Enol borinates

By employing optically active enol borinates instead of silylketene acetals, the Ireland-Claisen rearrangement has been further developed to an enantioselective... 

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. 

The enol acetates, in turn, can be prepared by treatment of the parent ketone with an appropriate reagent. Such treatment generally gives a mixture of the two enol acetates in which one or the other predominates, depending on the reagent. The mixtures are easily separable. An alternate procedure involves conversion of a silyl enol ether (see 12-22) or a dialkylboron enol ether (an enol borinate, see p. 560) to the corresponding enolate ion. If the less hindered enolate ion is desired (e.g., 126), it can be prepared directly from the ketone by treatment with lithium diisopropylamide in THE or 1,2-dimethoxyethane at —78°C. ... 

Regioselectivity in the halogenation of unsymmetrical ketones can be attained by treatment of the appropriate enol borinate of the ketone with NBS or NCS. The desired halo ketone is formed in high yield. Another method for achieving the... 

Among the preformed enol derivatives used in this way have been enolates of magnesium, lithium, titanium, zirconium, and tin, ° silyl enol ethers, enol borinates,and enol borates, R CH=CR"—OB(OR)2. The nucleophilicity of silyl enol ethers has been examined. In general, metallic Z enolates give the syn (or erythro) pair, and this reaction is highly useful for the diastereoselective synthesis of these products. The ( ) isomers generally react nonstereoselectively. However, anti (or threo) stereoselectivity has been achieved in a number of cases, with titanium enolates, with magnesium enolates, with certain enol bor-inates, and with lithium enolates at — 78°C. ... 

For conversion of ketones to either (Z) or ( ) enol borinates, see, for example, Evans,... 

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

The syntheses in Schemes 13.45 and 13.46 illustrate the use of oxazolidinone chiral auxiliaries in enantioselective synthesis. Step A in Scheme 13.45 established the configuration at the carbon that becomes C(4) in the product. This is an enolate alkylation in which the steric effect of the oxazolidinone chiral auxiliary directs the approach of the alkylating group. Step C also used the oxazolidinone structure. In this case, the enol borinate is formed and condensed with an aldehyde intermediate. This stereoselective aldol addition established the configuration at C(2) and C(3). The configuration at the final stereocenter at C(6) was established by the hydroboration in Step D. The selectivity for the desired stereoisomer was 85 15. Stereoselectivity in the same sense has been observed for a number of other 2-methylalkenes in which the remainder of the alkene constitutes a relatively bulky group.28 A TS such as 45-A can rationalize this result. 

Perhaps the best general method to date for preparing a-haloacyl silanes involves bromi-nation of silyl enol borinates (9) at 0 °C, a reaction which proceeds in good yield and involves no sensitive intermediates. This route offers a most convenient one-pot synthesis of a-haloacyl silanes from readily available starting materials, as the intermediate enol borinates are very easily prepared from silyl acetylenes (Scheme 35)7,117,118. 

E)- or (Z)-Enol borinates. These ends are useful for stereoselective preparation of anti- or jyn-aldols, respectively, and have usually been obtained stereoselec-tively by variation of the alkyl group attached to boron triflates. They can also be prepared in essentially quantitative yield by reaction with dialkylboron chlorides at... 

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