NONRACEMIC FRONTALIN VIA COPPER-BASED AND PALLADIUM-BASED SKELETAL CONSTRUCTION AND THE ASYMMETRIC DIHYDROXYLATION

The new higher order organocuprate, Li2Cu(CN)(C(OMe):CH2)2, prepared through a Sn --> Li --> Cu transmetalation sequence, was found to efficiently couple with allyl bromide producing 2-methoxy-1,4-pentadiene (3) which is efficiently converted to the ketal 4. Hydroboration of 4 with 9-BBN-H and the selective oxidation of this adduct with trimethylamine N-oxide (TMANO) produces 9-oxa-10-borabicyclo[3.3.2]decane 5, which undergoes Pd-catalyzed cross coupling with 2-bromopropene producing 6, thereby accomplishing the construction of the frontalin carbon skeleton in 63% overall yield from allyl bromide. The Os-based dihydroxylation of 6 followed by ketalization either produces the racemic pheromone ((+/-)-1) or, with the Sharpless catalytic asymmetric dihydroxylation (dihydroquinine ligand) procedure, results in the selective formation of the S enantiomer in 35% ee. The de determined from the C-13 NMR spectra of the diastereomeric Mosher's monoesters (9) was found to provide useful corroborative information on the optical purity of the intermediate 1,2-diols 7.