The First Stereoselective Synthesis of Racemic β-Multistriatin: A Pheromone Component of the European Elm Bark Beetle Scolytus multistriatus (Marsh.)

Article abstract of DOI:10.1021/jo00121a034

The first stereoselective synthesis of racemic β-multistriatin (4), a component of the pheromone of the European elm bark beetle Scolytus multistriatus (Marsh.), is described.Potassium glycerate (23) is alkylated with 2-bromopentanone (26) in the presence of the phase transfer catalyst TDA-1 to form the acyloin ester 29 in 75percent yield.After trimethylsilylation of 29, intramolecular acetalization of the product 30 in the presence of catalytic amounts of trimethylsilyl triflate and perchloric acid affords a mixture of diastereomers of the bicyclic lactone 28 in 65percent yield.Reduction of 28 with LiAlH4 gives the cis-disubstituted dioxolane diol 34 in 82percent yield.Oxidation of both the primary and secondary hydroxy groups present in 34 by a modified Swern oxidation affords the keto aldehyde 35 in 61percent yield.The change of the reagent ratio of oxalyl chloride:DMSO:NEt3 from 1.1:2.4:5 to 1.2:1.2:3 and a nonaqueous workup procedure are crucial for the success of this oxidation.Intramolecular aldol condensation of 35 catalyzed by triethylammonium chloride affords the aldol 38.Acetylation of 38 and pyrolytic distillation gave the bicyclic enone 37.Oxidation of 34, aldol reaction, and formation of the enone 37 can be combined in a one-pot procedure to give an overall yield of 21percent. 1,4-Addition of lithium dimethylcuprate to the enone 37 affords ketone 36 with the methyl group exclusively in the axial position.After Wittig methylenation of 36, the olefin 40 is obtained in 76percent yield.Catalytic hydrogenation of 40 with PtO2 gives a 92:8 mixture of β- and δ-multistriatin in 85percent yield.Acid-catalyzed epimerization of 4 affords δ-multistriatin (5).Some of the compounds prepared (28, 36, 37, and 40) are of interest as multistriatin analogues.Olefin 40 is also a valuable storage form for the unstable β-multistriatin (4).