Regio- and stereospecific synthesis of substituted cyclohexenediols from 7-oxabicyclo[2.2.1]hept-5-en-2-ols and organolithium reagents

Iridium/Copper Co-catalyzed Anti-Stereoselective Ring Opening of Oxabenzonorbornadienes with Grignard Reagents

Cooperative catalysis has been widely considered as one of the most powerful strategies to improve synthetic efficiency. A new iridium/copper cocatalyst was developed for the ring-opening reaction of oxabenzonorbornadienes with a wide variety of Grignard reagents, which afforded the corresponding anti-2-substituted 1,2-dihydronaphthalen-1-ols in high yields (up to 99% yield) under mild conditions. The effects of catalyst loading, Lewis acid, Grignard reagent loading, and reaction temperature on the yield were investigated. To the best of our knowledge, it represents the first example of ring-opening reactions of oxabicyclic alkenes with Grignard reagent nucleophiles in a trans-stereoselective manner.

New catalytic reactions of oxa- and azabicyclic alkenes

Bicyclic alkenes, including oxa- and azabenzonorbornadienes and their derivatives, can be readily activated by transition metal complexes face-selectively due to their unsymmetrical bicyclic structure and the intrinsic angle strain on the carbon-carbon double bond. We have developed several stereo-, regio-, and chemoselective reactions catalyzed by nickel and palladium complexes using these bicyclic alkenes as substrates, providing a unique means of constructing a variety of synthetically useful carbocycles and heterocycles with high efficiency not generally accessible by traditional methods. This Account outlines these new metal-catalyzed reactions that include couplings, cycloadditions, and cyclization reactions.

Rhodium(I)-catalyzed nucleophilic ring-opening reactions of oxabicyclo adducts derived from the [4 + 2]-cycloaddition of 2-imido-substituted furans

A series of 2-imido-substituted furans containing tethered unsaturation were prepared by the addition of the lithium carbamate of furan-2-ylcarbamic acid tert-butyl ester to a solution of the mixed anhydride of an appropriately substituted 3-butenoic acid. The initially formed imido furans undergo a rapid intramolecular [4 + 2]-cycloaddition at room temperature to deliver the Diels-Alder cycloadducts in good to excellent yield. Isolation of the highly labile oxabicyclic adduct is believed to be a consequence of the lower reaction temperatures employed as well as the presence of the extra carbonyl group, which diminishes the basicity of the nitrogen atom, thereby retarding the ring cleavage/rearrangement reaction generally encountered with related systems. By using a Rh(I)-catalyzed ring opening of the oxabicyclic adduct with various nucleophilic reagents, it was possible to prepare highly functionalized hexahydro-1H-indol-2(3H)-one derivatives in good yield. The major stereoisomer obtained possesses a cis-relationship between the nucleophile and hydroxyl group in the ring-opened product. The stereochemistry was unequivocally established by X-ray crystallographic analysis. Coordination of Rh(I) to the alkenyl pi-bond followed by a nitrogen-assisted cleavage of the carbon-oxygen bond occurs to furnish a pi-allyl rhodium(III) species. Addition of the nucleophile then occurs from the least hindered terminus of the resulting pi-allyl rhodium(III) complex. Proton exchange followed by rhodium(I) decomplexation ultimately leads to the cis-diastereomer.

Transition metal-catalyzed enantioselective ring-opening reactions of oxabicyclic alkenes

Over the past five years, several metal-catalyzed asymmetric ring-cleaving reactions have been developed that generate ring-opened products in high yield and enantiomeric excess. These reactions can be carried out with a range of nucleophiles including hydride, stabilized and nonstabilized carbanions, alcohols, amines, and carboxylates. To achieve these results, three different transition metals have been employed, namely nickel, palladium, and rhodium, leading not only to synthetically useful transformations but also to a greater understanding of the reaction manifolds possible with these metals. This Account covers the work by our group and by others toward the development of new metal-catalyzed asymmetric ring-opening reactions of oxabicyclic alkenes.

Remote electronic effects in the rhodium-catalyzed nucleophilic ring opening of oxabenzonorbornadienes

We report the application of our rhodium-catalyzed nucleophilic ring-opening methodology to unsymmetrically arene-substituted oxabenzonorbornadienes. The regioselectivity of the ring opening was investigated using a variety of nucleophiles that led to a broad selection of dihydronaphthalene products. It was found that good to excellent regioselectivities are obtained using strongly pi-donating substituents, whereas sigma-donating and electron-withdrawing functionalities have a minimal effect. Post ring-opening manipulations of functional groups in the dihydronaphthalene products were shown to give efficient access to mono- and diamine tetrahydronaphthalene building blocks.

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.

1994 Merck Frosst Award Lecture New strategies for the stereoselective synthesis of natural and unnatural products via organometallic reagents and catalysts

Samarium in the presence of diiodomethane selectively cyclopropanates an allenic alcohol to yield a methylenecyclopropane with modest to excellent diastereoselectivity. The effect of substituents at the carbinol carbon and the allenic carbon on the diastereoselectivity was investigated. A palladium catalyst was shown to promote the intramolecular cycloaddition of the methylenecyclopropane with an electron-deficient alkyne to yield a methylenecyclopentane. The reaction was stereospecific with retention of stereochemistry as proven by X-ray diffraction. Nickel catalysts accelerate the hydroalumination of oxabicyclo[3.2.1]alkenes, leading to bicyclic trialkylalanes. Upon treatment with a Lewis acid (diisobutylaluminum chloride), the trialkylalanes fragment to provide cycloheptenols. This reaction, followed by an enantioselective enzyme-catalyzed esterification, was used as a key step in a synthesis of the mevinic acid lactone. Keywords: methylenecyclopropane, palladium, stereocontrol, oxabicyclic, nickel.