GB2390557A

GB2390557A - A method for separating the enantiomers of a chiral compound from a mixture.

Info

Publication number: GB2390557A
Application number: GB0314705A
Authority: GB
Inventors: Peter J King; Robert J Boughtflower
Original assignee: Applied Photophysics Ltd
Current assignee: Applied Photophysics Ltd
Priority date: 2002-06-25
Filing date: 2003-06-24
Publication date: 2004-01-14
Also published as: GB0314705D0; GB0214617D0; AU2003236915A1; WO2004000761A1

Abstract

A method for separating the enantiomers of a chiral compound from a mixture comprises introducing the mixture into a separation device 16 which changes the distribution of the enantiomers in the mixture, so that a first part of the mixture includes the first enantiomer but substantially none of the second and a second part of the mixture includes the second enantiomer but substantially none of the first, analysing 18 the mixture to determine the boundary between the first and second parts of the mixture, directing at least a portion of the first part of the mixture to a collection point W, leaving the remainder of the portion in the fluid flow. The remaining portion may be reintroduced into the separation device so that the process may be repeated. The separation device may be a chromatography column suitable for use in high pressure liquid chromatography [HPLC]. The sensor may measure the optical activity of the fluid flow and may be an optical absorption meter and/or a circular dichroism meter.

Description

Separation of Chiral Compounds This invention relates to the separation of

enantiomers.

5 Chiral compounds contain at least one chiral centre, comprising a carbon atom that is bonded to 4 different atomic or molecular groups. Such a compound can exist in two distinct geometric configurations depending on the exact arrangement of the side groups and these are known as stereoisomers.

For a single chiral centre the stereo-isomers are non-superimposable mirror 10 images of each other and are called enantiomers. Such compounds are called enantiomeric. If more than one chiral centre exists (say n chiral centres) the compound has a maximum of 2n stereoisomers. Pairs of non-superimposable stereoisomers in this population are also called enantiomers.

15 Many pharmaceutically interesting compounds are chiral and therefore enantiomeric. However, usually only one enantiomer in a pair is pharmaceutically active. The other enantiomer may be less active, inactive, or may even possess a conflicting or harmful activity. It is important therefore to be able to isolate enantiomerically pure samples of such compounds.

Pairs of enantiomers have broadly identical physical properties which makes isolation of pure enantiomers from a mixture very difficult. However they do differ in me way each interacts with other chiral compounds and they also each interact differently with polarised light.

Routine separation of enantiomers is usually based on this differential interaction with a third chiral compound. This forms the basis of socalled enantioselective separation methods. One particular method is called chiral, or enantioselective, high performance liquid chromatography (HPLC); another 5 related technique is chiral supercritical fluid chromatography (SFC).

In one type of chiral HPLC a small quantity of a solution of an enantiomer mixture to be separated is injected into a solvent flow, this being called a mobile phase. The mobile phase is then passed through a column 10 packed with an immobile chiral material known as a stationary phase. Because of the different degree of interaction between the two enantiomers and the chiral stationary phase, one of the enantiomers passes through and elutes from the column faster than the other, resulting in a degree of separation of the enantiomers in the eluent. The slower-eluting enantiomer is said to have been 15 retained longer on the column due to its stronger interaction with the chiral stationary phase.

The degree of separation, and the amount of material that can be successfully separated is dependent on the degree of the differential interaction 20 of the enantiomers with the chiral stationary phase, and the relative amount of chiral stationary phase available to the mixture to interact with. The larger the amount of stationary phase and therefore the larger the physical dimensions of the column the more enantiomer mixture can be separated in one elusion run.

Small (<lcm diameter) so-called analytical columns can separate sub-

25 milligram quantities of enantiomers whereas larger (>lcm diameter) socalled

preparative columns can separate milligram to gram quantities. Significantly larger columns are used for industrial scale production.

Chiral stationary phases are very expensive and the quantity required 5 increases dramatically for larger scale separations using larger chiral columns.

Larger columns also require a much greater mobile phase flow rate and therefore consumption of solvent, which also becomes a significant expense, as well as requiring careful disposal.

