title: On the Diels-Alder reactions of pentadienyl maleates and citraconates.
abstract: Reactions between conjugated dienols and maleic anhydride provide either cis-fused or trans-fused bicyclic products as major products, depending upon how the reaction is carried out. Simply mixing the two reactants together generally leads to cis-fused lactone acids in thermal reactions which proceed viaintermolecular Diels-Alder reaction followed by intramolecular esterification. Pre-forming the maleate half ester derivative followed by heating affords predominantly trans-fused lactone acids in good yields by way of an intramolecular Diels-Alder (IMDA) reaction. Sorbyl citraconate half esters undergo a rapid thermolytic fragmentation in refluxing toluene to form the dienol and citraconic anhydride. The resulting diene-dienophile pair undergo an intermolecular cycloaddition followed by a rapid intramolecular esterification to give cis-fused bicyclic lactone acids as major products. The IMDA reaction of citraconic half esters is sufficiently rapid in DMSO to dominate over fragmentation: the exo-cycloadduct is formed almost exclusively. Nine literature reports of endo-selective IMDA reactions of triene acids are erroneous; the cycloadditions proceed in an intermolecular manner.
authors.affiliation: Research School of Chemistry, Australian National University, Canberra ACT 0200, Australia.
authors.forename: Tory N
authors.initials: TN
authors.lastname: Cayzer
authors.forename: Michael J
authors.initials: MJ
authors.lastname: Lilly
authors.forename: Rachel M
authors.initials: RM
authors.lastname: Williamson
authors.forename: Michael N
authors.initials: MN
authors.lastname: Paddon-Row
authors.forename: Michael S
authors.initials: MS
authors.lastname: Sherburn