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COMMUNICATIONS TO THE EDITOR
Nov. 20, 1961
ID-A
m-B
Fig. 2.
COOH FOLDED FORM
H
E X T E N D E D FORM
tion which is compatible with the chemical evidence ( R - D I ENE H E L I X ) (L-DIENE HELIX) for the 13a-configuration. A fundamental analysis6 of the rotatory disperB I-A sion of optically active 1,3-cyclohexadienes has Fig. 3. revealed that the skewness imposed on such diene systems by structural factors constitutes the major data.3 The signal caused by them is shifted -0.17 element qf asymmetry responsible f o r the Cotton p.p.m. downfield from its position in thirteen reeffect, its contribution to optical activity far out- lated compounds, including, e.g., neoabietic acid weighing that of adjacent asymmetric centers. I n ( A 7 ( I 8 ) and pimaric acid Hence, this addition, it has been derived theoretically and dem- shift appears to be due to a specific interaction, onstrated experimentally6 that the direction of the possible only in I, between the methyl protons and Cotton effect so produced depends u p o n the sense of the entire diene, rather than to the presence of the helicity of the diene, a left-handed helix producing a A8(14)bond, isolated or c0njugated.l‘ negative effect. On the basis of this rule, the negaWhile this interpretation is permissive rather tive Cotton effect of I requires that its dienic sys- than decisive, i t removes the otherwise unresolved tem form a left-handed helix. It is apparent from discrepancy between chemical evidence and rotaDreiding models that this requirement is met not tional data. I t also emphasizes the importance of only by the 13P-configuration of I, but also by one making due allowance for conformational in(I-A)’ of two possible conformations with 13a (the fluences not immediately obvious. other one, I-B, predicts a positive Cotton effect). !Are wish to thank Dr. R. V. Lawrence, Naval Since the first of these possibilities is contrary to the Stores Research Laboratory, Olustee, Florida, for available chemical evidence, we feel that the second samples of resin acids, and several colleagues for one with its 13a-configuration is likely to be correct. helpful discussions. An analogous situation obtains in (-)-a-phellanT h e downfield shift3 in compounds with a Ala bond (e.g., palusdrene (111), our only other case (out of eleven com- tric(11) acid) is normal for a double bond a t this distance and hence not pounds studied so far) of a seemingly wrong predic- relevant (cf. J. S . Shoolery and >T. I,Rogers, J . Am. Chem. SOL.,80, tion of the sign of a Cotton effect from the helicity 5121 (1968)). (12) (a) Visiting Scientist, X.I.H., Summer, 1961; (b) Department rule.8 Here, a positine Cotton effect is expected if the isopropyl group has the quasi-equatorial con- of Chemistry, T h e University of Kansas, Lawrence, Kansas. ISSTITUTE OF ALBERTilr.BURGSTAHLER~~ formation 111-A usually assumed to be the preferred NATIONAL ZIFFER ARTHRITIS AXD METABOLIC DISEASES HERMAX one. The observed negative effect suggests that the BETHESDA 14, MARYLAND VLRICHWEISS alternative conformation 111-B with a quasi-axial RECEIVED SEPTEMBER 18, 1961 isopropyl group is in fact correct; in this case the diene system assumes the required left-handed helicity . OPTICAL ACTIVITY IN SKEWED DIENES It seems significant that both in 111-B and in 1 - 4 S i r : the protons of an alkyl group (17-methyl in I, isoIn connection with our interest in dissymmetric propyl in 111) are spatially close to the center of the unsaturated system. This suggests some type of chromophores,l#’ we have considered the optical interaction which stabilizes an otherwise unfavor- activity associated with skewed dienes3 It will able c o n f o r m a t i ~ n . A ~ ~special ~~ condition of the be recognized that butadiene would be optically protons on C-17 of I is actually suggested by n.m.r. active in any of its nonplanar forms and that comparable dissymmetric diene conformations may (6) A. Moscowitz, E. Charney, U. Weiss and H. Ziffer, J . A m . Chem. be achieved permanently when the conjugated Soc., 83, 4081 (1961). moiety is part of a rigidly fixed ring system, e.g., (7) Models of I-A actually show a n almost planar diene; it is plausible t o assume, however, t h a t in reality this system is more in lumisterol (I).
I-
strongly twisted t o avoid serious crowding of t h e hydrogen atoms a t C-4 and C-5. (8) T h e absolute configuration of I11 appears well established; c.f., inlev alia, the discussion by K. Freudenherg and W. Lwowski, A n n . , 687,213 (19541, and literature quoted there. (9) Cf. also t h e case of isomenthone: (a) C. Djerassi, “Optical Rotatory Dispersion,” McGraw-Hill Book Co., New York, N. Y., 1960, PP. 106, 107; (b) C. Djerassi, el ol., J . A m . Chem. Soc., 83,3334 (1961). (10) T h e positive Cotton effect observed for I1 clearly eliminates a possible “folded” conformation resembling I-A (left-handed helix). I n I 1 t h e absence of a n alkyl group a t C-9 precludes t h e stabilizing interaction assumed for I-A a n d 111-B, a n d hence t h e extended conformation (cf. I-B)with a right-handed helix prevails.
(1) K . Mislow, iif. A. W . Glass, A. Moscowitz and C. Djerassi, J . Am. Chem. Sot., 83, 2771 (1961). ( 2 ) 4.Moxowitz, Tetuaheduon, 13, 48 (1961). (3) While this manuscript was in preparation, the publication of R . Deen a n d H. J. C. Jacobs, Koninkl. N e d e r l . Akadcmie Vart Wetenschappen Amstevdam, 64, 313 (1961), came t o our attention. From both theoretical and experimental considerations (see also R. Deen, Thesis, Leiden, September, 1961), these authors conclude t h a t “ a n asymmetrically situated butadiene system will either in itself or due t o its environment bring about optical activity correlated with its longest wavelength transition.” T h e present work is in agreement with and supports this conclusion.
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COMMUNICATIONS TO THE EDITOR
H3i
Vol. 83
R
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Fig. 1.-Cotton effect of rotatory dispersion of ( + I truns-9-methyl-1,4,9,10 tetrahydronaphthalene: . . . . experimental (c = 0 0073, cyclohexane); - - - - computed 2)/3 correction for refractive index of solvent; with (q2 computed without solvent correction.
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