10 EP 0569992 (Orion-Yhtma) uses such a technique to separate a racemic mixture of epoxypropionic acid, using carbon dioxide as a solvent, and detecting the separated enantiomers (using for example a flame ionisation detector). Here it is suggested that if the enantiomers are not fully resolved, different fractions may be removed, and a fraction containing the desired 15 enantiomer in an impure state can be returned to the column. It is stressed though that there should be at least some resolution for the process to be economic, and factors identified in affecting the resolution including not only the composition of the stationary phase but also the pressure, temperature and the quantity of racemic mixture introduced to the column.

US 5822067 (Yanik) shows an optical detector that measures optical absorption and rotation in order to identify enantiomers and calculate enantiomeric ratios. Thus, this instrument can identify an enantiomer separated by a chromatographic column, and check its purity for quality 25 control purpose and that the production and separation conditions have been optimised.

It is the object of the present invention to provide an improved means of separating quantities of enantiomers by chiral HPLC allowing smaller size columns and therefore less chiral stationary phase and mobile phase than 5 would be normally required.

Accordingly there is provided a method of separating the enantiomers of a chiral compound from a mixture, the mixture including a first enantiomer and a second enantiomer, including the steps of; introducing the mixture into a fluid flow leading to a separation means which changes the distribution of the enantiomer concentrations of the mixture so that the fluid flow includes at least a first region which includes the first enantiomer but substantially none of the second enantiomer, and a second I S region which includes the second enantiomer but substantially none of the first en ant iom er, sensing the mixture using a sensor means to produce a first sensor signal proportional to the sum of the concentration of the first enantiomer and the 20 concentration of the second enantiomer, sensing the mixture using a sensor means to produce a second sensor signal proportional to the difference between the concentration of the first enantiomer and the concentration of the second enantiomer,

processing the first and second sensor signals to detect the region in the fluid flow which includes the first enantiomer but substantially none of the second enantiomer and calculate its location, 5 directing at least part of said region in the fluid flow which includes the first enantiomer but substantially none of the second enantiomer to a collecting means, leaving a remaining portion in the fluid flow.

Preferably the remaining portion is reintroduced to the column so that 10 the above steps may be repeated.

Preferably, the first and second sensor signals are also processed to detect the region in the fluid flow which includes the second enantiomer but substantially none of the first enantiomer, and its location calculated, at least part of said region in the fluid flow which includes the second enantiomer but substantially none of the first enantiomer is directed to a collecting means, leaving a remaining portion in the fluid flow.

20 In this way, even small amounts of separation can be detected, and pure fractions can be thereby separated, as a fully automatic iterative process, until the mixture is completely separated, or as a continuous process. Therefore large quantities can be conveniently separated whilst minimising the amount (and therefore expense) of chiral environment. Measurement of characteristics 25 of the compounds proportional to the sum and to the difference are a s

convenient way of deriving the composition, concentration and purity of the material leaving the separation means.

The invention will now be described, by way of example, and with 5 reference to the accompanying drawings, of which; Figure 1 is a schematic view of the separating apparatus; Figure 2a is a graphical representation of the possible amounts of two 10 compounds to be separated; Figure 2b is a graphical representation of the corresponding sensing outputs; 15 Figure 3a is a graphical representation of another possible distribution of the amounts of two compounds to be separated; Figure 3b is a graphical representation of the corresponding sensing outputs; and Figure 4 is a schematic representation of the sensing apparatus.

Referring to Figure 1, a fluid circuit 10 comprises a loop 12 which includes a pump 14, column 16, flow cell 18 and a loop valve 20. A sample 25 syringe 22 allows a quantity of the mixture of the two enantiomers to be separated to be injected into the loop 12. The loop valve 20 allows

communication with a reservoir of solvent 24, and also leads to a collection valve 30 for collecting fractions from the fluid flow.

A volume of the mixture to be separated, is introduced by the sample 5 syringe 22 to the loop 12 and driven around the loop by the pump 14 in an anticlockwise direction. The mixture flows through the column 16, which contains the chiral stationary phase, where one enantiomer interacts more strongly with the stationary phase than the other enantiomer. The column is typically of a small size, and the volume of sample saturates it, so that the 10 volume is only partially separated. The sample leaving the column 16 therefore has one pure enantiomer at its forefront or 'leading edges (the enantiomer which interacts most weakly with the chiral stationary phase) the central portion of the sample being a mixture (as shown in Figure 2a), with a trailing edge' at its back comprising purely the other enantiomer which 15 interacts preferentially with the chiral stationary phase and which has been released later.

