Allylic alcohols Stereochemically controlled epoxidations

The oxygen that is transferred to the allylic alcohol to form epoxide is derived from tert-butyl hydroperoxide. The enantioselectivity of the reaction results from a titanium complex among the reagents that includes the enantiomerically pure tartrate ester as one of the ligands. The choice whether to use (+) or (-) tartrate ester for stereochemical control depends on which enantiomer of epoxide is desired. 

The Simmons-Smith cyclopropanation reaction Stereochemically controlled epoxidations Regio- and Stereocontrolled Reactions with Nucleophiles Claisen-Cope rearrangements Stereochemistry in the Claisen-Cope rearrangement The Claisen-Ireland rearrangement Pd-catalysed reactions of allylic alcohols Pd-allyl acetate complexes Stereochemistry of Pd-allyl cation complexes Pd and monoepoxides of dienes The control of remote chirality Recent developments Summary... 

Another approach in the study of the mechanism and synthetic applications of bromination of alkenes and alkynes involves the use of crystalline bromine-amine complexes such as pyridine hydrobromide perbromide (PyHBts), pyridine dibromide (PyBn), and tetrabutylammonium tribromide (BiMNBn) which show stereochemical differences and improved selectivities for addition to alkenes and alkynes compared to Bn itself.81 The improved selectivity of bromination by PyHBn forms the basis for a synthetically useful procedure for selective monoprotection of the higher alkylated double bond in dienes by bromination (Scheme 42).80 The less-alkylated double bonds in dienes can be selectively monoprotected by tetrabromination followed by monodeprotection at the higher alkylated double bond by controlled-potential electrolysis (the reduction potential of vicinal dibromides is shifted to more anodic values with increasing alkylation Scheme 42).80 The question of which diastereotopic face in chiral allylic alcohols reacts with bromine has been probed by Midland and Halterman as part of a stereoselective synthesis of bromo epoxides (Scheme 43).82... 

A major limitation to the Sharpless-Katsuki epoxidation is that its utility is largely confined to oxidation of allylic alcohols. Homoallylic alcohols are oxidized less cleanly and the oxidation of simple olefins shows little enantio-selectivity. This is presumably because the stereochemical control depends on anchoring the substrate to a particular site on the metal by means of an auxiliary coordinating function. 

The stereochemical outcome of Q [WZnM2(ZnW9034)2] [M = Zn(II), Mn(II), Ru(III), Fe(III)]-cafalyzed H2O2 epoxidation of various allylic alcohols with the OH group attached to a chiral center is controlled by allylic strain effects [23,24]. Thus, allylic alcohols with only 1,2-allylic strain were foimd to afford erythro-epoxides with excellent diastereoselectivity (Fig. 16.4, alkene A). In contrast, threo-epoxides predominate strongly in case of 1,3-allylic strain (alkene B). Diastereoselectivity drops to very low values in the absence of allylic strain (alkene C) or when both 1,2- and 1,3-allylic strain are present in the substrate (alkene D). 

The oxygen that is transferred to the allylic alcohol to form the epoxide is derived from tert-butyl hydroperoxide. The enantioselectivity ofthe reaction results from a titanium complex among the reagents that includes the enan-tiomerically pure tartrate ester as one of the ligands. The choice of whether to use the (-t)- or (-)-tartrate ester for stereochemical control depends on which enantiomer of the epoxide is desired. [The (-t)- and (-)-tartrates are either diethyl or diisopropyl esters.] The stereochemical preferences ofthe reaction have been well studied, such that it is possible to prepare either enantiomer of a chiral epoxide in high enantiomeric excess, simply by choosing the appropriate (-1-)- or (—)-tartrate stereoisomer as the chiral ligand ... 

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