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Formation of Mixed Crystals in Crystallization of 11r-Hydroxy-16r,17r-epoxyprogesterone and 16r,17r-Epoxyprogesterone Qiang Nie, Jun Bo Gong,* Jing Kang Wang, and Shi Wang School of Chemical Engineering and Technology, Tianjin UniVersity, Tianjin, 300072, People’s Republic of China
In the present study, the crystallization of two isomorphic steroid pharmaceutical intermediates 11R-hydroxy16R,17R-epoxyprogesterone (abbreviated as HEP) and 17R-epoxyprogesterone (abbreviated as EP) were studied. Formation of solid solution type mixed crystal system of two steroids in the crystallization process of corresponding mixture from solution was postulated and further confirmed. The experimental results indicate that forming mixed HEP/EP crystal takes place in a wide interval of HEP/EP ratios with the use of the powder X-ray diffraction, differential scanning calorimetry, and high-performance liquid chromatography techniques. Meanwhile, some thermal data (the melting point and enthalpy of melt) of HEP, EP, and their mixed crystals were determined. This discovery will helps us understand the distribution of EP in HEP crystals in the purification process of HEP by crystallization in more depth and will be very beneficial to develop and design the separation process of two steroids. 1. Introduction 11R-Hydroxy-16R,17R-epoxyprogesterone (abbreviated as HEP) and 16R,17R-epoxyprogesterone (abbreviated as EP) are two isomorphic steroid compounds; both of them serve as intermediates for many hormone pharmaceuticals.1 HEP is obtained from EP through bioconversion with very low conversion,2,3 so the product comprises a mass of unconverted starting materials (EP). It is very difficult to isolate and purify HEP from its mixture with EP. In industry, more than five times recrystallization was required to obtain the pure HEP product. More important, we have found that the composition of product by crystallization is not accord with the calculated estimation based on the solubility of two steroids in the solution system in our experiments. But there are few studies where the separation or purification of HEP or EP was reported, which may be due to technological confidentiality. The molecular mechanism of the crystallization process is still poorly understood. Through theoretical analysis and experimental observation, we postulated that the formation of mixed crystals is a potential route that can explain the difficult purification of HEP from its mixture with EP. As is well-known, when crystallization was carried out from solution with isomorphic impurities in the mother liquor, the separation and purification by crystallization becomes more difficult. This is because the isomorphic impurities can be incorporated into the crystal lattice desired product and forms solid solution type mixed crystals with the desired product. If these mixed crystals do form, there will be a thermodynamic equilibrium of the distribution of the impurity between the solid (crystal) phase and the solution phase.4-6 So in this case, the process design for separating two compounds should be focused on the modification or breakup of the equilibrium between solid and solution phases or resort to another separation method. If all these cannot be achieved, the crystallization should be conducted in a most favored solvent system.7 But before doing this, the formation of mixed crystals system between two compounds must be confirmed. Only after the * To whom correspondence should be addressed. E-mail:
[email protected]. Tel.: +86-22-27405754. Fax: +86-2227374971.
