Synthesis of 24,24-difluoro- and 24ξ-fluoro-25-hydroxyvitamin D3

Efficient Stereo-Selective Fluorination on Vitamin D3 Side-Chain Using Electrophilic Fluorination

Our research regarding side-chain fluorinated vitamin D3 analogues has explored a series of efficient fluorination methods. In this study, a new electrophilic stereo-selective fluorination methodology at C24 and C22 positions of the vitamin D3 side-chain was developed using N-fluorobenzenesulfonimide (NFSI) and CD-ring imides with an Evans chiral auxiliary (26,27,30).

The Centennial Collection of VDR Ligands: Metabolites, Analogs, Hybrids and Non-Secosteroidal Ligands

Since the discovery of vitamin D a century ago, a great number of metabolites, analogs, hybrids and nonsteroidal VDR ligands have been developed. An enormous effort has been made to synthesize compounds which present beneficial properties while attaining lower calcium serum levels than calcitriol. This structural review covers VDR ligands published to date.

Stereoselective Synthesis of 24-Fluoro-25-Hydroxyvitamin D3 Analogues and Their Stability to hCYP24A1-Dependent Catabolism

Two 24-fluoro-25-hydroxyvitamin D₃ analogues (3,4) were synthesized in a convergent manner. The introduction of a stereocenter to the vitamin D₃ side-chain C24 position was achieved via Sharpless dihydroxylation, and a deoxyfluorination reaction was utilized for the fluorination step. Comparison between (24R)- and (24S)-24-fluoro-25-hydroxyvitamin D₃ revealed that the C24-R-configuration isomer 4 was more resistant to CYP24A1-dependent metabolism than its 24S-isomer 3. The new synthetic route of the CYP24A1 main metabolite (24R)-24,25-dihydroxyvitamin D₃ (6) and its 24S-isomer (5) was also studied using synthetic intermediates (30,31) in parallel.

Design and Synthesis of Fluoro Analogues of Vitamin D

The discovery of a large variety of functions of vitamin D₃ and its metabolites has led to the design and synthesis of a vast amount of vitamin D₃ analogues in order to increase the potency and reduce toxicity. The introduction of highly electronegative fluorine atom(s) into vitamin D₃ skeletons alters their physical and chemical properties. To date, many fluorinated vitamin D₃ analogues have been designed and synthesized. This review summarizes the molecular structures of fluoro-containing vitamin D₃ analogues and their synthetic methodologies.

Introduction of fluorine atoms to vitamin D3 side-chain and synthesis of 24,24-difluoro-25-hydroxyvitamin D3

During our ongoing studies of vitamin D, we focused on the vitamin D₃ side-chain 24-position, which is the major metabolic site of human CYP24A1. In order to inhibit the metabolism of vitamin D₃, 24,24-difluorovitamin D₃analogues are important candidates. In this paper, we report the practical introduction of the difluoro-unit to the 24-position to synthesize 24,24-difluoro-CD ring (1) and 24,24-difluoro-25-hydroxyvitamin D₃ (2).

Synthesis and CYP24A1-Dependent Metabolism of 23-Fluorinated Vitamin D3 Analogues

Two novel 23-fluorinated 25-hydroxyvitamin D₃ analogues were synthesized using Inhoffen-Lythgoe diol as a precursor of the CD-ring, efficiently. Introduction of the C23 fluoro group was achieved by the deoxy-fluorination reaction using N,N-diethylaminosulfur trifluoride or 2-pyridinesulfonyl fluoride (PyFluor). Kinetic studies on the CYP24A1-dependent metabolism of these two analogues revealed that (23S)-23-fluoro-25-hydroxyvitamin D₃ was more resistant to CYP24A1-dependent metabolism than its 23R isomer.

Vitamin D and the kidney

The kidney is essential for the maintenance of normal calcium and phosphorus homeostasis. Calcium and inorganic phosphorus are filtered at the glomerulus, and are reabsorbed from tubular segments by transporters and channels which are regulated by 1α,25-dihydroxyvitamin (1α,25(OH)(2)D) and parathyroid hormone (PTH). The kidney is the major site of the synthesis of 1α,25(OH)(2)D under physiologic conditions, and is one of the sites of 24,25-dihydroxyvitamin D (24,25(OH)(2)D) synthesis. The activity of the 25(OH)D-1α-hydroxylase, the mixed function oxidase responsible for the synthesis of 1α,25(OH)(2)D, is regulated by PTH, 1α,25(OH)(2)D, fibroblast growth factor 23 (FGF23), inorganic phosphorus and other growth factors. Additionally, the vitamin D receptor which binds to, and mediates the activity of 1α,25(OH)(2)D, is widely distributed in the kidney. Thus, the kidney, by regulating multiple transport and synthetic processes is indispensible in the maintenance of mineral homeostasis in physiological states.

Vitamin D and skeletal tissues

It is now accepted that vitamin D is an integral part of a complex endocrine system, one with far-reaching implications in mineral metabolism. Reviews of the sources, functions and metabolism of vitamin D, as currently understood, are presented as a prelude to discussions of the role of vitamin D in calcium and phosphorous homeostatis and possible specific roles for vitamin D in mineralized tissues. Data describing a possible regulatory function for vitamin D in bone and bone protein metabolism are presented. Some of the controversy which presently exists regarding the biochemical mechanism of the action of this vitamin is discussed. Finally, the possible relationship of vitamin D and disorders of skeletal tissues is described.

