Single-isomer sulfated alpha-cyclodextrins for capillary electrophoresis: hexakis(2,3-di-O-methyl-6-O-sulfo)-alpha-cyclodextrin, synthesis, analytical characterization, and initial screening tests

The third, concluding member of the family of single-isomer, fully sulfated alpha-cyclodextrins, the sodium salt of hexakis(2,3-di-O-methyl-6-O-sulfo)-alpha-cyclodextrin (HxDMS), has been synthesized on the kilogram scale, completing the nine-member array of the single-isomer, 6-O-sulfo CDs now available. HxDMS was tested for the capillary electrophoretic (CE) resolution of the enantiomers of nonelectrolyte, weak acid and weak base analytes contained in our CD screening kit. HxDMS complexed differently with many of the analytes tested than either its larger-ring analogs, heptakis(2,3-di-O-methyl-6-O-sulfo)-beta-CD (HDMS) and octakis(2,3-di-O-methyl-6-O-sulfo)-gamma-CD (ODMS) or its same-ring, but differently substituted analogs, hexakis(6-O-sulfo)-alpha-CD (HxS) and hexakis(2,3-di-O-acetyl-6-O-sulfo)-alpha-CD (HxDAS). For all analytes, the effective mobilities and separation selectivities as a function of the background electrolyte concentration of HxDMS followed the trends that were found for the other single-isomer, 6-O-sulfo CDs.     

Use of the new, single-isomer, hexakis(2,3-diacetyl-6-O-sulfo)-alpha-cyclodextrin in acidic methanol background electrolytes for nonaqueous capillary electrophoretic enantiomer separations

The new, single-isomer, sulfated alpha-cyclodextrin, the sodium salt of hexakis(2,3-diacetyl-6-O-sulfo)-alpha-cyclodextrin (HxDAS), was used for the first time in acidic methanol background electrolytes (BGEs) to separate the enantiomers of weak base analytes by nonaqueous capillary electrophoresis (NACE). The concentration dependence of the effective mobilities and separation selectivities followed trends similar to those observed earlier in acidic methanol background electrolytes with heptakis(2,3-diacetyl-6-O-sulfo)-beta-cyclodextrin (HDAS) and octakis(2,3-diacetyl-6-O-sulfo)-gamma-cyclodextrin (ODAS). In general, interactions between the weak base analytes and HxDAS were weaker than with HDAS and ODAS. For some of the weak base analytes, separation selectivities observed in acidic aqueous and acidic methanol background electrolytes were complementary to each other, permitting the eventual separation of enantiomers that could not be achieved otherwise.     

Synthesis,analytical characterization and initial capillary electrophoretic use in acidic background electrolytes of a new,single-isomer chiral resolving agent: hexakis(2,3-di-O-acetyl-6-O-sulfo)-alpha-cyclodextrin

The sodium salt of hexakis(2,3-di-O-acetyl-6-O-sulfo)-alpha-CD (HxDAS), the first member of the family of single-isomer, fully sulfated alpha-CDs, has been synthesized and used for the initial capillary electrophoretic separation of the enantiomers of nonionic, weak acid, weak base, and ampholytic analytes. HxDAS complexes less strongly with many of the analytes tested than the analogous beta- and gamma-cyclodextrin derivatives, namely, heptakis(2,3-di-O-acetyl-6-O-sulfo)-beta-cyclodextrin (HDAS) and octakis(2,3-di-O-acetyl-6-O-sulfo)-gamma-cyclodextrin (ODAS). Nevertheless, it facilitated the separation of the enantiomers of a large number of weak electrolyte and nonelectrolyte analytes in acidic aqueous background electrolytes. For all analytes, the effective mobilities and separation selectivities as a function of the background electrolyte concentration of HxDAS followed the trends that were found for HDAS and ODAS.     