system was confirmed can the suitable separation process be designed based on the property and character of this kind of system. Although the solid solution type mixed crystals are not easy to form, we also can find some cases for steroid systems from the documents. Davis8 has detected the formation of mixed cholesterol:β-sitosterol crystals from the mixed solution of cholesterol and β-sitosterol (1:1 by weight) in methanol. Goetschel and Bar9 have described the formation of mixed steroid crystals during a microbial conversion process and explained that the decrease of conversion rate was caused by the formation of mixed crystals. Recently, Christiansen et al.10 have reported that solvent evaporation from mixed cholesterol: β-sitosterol solutions in ethanol leads to the formation of solid solutions in a wide interval of cholesterol:β-sitosterol ratios. However, to our best knowledge, there is no report on the possibility of the formation of mixed HEP:EP crystals in either the bioconversion process or the crystallization process. Can two steroids form mixed crystals? In this paper, we will demonstrate that mixed HEP/EP crystals do form in the crystallization process of corresponding mixtures from solutions. The definition of mixed crystals here refers to crystals containing an indefinite amount of two elements or a slight amount of one element replacing another (i.e., subtitutional solid solution type mixed crystal) according to the classification of Kitaigorodsky.11,12 In the present report, the experimental data on the process of crystallization and the structure of HEP/EP crystals were obtained with the use of the X-ray diffraction (XRD), differential scanning calorimetry (DSC), and highperformance liquid chromatography (HPLC) techniques. The XRD and DSC techniques can provide essential complementary information on the crystallization processes with participation of both HEP and EP in solution, and HPLC helps to exactly determine the composition content of the mixed crystals. These three characterizations together can identify the mixed crystal system effectively.9,13 All crystalline samples of HEP, EP, and their mixtures were prepared from DMF solution by the drowning out method. 2. Materials and Experimental Methods 2.1. Materials. HEP and EP (purity >99%) was provided by Tianjin Pharmaceuticals Group Corp. China. DMF (analytical
10.1021/ie050649i CCC: $33.50 © 2006 American Chemical Society Published on Web 11/09/2005
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grade) was purchased from Kewei Corp. of Tianjin University, China. Purified deionized water was used. 2.2. Preparation of Mixed Crystals of HEP/EP. Mixed crystals of HEP/EP were prepared as follows: a mixed preweighed amount of HEP/EP was dissolved in DMF solvent and then crystallized by gradual addition of pure dionized water as antisolvent at room temperature under the agitation. The crystals obtained from this method were then filtered and airdried at 40 °C before being used for thermodynamic and structure analysis. 2.3. Differential Scanning Calorimetry. DSC curves of the samples (4-6 mg) were recorded by a thermal analysis system (NETZSCH DSC 204,Germany). Following calibration with indium and lead as standards, samples were heated at 10 °C/ min in aluminum pans under a nitrogen atmosphere. The enthalpy of fusion, melting point, and onset temperatures were recorded automatically. Both individual components, their physical mixtures (premixed and grinded in an agate mortar), and mixed crystals were weighed and characterized. 2.4. Powder X-ray Diffraction. XRD experiments were performed as follows: Dried HEP/EP crystals prepared from DMF-water were ground with an agate mortar and pestle and positioned on the silicon plate at the sample holder. Data were collected with a Rigaku D/max 2500 powder X-ray diffractometer, operated at 40 kV and 100 mA; CuKR radiation was utilized in the measurements. The range of diffraction angles was 2θ e 3-50°. All XRD measurements were performed at ambient temperature. 