Isolation, Stereochemistry, and Configurational Lability of Optically Active Telluroxides

Abstract

Ever since an optically active enantiomeric selenoxide was obtained for the first time by Davis and co-workers in 1983, 1,2 there have been several reports on the isolation of optically active selenoxides 3 by various approaches, such as optical resolution of diastereomeric mixtures, 4 chromatographic resolution of enantiomeric mixtures using an optically active column, 5 complexation with a chiral ligand, 6 and asymmetric oxidation of selenides. 7,8 Optically active selenoxides have also been extensively studied as key intermediates in asymmetric synthesis. 3,8a-d,9,10 Optically active telluroxides have also been recently reported as transient key intermediates in asymmetric reactions by Uemura and co-workers. 11 However, optically active telluroxides have not yet been isolated, perhaps because telluroxides undergo racemization Via a hydrate, 12 formed by the addition of water, much faster than selenoxides. 13 We report here the first isolation of optically active telluroxides by means of optical resolution using an optically active column. Their stereochemistry and configurational labilities are also examined.Mesityl 2,4,6-triisopropylphenyl telluroxide (3) was subjected to several optically active columns 14 using high-performance liquid chromatography at an analytical scale. Two peaks corresponding to each enantiomer of 3 were observed separately when an optically active column such as AS, AD, OB, or OJ was used, and the best separation was obtained on a column packed with amylose carbamate derivative/silica gel (AS). Using the AS column, 2,4,6-tri-tert-butylphenyl mesityl telluroxide (1) and 2,4,6-tri-tert-butylphenyl phenyl telluroxide (2) were also resolved into two peaks corresponding to the enantiomers, as shown in Figure 1, while telluroxide 4, which has less bulky substituents, showed only one peak. Racemic telluroxide 2 was resolved into its enantioisomers better than 1. Deterioration of asymmetric recognition for 1 is probably due to steric hindrance around the telluroxide moiety or to the similarity of the bulkiness of the two aryl groups on the tellurium atom of 1. The chromatogram of telluroxide 3 showed an unusual shape which indicated that racemization was occurring in the column. These results show that a substituent more bulky than a 2,4,6-triisopropylphenyl group is needed to inhibit racemization.We attempted to resolve the racemic telluroxides 1 and 2 into their optical isomers at a preparative scale using mediumpressure liquid chromatography with the same type of column. 15 In the optical resolution of telluroxide 2, the first eluted enantiomer had a positive optical rotation {[R] D 123.0 (c 0.16, MeCN)} while the second enantiomer had a negative optical rotation {[R] D -68.8 (c 0.23, MeCN)}. However, the optical purities corresponding to the specific rotations could not be determined because racemization occurred readily. 16 Optically pure telluroxide (+)-1 17 was finally obtained (20 mg) from the (1) Davis, F. A.; Billmers, J. M.; Stringer, O. D. Tetrahedron Lett. 1983, 24, 3191. (2...

Polysaccharide Derivatives for Chromatographic Separation of Enantiomers

“…of the racemates shown in Figure 9, [65][66][67] including the chiral telluroxide 90, [66] are better resolved on the CSP than other phenylcarbamates of polysaccharides including CDMPC and ADMPC. …”

Optically active telluronium imides: Optical resolution, absolute configuration, and mechanism of racemization

“…These results indicate that telluronium imides and telluroxides exist in equilibrium through hydrolysis of the imides and imination of the oxides. We previously clarified that optically active telluroxides readily racemize in the presence of a trace amount of water remaining in the solvent despite careful purification [13,16]. Therefore, the mechanism involving the corresponding telluroxides, which are formed in situ by hydrolysis of the telluronium imides, is the most plausible for the racemization of telluronium imides.…”

“…Since the stereochemistry of the resulting telluroxide formed under basic conditions was R [13,16], the hydrolysis of telluronium imides proceeds with a retention of stereochemistry, at least under basic conditions. This result differs from that with sulfonium imide: sulfonium imides are known to be hydrolyzed with an inversion of stereochemistry under basic conditions [17].…”

“…Telluranes 10 and 11 exist in equilibrium. Elimination of amide from the tellurane yields telluroxide 12 with a retention of stereochemistry, which then racemizes via an achiral hydrate [13,16]. Imination of the resulting racemized telluroxide gives the racemic telluronium imide.…”

“…Phenyl 2,4,6-triisopropylphenyl telluroxide [16], mesityl 2,4,6-triisopropylphenyl telluroxide [13], 2,4,6-triethylphenyl 2 ,4 ,6 -triisopropylphenyl telluroxide, and phenyl 2,4,6-tri-tert-butylphenyl telluroxide [13] were prepared from corresponding tellurides by oxidation with tert-butyl hypochlorite in 69, 81, 62, and 86% yields, respectively [13].…”