A family of single-isomer,sulfated gamma-cyclodextrin chiral resolving agents for capillary electrophoresis: octa(6-O-sulfo)-gamma-cyclodextrin

The second member of the single-isomer, sulfated gamma-cyclodextrin family, the sodium salt of octa(6-O-sulfo)-gamma-cyclodextrin (OS) has been synthesized, characterized and used to separate the enantiomers of nonelectrolyte, acidic, basic, and ampholytic analytes by capillary electrophoresis in acidic aqueous background electrolytes. The anionic effective mobilities of the nonelectrolyte and anionic analytes increased with increasing concentration of OS. The effective mobilities of strongly complexing cationic analytes became anionic with very low OS concentrations and passed local anionic effective mobility maxima as the OS concentration, and along with it, the ionic strength, of the background electrolyte increased. The effective mobilities of the weakly binding cationic analytes became only slightly anionic at high OS concentration values and did not show the local anionic effective mobility maxima. For nonelectrolyte analytes, separation selectivities decreased with increasing OS concentration. For cationic analytes, separation selectivities were highest where the effective mobilities of the less mobile enantiomers approached zero. OS proved to be a broadly applicable chiral resolving agent.     

Synthesis,analytical characterization and use of octakis(2,3-di-O-methyl-6-O-sulfo)-gamma-cyclodextrin,a novel,single-isomer,chiral resolving agent in low-pH background electrolytes

The third member of the family of single-isomer, sulfated gamma-cyclodextrins, the sodium salt of octakis(2,3-di-O-methyl-6-O-sulfo)-gamma-cyclodextrin has been synthesized, analytically characterized and used for the capillary electrophoretic separation of the enantiomers of nonionic, weak acid and weak base analytes in low-pH aqueous background electrolytes. Though octakis(2,3-di-O-methyl-6-O-sulfo)-gamma-cyclodextrin complexes less strongly with many of the analytes tested than the other members of the single-isomer, 6-O-sulfo gamma-cyclodextrin family, such as octa(6-O-sulfo)-gamma-cyclodextrin and octakis(2,3-di-O-acetyl-6-O-sulfo)-gamma-cyclodextrin, it offers excellent separation selectivities, often complementary to those of both the single-isomer, 6-O-sulfo beta-cyclodextrins and 6-O-sulfo gamma-cyclodextrins. Rapid, efficient enantiomer separations were observed for a large number of structurally diverse analytes in acidic aqueous background electrolytes.     

Capillary electrophoretic separation of enantiomers in a high-pH background electrolyte by means of the single-isomer chiral resolving agent octa(6-O-sulfo)-gamma-cyclodextrin

The new, alkali-stable, single-isomer, sulfated gamma-cyclodextrin, the sodium salt of octa(6-O-sulfo)-gamma-cyclodextrin (OS) was used for the first time to separate the enantiomers of non-ionic, acidic, basic and ampholytic analytes by capillary electrophoresis in high-pH aqueous background electrolytes. The effective mobilities and separation selectivities were found to follow trends similar to those observed earlier in acidic aqueous background electrolytes. OS proved to be a broadly applicable chiral resolving agent and afforded adequate peak resolution values with short separation times for a number of non-ionic, weak acid, weak base and ampholytic analytes.     

Synthesis of a single-isomer sulfated beta-cyclodextrin carrying nonidentical substituents at all of the C2, C3, and C6 positions and its use for the electrophoretic separation of enantiomers in acidic aqueous and methanolic background electrolytes. Part 2: Heptakis(2-O-methyl-6-O-sulfo) cyclomaltoheptaose

The sodium salt of heptakis(2-O-methyl-6-O-sulfo)cyclomaltoheptaose (HMS), the second single-isomer, sulfated beta-CD carrying nonidentical substituents at all of the C2, C3, and C6 positions, has been synthesized, analytically characterized, and used for the capillary electrophoretic separation of the enantiomers of a group of 23 weak base analytes in acidic aqueous and methanolic BGEs. HMS interacted strongly with only about half of the analytes studied. The good separation selectivities and favorable normalized EOF mobilities allowed for rapid, efficient separation of the enantiomers of 19 of the 23 weak base analytes in the aqueous BGEs, often with separation selectivity values complimentary to those obtained with other single-isomer sulfated CDs. HMS did not prove to be as good a resolving agent in acidic methanolic BGEs as its counterpart, heptakis(2-O-methyl-3-O-acetyl-6-O-sulfo)cyclomaltoheptaose.     

