Conformational abnormalities in cyclohexane chemistry

Use of Circular Dichroism to Determine the Absolute Configuration of a Pimarane Diterpenoid from the Southern African Sclerocroton integerrimus (Euphorbiaceae)

“…General: 1D and 2D NMR spectra were recorded in CDCl 3 on a 500 MHz Bruker AVANCE NMR instrument at room temperature. Chemical shifts (δ) are expressed in ppm and were referenced to the solvent resonances at 7.26 and 77.23 ppm for 1 H and 13 C NMR spectra, respectively. ESI mass spectra were recorded on a Bruker MicroToF mass spectrometer using an Agilent 1100 HPLC to introduce samples (University of Oxford).…”

“…For ring A of cyclic di-and triterpenoids, that adopt the chair conformation, with a keto-group, a positive CE sign at ≈ 290 nm followed by a negative CE sign at ≈ 200-230 nm indicates the R and S configurations at C-5 (H-5α) and C-10 (3H-20β), respectively [12]. However, abnormalities attributable to boat and twisted chair conformations of the cyclohexanone ring on the sign and amplitudes of the CE at ≈ 290 nm have been observed [13] and, therefore, the octant rule is considered unreliable. The experimental electronic circular dichroism (ECD) spectrum of 1 gave a positive CE at 290 nm and a negative CE at 230 nm, therefore indicating the R and S configurations at C-5 (H-5α) and C-10 (3H-20β), respectively i.e.…”

Structure of 3-phenyl-1-oxacyclohexane-2,6-dione

“…A precise knowledge of the conformation of cyclohexane derivatives having one or more sp 2 hybridized carbon atoms is of great importance from both spectroscopic and crystallographic points of view. Cyclohexane derivatives normally present the chair conformation, nevertheless some alkyl derivatives do show a twisted boat conformation (Robinson & Thiobald, 1967). Moreover, it was found that 1,4-cyclohexanedione shows the boat conformation both in solution and in the solid state (Mossel & Romers, 1964;Allinger, Blatter, Freiberg & Karkowski, 1966), while 1,3-cyclohexanedione derivatives exist in the boat conformation in solution (Kwart, Rock, SanchezObregon & Walls, 1972), in the envelope conformation (Semmingsen, 1974;Singh & Calvo, 1975) or in the boat conformation in the solid (Rafferty, Benjamin, Raaen, Thiessen & Johnson, 1979).…”

Chiral spiranes. Optical activity and nuclear magnetic resonance spectroscopy as a proof for stable twist conformations

“…This may be due to the fact that in cyclohexane itself the twist conformation has an energy which lies about 5 kcal/mol above the energy of the chair conformation, while the energy of the boat conformation is some 6 kcal/mol higher than the energy of the chair conformation.1 Thus, the existence of stable twist conformations is ignored in several cases,2•33•4 although in the case of cyclohexanone the energy of the twist conformation is only 2.7 kcal/mol higher than the energy of the chair conformation. 1 It has been shown by X-ray analysis that 1,4-cyclohexanedione is a twisted molecule5 and in some ferf-butylcyclohexane derivatives a twist conformation is most favorable. 6 Most cyclohexane derivatives having a stable twist conformation are chiral molecules.…”