As this sample leaves the column 16 and flows through the flow cell 18, it is interrogated by a pair of suitable optical probes.

A particularly suitable pair of optical probes comprises an optical absorption meter and a circular dichroism meter. The optical absorption meter measures the amount of light absorbed by the fluid under investigation and is insensitive to chirality, whilst the measurement from the circular dichroism 25 meter is dependent upon optical activity (being influenced positively for one

enantiomer and negatively for the other enantiomer) and therefore dependent upon the relative proportions of the enantiomers.

A possible assembly for the optical probes is shown in Figure 4. An 5 intense light source (such as a Hg/Xe lamp) 40 is focussed by a concave reflector 41, where it passes through a slit 42, reflected by reflective grating 44, and passes through a slit 46. The slits and reflective surface are so configured that only a band of the spectrum from light source 40 is selected, so that the light passing through slit 46 is essentially monochromatic. The light 10 beam is collimated by lens 48 and polarised by a suitable polariser 50. The light beam is then focused by lens 54, before passing through a photoelastic modulator 52, onto a point in the observation cell 56 through which the fluid in the loop 12 flows. A photomultiplier tube detects the transmitted signal, with suitable electronics to separate the signals due to the circular dichroism and 15 total optical absorbance.

Referring to Figures 2a and 2b; a typical concentration profile for each enantiomer is shown in Figure 2a. As the sample leaves the column, and enters the flow cell, the leading edge of the sample contains pure enantiomer E, a 20 region denoted by the letter 'a'; the concentration of this enantiomer rises as shown by the curve in Figure 2a. The signal 1 from the optical absorption meter rises with the increase in proportion, as does the signal I2 from the circular dichroism meter (in this instance taking the signal resulting from the first enantiomer to be positive, although this is of course arbitrary). Plotting 25 the two signals against each other, as shown in Figure 2b, produces a straight line (indicated by the letter 'a') during this phase. In this example, the second

enantiomer E2 enters the flow cell and starts to increase in concentration before the peak of the first enantiomer E' has passed Trough the flow cell. The optical absorption signal l' increases further since it is affected by both the enantiomers, but the circular dichroism signal I2 decreases as the signal due to 5 the first enantiomer is cancelled by that of the second enantiomer. This results in a curved portion (indicated by letter 'b') on the plot of the two meters which crosses the vertical axis as the concentrations of the two enantiomers coincide.

As the concentrations of the two enantiomers decrease, the plot curves correspondingly. When all the enantiomer E' has passed through the flow cell, JO the plot becomes linear (indicated by letter 'c'), since the trailing edge containing pure enantiomer E2, and the plot returning to the origin.

Referring to Figures 3a and 3b, which again show relative enantiomer concentration with time and a plot of the optical absorption meter signal versus 15 the circular dichroism meter signal, it may be that the enantiomers are separated to a greater degree by the column whilst still not being completely resolved, the peak concentrations of me two enantiomers are separated sufficiently that the total concentration of the mixture falls before the peak of the second enantiomer is reached. Thus the plot shows a linear region 'a' as En 20 increases, a curved region 'b' where E2 also enters the flow cell, this curve including a region where the total absorption I' decreases whilst E2 increases, before increasing again near the peak concentration of E2, and a linear portion c' returning to the origin where all the E' has passed through the flow cell and the concentration of pure E2 decreases.

Analysing the signals produced from the optical absorption meter and the circular dichroism meter thus allows the proportions and purities of the enantiomers in the flow fluid to be accurately calculated by processing means connected to the meters. The regions in the flow fluid where pure enantiomer S exists are indicated by the ratio of 1, to 12 equalling a particular constant (i.e., the linear portion of the plot). The magnitude of the constant for the first enantiomer and the second enantiomer is the same, although the presence of the solvent (for example if a chiral solvent is used) may cause a discrepancy in the measurement which may be easily compensated for.