Synthesis of heptakis(2-O-methyl-3-O-acetyl-6-O-sulfo)-cyclomaltoheptaose, a single-isomer, sulfated beta-cyclodextrin carrying nonidentical substituents at all the C2, C3, and C6 positions and its use for the capillary electrophoretic separation of enantiomers in acidic aqueous and methanolic background electrolytes

The sodium salt of heptakis(2-O-methyl-3-O-acetyl-6-O-sulfo)cyclomaltoheptaose (HMAS), the first single-isomer, sulfated beta-cyclodextrin carrying nonidentical substituents at all of the C2, C3, and C6 positions, has been synthesized, analytically characterized, and used for the capillary electrophoretic separation of the enantiomers of a group of 24 weak base pharmaceuticals in acidic aqueous and acidic methanolic background electrolytes. HMAS interacted more strongly with most of the analytes studied than heptakis(2,3-di-O-methyl-6-O-sulfo)cyclomaltoheptaose, but less strongly than heptakis(2,3-di-O-acetyl-6-O-sulfo)cyclomaltoheptaose, the respective analogs with identical substituents at the C2 and C3 positions. The good separation selectivities and favorable normalized electroosmotic flow mobilities allowed for rapid, efficient separation of the enantiomers of 19 of the 24 weak base analytes in the aqueous and methanolic background electrolytes. The trends in the effective mobilities and separation selectivities as a function of the HMAS concentration closely followed the predictions of the ionic strength-corrected charged resolving agent migration model.     

Non-aqueous capillary electrophoretic enantiomer separations using the tetrabutylammonium salt of heptakis(2,3-O-diacetyl-6-O-sulfo)-cyclomaltoheptaose, a single-isomer sulfated beta-cyclodextrin highly-soluble in organic solvents

The tetrabutylammonium salt of heptakis(2,3-di-O-acetyl-6-O-sulfo)-cyclomaltoheptaose, a single-isomer sulfated beta-cyclodextrin that is adequately soluble in a number of protic and aprotic polar solvents was synthesized on the large scale and used for the capillary electrophoretic separation of the enantiomers of weak bases in acidic acetonitrile background electrolytes. The effective mobilities and separation selectivities observed for these analytes followed trends similar to those found with other single-isomer sulfated cyclodextrins in acidic methanol background electrolytes. Enantiomer separations obtained with the tetrabutylammonium and sodium salts of heptakis(2,3-di-O-acetyl-6-O-sulfo)-cyclomaltoheptaose were different indicating, for the first time, that selection of the counter ion of the single-isomer sulfated cyclodextrin is also of importance for the separation of enantiomers.     

Nonaqueous capillary electrophoretic separation of basic enantiomers using octakis(2,3-O-dimethyl-6-O-sulfo)-gamma-cyclodextrin, a new, single-isomer chiral resolving agent

The enantiomers of 34 pharmaceutical weak-base analytes were separated by nonaqueous capillary electrophoresis in acidic methanol background electrolytes using the sodium salt of the new, single-isomer chiral resolving agent, octakis(2,3-O-dimethyl-6-O-sulfo)-gamma-cyclodextrin (ODMS). The effective mobilities, separation selectivities and peak resolution values of the weak-base analytes were determined as a function of the ODMS concentration in the 0-40 mM range and were found to follow the theoretical predictions of the charged resolving agent migration model (CHARM model) modified for ionic strength effects. Fast, efficient separations were achieved for both comparatively small and large enantiomers.     

