Thermal rearrangement of functionalized 6-exo-(1-alkenyl)bicyclo\3.1.0]hex-2-enes application to the total synthesis of (+)-sinularene
Creators: name: Jung, Grace Lorena
Publisher: name: University of British Columbia
Year: 2010

description: This thesis describes firstly, a study involving the thermal rearrangement of substituted 6-e̲x̲o̲-(1-alkenyl) bicyclo-[3.1.0]hexenes, and secondly, the application of this type of transformation to a total synthesis of (±)-sinularene (1̲2̲5̲). The 6-e̲x̲o̲-(1-alkenyl)bicyclo[3.1.0]hexenes (1̲8̲7̲, 1̲8̲9̲, 1̲9̲2̲, 1̲9̲4̲, 2̲4̲0̲, 2̲7̲4̲ and 340) were prepared and thermolyzed in sealed tubes to afford the corresponding bicyclo[3.2.1]octa-2,6-dienes (1̲8̲8̲, 1̲9̲0̲, 1̲9̲3̲, 1̲9̲5̲, 2̲4̲1̲, 2̲7̲6̲ and 3̲4̲1̲) in generally excellent yields. With the exception of 1̲9̲0̲, the thermolysis products were subjected to acid-catalyzed hydrolysis to give the respective bicyclo[3.2.1]octenones. From this study, it is clear that a) the Cope rearrangement of substrates, such as 2̲7̲4̲ and 3̲4̲0̲, containing even sterically bulky substituents on the 6-alkenyl side chain presents a viable means of generating functionalized bicyclo [ 3.2.1] octa-2 , 6-dienes, b) this methodology provides for the placement of synthetically useful functionalities on any of the carbon bridges of the bicyclo-[3.2.1]octane skeleton, and c) the transformations 2̲4̲0̲→2̲4̲1̲ and 2̲7̲4̲→2̲7̲6̲ provide strong evidence for the stereospecificity of the rearrangement process. In the total synthesis of (±)-sinularene (1̲2̲5̲), the key step involved the thermal rearrangement of 3̲2̲2̲ to afford the bicyclo[3.2.1]octadiene 3̲2̲1̲. The compound 3̲2̲2̲̲ was readily prepared as follows. 1-Lithio-3-methyl-1-butyne was treated with methacrolein to furnish the allylic alcohol 3̲3̲1̲, which was transformed into the ester 3̲3̲2̲ v̲i̲a̲ an orthoester Claisen rearrangement (hot triethyl .orthoacetate, propionic acid). Hydrolysis of the ester 3̲3̲2̲, followed by reaction of the resultant acid with oxalyl chloride in refluxing hexane gave the corresponding acid chloride 3̲3̲4̲. Treatment of 3̲3̲4̲ with a cold, ethereal solution of diazomethane afforded the diazo ketone 3̲3̲5̲, which in the presence of copper (II) acetoacetonate in refluxing benzene, underwent an intramolecular carbenoid cyclization to furnish the bicyclic ketone 3̲3̲6̲. Semihydrogenation of 3̲3̲6̲ using Lindlar’s catalyst gave stereoselectively the c̲i̲s̲-alkenyl ketone 3̲3̲7̲. The enone 3̲3̲8̲ was obtained by oxidizing the trimethylsilyl enol ether of 3̲3̲7̲ using palladium (II) acetate in acetonitrile. When the enone 3̲3̲8̲ was treated with lithium divinylcuprate, the two epimeric products 3̲3̲9̲ and 3̲4̲6̲ were obtained in a ratio of 9:1, respectively, and were sus-sequently separated by column chromatography. Trapping the lithium enolate of 3̲3̲9̲ with t̲-butyldimethylsilyl chloride led to the required enol ether 3̲3̲2̲. Thermolysis (220°C, sealed tube) of 3̲3̲2̲ in benzene produced exclusively in 86% yield the desired bicyclic triene 3̲2̲1̲. Subjection of 3̲2̲1̲ to hydroboration using disiamylborane gave, after oxidative workup, the alcohol 3̲4̲7̲, which on treatment with p̲-toluenesulfonyl chloride in the presence of 4-dimethylaminopyridine, afforded the ketone 3̲4̲9̲. Successive hydrogenation of 3̲4̲9̲ and Wittig olefination of the resultant ketone 2̲8̲0̲ completed the total synthesis of (±)-sinularene (1̲2̲5̲). [formula omitted]