Patai S., Rappoport Z. 2000 The chemistry of functional groups. The chemistry of dienes and polyenes. V. 2 / 0059 196

conjugation in the O CDC moiety is significantly lowered by cross-conjugation of the CDC bond with the other olefinic linkage. In the related system of 2-methoxycyclohexa- 1,3-diene, however, the corresponding effect is much smaller, apparently because of weak conjugative – interaction in the olefinic system. On the other hand, the strength of the p – conjugation appears to be the same in the O CDC CDC moieties of both 1-methoxycyclohexa-1,3-diene and 1-methoxycyclohexa-1,4-diene. Moreover, as a transmitter of substituent effects, the unsaturated system of cyclohexa-1,3-diene is comparable to that of the benzene nucleus. The υ17O values, relative to external water, are given in Scheme 2, which shows that when the ˛-H atom of 18 is replaced with either a vinyl group (19) or a MeO-substituted vinyl group (20), a decrease of 5 – 6 ppm in υ17O is observed. For comparison, an Et-substituent in the ˛-position (21) causes an 8 ppm increase in υ17O . Thus there is a difference of 13 ppm in υ17O between 21 and 19, which suggests the p – conjugation to be less efficient in the latter compound. As it seems likely that the MeO group of 19 can readily adopt the planar s-cis conformation, the reduction in p – conjugation is likely to arise from electronic rather than steric factors.

As expected, υ17O of 24 is not significantly affected by the introduction of another CDC linkage into a nonconjugated position in the six-membered ring (25; υ D 56 ppm), or when the oxygen is at the center rather than at the terminus of the conjugated system (26; υ D 52 ppm). However, the presence of an O CDC CDC system in 27 increases υ17O by 9 ppm, with the shift value of 24 as a reference. This agrees with the 9 ppm difference in chemical shift between the respective open-chain compounds 22 and 23. The buta-1,3-diene skeleton of 22 is known to assume the planar s-trans conformation with a normal buta-1,3-diene stability while the 1,3-diene moiety of 27 behaves like that of the parent cyclohexa-1,3-diene, i.e. devoid of conjugative stabilization. Thus the 17O NMR data suggest that in an O CDC CDC system the oxygen chemical shift, and hence the strength of p – conjugation, do not essentially depend on whether or not there is any actual conjugative – interaction between the two CDC bonds.

The υ17O data in Scheme 2 show that replacement of the Et group of 23 by a vinyl substituent (22) leads to an increase of 9 ppm in υ17O . On the other hand, if the Et group of 29 is replaced with a MeOand Me-substituted vinyl group (leading to 30), the increase in υ17O is only 2 ppm. Clearly, in the latter case the smaller effect of substitution must be due to the combined electron-releasing power of the MeO and Me groups at the end of the 1,3-diene system of 30, which opposes the electron transfer due to the similar groups at the other end of the conjugated system. In 31, however, one of the MeO groups is forced by steric reasons to assume a nonplanar gauche conformation about the O C(sp2) bond, which leads to reduced p – conjugation between this MeO group and the adjacent CDC bond. The shift of the gauche MeO group is decreased by 22 ppm, whereas that of the other MeO group is increased by 3 ppm as a result of the weaker electron transfer to the 1,3-diene system by the gauche MeO group. It is noteworthy that the shift difference of 25 ppm between the two O atoms of 31 is in line with υ17O data observed previously for some related pairs of geometrical isomers, such as the 2-methoxybut-2-enes, for which the shift difference is also 25 ppm30. A comparison of the υ17O values of 27 and 33 (66 and 56 ppm, respectively) shows that introduction of an additional MeO group at the other end of the O CDC CDC system of 27 decreases the oxygen chemical shift by 10 ppm, making it comparable to that of 32, in which the CDC bonds are isolated.

Similarly, υ17O of 34 is 8 ppm lower than that of 28. Accordingly, although the 1,3- diene system of 33 does not possess the nature and thermochemical stability of ordinary conjugated 1,3-dienes, substituent effects are transmitted at least as efficiently through this system as they are transmitted through the aromatic system of 34.

SCHEME 2. 17O NMR chemical shift values (ppm) in CDCl3 solution for compounds 18 – 34

Lie and coworkers31 reported the synthesis and NMR properties of all geometrical isomers of conjugated linoleic acids. Pure geometric isomers of conjugated linoleic acid (CLA) were prepared from castor oil as the primary starting material. Methyl octadeca-9Z,11E-dienoate (36) and methyl octadeca-9Z,11Z-dienoate (38) were obtained by zinc reduction of methyl santalbate (35, methyl octadec-11E-en-9-ynoate) and methyl

82 Yoshito Takeuchi and Toshio Takayama

heteronuclear multiple bond correlation and 1H – 13C correlation spectroscopy techniques. Doubts remain in the absolute identification of the individual olefinic carbon atoms of the (9Z,11Z) (38) and (9E,11E) (42), expect for the fact that the shifts of the ‘inner’ (C10 and C11) and ‘outer’ (C9 and C12) olefinic carbon atoms of the conjugated diene system are distinguishable.

In order to assign the chemical shifts of the carbon atoms of the conjugated diene system of each CLA isomer, it was necessary to conduct INADEQUATE, HMBC (heteronuclear multiple bond correlation) and two-dimensional 1H – 13C correlation spectroscopy (COSY) techniques on the carbon signals of the diene system of the E,Z-isomers. The results of these experiments for the CLA isomers are summarized in Table 13.

The E,Z-, Z,E-, E,E- and Z,Z-isomers can be characterized in sufficient detail by a combination of NMR techniques.

Shtarev and coworkers32 established the structures of substituted F-polyenes on the basis of J(FF) coupling constants. 1-Aryl-1,3-butadienes-F5 44 (Figure 4) and ˛,ω-diaryl- F6-polyenes 45, 46 (Figure 5) were formed as mixtures of E and Z isomers, where E isomers predominated, as established on the basis of 19F NMR chemical shifts, J(FF) coupling constants and integration of the assigned signals. There are significant differences in coupling constants between the Fa and Fb, or the Fa and Fe nuclei, due to the specific configurations and conformation of the individual isomers. The long-range cou-

due to a significant contribution of the cisoid conformation (Figure 6).

The assumed out-of-plane s-cisoid conformations with the dihedral angles D 5 – 23°

aThe assignments a and a0 , b and b0 , c and c0 , d and d0 , e and e0 can be interchanged

FIGURE 4. Selected J(FF) for E- and Z-44. Reprinted with permission from Reference 32. Copyright (1997) American Chemical Society

32

(46)

FIGURE 5. Selected J(FF) for (E,E)-45 and (E,E,E)-46. Reprinted with permission from Reference 32. Copyright (1997) American Chemical Society

F F

FIGURE 6. Geometry of an (all-E)-˛,ω-diaryl-F4-polyene. Reprinted with permission from Reference 32. Copyright (1997) American Chemical Society

experiments were necessary for a correct assignment of the experimental frequencies to the calculated transitions. COSY spectra were helpful in estimating transitions and the proton chemical shift values used to start the iterative analysis. For compound 48f, decoupling of H4/H40 allowed the H3/H30 and H5/H50 resonances to be localized; these resonances strongly overlap with H6/H60, H7/H70 and H8/H80 resonances. Since the signals arising from these 10 protons are located in a range of 90 Hz, it was not possible to carry out the spectral analysis of the spin system H5/H50, H6/H60, H7/H70 and H8/H80. The stereochemistry of the fragment C7 – C8 – C80 – C70 was determined for the partially and selectively deuteriated octaene 48h, prepared from the tetradeuteriated 49d as shown in Scheme 5.