Regio- and stereoselective reductive coupling of bicyclic alkenes with propiolates catalyzed by nickel complexes:: A novel route to functionalized 1,2-dihydroarenes and γ-lactones

7-Oxabenzonorbornadienes derivatives 1a-d underwent reductive coupling with alkyl propiolates CH,(-CCO2CH3 (2a), PhCdropCCO(2)Et (2b), CH3(CH2)(3)CdropCCO(2)CH(3) (2c), CH3(CH2)(3)(CdropCO(2)CH(3) (2d), TMSCdropCCO(2)Et (2e), (CH3)(3)CdropCCO(2)CH(3) (2f) and HC-CCO2Et (2g) in the presence of [NiBr2(dppe)] (dppe=Ph(2)pCH(2)CH(2)PPh(2)), H2O and zinc powder in acetonitrile at room temperature to afford the corresponding 2-alkenyl-1,2dihydronapthalen-1-ol derivatives 3a - n with remarkable regio- and diastereoselectivity in good to excellent yields. Similarly, the reaction of 7-azabenzonorbornadienes derivative le with propiolates 2 a, b and d proceeded smoothly to afford reductive coupling products 2-alkenyl-1,2-dihydronapthalene carbamates 3o-p in good yields with high regio- and stereoselectivity. This nickel-catalyzed reductive coupling can be further extended to the reaction of 7-oxabenzonorbornene derivatives. Thus, 5,6-di(methoxymethyl)-7-oxabicy-clo[2.2.1]hept-2-ene (4) reacted with 2a and 2d to furnish cyclohexenol derivatives bearing four cis substituents 5 a and b in 81 and 84% yield, respectively. In contrast to the results of 4 with 2, the reaction of dimethyl 7-oxabicyclo[2.2.1]-hept-5-ene-2,3-dicarboxylate (6) with propiolates 2a-d afforded the corresponding reductive coupling/cyclization products, bicyclo[3.2.1]gamma-lactones 7a-d in good yields. The reaction provides a convenient one-pot synthesis of gamma-lactones with remarkably high regio- and stereoselectivity.