Acid-Catalyzed Rearrangements of 5,6-exo-Epoxy-7-oxabicyclo[2.2.1]hept-2-yl Derivatives. Migratory Aptitudes of Acyl vs. Alkyl Groups in Wagner-Meerwein Transpositions

In the presence of HSO₃F/Ac₂O in CH₂CL₂, 2-exo- and 2-endo-cyano-5,6-exo-epoxy-7-oxabicyclo[2.2.1]hept-2-yl acetates ( 6a , b ) gave products derived from the epoxide-ring opening and a 1,2-shift of the unsubstituted alkyl group (σ bond C(3)–C(4)). In contrast, under similar conditions, the 5,6-exo-epoxy-7-oxabicyclo[2.2.1]heptan-2-one ( 6c ) gave 5-oxo-2-oxabicyclo[2.2.1]heptane-3,7-diyl diacetates 20 and 21 arising from the 1,2-shift of the acyl group. Acid treatment of 5,6-exo-epoxy-2,2-dimethoxy-7-oxabicyclo[2.2.1]heptane ( 6d ) and of 5,6-exo-epoxy-2,2-bis(benzyloxy)-7-oxabicyclo[2.2.1]heptane ( 6e ) gave minor products arising from epoxide-ring opening and the 1,2-shift of σ bond C(3)–C(4) and major products ( 25 , 29 ) arising from the 1,3-shift of a methoxy and benzyloxy group, respectively. Under similar conditions, 5,6-exo-epoxy-2,2-ethylenedioxy-7-oxabicyclo[2.2.1]heptane ( 6f ) gave 1,1-(ethylenedioxy)-2-(2-furyl)ethyl acetate ( 32 , major) and a minor product 33 , arising from the 1,2-shift of σ bond C(3)–C(4). The following order of migratory aptitudes for 1,2-shifts toward electron-deficient centers has been established: acyl > alkyl > alkyl α-substituted with inductive electron-withdrawing groups. This order is valid for competitive Wagner-Meerwein rearrangements involving equilibria between carbocation intermediates with similar exothermicities.     

Organosulfur compounds: electrophilic reagents in transition-metal-catalyzed carbon-carbon bond-forming reactions

Transition-metal-catalyzed carbon-carbon bond-forming reactions are among the most powerful methods in organic synthesis and play a crucial role in modern materials science and medicinal chemistry. Recent developments in the area of ligands and additives permit the cross-coupling of a large variety of reactants, including inexpensive and readily available sulfonyl chlorides. Their desulfitative carbon-carbon cross-coupling reactions (Negishi, Stille, carbonylative Stille, Suzuki-Miyaura, and Sonogashira-Hagihara-type cross-couplings and Mizoroki-Heck-type arylations) are reviewed together with carbon-carbon cross-coupling reactions with other organosulfur compounds as electrophilic reagents.     

A New Total Synthesis of D-threo-L-talo-Octose

A new approach to the total, asymmetric synthesis of D -threo-L -talo-octose ((?)- 1 ) and its derivatives is presented. It is based on the chemoselective Wittig-Horner monoolefination of a 5-deoxy-D -ribo-hexodialdose derivative 4 obtained by selective reduction of (?)-5-deoxy-2.3-O-isopropylidene-/β-D -ribo-hexofuranurono-6,1-lactone ((?)- 3 ). Allylic bromination of the resulting methyl (E)-oct-6-enofuranuronate (+)- 5 followed by intramolecular nucleophilic displacement of the so-obtained bromides gave a 13.3:1 mixture of (?)-methyl (E)-l,4-anhydro-6,7-dideoxy-2,3-O-isopropylidene-β-L -talo-oct-6-enopyranuronate ((?)- 8 ) and methyl (E)-l,4-anhydro-6,7-dideoxy-2,3-O-isopropylidene-α-D -allo-oct-6-enopyranuronate ( 9 ). The double hydroxylation of the enoate (?)- 8 followed Kishi's rule and gave the corresponding D -threo-β-L -talo-octopyranuronate derivative (?)- 11 with a good diastereoselectivity. Reduction of ester (?)- 11 and deprotection led to pure (?)- 1 .     

