摘要：

A detailed investigation of the enolization of a variety of ketones and carboxylic acid derivatives with dialkylboryl triflates in the presence of a tertiary amine and the subsequent aldol condensations of these boron enolates was conducted. The stereochemistry of the enolates formed from acyclic ketones was found to be dependent on the structure of the ketone, the dialkylboryl triflate, and the tertiary amine. A mechanism for the enolization involving initial coordination of the boryl triflate to the ketone carbonyl with subsequent deprotonation by the amine is proposed to explain the results. The boron enolates derived from these acyclic ketones undergo aldol condensation with a number of aldehydes in good yield. Consistently good correlation was observed between the enolate geometry and the product aldol stereochemistry for these acyclic ketones regardless of the structure of the ketone or the boron ligands. However, for the boron enolate derived from cyclohexanone the aldol stereoselectivity was dependent on the boron ligands and the solvent. In this case, the use of the cyclopentylthexylboron enolate in tetrahydrofuran as solvent resulted in complete stereocontrol in the condensation. Although simple esters and amides cannot be enolized with the triflate reagents, tert-butyl thiopropionate was readily converted to the trans enolate. The stereoselectivity of the aldol condensations of this enolate is also dependent on the boron ligands and the solvent; again, the proper choice of these parameters allowed total stereocontrol of the condensation. It was found that carboxylic acids could be converted to the dialkylboryl enediolates, and the aldol condensations of these species were used to probe the relative reactivity of cis and trans enolates. Chiral boron enolates were studied for possible asymmetric induction in the aldol condensation. Methyl ketone enolates exhibited moderate levels of chirality transfer, while cis enolates gave only one detectable diastereoisomer. The sense of chirality transfer was proven by determination of the absolute configurations of newly created centers of asymmetry. A transition state model based on steric interactions is proposed for chirality transfer.

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