Boron enol ethers enolate geometry

Introduction and stereochemical control syn,anti and E,Z Relationship between enolate geometry and aldol stereochemistry The Zimmerman-Traxler transition state Anti-selective aldols of lithium enolates of hindered aryl esters Syn-selective aldols of boron enolates of PhS-esters Stereochemistry of aldols from enols and enolates of ketones Silyl enol ethers and the open transition state Syn selective aldols with zirconium enolates The synthesis of enones E,Z selectivity in enone formation from aldols Recent developments in stereoselective aldol reactions Stereoselectivity outside the Aldol Relationship A Synthesis ofJuvabione A Note on Stereochemical Nomenclature... 

Boron enolates (other names are vinyloxyboranes, enol borinates, or boron enol ethers) are often employed in the aldol reaction because they show higher stereoselectivity than alkali and magnesium enolates. Extensive developmental work in this area has been carried out by Evans, Masamune and Mukaiyama, and their respective coworkers. - - The correspondence between enolate geometry and aldol stereochemistry is exceptional (Z)-enolates give syn/erythro aldol products, whereas ( )-enolates give anti/threo aldol products, albeit with slightly lower selectivity. 

Only the E,3E stereoisomer of the 2-azadiene was found to be reactive under the reaction conditions. Consequently, each of the piperidine products was found to possess the cis 2-phenyl, 5-methyl relative stereochemistry. Moreover, the cycloaddition products derived from reactions of enol ethers possessed the all-cis 2-phenyl-4-alkoxy-5-methyl stereochemistry necessarily derived from exclusive endo cycloaddition. Dienophile geometry is maintained during the course of the [4 + 2] cycloaddition, and no products derived from a potential stepwise, addition-cyclization reaction were detected. Representative neutral and electron-deficient dienophiles failed to undergo cycloaddition. A related boron trifluoride etherate-catalyzed [4 + 2] cycloaddition of simple 2-aza-l,3-butadienes with carbonyl compounds provides 5,6-dihydro-2//-l,3-ox-azines and appears to proceed preferentially through an endo [4 + 2] transition state although evidence supporting a stepwise, addition-cycli-zation was occasionally detected.630... 

Enolate species 6, derived from 1-oxopropyl complex 5, reacts similarly with monosubstituted epoxides. Under the influence of diethylaluminum chloride, only the diastereomers 7 and 8 were observed in the reaction mixture 7 was the major product. The use of boron trifluoride - diethyl ether complex instead of diethylaluminum chloride caused a complete loss of stereocontrol at C , producing a 50 50 mixture of diastereomers 7 and 8, but stereocontrol at C was retained as no other diastereomers were produced. The major diastereomer produced is consistent with the intermediacy of a transition state like that represented in Newman projection C which has the usual anti-E-snolate geometry and lacks the R methyl gauche interaction of structure D. 

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