Pre calciferol

The action of heat on calciferol (Vitamin D2 10), or on pre-calciferol (2), with which it is in chemical equilibrium (see below), gives two further isomers, pyrocalciferol (ii) and isopyrocalciferol (12), both of which have sy i-9,io-configura-tions [ii]. Although these reactions were incomprehensible before the orbital-symmetry principles were developed, we can now see that the formation of sy i-isomers is the consequence of a preference for overlap of C<9) and C io) orbital envelopes on the same side of the molecule. The necessary conformations of the open-chain triene systems for these reactions are considerably strained, but no doubt the elevated temperature (180-200 ) at which cyclisation occurs makes the transition states accessible. 

The biologically important transformation of pre-calciferol (2) into calciferol (vitamin D2 10) is a thermsd and not a photochemical process [6]. By employing C(i9)-tritiated material it has been shown to proceed by a completely intramolecular (and reversible) hydrogen transfer (18) between C(i9) and C(9) [13]. A i i stribution of tritium between C(9) and C[Pg.215]

It would seem worthwhile, as a test for conformational rigidity in pre-calciferol, to carry out a controlled partial conversion of ig-H labelled pre-calciferol into calciferol, and determine the g /ga distribution of the label. 

Potassium cyanide, 129 Pre-calciferol, 414, 417-418 Prednisone acetate, 191 —, photolysis, 425, 428-429 Pregnane side chain, conformations, 12, 139 —degradation, 344, 346 Pregnane skeleton, 2. 3 Product development control , X36, 139... 

The above considerations leave one important question unanswered. This concerns the specific cyclisation of pre-calciferol to give ergosterol [8,g], rather than lumisterol (see above). At first sight the excited state of the pre-calciferol molecule appears to have the same electronic structure as excited tachysterol, into which a considerable part of the pre-calciferol is converted. Unless the answer is to be found in some subtle electronic difference between two excited states of pre-calciferol, it seems necessary to invoke a conformational restraint, perhaps associated with the a -configuration at the C(6)-C(7) bond, which may result in steric inhibition of... 

An especially important case is the thermal equilibrium between precalciferol (pre-vitamin Dj, 22) and calciferol (vitamin D2, 23). ... 

System Suitability Preparation Dissolve about 250 mg of USP Vitamin D Assay System Suitability Reference Standard in 10 mL of a 1 1 (v/v) mixture of toluen. Mobile Phase. Reflux this solution at 90° for 45 min, and cool. (This solution contains cholecalciferol, pre-cholecalciferol, and fraws-chole-calciferol.)... 

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