2.5. High Performance Liquid Chromatography. Agilent 1100 series components including a quaternary pump, an autosampler, and a variable UV detector were used to determine the composition content of the mixed crystals. The voltage signal from the detector was interpreted by an Agilent 1100 chemstation chromatography data system version 10.02. Chromatographic separations were achieved using a Zorbax 300SB C18 column (5 µm, 250.0 mm × 4.6 mm). The mobile phase consist of 4:6 acetonitrile:water with flow rate at 1.5 mL/min. The UV absorption was detected at 254 nm. 3. Results and Discussion 3.1. Possibility of Forming Mixed Crystal Between HEP and EP. In the crystallization process of desired product with the presence of impurities in solution, the impurities can be present in product crystal through three mechanisms:14 (1) If the initial impurity concentration in solution is sufficiently high, pure crystals of the impurities may form and be mixed physically with the desired product crystals. (2) At all impurity concentrations, the mother liquor can adhere to the crystal surface and contaminate the crystals. Washing the crystals can avoid or minimize contamination by this mechanism. (3) Similarities between the crystal structures of the solutes (primary species and impurities) can lead to the formation of a mixed crystal. In such instances, the impurity is substituted for the primary species in the crystal lattice. For the two studied steroids systems, the last mechanism is considered to be most probable and will be confirmed in the following sections. A necessary and sufficient condition for the formation of solid solution mixed crystals by two organic substances is a similarity of the shapes and sizes of the component molecules.11 Moreover, if two substances being mixed are isomorphous (identical space group, same number of molecules in the unit cell, and similar packing of molecules), a continuous series of solid solutions can be formed.11 Figure 1 shows the molecular structures of HEP and EP. It can be seen that the two structures are quite
Figure 1. Chemical structures of (a) HEP and (b) EP. Table 1. Unit Cell Parameters of HEP and EP unit cell parameters HEP EP
a (nm)
b (nm)
c (nm)
space group
0.7229(3) 1.3112(5) 1.9334(7) P212121 1.2154(1) 2.0320(1) 0.7336(1) P212121
no. of molecules in unit cell
cell vol (nm3)
4 4
1.833(1) 1.811(1)
similar, with only one difference in 11-OH. Single-crystal diffraction data of EP and HEP have been published in our previous work.15,16 From the crystal cell parameters data listed in Table 1, we can see that the crystal packing modes of two steroids are also very similar. This implies the feasibility of forming mixed crystals by a molecule substitution mechanism, just like the isomorphic amino acids series described in refs 6 and 14. 3.2. Characterization of the Crystal Structure, Thermal Properties, and Composition. XRD and DSC are undoubtedly powerful tools for identifying mixed crystals and discriminating mixed crystals from mixed single crystals or coated crystals.9 To investigate the possibility of the formation of mixed crystals between HEP and EP, a series of the XRD and DSC experiments have been performed. The ultimate goal of this study was to discover and confirm the formation of mixed crystals (or solid solution) in the crystallization of HEP/EP mixtures from solution, which is very important to design the separation and purification of two steroids. 3.2.1. X-ray Powder Diffraction Analysis. Figure 2 compares the X-ray powder diffractograms of pure HEP and EP as well as those of an equi-weight fraction physical mixture (HEP + EP) of HEP and EP and a recrystallized equi-weight fraction mixture (HEP:EP) of HEP and EP. From Figure 2, it can be seen that the individual diffractograms of HEP and EP are somewhat similar. The difference between two diffractograms is also clear: the primary reflection intensity peak (2θ ) 15.26°) of HEP does not appear in the diffractogram of EP, meanwhile the secondary intense diffraction peak (2θ ) 18.8°) of EP is hardly visible in the diffractogram of HEP. This observation
Figure 2. X-ray diffraction profiles of HEP, EP, and their physical 1:1 (w/w) mixture (HEP + EP) as well as of crystals (HEP:EP) precipitated from 1:1 (w/w) solution of HEP and EP.
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Table 2. Diffraction Angles (2θ) and Relative Intensities (in parentheses) of Major Reflections in the X-ray Diffraction Profiles of HEP, EP, Their Physical 1:1 (by weight) Mixture (without recrystallization) and Recrystallized 1:1 Mixture HEP
EP
8.12(0.45) 9.10(0.12) 14.64(0.47) 15.26(1.00) 16.64(0.20) 19.24(0.37) 19.56(0.37) 23.12(0.19) 28.46(0.13)
8.46(0.27) 12.74(0.34) 14.78(1.00) 15.66(0.12) 16.48(0.45) 17.34(0.16) 18.80(0.70) 22.96(0.17) 25.20(0.13) 25.96(0.13)
physical mixture (1:1 in wt)
coprecipitate (1:1 in wt)
7.98(0.20) 8.36(0.21) 11.28(0.10) 12.66(0.14) 14.68(0.78) 15.20(1.00) 16.42(0.28) 16.66(0.23) 17.26(0.10) 18.74(0.48) 19.50(0.34) 22.98(0.20)
8.16(0.17) 14.60(0.81) 15.10(1.00) 16.58(0.32) 19.28(0.41) 22.80(0.22)