Synthesis, analytical characterization and initial capillary electrophoretic use in an acidic background electrolyte of a new, single-isomer chiral resolving agent: heptakis(2-O-sulfo-3-O-methyl-6-O-acetyl)-β-cyclodextrin

The sodium salt of heptakis(2-O-sulfo-3-O-methyl-6-O-acetyl)cyclomaltoheptaose (HAMS), the first single-isomer sulfated β-CD that carries the sulfo group exclusively at the C2 position, has been synthesized. The purity of each synthetic intermediate and of the final product was determined by hydrophilic interaction (HILIC) and reversed-phase HPLC. The structural identity of each intermediate and of the final product was verified by 1-D and 2-D NMR spectroscopy and MALDI-TOF MS. HAMS was used for the capillary electrophoretic separation of the enantiomers of a set of non-ionic and weak base analytes in pH 2.5 background electrolytes. Rapid separations with satisfactory peak resolution values were obtained for most enantiomers using low concentrations of HAMS. The effective mobilities and separation selectivities were dependent on the concentration of HAMS according to the predictions of the charged resolving agent migration model. The separation selectivities observed with HAMS, heptakis(2-O-methyl-3-O-acetyl-6-O-sulfo)cyclomaltoheptaose and heptakis(2-O-methyl-3,6-di-O-sulfo)cyclomaltoheptaose were different for some of the analytes studied in detail.     

A new mixed acetal-type substitution pattern for alpha-cyclodextrin. Preparation of hexakis (3-O-benzyl)- alpha-cyclodextrin

alpha-CD was converted into hexakis[2,6-di-O-(methoxydimethyl)methyl]-alpha-CD by a proton-catalyzed reaction with 2-methoxypropene. Subsequent benzylation under Brimacombe conditions resulted in the fully protected compound, from which the acid-sensitive acetal groups were removed to obtain hexakis(3-O-benzyl)-alpha-cyclodextrin. The structure of all of the compounds synthesized was confirmed by 13C J-ECHO, COSY, HETCOR and HMBC NMR measurements.     

Synthesis of mono-6-deoxy-6-N,N,N',N',N'-pentamethylethylenediammonio-cyclomaltoheptaose, a single-isomer, monosubstituted, permanently dicationic beta-CD and its use for enantiomer separations by CE

The dichloride salt of mono-6-deoxy-6-N,N,N',N',N'-pentamethylethylenediammonio-cyclomaltoheptaose (PEMEDA-BCD), the first single-isomer, monosubstituted, permanently dicationic beta-CD has been synthesized, analytically characterized, and used for the capillary electrophoretic separation of the enantiomers of a group of analytes in acidic and basic BGEs. When the concentration of PEMEDA-BCD was changed in the BGEs, the resulting effective mobilities of the analytes and the respective separation selectivities followed the predictions of the ionic strength-corrected charged resolving agent migration model. Good separation selectivities and favorable normalized EOF mobilities allowed for the rapid, efficient separation of the enantiomers of anionic, weak acid and nonionic analytes in the low- and/or high-pH BGEs.     

Selectivity comparisons of monolithic silica capillary columns modified with poly(octadecyl methacrylate) and octadecyl moieties for halogenated compounds in reversed-phase liquid chromatography

Stationary phase selectivities for halogenated compounds in reversed-phase HPLC were compared using C18 monolithic silica capillary columns modified with poly(octadecyl methacrylate) (ODM) and octadecyl moieties (ODS). The preferential retention of halogenated benzenes on ODM was observed in methanol/water and acetonitrile/water mobile phases. In selectivity comparison of selected analytes on ODM and ODS, greater selectivities for halogenated compounds were obtained with respect to alkylbenzenes on an ODM column, while similar selectivities were observed with a homologous series of alkylbenzenes on ODM and ODS columns. These data can be explained by greater dispersive interactions by more densely packed octadecyl groups on the ODM polymer coated column together with the contribution of carbonyl groups in ODM side chains. For the positional isomeric separation of dihalogenated benzenes (ortho-, meta-, para-), the ODM column also provided better separation of these isomers for the adjacently eluted isomers that cannot be completely separated on an ODS column in the same mobile phase. These results imply that the ODM column can be used as a better alternative to the ODS column for the separation of other halogenated compounds.     

