Discrimination of Enantiofaces and Stereoselective Electrophilic Addition Reactions for η²-Pyrrole Complexes

A series of asymmetric N-substituted pyrroles (1−7) has been synthesized from amino acid derivatives and complexed to the pentaammineosmium(II) fragment. Many of these pyrrole complexes (8−14) show a thermodynamic preference for one coordination diastereomer according to their NMR spectra (−5 to −20 °C). This stereoselective coordination results in stereoselective electrophilic addition at the uncoordinated β carbon (C3) when the complexes are treated with trifluoromethanesulfonic acid (HOTf), methyl triflate, or dimethoxymethane. These stereoselective reactions at C3 are a direct result of differentiation of the pyrrole enantiofaces

Discrimination of Enantiofaces and Stereoselective Electrophilic Addition Reactions for η²-Pyrrole Complexes

A series of asymmetric N-substituted pyrroles (1−7) has been synthesized from amino acid derivatives and complexed to the pentaammineosmium(II) fragment. Many of these pyrrole complexes (8−14) show a thermodynamic preference for one coordination diastereomer according to their NMR spectra (−5 to −20 °C). This stereoselective coordination results in stereoselective electrophilic addition at the uncoordinated β carbon (C3) when the complexes are treated with trifluoromethanesulfonic acid (HOTf), methyl triflate, or dimethoxymethane. These stereoselective reactions at C3 are a direct result of differentiation of the pyrrole enantiofaces

[2+2] Cycloaddition Reactions with a Tungsten-Stabilized 2<i>H</i>-Phenol

The phenol ligand of the complex TpW(NO)(PMe3)(η2-phenol) exists in solution as its 2H tautomer. This normally inaccessible isomer of phenol undergoes a [2+2] cycloaddition with several ketenes across the C3−C4 bond of the phenol. The resulting cyclobutanone can be selectively reduced with sodium borohydride without disturbing the enone functionality, and the resulting hydroxy[4.2.0]bicyclooctenone, which contains four new stereocenters, may be removed from the metal

Polarization of the Pyridine Ring: Highly Functionalized Piperidines from Tungsten−Pyridine Complex

The N-acetylpyridinium complex of {TpW(NO)(PMe3)} undergoes regio- and stereoselective reactions with a broad range of common organic nucleophiles, providing a family of 1,2-dihydropyridine (DHP) complexes of the form TpW(NO)(PMe3)(3,4-η2-DHP). The present study explores the elaboration of these systems into novel piperidines. The addition of an acid to the DHP complexes generates highly asymmetric π-allyl complexes that in turn react with a second nucleophile at either C3 or C5. The subsequent oxidative decomplexation of these materials yields several piperidinamides with unconventional substitution patterns

Polarization of the Pyridine Ring: Highly Functionalized Piperidines from Tungsten−Pyridine Complex

The N-acetylpyridinium complex of {TpW(NO)(PMe3)} undergoes regio- and stereoselective reactions with a broad range of common organic nucleophiles, providing a family of 1,2-dihydropyridine (DHP) complexes of the form TpW(NO)(PMe3)(3,4-η2-DHP). The present study explores the elaboration of these systems into novel piperidines. The addition of an acid to the DHP complexes generates highly asymmetric π-allyl complexes that in turn react with a second nucleophile at either C3 or C5. The subsequent oxidative decomplexation of these materials yields several piperidinamides with unconventional substitution patterns

[2+2] Cycloaddition Reactions with a Tungsten-Stabilized 2<i>H</i>-Phenol

The phenol ligand of the complex TpW(NO)(PMe3)(η2-phenol) exists in solution as its 2H tautomer. This normally inaccessible isomer of phenol undergoes a [2+2] cycloaddition with several ketenes across the C3−C4 bond of the phenol. The resulting cyclobutanone can be selectively reduced with sodium borohydride without disturbing the enone functionality, and the resulting hydroxy[4.2.0]bicyclooctenone, which contains four new stereocenters, may be removed from the metal

Polarization of the Pyridine Ring: Highly Functionalized Piperidines from Tungsten−Pyridine Complex

The N-acetylpyridinium complex of {TpW(NO)(PMe3)} undergoes regio- and stereoselective reactions with a broad range of common organic nucleophiles, providing a family of 1,2-dihydropyridine (DHP) complexes of the form TpW(NO)(PMe3)(3,4-η2-DHP). The present study explores the elaboration of these systems into novel piperidines. The addition of an acid to the DHP complexes generates highly asymmetric π-allyl complexes that in turn react with a second nucleophile at either C3 or C5. The subsequent oxidative decomplexation of these materials yields several piperidinamides with unconventional substitution patterns

Tungsten(0) <i>η</i><i>²</i>-Thiophene Complexes: Dearomatization of Thiophene and Its Facile Oxidation, Protonation, and Hydrogenation

Complexes of the form TpW(NO)(PMe3)(η2-thiophene) have been synthesized in 31−40% yield by reduction of TpW(NO)(PMe3)(Br) in the presence of the thiophene ligand. The dynamics of protonation and the subsequent deprotonation for the corresponding 2H-thiophenium complexes have been investigated. Hydrogenation of the uncoordinated double bond was accomplished, and sulfur−carbon, sulfur−oxygen, and carbon−carbon bond-forming reactions were explored with these complexes

Synthesis of a Lactone Diastereomer of the Cembranolide Uprolide D

A convergent stereoselective synthesis of a C1/C14 bis-epimer of uprolide D is described in which an intramolecular Barbier-type reaction was employed for macrocyclization with concomitant introduction of the C1 and C14 stereocenters of a fused α-methylene lactone ring through an anti-Felkin−Anh transition state. Unlike previous examples of allyl chromium additions, none of the Felkin−Anh derived adduct could be detected

Synthesis of a Lactone Diastereomer of the Cembranolide Uprolide D

A convergent stereoselective synthesis of a C1/C14 bis-epimer of uprolide D is described in which an intramolecular Barbier-type reaction was employed for macrocyclization with concomitant introduction of the C1 and C14 stereocenters of a fused α-methylene lactone ring through an anti-Felkin−Anh transition state. Unlike previous examples of allyl chromium additions, none of the Felkin−Anh derived adduct could be detected