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Asymmetric Oxidation of Thioethers .4. Optical Resolution of [1,1'-binaphthalene]-2,2'-dithiol
No full textAlmost optically pure (e.e. > 98%) [1,1′-binaphthalene]-2,2′-dithiol (2) is obtained by resolution of racemic 2via the transformation of the sulfidryl functions into the corresponding thioethers 3 which are asymmetrically oxidized to diastereomeric chiral monosulfoxides 4 and then reconverted into 2. The diastereo and enantioselectivity is dependent on the structure of the thioether; i.e. the dimethylthioether 3a gives two diastereomeric sulfoxides 4a in ca. 1:1 ratio, each of them in > 98% e.e., while cyclic dithioethers 3b–d afford a single diastereomeric sulfoxide 4b–d in 46–78% e.e.
Enantioselective Oxidation of Thioethers - A Route To the Resolution of [1,1'-binaphthalene]-2,2'-dithiol
No full textA route to the resolution of racemic [1,1'-binaphthalene]-2,2'-dithiol 2 is described, involving the transformation of 2 into a series of dithioethers. The latter are subjected to asymmetric oxidation to monosulfoxides by a procedure developed in our laboratory. For some of the thioethers examined the oxidation is both highly diastereo- and/or enantioselective. The resolved 2 is then obtained from the optically pure monosulfoxides either by oxidation to the bis-sulfoxides followed by a Pummerer reaction or by a deoxygenation to the dithiother and subsequent reduction. In both cases enantiomerically pure 2 (e.e. > 98%) was obtained
Synthesis and Diels-Alder reactions of enantiopure (-)-trans-benzo[d]-dithiine-S,S'-dioxide
No full textEnantiopure (-)-trans-benzo[d] dithiine-S,S'-dioxide 4 was obtained by enantioselective oxidation of the parent benzo[d]dithiine. The reaction of the bis-sulfoxide with a series of cyclic dienes affords the corresponding Diels-Alder cycloadducts with diastereoselectivities ranging from fair to high and good chemical yields
Enantioselective Oxidation of Thioethers1 - An Easy Route To Enantiopure C-2 Symmetrical Bis-methylsulfinylbenzenes
No full textThe direct oxidation of bis-methylthioethers 1 by t-butyl hydroperoxide, titanium tetra-iso-propilate, and (+)-diethyltartrate, affords the almost enantiomerically pure dl bis-methylsulfinylbenzenes 2 (e.e. greater-than-or-equal-to 99%) in a process which is also characterized by a very high diastereoselectivity
Enantioselective Oxidation of Thioethers .7. Synthesis of Trans-2-n,n-dialkylacetamide-1,3-dithiolanes-s-oxide and Their Use In Asymmetric Aldol-type Reactions
No full text(-)-trans-2-N,N-dialkylacetamide-1,3-dithiolanes-S-oxide 2 have been obtained in high d.e. (> 99:1) and ee. (up to 94%, > 98% after crystallization) by enantioselective oxidation [Ti(i-Pro)4, (+)-DET, t-BuOOH]. The aldol-type addition of the magnesium enolate of the N,N-diethyl derivative (-)-2b to iso-butyraldehyde afforded good chemical yields of a single diastereomer
Asymmetric Oxidation of Thioethers .5. Enantioselective Synthesis of Beta-hydroxysulfoxides By Direct Oxidation
No full textS-methyl β-hydroxysulfoxides of fairly high optical purity (up to 80%) may be prepared by direct asymmetric oxidation [ButO2H, Ti(OPri)4, (+)-DET] of acetylated or silylated S-methyl β-hydroxysulfides and subsequent deprotection. The upgrading of the optical purity from 78% up to >98% by simple crystallization has been obtaine
Enantioselective Oxidation of Beta-hydroxythioethers .1. Synthesis of Optically-active Alcohols and Epoxides
No full textOptically pure beta-dydroxysulfoxides may be obtained by direct oxidation of the parent thioethers with the modified Sharpless reagent developed in our laboratory. Necessary requisites for a succesful asymmetric oxidation are both the protection of the OH function and the largest possible difference in size of the two groups linked to the thioether sulfur. This latter condition is fulfilled by using S-methyl derivatives. Examples referring to the oxidation of various S-methyl-beta-dydroxythioethers, characterized by e.e. values up to 80%, which may be further increased (> 98%) by crystallization, are presented. Results concerning the synthesis of optically active epoxides and alcohols starting from beta-hydroxysulfoxides are also presented which, in some cases, allows the absolute configuration of the parent sulfoxide to be established
Asymmetric Oxidation of Beta-hydroxysulfides - the Role of the Hydroxy Group
No full textIn the asymmetric oxidation of prochiral β-hydroxysulfides to the corresponding sulfoxides, by using a recently developed modified Sharpless reagent, it has been observed that the presence of the hydroxy group in the molecule of the substrate is not playing a significant role in determining the extent of the enantioselection, at variance with the asymmetric epoxidation of allylic alcohols where formation of alkoxytitanium complexes is believed to be a crucial process, thus indicating some important difference in the oxidation mechanisms
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