allows discrimination between individual HEP and EP crystals. The third diffractogram in Figure 2 corresponds to the physical mixture of equal weight fractions of HEP and EP. Obviously, the HEP + EP diffractogram is an additive collection of all reflections of the individual constituents, notably the two intense reflections at 2θ ) 15.26° and at 14.78°. To favor cocrystallization of crystals, an equi-weight of the steroids was first dissolved in DMF solvent and then totally recrystallized to recover crystals (HEP:EP) at virtually the same as the initial composition (which can be confirmed by HPLC results, see Table 2). The diffractogram of HEP:EP is different from pure EP but is similar to that pure HEP in some degree. It is not identical to the physical mixeture (HEP + EP) with one significant difference: one of the typical diffraction peaks of EP (at 2θ ) 18.8°) that was present in the diffractogram of HEP + EP did not appear in HEP:EP. Since this observation was confirmed in two other similar experiments, we were intrigued to further investigate of incorporation of EP and HEP into the crystal lattice of each other. The angles (2θ) and relative intensities of the major reflections in these diffractograms are tabulated in Table 2. Cocrystallization behavior of HEP and EP was studied in more detail by varying the HEP:EP ratio in the mixture. Figure 3 shows the X-ray powder diffractograms of the crystals that coprecipitated from solutions at initial HEP:EP weight ratios of 5:1, 2:1, 1:1, 1:2, and 1:5. The precipitated crystals have virtually the same with initial composition, as confirmed by HPLC (see Table 4). It can be seen that the typical EP reflection at 2θ e 18.8° is hardly visible or nonexistent in the diffractograms of HEP:EP at 1:2, 1:1, 2:1, and 5:1 weight ratios, but certainly visible in that of HEP:EP ) 1:5. These indicate that the EP molecules have been incorporated into the HEP crystals, and then no typical diffractions of EP were observed for the HEP:EP crystals at 1:2, 1:1, 2:1, and 5:1 weight ratios. We also found the presence of excess HEP results in a crystal lattice very similar to pure HEP. This similarity increases with
Figure 3. XRD profiles of coprecipitate of HEP and EP from solution with various HEP:EP weight ratio.
increasing the HEP content, as can be seen from Figure 3. The incorporation of EP into HEP only caused a small change of the HEP crystal lattice, but the small changes do exist. Figure 4 compared the diffractograms of pure HEP and coprecipitate of HEP and EP at 9:1 weight ratio. Although great similarity was obtained between these two profiles, we also have observed that two profiles have an obvious difference in the region of 2θ e 19-20°. This indicates that the addition of 10% EP into HEP crystal leads to a small change of the HEP crystal structure, and the product crystals composition examined by HPLC and following DSC experiments result can be as another proof to prove the formation of true mixed crystals in the system. For crystals of HEP:EP ) 1:5, the position of the main diffraction peak changes to 2θ ) 8.36° from the pure EP 2θ ) 18.8°, and two peaks emerge at the range of 2θ ) 17-19°, which is quite different with pure EP crystal. This indicates that the addition of 17% HEP has changed the lattice structure of pure EP. We also observed that the reflection peak character of the HEP:EP ) 1:5 in range of 2θ from 14.0° to 20.0° is more similar to the physical mixture of HEP and EP at 1:5 weight ratio, which may be evidence that the pure EP and HEP components coexist with mixed crystals HEP:EP. The diffraction angles (2θ) and relative
Table 3. Diffraction Angles (2θ) and Relative Intensities (in parentheses) of Major Reflections in the X-ray Diffraction Profiles of HEP, EP, and Their Mixtures Recrystallized from Solution HEP
HEP:EP ) 9:1
HEP:EP ) 5:1
HEP:EP ) 2:1
HEP:EP ) 1:1
HEP:EP ) 1:2
HEP:EP ) 1:5
EP
8.12(0.45) 9.10(0.12) 14.64(0.47) 15.26(1.00) 16.64(0.20) 19.24(0.37) 19.56(0.37) 23.12(0.19) 28.46(0.13)
8.08(0.12) 11.30(0.13) 14.60(0.32) 15.22(1.00) 16.62(0.13) 19.30(0.22) 19.50(0.19) 22.84(0.11)
8.12(0.20) 9.05(0.11) 11.28(0.31) 14.66(0.91) 15.20(1.00) 16.66(0.48) 18.26(0.18) 19.26(0.37) 19.52(0.40) 22.9 (0.26)
8.22(0.49) 14.68(0.30) 15.24(1.00) 16.44(0.16) 16.68(0.11) 19.38(0.22) 19.56(0.19) 22.86(0.11)
8.16(0.17) 14.60(0.81) 15.10(1.00) 16.58(0.32) 19.28(0.41) 22.80(0.22)
8.18(0.12) 14.58(0.63) 15.12(1.00) 16.60(0.33) 19.32(0.34) 22.78(0.22)
8.36(1.00) 12.66(0.14) 14.30(0.21) 14.64(0.65) 15.10(0.27) 16.44(0.30) 18.64(0.25) 19.44(0.28) 22.70(0.21) 25.30(0.12)
8.46(0.27) 12.74(0.34) 14.78(1.00) 15.66(0.12) 16.48(0.45) 17.34(0.16) 18.80(0.70) 22.96(0.17) 25.20(0.13) 25.96(0.13)
Ind. Eng. Chem. Res., Vol. 45, No. 1, 2006 435 Table 4. Fraction of EP in the Initial Mixture and in the Mixed Crystals, As Determined with the High-Performance Liquid Chromatography (HPLC) Technique HEP:EP (wt ratio) EP fraction in Initial mixture EP fraction in mixed crystal
Figure 4. X-ray diffraction profiles of pure HEP and crystals (HEP:EP) precipitated from solution with HEP:EP ) 9:1 (w/w).
