I. The Stereochemistry of the [3,3] Sigmatropic Rearrangement of 1,5-Diene-3-Alkoxides. II. Stereoselective Aldol Condensations via Dialkylboron Enolates

Author: Nelson, John Victor

Year: 1981

Degree: Dissertation (Ph.D.)

Advisor: Evans, David A.

Committee Member: Unknown, Unknown

Option: Chemistry

DOI: 10.7907/93mv-cf69

Abstract

A study was carried out on the [3,3] sigmatropic rearrangement of the potassium salts of the four diastereomers (both cis,trans and erythro,threo) of 1-(1-methoxy- 2-butenyl)-2-cyclohexen-l-o1. It was found that these rearrangements proceed in a concerted fashion predominately via chair-like transition states to give diastereomers of 3-(3-methoxy-l-methyl-2-propenyl)-cyclohexanone. The application of these modified oxy-Cope rearrangements to the synthesis of (±)-erythro-juvabione is reported.

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 dialkyl-boryl 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 a 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 thio-propionate was readily converted to the trans-enolate. The stereoselectivity of the aldol condensations of this enolate are 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 dialkyl boron enediolates and the aldol condensations of these species were used to probe the relative reactivity of cis- and trans-enolates.

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