Cholesteryl iodide

FIGURE 1.11. The derivation of the chemical formulae of steroids resulting from Bernal s X-ray crystallographic work. I and II had been put forward as possible fcrmulae, but III seemed to provide a better fit to the experimental data on molecular ine from refractive index data combined with unit cell dimensions. Bernal listed ses 8.5 x7.0 xl8 A for I, 11.0 x7.5 xl5 A for II, 7.5 x4.5 x20 A for III (from models built to scale), and 7.2 x5.0 x 17-20 A from observed data from X-f .y studies and refractive index measiu-ements. The correct formula, which is very iiiniiar to formula III, is shown in formula IV (cholesterol). The crystal structure of cholesteryl iodide is shown as V. (The value of the parameter x reported for C15 apparently should be 0.585, not that reported in ref. 85.) Iodine atom, filled circle. 

In the 1960s, there was already a forerunner of photochemically induced phase transition in LC (20). When mixtures of cholesteryl iodide and cholesteryl bromide with cholesteryl nonanoate were exposed to UV irradiation, the helical pitch of cholesteric LC changes as a result of photodecomposition of the halides. The reflected color shifts gradually to red with progression of photodecomposition. Pattern-wise imaging was demonstrated but the image was blurred within 15 min. since LC of small molecules is a viscous fluid. 

Cyclohexene exists in the half chair [1] conformation la as confirmed from the X-ray structures of many species including morphine [2] and cholesteryl iodide [3]. Relative to cyclohexane, cyclohexene is flattened and only C4 and C5 bear the tmly axial and equatorial bonds. The bonds on C3 and C6, which are slightly off from the tmly axial and equatorial positions, are called pseudo-axial (a ) and pseudo-equatorial (e ), respectively. The dihedral angle between a substituent on Cl (like R1 in the structure lb) and an equatorial substituent on position C6 (like equatorial R2 in the structure lb) is about 43-44° (significantly less than the normal value of 60 ). This allows them to interfere with each other sterically. 

The measured agrees with the value calculated assuming skeletal carbon-atom arrangements as described by Carlisle and Crowfoot (1945), for cholesteryl iodide... 

As shown in Figure 13.4, and as reported in 1940, an extensive survey of the structures of steroids as determined by X-ray crystallography was undertaken and shortly thereafter, the X-ray crystal structure of cholesteryl iodide (3-iodocholestane) was reported. The structure was largely that anticipated on the basis of a vast amount of chemical evidence (that was subsequently summarized). This established the relative stereochemistry of the side chain and rings. 

Tosyl cholesteryl acetate (V) (Akhtar and Barton, 1964) was first converted to the corresponding 19-iodide (VI) by treatment with sodium iodide in boiling ethyl methyl ketone. Reduction of the iodide with zinc and acid in the presenee of tritiated water provided cholesteryl acetate-19-H (VII) which was then converted to 7-dehydro-cholesterol-19-H (VIII), presxunably via the N-bromosuccinimide reaction. Ultraviolet irradiation and thermal rearrangement of the provitamin yielded cholecalciferol-9,19-H (IX). 

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