title: Synthesis of (+/-)-bakkenolide-A and its C-7, C-10, and C-7,10 epimers by means of an intramolecular Diels-Alder reaction.
abstract: (+/-)-bakkenolide-A (1) was prepared in five steps from ethyl 4-benzyloxyacetoacetate by sequential alkylations with tiglyl bromide and (Z)-5-bromo-1,3-pentadiene, followed by an intramolecular Diels-Alder reaction of (E,Z)-triene 25b as the key step. The hydrindane cycloadduct 28 was subjected to hydrogenation and spontaneous or acid-catalyzed lactonization, followed by a Witttig reaction to introduce the exocyclic methylene group of 1. The known 7-epibakkenolide-A (2) and novel 10-epi- and 7,10-diepibakkenolide-A (3 and 4, respectively) stereoisomers were obtained as minor byproducts. When (E)-5-bromo-1,3-pentadiene was used instead of the Z-isomer, the 10-epi- and 7,10-diepibakkenolides were the major products. In both cases exo cyclization was preferred over endo. An alternative approach was based on a similar intramolecular Diels-Alder cycloaddition, using dimethyl malonate instead of ethyl 4-benzyloxyacetoacetate as the starting material for the double alkylation preceding the cycloaddition step. The cycloadduct was then converted into the corresponding alpha-phenylseleno propargyl esters 16 or 22. However, attempted formation of the spiro center by a radical cyclization resulted chiefly in reductive deselenization.
authors.affiliation: Department of Chemistry, University of Calgary, Calgary, Alberta, Canada T2N 1N4. tgback@ugalgary.ca
authors.forename: T G
authors.initials: TG
authors.lastname: Back
authors.forename: V O
authors.initials: VO
authors.lastname: Nava-Salgado
authors.forename: J E
authors.initials: JE
authors.lastname: Payne
mesh_terms: D015107:4-Butyrolactone / Q000031:analogs & derivatives* / Q000138:chemical synthesis* / Q000737:chemistry; D007536:Isomerism; D009682:Magnetic Resonance Spectroscopy; D013058:Mass Spectrometry; D015394:Molecular Structure; D012717:Sesquiterpenes
chemical_list: D012717:Sesquiterpenes; D015107:4-Butyrolactone; C016257:bakkenolide A