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For many years our scientific interest concentrated on the synthesis of vitamin D analogues having potential applications in therapy. The literature describes more than three thousands of vitamin D3 analogues and metabolites, some of which are commonly used in the clinical practice. Making use of so extensive base of compounds, having known biologic alactivity in most cases, we tried to draw conclusions regarding changes in the molecular structure of the parent vitamin that could result in enhanced, preferably selective, biological action. For a scientist, the fascinating problem of relating the compound structures to their activities is a challenge even greater in the case of vitamins D than in the case of many biologically active substances. It turned out the most active, dihydroxylated meta-

-bolite of vitamin D3, generally regarded a hormonal form there of, fulfills a number of essential regulatory functions in organisms of animals and humans, that exceed classical control of calcium-phosphate homeostasis. The progress of investigation of this compound and its protein receptors demonstrated that it affects almost the whole organism - the protein receptor for vitamin D has been found in dozens of tissues, including brain. Thus we are still far away from learning all functions of these unusual compounds and understanding mechanisms of their action. The vitamins D, being the B-seco-steroid compounds, are also a graceful object of conformation studies. These, in turn, make it possible to relate the energetically favoured structures of vitamin analogues to their activity in living organisms. Our studies concentrated on the syntheses of vitamin D3 analogues and vitamin hormones of modified structure, often having an altered carbon skeleton, hopefully to be used in the medicine. While carrying out these tasks, I employed the methods of molecular modelling, as well as - later - docking to the protein receptor of vitamin D in order to determine the structures and preferential conformations of both intermediates in the synthetic path and the final vitamin analogues. Initially, we have carried these studies unaided, but gradually we included postgraduate students and undergraduates.

 

Introduction

In the organisms of birds and mammals, vitamin D3 (3) is formed from 7-dehydro­cholesterol (1), provided as a dietary component. Opening of the cyclohexadiene ring B that is induced by ultraviolet radiation leads to formation of three major, isomeric products of triene nature, namely previtamin D3 (2), lumisterol3 and

tachysterol3. As a result of the subsequent thermal process, namely antarafacial 1,7-rearrangement of hydrogen atom, previtamin D3 is rearranged to the 5,7,10(19)-triene being vitamin D3 (3) itself. 

Synthesis and evaluation of Vitamin D analogues

The synthesis of vitamin D analogues with missing or added hydrophobic substituents

 

Our investigation on the synthesis of vitamins D lacking angular methyl group at C-13 started several years before publication of the paper by Moras, which presented the crystallographic structure of a complex of 1α,25-dihydroxyvitamin D3 with its mutated protein receptor (human vitamin D receptor, hVDR). Taking the structure of this most active vitamin D3 metabolite into account, we speculated that its middle fragment (the disrupted B ring, C and D rings as well as a significant part of the side chain) should be likely surrounded by hydrophobic amino acids in the receptor pocket.

 

 

 

Effect of the A ring conformation on the activity of 19-norvitamins D

Taking into account the significant biological activity of 2-methylene analogues of 19-norvitamins, which were described by us previously, we decided to obtain also the homologous compounds containing a 2-ethylidene moiety, that would allow to expect strong shift of the conformational equilibrium of the A ring towards one chair form. The results of molecular modelling indicated that, in the 2-ethylidene vitamins, the steric interaction of methyl of the ethylidene group with the allyl hydroxy at C-1

 

(or C-3) should be very important. In the stiffened allyl systems, such strong interactions occur between the substituents of terminal carbons, and these are called A-strain(1,3). We started the synthesis of the planned vitamins from obtaining the appropriate building blocks containing the modified A ring.

 

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The synthesis of analogues of vitamins D having two exo-methylene moieties in the A ring

The studies on obtaining analogues of vitamins D of potential therapeutic use comprised hitherto unknown structural modifications within the cyclohexane A ring. Taking into account the unusual activity of 2MD, i.e., the 20S-calcitriol derivative in which the exo-methylene moiety has been formally translated from C-10 position (characteristic to all vitamins D) to C-2 position, it was an interesting idea to introduce additional 2-methylene group into calcitriol.

 

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