 1675 unique reflections with 8 I55” were measured on a fully automated Hilger-Watts four;circle diffractometer with Ni-filtered Cu K a (1.5418 A) radiation. After correction for background, Lorentz, and polarization effects, a total of 1189 reflections were judged observed ( I 2 3a(Z)). Phases were assigned to 183 E‘s with magnitudes greater than 1.50 with a multiple solution, weighted tangent formula approach.’ Three solutions with essentially identical phasing were strongly indicated. Inspection of the phased, three-dimensional E synthesis showed all 22 nonhydrogen atoms of the diterpene. Full-matrix least-squares refinement, with all atoms anisotropic and no hydrogens included, has lowered the crystallographic discrepancy index to 0.104.* Figure 1 is a computer generated drawing of the X-ray model indicating the structure and conformation of s t e m ~ d i n o n e . ~All bond distances and angles Fgree well within estimated experimental error (0.02 A and 2”) with generally accepted values.’O There were no abnormally short intermolecular contacts. As can be seen from Figure 1 each of the sixmembered rings is in the chair conformation and the five-membered ring in the envelope conformation with C( 16) serving as the flap. The structure of stemodinone (5), and thus stemodin (l),bears a close resemblance to that of the antibiotic aphidicolin, recently isolated from Cephalosporium uphidicola. However, the stemodin stereochemistry differs from that reported for aphidicolin at C-9, 13, and 14 (stemodane numbering). The O R D of stemodinone exhibits a positive Cotton effect ([ala08 = 2460), in agreement with an absolute configuration contain(7) G. Germain, P. Main, and M. M. Woolfson, Acta Crysrallogr., Secr. A, 27, 368 (1971); J. Karle, Acta Crystallogr., 9, 635 (1968). (8) W. R. Busing, K. 0. Martin, and H. F. Levy, “A Fortran Crystallographic Least-Squares Program,” USAEC Report ORNL-TM-305, Oak Ridge National Laboratory, Oak Ridge, Tenn., 1964. (9) C. K. Johnson, “ORTEP, A Thermal-Ellipsoid Plot Program for Crystal Structure Illustrations,” USAEC Report ORNL-3794, Oak Ridge National Laboratory, Oak Ridge, Tenn., 1965. (lo? 0. Kennard and D. G. Watson, “Molecular Structure and Dimensions,” Crystallographic Data Centre, Cambridge, 1970. (11) K. M. Brundret, W. Dalziel, B. Hesp, J. A. J. Jarvis, and S . Neidle, J . Chem. Soc., Chem. Commun., 1027 (1972).
Journal of the American Chemical Society / 95:8
/
J. D. White* Department of Chemistry, Oregon State University Corvallis, Oregon 97331
Hal Wright, Jon Clardy’a Ames Laboratory-USAEC and Department of Chemistry Iowa State University Ames, Iowa 50010 Received February 14, I973
Effect of Solvation on the Intrinsic Relative Acidity of Methanol and Ethanol Sir. It has been well known for many years that in solution tert-butoxide is a stronger base than ethoxide, and ethoxide in turn is a stronger base than methoxide. These observations lead to the conclusion that the relative order of acidity is CHIOH > C2H,0H > t-C4H90H. Recently, however, Brauman and Blair reported that in the gas phase the relative order of acidities is reversed from that in solution.2 This dramatic reversal in acid strengths indicates that the solvent assumes a major role in determining relative acidities. Using the pulsed ion cyclotron resonance technique,3 we have been able to determine quantitatively the relative gas-phase acidity of CHIOH and C2HsOH. Methoxide ions, generated by electron impact on C H 3 0 N 0 , 4 react rapidly in the gas phase with CzHjOH to produce ethoxide ions. If a gaseous CH30H-C2HjOH mixture of known relative concentration is added to the spectrometer, a chemical equilibrium is established with respect to the reaction in eq l., This result confirms CHaO-
+ CzH50H = C2HaO- + CHaOH
KI = 23 i 2
(1)
that CzHjOH is a stronger acid than C H 3 0 H in the gas phase and is in agreement with the qualitative order reported by Brauman and Blair. By using the relation AGO = -RT In IC, the free energy change for reaction 1 at 298OK can be calculated as - 1.9 f 0.2 kcal/mol. (1) (a) J. Hine and M. Hine, J . Amer. Chem. Soc., 74, 5266 (1952); (b) W. K. McEwen, ibid., 58,1124(1936). (2) (a) J. I. Brauman and L. .(I Blair, ibid., 92, 5986 (1970); (b) J. I. Brauman and L. K. Blair, ibid., 90,6561 (1968). (3) (a) R. T. McIver, Jr., and J. R. Eyler, ibid., 93, 6334 (1971); (b) R . T. McIver, Jr., and R. C. Dunbar, In?. J . Mass Spectrom. Ion Phys., 7, 471 (1971); (c) R. T. McIver, Jr., Reu. Sci. Instrum., 41, 555 (1970). (4) I
