Regio- and diastereoselective tandem rhodium-catalyzed allylic alkylation/Pauson-Khand annulation reactions

Regioselective and Enantiospecific Rhodium-Catalyzed Allylic Alkylation Reactions Using Copper(I) Enolates: Synthesis of (−)-Sugiresinol Dimethyl Ether

The transition metal-catalyzed allylic alkylation represents a fundamentally important cross-coupling reaction for the construction of ternary carbon stereogenic centers. We have developed a regioselective and enantiospecific rhodium-catalyzed allylic alkylation of acyclic unsymmetrical allylic alcohol derivatives using copper(I) enolates to prepare beta-substituted ketones. This protocol represents a convenient asymmetric Claisen rearrangement surrogate in which alpha-substituted enolates permit the introduction of an additional stereogenic center. The synthetic utility of this transformation was highlighted in the construction of a trans-1,2-disubstituted cyclohexene and the total synthesis of (-)-sugiresinol dimethyl ether. Finally, we anticipate that copper(I) enolates may prove useful nucleophiles in related metal-catalyzed reactions.

Regio- and enantiospecific rhodium-catalyzed arylation of unsymmetrical fluorinated acyclic allylic carbonates: inversion of absolute configuration

The transition metal-catalyzed allylic substitution with unstabilized carbon nucleophiles represents an important cross-coupling reaction for the construction of ternary carbon stereogenic centers. We have developed a new regio- and enantiospecific rhodium-catalyzed allylic alkylation of acyclic unsymmetrical chiral nonracemic allylic alcohol derivatives with aryl zinc bromides. This study demonstrates that the hydrotris(pyrazolyl)borate rhodium catalyst and zinc(II) halide salt are crucial for efficiency, while the addition of lithium bromide to the catalyst is necessary for obtaining optimal regiospecificity. The stereochemical course of this reaction was established through the synthesis of (S)-ibuprofen, which demonstrated that the alkylation proceeds with net inversion of absolute configuration consistent with direct addition of the nucleophile to the metal center followed by reductive elimination.

High enantioselectivity in rhodium-catalyzed allylic alkylation of 1-substituted 2-propenyl acetates

[reaction: see text] Rhodium-catalyzed asymmetric allylic alkylation of 1-substituted 2-propenyl acetates with dimethyl malonate proceeded with high enantioselectivity in the presence of cesium carbonate as a base and a rhodium catalyst generated from Rh(dpm)(C(2)H(4))(2) (dpm = dipivaloylmethanato) and a chiral phosphino-oxazoline whose basic skeleton is axially chiral binaphthyl to give branch alkylation products in greater than 90% ee.

Hydrogen bond directed highly regioselective palladium-catalyzed allylic substitution

The palladium-catalyzed allylic substitution of 5-vinyloxazolidinones and derivatives was investigated. Unusual and high regioselectivity for the branched product was observed. The regioselectivity was influenced by the type of substrate, the solvents, and the nucleophile. Imide-type nucleophiles were found to be directed to the internal carbon (branched:linear, 75:25 to >98:2), whereas sulfonamides, amines, and malonates added only to the terminal carbon of the allyl complex. Relatively nonpolar solvents such as toluene and THF favored the branched product (97:3 and 95:5, respectively). Acetonitrile and dichloromethane afforded lower regioselectivity (50:50 and 67:33, respectively), and the use of the protic solvent ethanol resulted in reversal of the regioselectivity (12:88). The directing group on the substrate was important. Amides afforded almost complete formation of the branched product, and carbamates gave a 50:50 mixture of regioisomers with phthalimide as the nucleophile. Evidence for direction of the nucleophile via hydrogen bonding was obtained by replacing the hydrogen of the amide with a methyl, resulting in the production of only the normal linear product.

Intermolecular transition metal-catalyzed [4 + 2 + 2] cycloaddition reactions: a new approach to the construction of eight-membered rings

Transition metal-catalyzed cycloaddition reactions represent powerful methods for the construction of complex polycyclic systems. We have developed a new intermolecular metal-catalyzed [4 + 2 + 2] cycloaddition of heteroatom-tethered enyne derivatives with 1,3-butadiene. This study demonstrates that excellent selectivity can be obtained for the heterocycloaddition adducts through the judicious choice of silver salt. The development of the tandem rhodium-catalyzed allylic substitution [4 + 2 + 2] cycloaddition provides a convenient three-component coupling that circumvents the prior formation of the enyne derivative. Finally, the introduction of a stereogenic center at C-2 leads to a diastereoselective cycloaddition, which provides a powerful new method for the construction of bicyclic octanoid ring systems applicable to target directed synthesis.

Regio- and enantiospecific rhodium-catalyzed allylic etherification reactions using copper(I) alkoxides: influence of the copper halide salt on selectivity

The transition metal-catalyzed allylic etherification represents a fundamentally important cross-coupling reaction for the construction of allylic ethers. We have developed a new regio- and enantiospecific rhodium-catalyzed allylic etherification of acyclic unsymmetrical allylic alcohol derivatives using copper(I) alkoxides derived from primary, secondary and tertiary alcohols. This study demonstrates that the choice of copper(I) halide salt is crucial for obtaining excellent regio- and enantiospecificity, providing another example of the effect of halide ions in asymmetric transition metal-catalyzed reactions. Finally, the ability to alter the reactivity of the alkali metal alkoxides in this manner may provide a useful method for related metal-catalyzed cross-coupling reactions involving heteroatoms.

Sequential actions of cobalt nanoparticles and palladium(II) catalysts: three-step one-pot synthesis of fenestranes from an enyne and an alkyne diester

A three-step one-pot synthesis of fenestranes from a readily available enyne and an alkyne diester has been carried out with cobalt nanoparticles and palladium(II) as catalysts.

A new two-step four-component synthesis of highly functionalized cyclohexenols by sequential nickel-catalyzed couplings

A new two-step procedure for the synthesis of cyclohexenols has been developed. A nickel-catalyzed three-component addition of an enal, alkyne, and acetylenic tin affords substituted hept-4-en-6-ynals. The products of this first step then undergo a second nickel-catalyzed reaction with organozincs or organoboranes to afford densely functionalized cyclohexenols. Variation in each of the four components is tolerated to provide access to a wide range of versatile building blocks.

Regioselective and enantiospecific rhodium-catalyzed allylic amination with N-(arylsulfonyl)anilines

[reaction: see text]. The regioselective and enantiospecific rhodium-catalyzed allylic amination of secondary allylic carbonates 1 with N-(arylsulfonyl)anilines provides a convenient process for the construction of arylamines 2. This method, in conjunction with ring-closing metathesis and radical cyclization reactions, allows the direct construction of biologically relevant pharmacophores as exemplified by the construction of dihydroquinoline and dihydrobenzo[b]indoline derivatives.