Chiral recognition of helical metal complexes by modified cyclodextrins.

Chirality of metal complexes M(phen)3(n+) (M = Ru(II), Rh(III), Fe(II), Co(II), and Zn(II), and phen = 1,10-phenanthroline) is recognized by heptakis(6-carboxymethylthio-6-deoxy)-beta-cyclodextrin heptaanion (per-CO2(-)-beta-CD) and hexakis(2,3,6-tri-O-methyl)-alpha-cyclodextrin (TMe-alpha-CD) in D2O. The binding constant (K) for the Delta-Ru(phen)3(2+) complex of per-CO2(-)-beta-CD (K = 1250 M(-1)) in 0.067 M phosphate buffer at pD 7.0 is approximately 2 times larger than that for the Lambda-isomer (590 M(-1)). Definite effects of inorganic salts on stability of the complexes indicate a large contribution of Coulomb interactions to complexation. The fact that hydrophilic Ru(bpy)3(2+) (bpy = 2,2'-bipyridine) does not form a complex with per-CO2(-)-beta-CD suggests the importance of inclusion of the guest molecule into the host cavity for forming a stable ion-association complex. The positive entropy change for complexation of Ru(phen)3(2+) with per-CO2(-)-beta-CD shows that dehydration from both the host and the guest occurs upon complexation. Similar results were obtained with trivalent Rh(phen)3(3+) cation. Pfeiffer effects were observed in complexation of racemic Fe(phen)3(2+), Co(phen)3(2+), and Zn(phen)3(2+) with per-CO2(-)-beta-CD with enriched Delta-isomers. Native cyclodextrins such as alpha-, beta-, and gamma-cyclodextrins as well as heptakis(2,3,6-tri-O-methyl)-beta-cyclodextrin do not interact with Ru(bpy)3(2+). However, hexakis(2,3,6-tri-O-methyl)-alpha-cyclodextrin (TMe-alpha-CD) interacts with Ru(phen)3(2+) and Ru(bpy)3(2+) and discriminates between the enantiomers of these metal complexes. The K values for the Delta- and Lambda-Ru(phen)3(2+) ions are 54 and 108 M(-1), respectively. Complexation of the Delta- and Lambda-isomers of Ru(phen)3(2+) with TMe-alpha-CD is accompanied by negative entropy changes, suggesting that cationic Ru(phen)3(2+) is shallowly included into the cavity of the neutral host through van der Waals interactions. The Delta-enantiomer, having a right-handed helix configuration, fits the primary OH group side of per-CO2(-)-beta-CD (SCH2CO2(-) side) well, while the Lambda-enantiomer, having a left-handed helix configuration, is preferably bound to the secondary OH group side of TMe-alpha-CD. The asymmetrically twisted shape of a host cavity seems to be the origin of chiral recognition by cyclodextrin.     

Inclusion complexes of trivalent lutetium cations with an acidic derivative of per(3,6-anhydro)-alpha-cyclodextrin

The cyclodextrin derivative (hexakis (2-O-carboxymethyl-3,6-anhydro)-alpha-cyclodextrin) forms mono- and bimetallic complexes with lutetium(III) in aqueous solution; the X-ray structure of the binuclear complex [Lu2(ACX)(H2O)2] is the first example of a lanthanide-cyclodextrin inclusion complex.     

Nonaqueous capillary electrophoretic separation of basic enantiomers using heptakis(2,3-dimethyl-6-sulfato)-beta-cyclodextrin.

