Syntheses with Organoboranes. XI. Allylboration of Vinylic Epoxides with Allylic Dialkylboranes

Allylboration of representative vinylic epoxides with allyldiethylborane (1) and (2-cyclohexenyl)dicyclohexylborane (2) affords the corresponding 1,2- and 1,4-addition products. cis-1,2-Addition is favored in the reaction of 1 with 3,4-epoxycycloalkenes of six- to eight-membered rings. 3,4-Epoxycyclopentene (3a) and 5,5-dimethyl-3,4-epoxycyclopentene (3b) undergo five-membered ring opening during allylboration with 1 and 2, producing the corresponding (Z)-trienols (4a and 4b) with high stereoselectivity. 1,4-Addition of 1 and 2 to monoepoxides of 1,3-butadiene and isoprene is favored, producing predominantly the corresponding (E)-alcohols.