intensities (in parentheses) of major reflections in the X-ray diffraction profiles of HEP, EP, and their mixtures recrystallized from solution are listed in Table 3. To further confirm the mixed crystal system of HEP and EP, the diffractograms between the coprecipitates and physical mixtures of HEP and EP were also compared. They are shown in Figure 5. It can be seen that there are obvious differences between the crystals and the physical mixtures for HEP:EP ) 1:1 and 1:2. For coprecipitate at the other three weight ratios, the difference also can be observed, although they are not so obvious. The DSC thermal analysis and HPLC analysis in the
9:1
5:1
2:1
1:1
1:2
1:5
0.10 0.09
0.17 0.16
0.33 0.31
0.50 0.45
0.67 0.61
0.83 0.80
following sections will afford more proof to confirm our conclusion. We have to mention that the XRD patterns of cocprecipitate of HEP and EP with low fraction of EP are similar to that of pure HEP, while at the low fraction of HEP, the diffraction pattern is far from that of pure EP. This effect may be due to the difference in the molecular structure of HEP and EP. The extra OH group of HEP may lead to the different spatial organization of HEP crystals from that of EP crystals, such as special effect and possibility of formation of intermolecular hydrogen bond. The single-crystal structure of HEP determined in our previous work shows that there are intermolecular hydrogen bonds between the 3-CdO and 11-OH.16 As 3-CdO (relative active due to the electron delocalization effect with the adjacent -CdC- double bond) is both in HEP and EP molecule, then the intermolecular hydrogen bond between HEP and EP also can be formed. In pure EP crystal, van der Waals
Figure 5. Comparison of diffractograms between physical mixture (HEP + EP) and coprecipitate (HEP:EP) with various HEP:EP weight ratios.
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Ind. Eng. Chem. Res., Vol. 45, No. 1, 2006 Table 5. Thermodynamic Parameters Derived from the DSC Thermograms of Free HEP, EP, and Their Mixtures
EP HEP:EP ) 1:5 HEP:EP ) 1:2 HEP:EP ) 1:1 HEP:EP ) 2:1 HEP:EP ) 5:1 HEP
Figure 6. DSC thermograms of coprecipitate and physical mixture of HEP and EP with various HEP:EP weight ratios.