The volume of fluid between the flow cell and the loop valve is precisely known, and the flow rate produced by the pump and the introduction

of the sample volume (and due to the introduction of solvent and extraction of

fractions to be described below) is also accurately measured or calculated.

15 From this, the composition of the fluid at the loop valve can be known precisely, calculated simply from compensating for the time taken for a portion of fluid to travel from the flow cell to the loop valve.

The loop valve 20 is controlled by the processing means such that, 20 when the fluid contains only the first enantiomer and the solvent, the flow is diverted to the collection valve (the paths of the loop valve here being indicated by the solid lines). The collection valve 30 is also controlled by the processing means and again the volume and flow rate between the loop valve and the collection valve are known and/or calculated so that the fluid's 25 composition at the collection valve is precisely known. The pure portion of the first enantiomer is collected into a first collection container F'. As this portion

of the flow is removed, solvent from the reservoir is permitted by the loop I valve 20 to enter the loop to balance the flow.

Before the second enantiomer reaches the loop valve, the loop valve S remakes the circuit (the paths of the loop valve here being indicated by the dotted lines) so that the enantiomer mixture is retained on the loop heading towards the column.

When the fluid flowing through the loop valve contains purely the 10 second enantiomer, the loop valve once again diverts the flow from the loop to the collector valve. The collector valve is switched to divert the fluid into a second collection container F2.

When there is no further, or only an insignificant amount, of the second 15 enantiomer present in the fluid, the loop valve redirects the fluid flow back to the loop returning to the column.

The fluid that has been redirected by the loop valve is passed through the column a second time. As for the first time through the column, the first 20 enantiomer is preferentially allowed through the column whilst the second enantiomer is preferentially bound and therefore delayed in its passage. The proportions of the two enantiomers in the remaining mixture before entering the column are not equally distributed but the leading edge is now richer in the first enantiomer than the original mixture, and the trailing edge is richer in the 25 second enantiomer.

The eluent exiting the column will contain similar enantiomer distribution as described in relation to Figures 2a and 2b, although the separation will have been enhanced by the above-mentioned altered distribution between the concentrations of the first and second enantiomers in 5 the mixture entering the column and also enhanced by the smaller amounts of enantiomer mixture. The enantiomer concentrations in the eluent are again calculated by the optical absorption meter and circular dichroism meter. Pure enantiomer is diverted from the loop and collected in the relevant receptacle by the loop valve and collection valve.! In general, the efficiency with which the enantiomer mixture is separated increases with the number of passes through the column, since amounts of enantiomer mixture decrease and the chiral stationary phase becomes either less saturated or unsaturated and the relative proportions of the 15 enantiomers in any particular portion of mobile phase become less equal. The processing of the enantiomer mixture into its separate components continues automatically with as many passes through the chiral stationary phase as necessary until the entire mixture has been resolved. As the processing continues, at some stage the enantiomers will be completely separated; in this 20 case, a plot of the optical absorption meter signal against the circular dichroism meter signal would produce two linear portions corresponding to the fully separated enantiomers and all the remaining enantiomer would be removed from the fluid loop in that pass.

25 At the collection valve, the portion where the two enantiomers meet may be diverted into separate receptacles W. so that the different collected

enantiomers are pure and uncontaminated. This mixture may be later reintroduced to the loop for separation. Alternatively, if one enantiomer is to be discarded, the portion including the interface between the two enantiomers may each time be separated into the receptacle of the enantiomer to be 5 discarded.

After the enantiomers have been separated and collected, farther quantities of enantiomer mixture may be introduced to the loop. Such introduction of enantiomer mixture may also take place whilst previously

10 introduced mixture is still present in the loop. Conveniently, the mixture may be introduced into the slug of mixture that has exited from the loop valve. This method will be particularly suitable to conveniently separate the mixture in a continuous and automatic fashion.

15 It will of course be realised that there are many ways of implementing the sensor assembly described above, particularly with alternative focusing, polarising and collimating elements etc. Other types (particularly similar sensor pairs, one sensitive to the summation of the two enantiomers' concentration, the other sensitive to the difference in concentration) of sensor 20 may be substituted for the optical absorption meter and circular dichroism meter described above, depending upon the enantiomers to be separated. For example, the sensors may be an absorption meter and a polarimeter Alternatively, the sensors may be a fluorescence meter and fluorescence detected circular dichroism meter. A polychromatic absorption meter could be 25 substituted for the optical absorption meter. The sources may of course be changed to correspond to whichever meter is used.

