Method For Synthesizing Vitamin D Analogs
Processes for preparing vitamin D 2 derivatives and intermediates to vitamin D 2 derivatives are disclosed. An improved photolysis process for the preparation of cis intermediate (I) from the trans starting material (II) are disclosed. Also disclosed is an improved process for the formation of a trans double bond at C 22 -C 23 of the vitamin D 2 derivative, which provides high selectivity of the desired trans double bond of compound (III) over the undesired cis double bond of compound (IIIA).
1 . A method of synthesizing a compound of formula (I)
comprising exposing a compound of formula (II) to light form the compound of formula (I),
wherein P 1 and P 2 are each independently selected from the group consisting of hydrogen and a hydroxyl protecting group; the light has a wavelength of greater than 360 nm, and the exposing step is performed at a temperature below about 15° C.
2 . The method of claim 1 , wherein the light has a wavelength of greater than 360 nm to 400 nm.
3 . The method of claim 1 , wherein the exposing of the compound of formula (II) to light is performed in the presence of 9-acetylanthracene, acridine, phenazine, anthracene, or a combination thereof.
4 . The method of claim 1 , wherein the exposing of the compound of formula (II) to light is performed in the presence of an organic base.
5 . The method of claim 4 , wherein the organic base comprises an alkyl amine.
6 . The method of claim 5 , wherein the alkyl amine comprises triethylamine.
7 . The method of claim 1 , wherein at least one of P 1 and P 2 is a silyl protecting group.
8 . The method of claim 1 , wherein P 1 and P 2 are the same.
9 . The method of claim 1 , wherein the light is filtered through a uranium filter.
10 . The method of claim 1 , wherein the compound of formula (II) is exposed to light for less than one hour and the compound of formula (I) is formed in greater than 95% yield.
11 . The method of claim 10 , wherein the compound of formula (II) is exposed to light for less than 45 minutes and the compound of formula (I) is formed in greater than 98% yield.
12 . A method for preparing a compound of formula (III)
and optionally a compound of formula (IIIA)
comprising admixing a compound of formula (IV) and a compound of formula (V) to form the compound of formula (III) and optionally the compound of formula (IIIA),
wherein each R is independently an alkyl group or an aryl group, P 1 , P 2 , and P 3 are each independently selected from the group consisting of hydrogen and a hydroxyl protecting group, and the ratio of the compound of formula (III) to the compound of formula (IIIA) is at least 95:5.
13 . The method of claim 12 , wherein at least one R is phenyl.
14 . The method of claim 12 , wherein P 1 and P 2 are the same.
15 . The method of claim 12 , wherein at least one of P 1 , P 2 , and P 3 is a silyl protecting group.
16 . The method of claim 12 , wherein the ratio of the compound of formula (III) to the compound of formula (IIIA) is at least 98:2.
17 . The method of claim 16 , wherein the ratio of the compound of formula (III) to the compound of formula (IIIA) is at least 99:1.
18 . The method of claim 12 , wherein the compound of formula (V) has a stereochemistry of
19 . The method of claim 18 , wherein the compound of formula (III) has a stereochemistry of
20 . The method of claim 12 , when at least one of P 1 , P 2 , or P 3 is not hydrogen, further comprising removing the non-hydrogen hydroxyl protecting groups of P 1 , P 2 , and P 3 to form the compound of formula (III) such that each of P 1 , P 2 , and P 3 is hydrogen.
21 . The method of claim 20 , further comprising crystallizing the compound of formula (III) from a solvent mixture comprising acetone and water to provide crystals of the compound of formula (III) having at least 99% purity by weight in a single crystallization step.
22 . The method of claim 20 , further comprising crystallizing the compound of formula (III) from t-butyl methyl ether to provide crystals of the compound of formula (III) having at least 99% purity by weight in a single crystallization step.
23 . The method of claim 21 , wherein the crystals of the compound of formula (III) have a purity of at least 99.5% by weight.
24 . The method of claim 21 , wherein the crystals of the compound of formula (III) are free of methyl formate.
25 . The method of claim 21 , further comprising drying the crystals of the compound of formula (III) under vacuum and at a temperature greater than 35° C.
26 . The method of claim 25 , wherein the temperature is about 40° C.
27 . The method of claim 22 , further comprising drying the crystals of the compound of formula (III) under vacuum at ambient temperature.
28 . A compound of formula (V)
wherein each R is independently an alkyl group or an aryl group and P 3 is hydrogen or a hydroxyl protecting group.
29 . The compound of claim 28 , wherein P 3 is a silyl group.
30 . The compound of claim 28 , wherein P 3 is hydrogen.
31 . The compound of claim 28 , wherein at least one R is phenyl.
32 . The compound of claim 28 having the stereochemistry
33 . The method of claim 22 , wherein the crystals of the compound of formula (III) have a purity of at least 99.5% by weight.
34 . The method of claim 22 , wherein the crystals of the compound of formula (III) are free of methyl formate.