Medium-pressure column chromatography

On the other hand, telluronium imides 13 were isolated for the first time in 2002 by optical resolution of their racemic samples on an optically active column by medium-pressure column chromatography.27 The relationship between the absolute configurations and the chiroptical properties was clarified on the basis of their specific rotations and circular dichroism spectra. The racemization mechanism of the optically active telluronium imides, which involved the formation of corresponding telluroxides by hydrolysis of the telluronium imides, was proposed (Scheme 6). 

A mixture of 1 mmol of optically active sulfonium salt, 1 mmol of cyclic /J-oxo ester and 1.3 mmol of anhyd potassium carbonate is stirred in 10 mL of CH2C12 at r.t. for 2 d. After filtration of insoluble materials, the alkylated products are purified by preparative TLC (Merck, silica gel 60F254) and medium-pressure column chromatography. The ee is determined by H NMR analysis [Eu(hlc)3] or by HPLC (Oaicel Chiralcel CA-1. methanol). 

The sulfone (9) (mixture of two diastereoisomers, 318 mg, lmmol) was dissolved in di-n-butyl phthalate (1.62 ml) and degassed with argon. The solution was heated at 210°C under argon for 8h (TLC monitoring). The reaction mixture was purified by medium-pressure column chromatography (silica, 3% ether in benzene), followed by recrystallization (hexane), to yield the product as a white crystalline solid (215 mg, 85%), m.p. 109-110°C. 

The helical compounds 198a and 199 were separated into the enantiomers by inclusion chromatography (medium-pressure column chromatography on cellulose triacetate). 200 was enantiomerically enriched (—)-200 was only weakly enrich-... 

The newer LC techniques of flash column chromatography and short path or medium pressure column chromatography though offering resolution approaching that of thin layer are still comparatively slow and expensive in terms of packing and solvents. The principal advantage of these techniques is in their preparative capability. 

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