Process for Preparing 24-Fluoro-25-Hydroxycholecalciferol

This invention relates to a compound which is characterized by vitamin D-like activity.

More specifically this invention relates to a method for preparing a derivative of vitamin D.sub.3 and the new and novel intermediates in such process.

Vitamin D.sub.3 is a well-known agent for the control of calcium and phosphorous homeostasis. In the normal animal or human this compound is known to stimulate intestinal calcium transport and bone-calcium mobilization and is effective in preventing rickets.

It is also now well known that to be effective vitamin D.sub.3 must be converted in vivo to its hydroxylated forms. For example, the vitamin D.sub.3 is first hydroxylated in the liver to form 25-hydroxy vitamin D.sub.3 and is further hydroxylated in the kidney to produce 1.alpha.,25-dihydroxyvitamin D.sub.3 or 24,25-dihydroxy vitamin D.sub.3. The 1.alpha.-hydroxylated form of the vitamin is generally considered to be the physiologically active or hormonal form of the vitamin and to be responsible for what are termed the vitamin D-like activities, such as increasing intestinal absorption of calcium and phosphate, mobilizing bone mineral, and retaining calcium in the kidneys.

References to various of vitamin D derivatives are extant in the patent and other literature. See, for example, U.S. Pat. Nos. 3,565,924 directed to 25-hydroxycholecalciferol; 3,697,559 directed to 1,25-dihydroxycholecalciferol; 3,741,996 directed to 1.alpha.-hydroxycholecalciferol; 3,907,843 directed to 1.alpha.-hydroxyergocalciferol; 3,715,374 directed to 24,25-dihydroxycholecalciferol; 3,739,001 directed to 25,26-dihydroxycholecalciferol; 3,786,062 directed to 22-dehydro-25-hydroxycholecalciferol; 3,847,955 directed to 1,24,25-trihydroxycholecalciferol, 3,906,014 directed to 3-deoxy-1.alpha.-hydroxycholecalciferol; 4,069,321 directed to the preparation of various side chain fluorinated vitamin D.sub.3 derivatives and side chain fluorinated dihydrotachysterol.sub.3 analogs; 4,188,345 directed to a process for preparing fluorinated vitamin D compounds; 4,196,133 directed to 24,24-difluoro-25-hydroxycholecalciferol.

A new method for preparing 24-fluoro-25-hydroxycholecalciferol (24-fluoro-25-hydroxyvitamin D.sub.3 or 24-F-25-OH-D.sub.3) has now been found by which said compound can be readily prepared. 24-F-25-OH-D.sub.3 expresses excellent vitamin D-like activity as measured by its ability to stimulate calcium transport in intestine and its ability to increase serum calcium concentration. Such compound, therefore, could serve as a substitute for vitamin D in its various metabolic bone diseases such as ostenmalacia, osteodystrophy and hypoparathyroidism.

24-F-25-OH-D.sub.3 was synthesized in accordance with the following description and abbreviated schematic: ##STR1##

For the synthesis of 24F-25-hydroxyvitamin D.sub.3 11, cholenic acid is successively treated with dihydropyran-p-TsOH, LiAlH.sub.4, and p-TsCl-pyridine to afford the tosylate 1, which in turn is reacted with KCN/18-crown-6 in dimethylformamide at 70.degree. C. for 3 hr. to give the cyanide 2 (48% yield, mp 142-143). Hydrolysis of 2 with KOH in aqueous ethanol (140.degree., 48 hr.) yielded the carboxylic acid 3 (63% yield, mp. 171.degree.-172.degree. C.), which on treatment with CH.sub.2 N.sub.2 gives the methyl ester 4 (79% yield, mp 159.degree.-161.degree.). The enolate of 4 (generated by treatment with lithium dicyclohexylamide at -78.degree. C.) was treated with iodine in THF at -78.degree. C. to yield the idodide 5. Substitution of iodine of 5 with hydroxyl was accomplished by reaction with CF.sub.3 COOAg-Ag.sub.2 O in CH.sub.3 CN-ethyl ether (20.degree. C., 16 hr.). Reesterification with CH.sub.2 N.sub.2 gave the hydroxy ester 6 (mp 114.degree.-117.degree. C., .delta.4.16 (1H, m, C-24), m/e 418 (M-84)) in 63% overall yield from 4. (The 3,24-dibenzoyl ester derived from 6 showed a twin peak on high pressure liquid chromatography (HPLC) (Zorbax-SIL, 10% CH.sub.2 Cl.sub.2 in n-hexane), indicating that 6 is a 1:1 epimeric mixture at C-24 position. This was further corroborated by transformation of 6 into a 1:1 mixture of (24R)- and (24S)-3,24-dibenzoyloxycholest-5-ene-25-ol trimethylsilyl ether which was cochromatographed on HPLC with authentic samples (M. Seki, N. Koizumi, M. Morisaki and N. Ikekawa, Tetrahedron Letters, 15 (1973)).)

