Enantioseparation of chiral thiobarbiturates using cyclodextrin-modified capillary electrophoresis

The racemates of several chiral thiobarbiturates were separated by using different cyclodextrins in capillary electrophoresis (CE). Six neutral and negatively charged cyclodextrins 1 (CDs) were employed as chiral separators whereof five led to successful separation of enantiomeric thiobarbiturate pairs. The CDs used were the native alpha-CD, beta-CD, gamma-CD, and heptakis-(2,6-di-O-methyl)-beta-cyclodextrin (HDM) as well as heptakis-(2,3-di-O-methyl-6-sulfato)-beta-cyclodextrin (HDMS) and heptakis-(2,3-di-O-acetyl-6-sulfato)-beta-CD (HDAS). Five of the six chiral thiobarbiturates studied could be resolved at a basic pH value of 9.4 and a phosphate buffer concentration of 100 mM in a fused-silica capillary. Structurally related substances showed a similar behavior in separation: 1 and 2 bearing the center of chirality in the side chain at C5 can be best separated using gamma-CD, the N-alkyl-substituted compounds 3 and 4 as well as the N/S-dialkyl-substituted compound 5 could be resolved with HDM. Using the neutral CDs, the migration times were relatively small (< 11 min). 3 and 4 could be also resolved by means of the negatively charged HDMS. In the latter case, the migration time is twice as long as with HDM.