The change in the distribution of the enantiomers in the enantiomer mixture described above could be effected by chiral (i.e. enantioselective) high performance liquid chromatography (HPLC), by chiral supercritical fluid 5 chromatography (SFC), or indeed some other enantioselective separation method. The technique may also be used with several columns having different chiral stationary phases in series. Further, columns of different chiral 10 stationary phases may be attached in parallel, with enantiomer mixture being directed in turn to each column, in order to assess which chiral stationary phase most effectively separates the mixture; the mixture may thereafter be directed solely to the most effective column or columns.

15 It will be seen that large amounts of enantiomer mixture may be separated using a relatively small amount of chiral stationary phase, typically a quantity of chiral stationary phase that is currently suitable only for analytical separation may be used to produce preparative quantities of pure enantiomer.

Furthermore, the technique may also be scaled up to increase the processing 20 capacity and efficiency of industrial scale chiral separation installations.

The system could also be used to determine what proportion of the mixture elating from the stationary phase will be pure, and then to use this information in an automated process to take a preset amount, without 25 necessarily having to make dual measurements and so calculate the purity on each or any pass, particularly if new mixture is injected into the recirculated

eluent on each pass, so that the passes reach a steady state wim a similar concentration distribution in each eluent pass. A single sensor would then ideally be used (not necessarily on each pass) to detect the front part of the eluted mixture, and portions of the mixture may then be drawn off for 5 predetermined times after detection to extract predetermined volume of the pure enantiomers.

Although the principles of the system described herein are particularly suited to recirculation of any portion of the eluent comprising an unseparated 10 enantiomer mixture, the principles could also be used to separate pure enantiomers from eluted enantiomeric mixtures without recirculating the remaining enantiomer mixture back into the stationary phase, but merely discarding the remaining mixture; such an embodiment represents a useful separation technique, particular where the nature of the enantiomer mixture 15 samplesis variable or uncertain.

Claims

. CLAIMS

1. A method of separating the enantiomers of a chiral compound from a 5 mixture, the mixture including a first enantiomer and a second enantiomer, including the steps of, introducing the mixture into a fluid flow leading to a separation means which changes the distribution of the enantiomer concentrations of the mixture so that 10 the fluid flow includes at least a first region in which includes the first enantiomer but substantially none of the second enantiomer and a second region which includes the second enantiomer but substantially none of the first enantiomer, 15 sensing the mixture using a sensor means to produce a first sensor signal proportional to the sum of the concentration of the first enantiomer and the concentration of the second enantiomer, sensing the mixture using a sensor means to produce a second sensor signal 20 proportional to the difference between the concentration of the first enantiomer and the concentration of the second enantiomer, processing the first and second sensor signals to detect the region in the fluid I flow which includes the first enantiomer but substantially none of the second 25 enantiomer and calculate its location,

directing at least part of said region in the fluid flow which includes the first enantiomer but substantially none of the second enantiomer to a collecting means, leaving a remaining portion in the fluid flow.

5 2. A method according to claim I wherein the remaining portion is reintroduced to the column so that the above steps may be repeated.

3. A method according to either previous claim wherein the first and second sensor signals are also processed to detect the region in the fluid flow 10 which includes the second enantiomer but substantially none of the first enantiomer and its location calculated, at least part of said region in the fluid flow which includes the second enantiomer but substantially none of the first enantiomer is directed to a 15 collecting means, leaving a remaining portion in the fluid flow.

4. A method according to any previous claim, wherein the sensor means measures the optical activity of the fluid flow.

20 5. A method according to claim 4, wherein the sensor means includes an optical absorption meter.

6. A method according to either of claims 3 or 4, wherein the sensor means includes a circular dichroism meter.

7. A method and apparatus substantially as herein described and illustrated. 8. Any novel and inventive feature or combination of features specifically 5 disclosed herein within the meaning of Article 4H of the International Convention (Paris Convention).