The tosylate 7 derived from 6 was treated with KF/18-crown-6 in dimethylformamide (70.degree. C., 15 hr.) to give the fluoride 8, which on hydrolysis (p-TsOH in methanol-CH.sub.2 Cl.sub.2, 0.degree., 4 hr.) affords the fluoro ester 9 (73% yield, mp 104.degree.-105.degree. C. .delta.4.86 (1H, dm, J.sub.ht =48 Hz, C-24), m/e 420 (M.sup.+)). Treatment of 9, which, if desired can be reprotected at C.sub.3 by acylation (e.g. acetylation, benzoylation) by known procedures, with an excess of CH.sub.3 MgI in ethyl ether (20.degree. C., 20 min) followed by acetylation furnished in 80% yield, the fluorohydrin 10 (mp 153.degree.-154.degree. C., .delta.4.14 (1H, dm, J.sub.Hf =48 Hz, C-24), 1.20 (6H, S, C-26,27), m/e 402 (M-60)). Conversion of 10 to the corresponding vitamin D.sub.3 11 was carried out essentially as described for 24,24-difluoro-25-hydroxycholecalciferol in U.S. Pat. No. 4,196,133, issued Apr. 1, 1980 and in Y. Kobayashi, T. Taguchi, T. Terada, J. Oshida, M. Morisaki and N. Ikekawa. Synthesis of 24,24-difluoro and 24-fluoro-25-hydroxyvitamin D.sub.3. Tetrahedron Letters, No. 22, pp. 2020 (1979).

Compound 11 showed the expected spectral properties (.lambda..sub.max 263, nm, .lambda..sub.min 228, nm, m/e 418 (M.sup.+), 402 (M-15), 400 (M-18), 385 (M-15-18), 359, 271, 253, 136, 118).

Male weanling rats were purchased from Holtzman Company, Madison, Wis., were housed in hanging wire cages and were fed ad libitum water and the low calcium vitamin D-deficient diet as described by Suda et al. (J. Nutrition 100, 1049, 1979) for three weeks prior to their use in the following assays.

Rats that had been fed the low-calcium, vitamin D-deficient diet described above were divided into three groups of 5 rats each. The rats in each group were then respectively given a single dose of 650 pmole of either 24-F-25-OH-D.sub.3 or 25-OH-D.sub.3 dissolved in 0.1 ml 95% ethanol intrajugularly 23 hours prior to sacrifice. Rats in the control groups were given the ethanol vehicle only in the same manner. The rats in the groups were killed by decapitation and their blood was collected for the measurement of serum calcium concentration. Their duodena were used to measure intestinal calcium transport activity in accordance with the technique of Martin and DeLuca (Am. J. Physiol. 216, 1351, 1969) with the results shown in Table I, second column.

The blood collected as described above was centrifuged to obtain the serum. One-tenth ml of serum was mixed with 1.9 ml of 0.1% lathanum chloride solution and the calcium concentration was measured with an atomic absorption spectrophotometer (Perkin-Elmer Model HO-214). Results are shown in Table 1, third column.

It is evident from Table I, that 24-fluoro-25-hydroxyvitamin D.sub.3 exhibits pronounced vitamin D-like activity as evidenced by the increase of intestinal calcium transport and increase in the concentration of serum calcium and appears to be wholly as effective in this regard as 25-hydroxyvitamin D.sub.3 (see U.S. Pat. No. 3,565,924).