force is the mainly intermolecular interaction. So the molecular packing of EP is somewhat different from HEP. We have attempted to replace one EP molecule by using a HEP molecule in a EP unit cell in a simulation software, found that the replacement of HEP molecule will leading to the impulsion between the oxygen atom in the hydroxyl of the HEP molecule and EP molecule, and then will make the conformation or molecular packing of EP changed in some degree, consequently leading to the increase of the system energy. 3.2.2. Thermal Behaviors of the Crystals. To further testify our conclusion, corresponding to the XRD studies, a series of thermal analysis experiments to study the structure change with HEP:EP composition variety were performed. Meanwhile, thermal behavior between the cocrystallized and physical mixture of HEP/EP were determined and compared. Figure 6 shows the comparison of DSC curves of HEP, EP, and their equi-weight physical mixture and crystals cocrystallized from DMF-water with various weight ratios. The DSC curves of pure HEP and EP show single endothermic peaks, with peak temperature at 249 and 209 °C, respectively, which is in accord with the reported literature.2 From the DSC thermograms, the visible difference also can be observed between the cocrystallized crystals and the physical mixture of HEP and EP. The cocrystallized crystals show a single peak in a wide interval of HEP:EP weight ratios, the whole ratio range
onset melting T (°C)
top melting T (°C)
end melting T (°C)
heat of fustion (J/g)
205.4 198.1 198.6 207.4 220.2 236.3 245.5
209.0 201.1 207.8 220.1 231.5 241.8 249.0
211.2 204.6 212.2 224.4 234.9 244.4 251.0
80.51 87.31 89.02 90.66 97.60 117.1 130.2
in this study. However, the DSC curves of the HEP/EP physical mixture show two endothermic peaks and reveal that EP preferentially melted with a melting point at 209 °C followed by HEP with a melting point at 249 °C. The presence of only one melting point, which is higher than the melting point of EP but lower than that of HEP, indicates the formation of mixed crystals of HEP and EP. When EP is the primary component in the crystals, the contents of HEP are 17% and 33%. The peak melting point decreased to 201.1 and 207.8 °C, respectively, which is below the melting point of pure EP. This indicates that a small amount of HEP acts as an impurity with eutectic behavior with EP. We also found that the DSC curve of crystals obtained from HEP:EP ) 1:5 (w/w) solution has an evident shoulder, which is different from others. The steep peak shape and the existence of a shoulder in the DSC curve also further proved the coexistence of pure EP crystals and mixed HEP/EP crystals in the coprecipitated product from 1:5 (w/w) HEP/EP solution, which is in accord with our previous opinion obtained by XRD analysis. At the same time, the melting points and enthalpies of melting of two steroids and the cocrystallized mixture with various HEP: EP weight ratios were determined. To our knowledge, these data are reported for the first time. Table 5 summarizes the thermodynamic data of HEP, EP, and cocrystallized crystals of their mixtures from solution. Figure 7 presents the summary of peak temperatures from DSC profiles of mixtures between HEP and EP. On the basis of the results of Figure 7, we also can support our conclusion made after the XRD experiments that HEP and EP form mixed crystals in a wide range of HEP:EP ratios. 3.2.3. Composition Analysis of Mixed Crystals by HPLC. The HPLC results also can be as proof to characterizing the formed mixed crystals system. Table 4 shows the comparison of the composition contents of product crystals and those of the initial materials. To examine the uniformity of distribution of one steroid molecule in the other, the solid used to determine the composition by HPLC is only one piece of single crystal; the HPLC result shows that the crystal comprises two com-
Figure 7. Onset and Peak melting in the mixed crtstals of HEP and EP prepared in a wide interval of HEP:EP weight ratios (HEP + EP ) 1.0).
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pounds. Moreover, for a crystallization product, three HPLC determinations were performed. One piece of single crystal was used for each time, and examination results three times showed no substantial difference in crystal composition. This indicates that the molecules of the impurity (EP) are uniformly distributed in the crystal lattice of the desired product (HEP) (i.e., two steroids form subtitutional solid solution). Up to now, we can draw a conclusion that HEP and EP formed mixed crystals in the recrystallization process from the DMF-water system in a wide HEP:EP weight ratio interval. These three kinds of characterizations (XRD, DSC, and HPLC) have supported and confirmed our conclusion. 4. Conclusion Examination of the crystalline structure of the materials obtained by drowning out crystallization from the HEP:EP mixture in DMF solution revealed that in this case the formation of solid solution mixed crystals takes place in a wide interval of HEP:EP proportions. Our experimental data (Figures 2-7, Tables 2-4) clearly show that variation of HEP content in the initial mixture from ∼17 up to 90 wt % results in the formation of mixed HEP:EP crystals. This indicates that when the desired product HEP was crystallized from solution with the presence of EP, there is an thermodynamic equilibrium to limit the distribution of EP in the crystal phase and the solution phase. Then the isolation and purification of two compounds should be focused on the modification or breakup of the equilibrium between the solid and the solution phases or resort to another separation method. This discovery will be beneficial to design the separation and purification process of HEP and EP.