The enantiomers of 40 basic analytes, mostly pharmaceuticals, were separated by nonaqueous capillary electrophoresis in acidic methanol background electrolytes using the sodium salt of heptakis(2,3-dimethyl-6-sulfato)-beta-cyclodextrin (HDMS-beta-CD). The effective mobilities, separation selectivities, and peak resolution values were determined as a function of the HDMS-beta-CD concentration in the 0-40 mM range and were found to follow the theoretical predictions of the charged resolving agent migration model (CHARM model). Fast, efficient enantiomer separations were achieved for a large number of both very hydrophobic and hydrophilic weak bases.     

Enantiomeric separation of basic compounds using heptakis(2,3-di-O-methyl-6-O-sulfo)-beta-cyclodextrin in combination with potassium camphorsulfonate in nonaqueous capillary electrophoresis: optimization by means of an experimental design

The enantiomeric separation of a series of basic pharmaceuticals (beta-blockers, local anesthetics, sympathomimetics) has been investigated in nonaqueous capillary electrophoresis (NACE) systems using heptakis(2,3-di-O-methyl-6-O-sulfo)-beta-cyclodextrin (HDMS-beta-CD) in combination with potassium camphorsulfonate (camphorSO3-). For this purpose, a face-centered central composite design with 11 experimental points was applied. The effect of the concentrations of HDMS-beta-CD and camphorSO3- on enantioresolution was statistically evaluated and depended largely on the considered analyte. The presence of camphorSO3- was found to be particularly useful for the enantioseparation of compounds with high affinity for the anionic CD. CamphorSO3- seems to act as a competitor, reducing the affinity for the CD, probably by ion-pair formation with these analytes. For compounds with lower affinity for HDMS-beta-CD, the combination of camphorSO3- and the CD appeared to have a favorable effect on enantioresolution only if the optimal CD concentration could be reached. On the other hand, for compounds characterized by a very low affinity for the anionic CD, the association of camphorSO3- and HDMS-beta-CD is always unfavorable. Finally, experimental conditions were selected by means of the multivariate approach in order to obtain the highest resolution (Rs) value for each studied compound.     

Separation selectivity patterns of fully charged achiral compounds in capillary electrophoresis with a neutral cyclodextrin

Based on the separation selectivity equation, related to the dimensionless parameters for fully charged achiral analytes using a neutral CD, the separation selectivity can be classified into seven patterns. With respect to CZE without CD, the presence of CD in the buffer may improve, or reduce, the separation selectivity with this effect being accompanied by the same or reversed electrophoretic mobility order for charged analytes. This can depend on the separation selectivity of the two analytes in free solution, the binding selectivity, the separation selectivity of analyte–CD complexes and the ratio of electrophoretic mobility of the analytes in free, and complexed forms. Using positional isomers of benzoic acids and phenoxy acids as test analytes and α‐CD as a selector, the observed separation selectivity shapes were found to be in excellent agreement with the predicted separation selectivities.     

Cyclodextrins containing an acetone bridge. Synthesis and study as epoxidation catalysts

Three cyclodextrine derivatives (6A,6D-di-O-(prop-2-one-1,3-dienyl)-alpha-cyclodextrin (1), 6-O-(prop-2-one-1-yl)-alpha-cyclodextrin (2) and 6A,6D-di-O-(prop-2-one-1,3-dienyl)-beta-cyclodextrin (3)) were synthesised and investigated as epoxidation catalysts. The three compounds were synthesised from the corresponding perbenzylated cyclodextrins which were mono- or didebenzylated in the 6-position using Sina?'s method. Reaction with NaH and methallyl chloride in the case of 2, or methallyl dichloride in the case of 1 and 3, followed by dihydroxylation, periodate cleavage and protection group removal gave the target compounds. All three compounds catalysed, in the presence of oxone, the epoxidation of a series of alkenes. Epoxidation was compared to the reaction catalysed by simple ketones and inhibition was studied.