Base-induced bridge cleavage of 1,5-dimethyl-7-oxabicyclo-[2.2.1]hept-5-ene systems

Nickel-Catalyzed Addition of Alkenylzirconium Reagents to Bicyclic Olefins: A Highly Regio- and Stereoselective Ring-Opening Reaction

A highly regio- and stereoselective ring-opening addition of alkenylzirconium reagents to bicyclic olefins catalyzed by nickel complexes was described. Treatment of 7-oxa- and 7-azabenzonorbornadienes (1a-e) with various terminal alkenylzirconium reagents 2a-f (Cp(2)ZrClCH=CHR; R = t-Bu, n-Pr, n-Oct, 1-cyclohexenyl, SiMe(3), and Ph) in the presence of Ni(PPh(3))(2)Cl(2) and Zn powder (or a combination of ZnCl(2) and NEt(3)) in dry THF at 50 degrees C afforded the corresponding cis-2-alkenyl-1,2-dihydronaphthalene derivatives 3a-l in moderate to excellent yields. Under similar reaction conditions, internal alkenylzirconium reagents 2g,h (Cp(2)ZrClCR=CHR: R = Et and n-Pr) also undergo ring-opening addition to oxanorbornadienes 1a and 1d to give cis-2-alkenyl-1,2-dihydronaphthalene derivatives 4a-c in good yields. Possible pathways involving the transfer of alkenyl group in the alkenylzirconium reagent to the Ni(II) center followed by migration of the alkenyl group from the Ni(II) center to the carbon-carbon double bond of 7-oxanorbornadiene or the reaction of 7-oxanorbornadiene with Ni(0) to form a Ni(II)-pi-allyl prior to the transfer of the alkenyl group as key steps for the catalytic reaction were proposed and discussed.

Asymmetric reductive ring-opening of bicyclic olefins catalyzed by palladium and nickel complexes

[reaction: see text] Asymmetric reductive ring opening of oxa- and azabenzonorbornadienes with organic acids and zinc powder under mild conditions catalyzed by Ni(binap)Cl(2) or Pd(binap)I(2) produces the corresponding 1,2-dihydronaphth-1-ols in good to excellent yields with high enantioselectivity.

Highly stereoselective ring-opening addition of terminal acetylenes to bicyclic olefins catalyzed by nickel complexes

[reaction: see text] Treatment of 7-oxa- and 7-azabenzonorbornadienes with terminal acetylenes in the presence of Ni(dppe)Cl2, ZnCl2, and Zn powder in toluene at 90 degrees C afforded the corresponding cis-2-alkynyl-1,2-dihydronaphthalene derivatives in moderate to excellent yields with remarkably high stereoselectivity.

Unmasking the 6,7,5-tricarbocyclic frame of

Opening of the bridging ether of 8-oxabicyclo[3.2.1]oct-6-ene systems 1,7-fused to a five-membered carbocycle that bears a double bond conjugated to that of the oxabicycle can be induced at low temperaure by the addition of an organolithium reagent. If this double bond is unsubstituted, the opening occurs via an anti-1,6-addition pathway, and if substituted it takes place by means of a more classical syn SN2' addition.

Nickel-Catalyzed Highly Stereoselective Ring Opening of 7-Oxa- and Azanorbornenes with Organic Halides

Nickel-catalyzed ring-opening reactions of 7-heteroatom norbornadienes and norbornenes with various organic halides to give products with multiple stereocenters are described. Treatment of 7-oxabenzonorbornadiene (1a) and 7-carbomethoxy-7-azabenzonorbornadiene (1b) with aryl iodides (ArI) in the presence of NiCl(2)(PPh(3))(2) and Zn powder gave the corresponding ring-opening addition products cis-1,2-dihydro-2-aryl-1-naphthol (2a-m) and methyl N-[cis-1,2-dihydro-2-aryl-1-naphthyl]carbamate (3a-e) completely stereoselectively in 40-99% yields. The nickel system also catalyzes the reaction of highly substituted oxabicyclic [2.2.1] compounds (1c-e) with organic halides (PhI, PhCH(2)Br, PhCHCHBr, and PhCBrCH(2)) to give the corresponding ring-opening products (4a-d, 5, 6a,b) that consist of four fixed stereocenters. Studies on the effect of solvent on the reaction of 1a with PhI show that CH(3)CN gives the highest yield of product 2a; no product 2a is observed when toluene, dichloromethane, methanol, DMF, or DMSO is used as solvent. Addition of extra PPh(3) to the reaction mixture reduced the yield of 2a. A mechanism is proposed to account for the formation of these nickel-catalyzed ring-opening addition products.

Exploring the Reactivity of Dioxacyclic Compounds as a Route to Polysubstituted Decalins and Fused Polycycles

We have described a chemo-, regio-, and stereocontrolled methodology for the simple and efficient synthesis of a large variety of cis-decalins and related fused polycyclic systems with control at up to six stereocenters, based on the sequential ring-opening of dioxacyclic templates. We have established that the most useful feature of the reactivity of the dioxacyclic compounds toward the nucleophilic ring-opening reaction is that the first ring-opening reaction is significantly faster than the second allowing the sequential transformation of the oxabicyclic moieties. The flexibility of the sequential ring-opening process and its limitations have been demonstrated and a new enantioselective mode of opening was reported. The enantioselective base-induced desymmetrization was successfully applied to a thiadioxapentacycle to give the product in >95% ee using a chiral lithium amide base.