Lipid modifications of a Ras peptide exhibit altered packing and mobility versus host membrane as detected by 2H solid-state NMR

The human N-ras protein binds to cellular membranes by insertion of two covalently bound posttranslational lipid modifications, which is crucial for its function in signal transduction and cell proliferation. Mutations in ras may lead to unregulated cell growth and eventually cancer, making it an important therapeutic target. Here we have investigated the molecular details of the membrane binding mechanism. A heptapeptide derived from the C-terminus of the human N-ras protein was synthesized including two hexadecyl modifications. Solid-state 2H NMR was used to determine the packing and molecular dynamics of the ras lipid chains as well as the phospholipid matrix. Separately labeling the chains of the peptide and the phospholipids with 2H enabled us to obtain atomically resolved parameters relevant to their structural dynamics. While the presence of ras only marginally affected the packing of DMPC membranes, dramatically lower order parameters (S(CD)) were observed for the ras acyl chains indicating modified packing properties. Essentially identical projected lengths of the 16:0 ras chains and the 14:0 DMPC chains were found, implying that the polypeptide backbone is located at the lipid-water interface. Dynamical properties of both the ras and phospholipid chains were determined from spin-lattice 2H relaxation (R1Z) measurements. Plots of R1Z rates versus the corresponding squared segmental order parameters revealed striking differences. We propose the ras peptide is confined to microdomains containing DMPC chains which are in exchange with the bulk bilayer on the 2H NMR time scale (approximately 10(-5) s). Compared to the host DMPC matrix, the ras lipid modifications are extremely flexible and undergo relatively large amplitude motions. It is hypothesized that this flexibility is a requirement for the optimal anchoring of lipid-modified proteins to cellular membranes.     

Specific labeling of cell surface proteins with chemically diverse compounds

The specific and covalent labeling of fusion proteins with synthetic molecules opens up new ways to study protein function in the living cell. Here we present a novel method that allows for the specific and exclusive extracellular labeling of proteins on the surfaces of live cells with a large variety of synthetic molecules including fluorophores, protein ligands, or quantum dots. The approach is based on the specific labeling of fusion proteins of acyl carrier protein with synthetic molecules through post-translational modification catalyzed by phosphopantetheine transferase. The specificity and versatility of the labeling should allow it to become an important tool for studying and manipulating cell surface proteins and for complementing existing approaches in cell surface engineering.     

Control of the Diels-Alder-Addition Regioselectivity by Remote Olefins. Syntheses and Cycloadditions of 2,3,5-Trimethylidenebicyclo[2.2.1]heptane and 2,3,5,6,7-Pentamethylidenebicyclo[2.2.2]octane

The syntheses of 2,3,5-trimethylidenebicyclo[2.2.1]heptane ( 1 ) and 2,3,5,6,7-pentamethylidenebicyclo[2.2.2]-octane ( 2 ) are reported. The Diels-Alder additions of the diene moieties of these polyenes can be regioselective, probably because of a possible transannular interaction between the homoconjugated methylidene and s-cis-buta-diene groups.     

The expected magnitude of simultaneous parity and time violation in nuclear systems

Time (T) and simultaneous parity-time (PT) symmetry violations in the nuclear system generally cause the gamma ray multipole mixing ratios to acquire imaginary components. These complex phases may then be measured experimentally by virtue of the resulting gamma ray distributions. However, the true significance of such experiments may only be assessed once the imaginary mixing ratios have been related theoretically to the coupling constant of some fundamental, symmetry violating, interaction. We discuss, in a quantitative way, the various aspects of this relationship. To illustrate this further we examine the case of the 1189 keV transition in¹⁸²W. For this transition we predict the experimentally observable mixing ratio to have a parity (P) violating real part of ≅5×10⁻⁵ and a PT violating imaginary part of , where is the strength of the isovector PT violating pion-nucleon coupling. An upper limit to this coupling of ≲3×10⁻¹⁰ may be obtained from the electric dipole moment of the neutron.     