transformation of steroids. XIII. Oxygenation of 16R,l7R-oxidoprogesterone to 11R-hydroxy-l6R,17R-oxidoprogesterone by Rhizopus nigricans. J. Am. Chem. Soc. 1955, 77, 4428. (3) Ercoli, A.; De Ruggieri, P.; Della Morte, D.; Vister Labs., C. Corticosteroids. III. Microbiological oxidation of 16R,17-oxidoprogesterone to 11R-hydroxy-16R,17-oxidoprogesterone. Successive conversion into 17Rhydroxy-11-oxoprogesterone (21-deoxycortisone). Gazz. Chim. Ital. 1955, 85, 628. (4) Rosenberger, F.; Riveros, H. G. Segregation in alkali halide crystallization from aqueous solution. J. Chem. Phys. 1974, 60, 668-674. (5) Teja, A. S.; Givand, J. C.; Rousseau, R. W. Correlation and prediction of crystal solubility and purity. AIChE J. 2002, 48, 2629-2634. (6) Givand, J. C.; Chang, B.-K.; Teja, A. S.; Rousseau, R. W. Distribution of isomorphic amino acids between a crystal phase and an aqueous solution. Ind. Eng. Chem. Res. 2002, 41, 1873. (7) Givand, J. C.; Teja, A. S.; Rousseau, R. W. Effect of relative solubility on amino acid crystal purity. AIChE J. 2001, 47, 2705-2712. (8) Davis, W. W. The physical chemistry of cholesterol and β-sitosterol related to the intestinal absorption of cholesterol. Ann. N.Y. Acad. Sci. 1955, 18, 123-127. (9) Goetschel, R.; Bar, R. Formation of mixed crystals in microbial conversion of sterols and steroids. Enzyme Microb. Technol. 1992, 14, 462. (10) Christiansen, L.; Karjalainen, M.; Serimaa, R.; Lo¨nnroth, N.; Paakkari, T.; Yliruusi, J. Phase behaviour of β-sitosterol-cholesterol and β-sitostanol-cholesterol coprecipitates. S.T.P. Pharma Sci. 2001, 11, 167. (11) Kitaigorodsky, A. I. Molecular Crystals and Molecules; Academic Press: New York, 1973. (12) Kitaigorodsky, A. I. Mixed Crystals; Springer-Verlag: Berlin, 1984. (13) Mel’nikov, S. M.; Seijen ten Hoorn, J. W. M.; Bertrand, B. Can cholesterol absorption be reduced by phytosterols and phytostanols via a cocrystallization mechanism? Chem. Phys. Lipids 2004, 127, 15. (14) Koolman, H. C.; Rousseau, R. W. Effects of isomorphic compounds on the purity and morphology of L-isoleucine crystals. AIChE J. 1996, 42, 147. (15) Wang, Y.; Wang, S.; Hao, H.; Chen, W.; Nie, Q. 11-Hydroxy16,17-epoxypregn-4-ene-3,20-dione. Acta Crystallogr. 2004, E60, o1338. (16) Wang, S.; Wang, Y.; Nie, Q.; Zhou, L.; Wu, J. 16R,17-Epoxy-4pregnene-3,20-dione. Acta Crystallogr. 2004, E60, o2337.
Literature Cited (1) Xu, G. Y. Handbook of Intermediate Pharmaceuticals; Chemical Industry Press: Beijing, 2001. (2) Peterson, D. H.; Meister, P. D.; Weintraur, A.; Reineke, L. M.; Eppstein, S. H.; Murray, H. C.; Leigh Osborn, H. M. Microbiological
ReceiVed for reView June 6, 2005 ReVised manuscript receiVed September 25, 2005 Accepted October 12, 2005 IE050649I
