Boron enolates from enones

It was anticipated that two of the three stereochemical relationships required for intermediate 12 could be created through reaction of the boron enolate derived from imide 21 with a-(benzyloxy)ace-taldehyde 24. After conversion of the syn aldol adduct into enone 23, a substrate-stereocontrolled 1,2-reduction of the C-5 ketone car-... 

The synthesis of the polyol glycoside subunit 7 commences with an asymmetric aldol condensation between the boron enolate derived from imide 21 and a-(benzyloxy)acetaldehyde (24) to give syn adduct 39 in 87 % yield and in greater than 99 % diastereomeric purity (see Scheme 8a). Treatment of the Weinreb amide,20 derived in one step through transamination of 39, with 2-lithiopropene furnishes enone 23 in an overall yield of 92 %. To accomplish the formation of the syn 1,3-diol, enone 23 is reduced in a chemo- and... 

Further insight into the P-borylation reaction of a,P-enones (Scheme 2.32) showed that the reaction can be carried out in THF, and the catalyst generated in situ from CuCl (5 mol%), the imidazolium salt (5 mol%), and NaO Bu (10 mol%), to form the [Cu(O Bu) (NHC)] as the catalysis initiating species. In this case, stable boron enolate products are formed which need to be hydrolysed by methanol to the ketone products [114]. 

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... 

The chain is completed by displacement of another alkyl radical from a trialkylborane at oxygen by the delocalized radical and the formation of the same boron enolate proposed in the ionic reaction. This alkyl radical adds in its turn to the enone, and the boron enolate which forms is hydrolysed to the ketone product. Only small amounts of oxygen are needed to initiate the chain and it is not surprising that the air around the reaction mixture is enough to start a typical reaction. The water (which is inert to radicals, so can be present in the reaction mixture) again hydrolyses the boron enolate. 

A model proposed for rationalizing the stereochemical outcome is also shown in Scheme 5.112. It seems plausible that the skewed structure 441, typical for transition metal BINAP complexes, has just one open space that is filled by a coordination of rhodium to the carbon - carbon bond of cyclohexenone. As a consequence, the insertion of the phenyl group then occurs from the Sf-face to the enone to give the / -rhodium complex 442 that subsequently tautomerizes to the more stable oxallyl-type enolate [205a]. It seems that a transmetallation at the stage of the enolate - from rhodium to boron enolate, as indicated in cycle B (Scheme 5.111) - doesnot occur in all these rhodium-mediated domino reactions without exception. Thus, Hayashi and coworkers produced evidence in support of a rhodium enolate as an active nucleophile in the aldol step when phenyl-9-BBN 438 was reacted with acyclic enones in the presence of [Rh(OH)-(S)-BINAP]2. In this case, the catalytic cycle is maintained by a transmetallation of the rhodium to the boron aldolate [221]. 

Allyl silanes are rather like silyl enol ethers they react with electrophiles, provided they are activated, for example by a Lewis acid. Titanium tetrachloride is widely used but other successful Lewis acids include boron trifluoride, aluminium chloride, and trimethylsilyl triflate. Electrophiles include acylium ions produced from acid chlorides, carbocations from tertiary halides or secondary benzylic halides, activated enones, and epoxides all in the presence of Lewis acid. In each case the new bond is highlighted in black. 

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