Enol borinates formation

Dibutylboron triflate promotes both enol borinate formation and addition.70... 

Selective trans deprotonation of the ketone-L2BOTf complex leads to formation of (Z)-enolborinate see Evans, D. A. Nelson, J. V. Vogel, E. Taber, T, R. J. Am. Chem. Soc. 1981, 103, 3099. An alternative explanation for L2BCI to ( )-enol borinate and L2BOTf to (Z)-enol borinate has been proposed by Corey and Kim see Corey, E. J. Kim,... 

Although stereoselective formation of enolates from acyclic ketones with bases such as LDA is rather difficult, stereodefined boron enolates are more readily accessible. In the Mukaiyama method, an ethyl ketone is treated with a dialkylboron triflate and a tertiary amine, usually i-Pr2NEt. The resultant Z-(0) boron enolates (also known as enol borinates) are believed to be formed under kinetic control by deprotonation of the Lewis acid-complexed substrate. Brown and co-workers have shown that E- 0) boron enolates may be prepared by treatment of ethyl ketones with dicyclohexylboron chloride in the presence of Et3N. ... 

Ethyl ketones (/ )- and (5)-18 (Scheme 9-8), as introduced by our group, have been u.sed extensively as versatile dipropionate reagents in polyketide synthesis [11]. Selective formation of the (ZO-enol borinate 19 is possible using c-Hex2BCl and the resulting anti aldol products, e.g, 20, are formed with ca. 97% ds [6a]. Several different hydroxyl protecting groups can be accommodated, but benzyl or... 

Our final highlight in the discodermolide synthesis is the use of reagent control to get what we want and not what the molecules want. The combination of enol borinate 276 and an aldehyde 277 featuring a cis double bond, led to the formation of aldols anti- and syn-278. A model study had shown that the inherent selectivity with these enals could be improved by the use of ( )-Ipc groups on boron instead of cyclohexyl but it is not the selectivity that was wanted. (+)-Ipc groups were able to turn around the selectivity and improve the yield of the reaction while they were at it.50... 

Scheme 33 illustrates the use of two standard persistent auxiliaries. The Evans oxazolidinone 33-1 [83] is highly versatile, i.e., suitable for enolate reactions and double bond additions alike. In the enolate alkylation case [reaction (99)] the high diastereoselectivity depends on the formation of a chelate 33-2 which fixes the reaction site in a defined conformation in which one of the diastereofaces is efficiently shielded. The removal of the auxiliary requires the chemoselective cleavage of the exo cyclic amide bond which is sometimes difficult to achieve. In boron mediated aldoltype additions [Scheme 34, reaction (100)] no chelate can be formed so that the extremeley high diastereoselectivity with which the syn-adduct 34-1 is formed must be due to some other effect, presumably allyl 1,3-strain on the stage of the enol borinate 34-1. 

Enolboration. With EtjN the enolboration of ketones proceeds with stereoselectivities following the leaving abilities of the substituents on boron. A better leaving group favors formation of (Z)-enol borinates, whereas a poorer leaving group leads to more ( )-enol borinates. 

Reaction C in Fig. 4 is an aldol condensation between an achiral aldehyde and an ester enol borinate featuring a bidentate chiral substituent at the boron atom [24]. Upon enolate-boron/aldehyde-oxygen co-ordination, two chair-like TS can be formed, both featuring the aldehyde phenyl group in a pseudo-equatorial position. Preferential attack on the aldehyde Si face is determined by the spatial arrangement of the metal ligand. The almost exclusive formation of the anti diastereoisomers arises from control of the enolate geometry. 

The enantiomeric /1-hydroxy ketones are available in an analogous way using the corresponding enantiomeric borinates. The reaction is plagued by low regioselectivity in the formation of the boron enolates, except when R1 is phenyl or isobutyl53,57. 

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