Face Selectivity of the Diels-Alder Additions of Sulfur-Substituted Dienes and Tetraenes Grafted onto 7-Oxabicyclo[2.2.1]heptanes

Stereoselective synthesis of 2-methylidene-3-[(Z)-(2-nitrophenylsulfenyl)methylidene]-7-oxabicyclo[2.2.1]-heptane ( 16 ), 1,4-epoxy-1,2,3,4-tetrahydro-5,8-dimethoxy-2-methylidene-3-[(Z)-(2-nitrophenylsulfenyl)methylidene]anthracene ( 18 ), and 1,4-epoxy-1,2,3,4-tetrahydro-5,8-dimethyoxy-2-methylidene-3-[(Z)-(phenylsulfenyl)-methylidene]anthracene ( 19 ) are presented. The Diels-Alder additions of these S-substituted dienes and those of 2,5-dimethylidene-3,6-bis{[(Z)-(2-nitrophenyl)sulfenyl]methylidene}-7-oxabicyclo[2.2.1]heptane ( 17 ) have been found to be face selective and ‘ortho’ regiospecific. The face selectivity depends on the nature of the dienophile. It is exo-face selective with bulky dienophiles such as ethylene-tetracarbonitrile (TCNE) and 2-nitro-1-butene and endo-face selective with methyl vinyl ketone, methyl acrylate, and 3-butyn-2-one. In the presence of a Lewis acid, the face selectivity of the Diels-Alder reaction can be reversed. The addition of the first equivalent of a dienophile to tetraene 17 is at least 100 times faster than the addition of the second equivalent of the same dienophile to the corresponding mono-adduct. The X-ray structure of the crystalline bis-adduct 43 , a 7-oxabicyclo[2.2.1]hepta-2,5-diene system annellated to two cyclohexene rings, resulting from the successive additions of methyl acrylate and methyl vinyl ketone to tetraene 17 is presented. Only one of the two endocyclic double bonds of the 7-oxabicyclo[2.2.1]hepta-2,5-diene deviates from planarity, the substituents bending towards the endo face by 5.7°.     

Stereoselective Double Functionalization of Iron-Carbonyl Complexes of 5,6,7,8-Tetramethylidenebicyclo[2.2.2]oct-2-ene. Crystal Structure and Absolute Configuration of (–)-trans-μ-[(2S,5R,7S)-C,5,6,C-η:C,7,8,C-η-(6,7,8-trimethylidene-5-((Z)-2-oxopropylidene)-2-bicyclo[2.2.2]octyl acetate)]-bis(tricarbonyliron)

The Friedel-Crafts monoacylation of trans-η-[(1RS,2RS,4SR,5SR,6RS,7SR,8SR)-C,5,6,C-η:C,7,8,C-η-(5,6,7,8-tetramethylidene-2-bicyclo[2.2.2]octyl acetate)]-bis(tricarbonyliron) ((±)- 5 ) is highly stereoselective and yields trans-η-[(1RS,2RS,4RS,5SR,6RS,7RS,8SR)-C,6-η,oxo-σ:C,7,8,C-η-(6,7,8-trimethylidene-5-((Z)-2-oxopropylidene)-2-bicyclo[2.2.2]octyl acetate)]-bis(tricarbonyliron) ((±)- 8 ) which equilibrates with the trans-η-[(1RS,2RS,4RS,5SR,6RS,7RS,8SR)-C,5,6,C-η:C,7,8,C-η-(6,7,8-trimethylidene-5-((Z)-2-oxopropylidene)-2-bicyclo[2.2.2]octyl acetate)]-bis(tricarbonyliron) ((±)- 9 ) on heating. Optically pure (–)- 9 has been prepared from the corresponding optically pure alcohol (+)- 4 . The structure and absolute configuration of (–)- 9 was established by single-crystal X-ray diffraction.