Cyclohexene, conformational analysis

The principles involved in the conformational analysis of six-membered rings containing one or two trigonal atoms, for example, cyclohexanone and cyclohexene are similar. The barrier to interconversion in cyclohexane has been calculated to be 8.4-12.1 kcal mol . Cyclohexanone derivatives also assume a chair conformation. Substituents at C2 can assume an axial or equatorial position depending on steric and electronic influences. The proportion of the conformation with an axial X group is shown in Table 4.4 for a variety of substituents (X) in 2-substituted cyclohexanones. 

P. W. Rabideau (Ed.) The Conformational Analysis of Cyclohexenes, Cyclohexadienes and Related Hydroaromatic Compounds, VCH, New York, 1989. 

The principles involved in the conformational analysis of six-membered rings containing one or two trigonal atoms, e.g., cyclohexanone and cyclohexene, are similar.242... 

Rabideau PW (ed) (1989) Conformational analysis of cyclohexenes, and cyclohexadie-nes. Verlag-Chemie, New York... 

Rabideau PW (1989) The conformational analysis of cyclohexenes, cyclohexadienes, and related hydroaromatic compounds, VCH, New York... 

The compounds with one double bond are dihydro-1,3-oxazines, and the structures shown in Fig. 1 are possible. The conformations of dihydro-1,3-oxazine rings are based on analogy with cyclohexene and also on the conformational analysis of several benzo-1,3-oxazine derivatives. 

The main structural features of compound 1 are the seven-membered ring and the orientation of the dimethylallyl group. We believe that it is not possible by calculation accurately to predict the exact conformation of the side chain the usefulness of a crystal structure is also limited concerning this flexible group. The seven-membered ring contains three planar atoms and has some similarity with cyclohexene we expect therefore the existence of two conform-ers that are related to the two twist forms of cyclohexene. A conformational analysis using MNDO [5] supports this assumption. For our model we have used the conformer with the lowest MNDO heat of formation, which is also in agreement with a published X-ray structure of a chloro derivative of TIBO [6]. 

An A-ray crystallographic analysis of 17jS-hydroxyoestr-5(10)-en-3-one 17-iodoacetate (2) shows ring a in the crystal to have a semiplanar conformation (3), with all carbon atoms except C-2 essentially coplanar. Recent computer treatments have suggested the half-chair conformation (4), so the new experimental finding shows the need for further study in this field (for a recent survey of conformational analysis in cyclohexenes see ref. 6). 

Unusual Properties of Usual Molecules. Conformational Analysis of Cyclohexene, Its Derivatives and Heterocyclic Analogues... 

K. B. Lipkowitz, in Conformational Analysis of Cyclohexenes, Cyclohexadienes, and Related Hydroaromatic Compounds. P. W. Rabideau, Ed., VCH Publishers, New York, 1989, Appendix, p. 299. Molecular Mechanics What It Is and How to Use It. (Tliis reference also contains a list of review articles on molecular mechanics.)... 

Prepared phosphono-1-cyclohexene derivatives, (Diels-Alder [4-1-2] cycloadducts) (41a-c), have been analysed by X-ray diffraction and NMR spectroscopy and their structural and conformational analysis fully described. The absolute configuration of C(3) was proved to be R by XRD... 

FIGURE 47. Three-dimensional structure of the cyclohexene ring of retinal in bR as determined by 2H NMR, relative to the membrane surface in the x-y plane. Analysis of the orientations of the three deuterium labeled methyl groups on the puckered ring (skew around C1-C6) indicates that the chromophore has a 6s-trans conformation around the C6-C7 bond. Reprinted with permission from Reference 60. Copyright (1997) American Chemical Society... 

Analysis of the far IR-spectra of 3,4-dihydro-2//- pyran (13) (72JCP(57)2572> and 5,6-dihydro-2/f- pyran (14) (81JST(71)97> indicates that for both molecules the most stable conformation is a half-chair form. The barrier to planarity is greater for the former compound. These preferred structures are in accord with the half-chair conformation established for cyclohexene and its derivatives. The conformational mobility of cyclohexene is greater than that of the 3,4-dihydropyran. The increased stabilization of the pyran has been attributed to delocalization of the v- electrons of the alkenic carbon atoms and the oxygen lone-pairs (69TL4713). 

The absolute stereochemistry of the Al(6)-alkene alkaloids cocculine (56) and coccutrine (52) has also been established by X-ray analysis. It was found that the cyclohexene ring A exists preferentially in an approximate half-chair conformation in the free base, but this was altered to an envelope conformation on protonation of the nitrogen atom (49). 

Fig. 14. X-Ray crystallographic analysis of 44 reveals the presence of two crystailogra-phically-independent molecules of the same relative configurations [38, 112]. The major difference in the solid state structures of 44a and 44b is the conformation adopted by the cyclohexene rings wherein the hinge angles (0) differ...

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