Stereoselectivity isomerization

Electrophilic attack on the sulfur atom of thiiranes by alkyl halides does not give thiiranium salts but rather products derived from attack of the halide ion on the intermediate cyclic salt (B-81MI50602). Treatment of a s-2,3-dimethylthiirane with methyl iodide yields cis-2-butene by two possible mechanisms (Scheme 31). A stereoselective isomerization of alkenes is accomplished by conversion to a thiirane of opposite stereochemistry followed by desulfurization by methyl iodide (75TL2709). Treatment of thiiranes with alkyl chlorides and bromides gives 2-chloro- or 2-bromo-ethyl sulfides (Scheme 32). Intramolecular alkylation of the sulfur atom of a thiirane may occur if the geometry is favorable the intermediate sulfonium ions are unstable to nucleophilic attack and rearrangement may occur (Scheme 33). 

S-methyl dithiocarbonate (76S413). Stereoselective isomerization of 1,2-disubstituted alkenes may be achieved by a sequence such as the following fr<2n5-alkene bromo-hydrin -> /3-hydroxythiocyanate -> cw-thiirane -> cw-alkene (75TL2709). 

The stereoselective isomerization of allyl silyl ethers to (E)- or (Z)-silyl enol ethers was carried out in the presence of a cationic iridium(i) catalyst. The complex, prepared in situ by treating [Ir(cod)2]PFf,/2PPi3 with hydrogen was... 

Some synthetically useful isomerization reactions of alkenes, other than nitrogen- or oxygen-substituted allylic compounds, were reported by the use of a catalytic amount of transition metal complexes. The palladium complex, /ra r-Pd(C6HsCN)2Gl2, effectively catalyzed the stereoselective isomerization of /3,7-unsaturated esters to a,/3-unsaturated esters (Equation (26)). 

The preparation of polymer-supported iridium catalysts (61) and (62) for the stereoselective isomerization of double bonds using polystyrene based immobilized triphenyl phosphine were recently reported by Ley and coworkers (Fig. 4.5). The immobilized catalyst is potentially useful for deprotection strategies of aUyl ethers [130]. 

New Aldonolactones by Stereoselective Isomerization at One or More Carbon Atoms... 

Giles and coworkers have recently reported the use of TiCU in CH2CI2 at -78°C for the stereoselective isomerization of naphthyl- and phenyl-dioxolanes to yield pyrans that are important in aphid pigments. 13 Other Lewis acids besides TiCl4 such as BF3, SnCU, Ti(IsoPr)2Cl2 and Ti(IsoPr)3Cl are not effective in such reactions. NMR and nuclear Overhauser effect spectroscopies were used to establish stereochemistry. Some diol side product (13 %) was also formed. The major effort in this research was to clearly identify products of the isomerization. 

Heeb, N.V. Schweizer, W.B. Mattrel, R Haag, R. Gerecke, A.C. Schmid, R Zennegg, M. Vonmont, H., Regio- and stereoselective isomerization of hexabromocyclododecanes (HBCDs) Kinetics and mechanism of y- to a-HBCD isomerization Chemosphere 2008, 73, 1201-1210. 

Iwano et al. reported the stereoselective isomerization of geometrically constrained oxaziridines to the corresponding nitrones by photosensitized electron transfer <1995CL67>. For example, irradiation of oxaziridine 10 in 4-methanol in the presence of 9,10-dicyanoanthracene (DCA) gave (i4)-nitrone 11 in 97% yield. 

Stereoselective Isomerization Catalyzed by SnCl4-BIPOL Derivatives... 

The two main problems in the preparation of silyl enol ethers are control of regios-electivity, kinetic and thermodynamic, and stereoselectivity, (E) and (Z). Although many useful procedures are now available for the kinetic deprotonation of ketones by use of alkali metal dialkylamides, there are few practical procedures for thermodynamic deprotonation. Recently, the author and Yamamoto et al. found that the regio- and stereoselective isomerization of a kinetic silyl enol ether to a thermodynamic ether was catalyzed by LBA [138]. 

A few molecular orbital calculations have been reported for these metallacycles. An ab initio study of a postulated azoniumrhodacyclopropane produced a structure in good agreement with known azoniummetallacyclopropanes. This as yet unknown heterocycle was studied because it has been proposed as an intermediate in the stereoselective isomerization of allyl amines to enamines... 

Isomerization. The Baylis-Hillman adducts from aromatic aldehydes and acrylic esters undergo stereoselective isomerization to ( )-2-hydoxymethylcinnamic esters. Adducts derived from o-nitroaraldehydes behave differently due to intervention of an intramolecular redox reaction, leading eventually to N-oxides of 4-hydroxyquinoline-3-carboxylic esters. ... 

The observation by Ma and Lu [53] that 2-alkynylamides, when heated in the presence of /2-BU3P and transition metal complexes, undergo efficient and stereoselective isomerization to 2 , 4 -dienamides was exploited for the synthesis of several naturally occurring aliphatic... 

Stereoselective Isomerization of Vinyl Group to Ethylidene Group... 

Shanmugam et al. have demonstrated the usefulness of a montmorillonite KIO clay microwave combination as an alternative, useful, speedy and efficient catalyst for the stereoselective isomerization of various acetates of MBH adducts to provide densely functionalized ( )-alkenes 118 in high yields (Scheme 3.43). They also demonstrated the usefulness of the same catalyst system for a one-pot protection isomerization of various MBH adducts with trimethyl orthoformate and alcohols (Scheme 3.44). ... 

Cook GR, Shanker PS (1998) Palladium-catalyzed formation and stereoselective isomerization of 5-vinyloxazolines. Tetrahedron Lett 39 3405-3408. doi 10.1016/S0040-4039(98)00534-6... 

Rh-catalyzed stereoselective isomerization of tert-cyclobutanols into chiral indanoles 2.2.1 Overview... 

The influence of steric factors on reactivity and on selectivity has been discussed for the catalysis, by a variety of phosphine complexes of ruthenium, iridium, and platinum, of the isomerization of 4-vinylcyclo-hexane. Titanium tetra-( - )-menthoxide plus tri-isobutylaluminium catalyse stereoselective isomerization of racemic alk-l-enes. ... 

Hayashi R, Hsung, RP, Feltenherger JB, Lohse AG. Regio- and stereoselective isomerizations of allenamides ... 

The stereoselective isomerization of disubstituted olefins via thiirans has been reported. 0-Alkyl 5 -jS-keto-alkyl dithiocarbonates (5) eliminate sulphur on... 

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