No abstract

Both enantiomers of the novel amino alcohol (R)-and (S)-2 are prepared from the corresponding enantiomer of the mandelic acid-derived ethanediol 3. The regioisomeric amino alcohols 1 and 2 are converted into the imines 7 and 8, respectively. Titanium complexes 9 and 10 derived therefrom are used as catalysts for the addition of diethylzinc to benzaldehyde and yield the alcohol 11 in up to 92% ee. On the other hand, the chloro-substituted titanium complexes 14 and 15 are able to mediate the Torgov cyclization reaction of the diketone 16 to give the estrone derivative 17. In both reactions titanium complexes 10 and 15 derived of the novel amino alcohol 2 give higher enantioselectivities than the complexes 9 and 14 that are based on the regioisomeric amino alcohol 1.

Enantiomerically and diastereomerically pure bis-chelated imine-alkoxytitanium complexes 6 and 7 have been synthesized and used as chiral dopants for converting nematic into cholesteric phases. The dopants were tested in mainly commercially available nematic liquid crystalline compounds or mixtures: LC1 (BASF), ZLI-1695 and ZLI-1840 (Merck), as well as N-(4-methoxybenzylidene)-4'-butylaniline (MBBA). The values of the helical twisting power (HTP) were determined by the Grandjean-Cano method. Exceptionally high helical twisting powers were obtained. Thus, the titanium complex 6 h displayed a HTP value of 740 microm(-1) in MBBA, the highest HTP value reported. The helical twisting power has been found to depend strongly on the structure of the nematic phase and the substitution pattern of the chiral ligand in the titanium complexes 6 and 7. Crystal structure analysis of 6 f confirmed the A,R,R configuration of the metal complex. The chiral imine ligands 4 and 5 were derived from the regioisomeric amino alcohols 1 and 2.

The novel chiral amino alcohols (R)-and (S)-1 are prepared from the corresponding enantiomer of ethanediol 2. Alkoxytitanium complexes of the imines 8 derived from 1 are suitable to catalyze the addition of diethylzinc to benzaldehyde in up to 92% e.e.In recent developments of asymmetric synthesis, the structural unit of the diarylhydroxymethyl group was frequently applied, not only in covalently bound chiral auxiliary reagents, but also in ligands and catalysts. 1 It led to an enhancement of stereoselectivity in many cases, although it is not a stereogenic group. After the application of the diarylhydroxymethyl motif in TADDOLs 2 and triphenylglycol-derived carboxylic esters, 3 it was also used in β-amino alcohols, most of which are based on valine and proline. 4 In this communication, we present a new chiral auxiliary reagent, (R)-and (S)-2-amino-1,2,2-triphenylethanol 1, which contains the diphenylaminomethyl moiety, hitherto unexplored in asymmetric syntheses. The preparation and the first application of the novel reagent 1 in enantioselective additions of diethylzinc are reported. Scheme 1(R)-and (S)-triphenylglycol 2, readily available from the corresponding enantiomers of mandelic acid, 3,5 were chosen as starting materials for the amino alcohols (R)-and (S)-1, respectively. As described recently, 6 the cyclic sulfite 3 was generated from the diol (R)-2 by treatment with thionyl chloride in the presence of triethylamine. Although the pure R c ,S s stereoisomer can be readily isolated, the diastereomeric mixture of 3 was used in the following step: in an unprecedented variant of the Ritter reaction, the sulfite 3 delivered the oxazoline 4 upon treatment with triflic acid in acetonitrile. 7,8 Finally, the amino alcohol (R)-1 was liberated from the oxazoline (R)-4 by methanolysis in the presence of sulfuric acid, whereas (S)-1 was prepared from (S)-oxazoline 4 by the same protocol (Scheme 2). 9

by an unexpected SN1-type ring cleavage. Enantiopure triphenyloxirane (R)-10 arises under inversion of configuration upon treatment of 2a/b with sodium azide. The configuration of the major diastereomer 2a is determined by NMR spectroscopy and by conversion into the sulfoxide (S)-15.The reaction of triphenylglycol (S)-1 with thionyl chloride leads to the cyclic sulfites 2a and 2b in a ratio of 9 O : l O . When the mixture of diastereomers 2a/b is treated with alcohols 3 or diols 6 , enantiomerically pure ethers 5 and ? are obtainedThe diarylhydroxymethyl motif plays an imporant role in recent asymmetric syntheses. Though it is not a permanent stereogenic unit it often leads to an enhancement in selectivity"]. Chiral cyclic sulfites containing the geminal diphenyl pattern have been used for the preparation of enantiomerically pure sulfoxides by means of stereoselective nucleophilic substitutions at the sulfur a t~m [~,~] .In a program aimed at obtaining novel chiral ligands for enantioselective catalyses, we are trying to synthesize various derivatives of the versatile chiral auxiliary reagent 1,1,2-triphenyIethane-1,2-diol (l) [435].In this paper, we report on derivatizations of ( 8 -1 taking advantage of an unexpected ring opening of the cyclic sulfite 2L61. Formation of Sulfite 2 and Ring Opening with Alcohols 3 and Diols 6The reaction of (8-diol 1 and thionyl chloride in the presence of two equivalents of triethylamine led to a mixture of diastereomeric sulfites 2a and 2b, which were formed in a ratio of 90:lO. The main product 2a was isolated as a pure stereoisomer in 47% yield. Since in the following ring cleavage the configuration at the sulfur atom is irrelevant, the reactions were performed conveniently with the diastereomeric mixture 2a/b, which was obtained in 79% yield.When the heterocycle 2 is submitted to a reaction with nucleophiles, one may anticipate an attack at the sulfur atom (path a) or at the lower substituted carbon atom (path b)[7-91. Neither one, however, was observed when the mixture of 2 a h was dissolved in the alcohols 3a-e and heated with triethylamine: In a completely regioselective manner, the tertiary carbon-oxygen bond was cleaved so that the ethers 5a-e were obtained in 17-71941 isolated yield. The unexpected substitution at the tertiary carbon atom (path c) is explained in the following way: Under the protic, typical SN1 conditions, cleavage occurs of that carbon-oxygen bond that leads to the diphenyl-substituted carbenium ion, so that the zwitterionic intermediate 4 forms regioselectively. The positively charged carbon atom is trapped by the corresponding alcohol and the subsequent spontaneous cleavage of the sulfite moiety liberates the ethers 5a-e. Presumably due to steric hindrance 2-propanol3e afforded the ether 5e in low yield, whereas tert-butyl alcohol failed to give the corresponding substitution product. Glycols 6a-c reacted with the cyclic sulfite 2alb in the presence of triethylamine and tetrahydrofuran as cosolvent according to the reaction path c outlined above. T...