Asymmetric Diels-Alder studies involving chiral acetylenic diesters and investigations of an intramolecular Diels-Alder approach to the pentalenolactones

Abstract

Asymmetric Diels-Alder reactions of 2-(trimethylsilyloxy)cyclohexa-1,3-diene 17a, 6,6-dimethyl-2-(trimethylsilyloxy)cyclohexa-1,3-diene 17b, and 5,5-dimethyl-2-(trimethylsilyloxy)cyclohexa-1,3-diene 17c with several chiral acetylenedicarboxylates were conducted. Modest levels of diastereoselectivity were obtained for several examples. -- Semiempirical molecular orbital calculations for the Diels-Alder reactions of 2-hydroxy analogues of these dienes with di-t-butyl acetylenedicarboxylate were carried out to help interpret the experimental results. Four transtion states having very similar relative energies were obtained for each diene-dienophile combination. The calculations also supported an asynchronous transition state with the shorter incipient bond nearest the electron-donating trimethylsiloxy group. It follows that the ester group further from the trimethylsiloxy group most likely adopts a fixed, parallel-planar conformation to activate the triple bond for attack, whereas the other ester is perpendicular to the incoming diene. -- (3-Phenylsulfonyl-2-propynyl) 2-(5,5-dimethyl-2-oxocyclohex-3-enyl) ethanoate 223 and ((E)-3-phenylsulfonyl-2-propenyl) 2-(5,5-dimethyl-2-(((1,1-dimethylethyl)dimethylsilyl)oxy)cyclohexa-1,3-dienyl)ethanoate 231 could serve as precursors in the synthesis of pentalenolactones. Our synthesis of these compounds was based on alkylation of 4,4-dimethyl-2-cyclohexen-1-one 20c with 2-halo esters similar to compound 146. -- Various 2-bromo esters were prepared and converted to their corresponding 2-iodo equivalents via the Finkelstein reaction. These 2-iodo esters were found to undergo alkylation in good yield with 4,4-dimethyl-2-cyclohexen-1-one 20c. In this manner, (3-phenylthio-2-propynyl) 2-(5,5-dimethyl-2-oxocyclohex-3-enyl)ethanoate 205 was synthesized from (3-phenylthio-2-propynyl) 2-iodoethanoate 204. Treatment under kinetic conditions gave the desired diene, (3-phenylthio-2-propynyl) 2-(5,5-dimethyl-2-(((1,1-dimethylethyl)dimethylsilyl)oxy)cyclohexa-1,3-dienyl)ethanoate 220. Activation of the alkyne, by oxidation to the sulfone, gave (3-phenylsulfonyl-2-propynyl) 2-(5,5-dimethyl-2-oxocyclohex-3-enyl)ethanoate 223. -- A similar route involving (E)-3-phenylsulfonyl-2-propen-1-ol 227 as the starting alcohol resulted in ((E)-3-phenylsulfonyl-2-propenyl) 2-(5,5-dimethyl-2-oxocyclohex-3-enyl)ethanoate 230. Diene formation under thermodynamic conditions gave ((E)-3-phenylsulfonyl-2-propenyl) 2-(5,5-dimethyl-2-(((1,1 dimethylethyl)dimethylsilyl)oxy)cyclohexa-1,3-dienyl)ethanoate 231.