26,26,26,27,27,27-hexafluoro-1,25-dihydroxyvitamin D3: a highly potent, long-lasting analog of 1,25-dihydroxyvitamin D3

A new fluoro analog of 1,25-dihydroxyvitamin D3, i.e., 26,26,26,27,27,27-hexafluoro-1,25-dihydroxyvitamin D3, has been compared with the native hormone, 1,25-dihydroxyvitamin D3, in its biological potency, duration of action, and binding to the vitamin D transport protein and intestinal receptor protein. The fluoro analog is about 5 times more active than the native hormone in healing rickets and elevating serum inorganic phosphorus levels of rachitic rats. It is about 10 times more active than 1,25-dihydroxyvitamin D3 in increasing intestinal calcium transport and bone calcium mobilization of vitamin D-deficient rats fed a low-calcium diet. Furthermore, the higher biopotency is manifested in animals after oral dosing. Of great importance is that the action of the fluoro analog is longer lasting than that of 1,25-dihydroxyvitamin D3. This is especially apparent in the elevation of serum phosphorus and bone mineralization responses. The fluoro analog is only slightly less competent than 1,25-dihydroxyvitamin D3 in binding to the vitamin D transport protein in rat blood, and is one-third as competent as 1,25-dihydroxyvitamin D3 in binding to the chick intestinal cytosol receptor for 1,25-dihydroxyvitamin D3. These results suggest that the basis for increased potency of this analog is likely the result of less rapid metabolism.

Metabolism and binding properties of 24,24-difluoro-25-hydroxyvitamin D3

To evaluate possible functional roles for 24,25-dihydroxyvitamin D3, 24,24-difluoro-25-hydroxyvitamin D3 has been synthesized and shown to be equally as active as 25-hydroxyvitamin D3 in all known functions of vitamin D. The use of the difluoro compound for this purpose is based on the assumption that the C-F bonds are stable in vivo and that the fluorine atom does not act as hydroxyl in biological systems. No 24,25-dihydroxyvitamin D3 was detected in the serum obtained from vitamin D-deficient rats that had been given 24,24-difluoro-25-hydroxyvitamin D3, while large amounts were found when 25-hydroxyvitamin D3 was given. Incubation of the 24,24-difluoro compound with kidney homogenate prepared from vitamin D-replete chickens failed to produce 24,25-dihydroxyvitamin D3, while the same preparations produced large amounts of 24,25-dihydroxyvitamin D3 from 25-hydroxyvitamin D3. Kidney homogenate prepared from vitamin D-deficient chickens produced 24,24-difluoro-1,25-dihydroxyvitamin D3 from 24,24-difluoro-25-hydroxyvitamin D3 and 1,25-dihydroxyvitamin D3 from 25-hydroxyvitamin D3. In binding to the plasma transport protein for vitamin D compounds, 24,24-difluoro-25-hydroxyvitamin D3 is less active than 25-hydroxyvitamin D3 and 24R,25-dihydroxyvitamin D3. In binding to the chick intestinal cytosol receptor, 24,24-difluoro-25-hydroxyvitamin D3 is more active than 25-hydroxyvitamin D3 which is itself more active than 24R,25-dihydroxyvitamin D3. The 24,24-difluoro-1,25-dihydroxyvitamin D3 is equal to 1,25-dihydroxyvitamin D3, and both are 10 times more active than 1,24R,25-trihydroxyvitamin D3 in this system. These results provide strong evidence that the C-24 carbon of 24,24-difluoro-25-hydroxyvitamin D3 cannot be hydroxylated in vivo, and, further, the 24-F substitution acts similar to H and not to OH in discriminating binding systems for vitamin D compounds.

Synthesis and biological activity of vitamin D3 analogues

Many analogues of vitamin D metabolites including stereochemical isomers and fluorine substituents were synthesized, and their biological activities were determined by in vitro and in vivo assays. Among those analogues, 24,24-F2-1,25-(OH)2D3 and 26,26,26,27,27,27-F6-1,25-(OH)2D3 are more active than the most potent naturally occurring vitamin D3 metabolite, 1,25-(OH)2D3.

The vitamin D system: a view from basic science to the clinic

Vitamin D produced in the skin and absorbed in the small intestine must be modified metabolically before it can function. It is ultimately converted to a hormone in the kidney that stimulates intestine, bone and kidney to mobilize calcium and phosphorus. This results in normal bone development and normal neuromuscular function. The vitamin D hormone appears to act by a nuclear mechanism to facilitate a target organ response. Finally the vitamin D hormone is produced in response to the need for calcium and phosphorus. The calcium need is interpreted by the parathyroid gland that in turn secretes parathyroid hormone. The parathyroid hormone stimulates production of the vitamin D hormone. This constitutes the vitamin D endocrine system that plays an important role not only in calcium homeostasis but in phosphate homeostasis and in calcium economy of the body. A number of disease states including hypoparathyroidism, pseudohypoparathyroidism, renal osteodystrophy, certain types of vitamin D-resistant rickets and osteoporosis can in part be related to disturbance in the vitamin D endocrine system. Thus measurement of the vitamin D hormone and its precursor will be of great value in diagnosis of metabolic bone disease and most importantly, the availability of new vitamin D compounds will play an important role in the treatment of these bone diseases.