Stereoselective synthesis of the cyclic ether core of (+)-laurenyne

A concise enantioselective synthesis of the cyclic ether core of the marine natural product (+)-laurenyne has been accomplished using ring-closing metathesis for medium-ring construction.     

A convenient synthesis of cis and trans 4-tert-butoxycarbonyl-substituted cyclohexylglycine

A novel synthesis of cis and trans substituted 4-tert-butoxycarbonyl cyclohexylglycines via asymmetric aminohydroxylation of vinyl styrene followed by reduction of the aromatic ring and subsequent oxidation is reported.     

Effects of phosphine ligand chelation on the reactivity of monomeric parent amido ruthenium complexes: synthesis and reactivity of such a complex bearing monodentate ligands

The parent amido complex cis-(PMe(3))(4)Ru(H)(NH(2)) (2) has been prepared via the deprotonation of [cis-(PMe(3))(4)Ru(H)(NH(3))(+)][BPh(4)(-)]. The amido complex is a somewhat weaker base than the DMPE analogue trans-(DMPE)(2)Ru(H)(NH(2)) but is still basic enough to quantitatively deprotonate fluorene and reversibly deprotonate 1,3-cyclohexadiene and toluene. Complex 2 exhibits very labile phosphine ligands, two of which can be replaced by DMPE to yield the mixed complex cis-(PMe(3))(2)(DMPE)Ru(H)(NH(2)). Because of the ligand lability, 2 also undergoes hydrogenolysis and rapid exchange with labeled NH(3). The amide complex reacts with alkyl halides to yield E2 and S(N)2 products, along with ruthenium hydrido halide complexes including the ruthenium fluoride cis-(PMe(3))(4)Ru(H)(F). Ruthenium hydrido ammonia halide ion pair intermediates [cis-(PMe(3))(4)Ru(H)(NH(3))(+)][X(-)] are observed in some deprotonation and E2 reactions, and measurement of the equilibrium constants for NH(3) displacement from these complexes suggests that they benefit from significant hydrogen bonding between X(-) and NH(3) groups. Cumulenes also react with complex 2 to afford the products of insertion into an NH bond. The rates of neither these NH insertion reactions nor the reversible deprotonation reactions show any dependence on the concentration of PMe(3) present, suggesting that these reactions take place directly at the NH(2) group and do not involve precoordination of substrate to the metal center.     

Templating and selection in the formation of macrocycles containing [[P(micro-NtBu)(2)](micro-NH)](n) frameworks: observation of halide ion coordination

Amination of [ClP(micro-NtBu)](2) (1) using NH(3) in THF gives the cyclophospha(III)zane dimer [H(2)NP(micro-NtBu)](2) (2), in good yield. (31)P NMR spectroscopic studies of the reaction of 1 with 2 in THF/Et(3)N show that almost quantitative formation of the cyclic tetramer [[P(micro-NtBu)](2)(micro-NH)](4) (3) occurs. The remarkable selectivity of this reaction can (in part) be attributed to pre-organisation of 1 and 2, which prefer cis arrangements in the solid state and solution. The macrocycle 3 can be isolated in yields of 58-67 % using various reaction scales. The isolation of the major by-product of the reaction (ca. 0.5-1 % of samples of 3), the pentameric, host-guest complex [[P(micro-NtBu)(2)](2)(micro-NH)](5)(HCl).2 THF] (4.2 THF), gives a strong indication of the mechanism involved. In situ (31)P NMR spectroscopic studies support a stepwise condensation mechanism in which Cl(-) ions play an important role in templating and selection of 3 and 4. Amplification of the pentameric arrangement occurs in the presence of excess LiX (X=Cl, Br, I). In addition, the cyclisation reaction is solvent- and anion-dependent. The X-ray structures of 2 and 4.2 THF are reported.     

Pyridine N-alkylation by lithium, magnesium, and zinc alkyl reagents: synthetic, structural, and mechanistic studies on the bis(imino)pyridine system

The 2,6-bis(alpha-iminoalkyl)pyridines 2,6-[ArNC(CR(3))](2)C(5)H(3)N [R = H, D; Ar = 2,6-i-Pr(2)C(6)H(3) (DIPP), 2,6-Me(2)C(6)H(3) (DMP)] react with MeLi in Et(2)O to give a binary mixture of products: the pyridine N-methylated species 2,6-[ArNC(CR(3))](2)C(5)H(3)N(Me)Li(OEt(2)) and the deprotonated/dedeuterated species 2-[ArNC(CR(3))],6-[ArNC(=CR(2))]C(5)H(3)NLi(OEt(2)). For R = D, the product ratio is 2:1 in favor of the N-methylated product, while, for R = H, the deprotonated product is favored by 5:1, increasing to 8:1 in toluene solvent. Warming solutions of the N-methylated species leads to clean conversion to the thermodynamically preferred deprotonated species. Crossover experiments show that MeLi is re-formed and dissociates from the terdentate ligand before deprotonating the ketimine methyl unit. For MgR(2) (R = Et, i-Pr) and ZnR(2) (R = Et) reagents, N-alkylation products are formed exclusively, but derivatives containing bulky aryl substituents are found to undergo further rearrangement to 2-alkylated species, arising by migration of the alkyl group of the N-alkyl moiety to the adjacent ring carbon atom. The reversibility of the N-alkylation process has been probed using deuterio-labeled Mg alkyl reagents and mixed alkyl zinc species. A cationic zinc derivative is shown to undergo "reverse" alkyl migration, from the heterocycle nitrogen atom to the zinc center. EPR spectroscopy reveals a paramagnetic intermediate in which the unpaired electron is delocalized over the heterocycle and di-imine moieties of the ligand, indicating that the N-alkylation reactions proceed via single electron-transfer processes.     

Stereoselective synthesis of (8R,8aS)-8-methylhexahydroindolizin-5-one

Catalytic hydrogenation of dihydroindolizidinone occurred preferentially from the endo-face giving rapid entry to (8R,8aS)-8-methylhexahydroindolizin-5-one, a key intermediate in the synthesis of 5,8-disubstituted indolizidines and deoxypumiliotoxin 251H. The selectivity could be improved further by diimide reduction though this also resulted in some oxidation of the alkene to the diene. The basis of the unusual stereoselectivity in the diimide reduction is believed to be stereoelectronic in origin.     

Reactions of organoboranes and 2-lithio-2-alkyl-1,3-benzodithioles. A new,improved synthesis of ketones

Reactions of trialkylboranes with 2-lithio-2-alkyl-1,3-benzodithioles followed by oxidation give ketones in good yields. This method is far less subject to steric hindrance than the analogous reactions using anions derived from bis(phenylthio)alkanes.     

Eu(III)-cyclen-phen conjugate as a luminescent copper sensor: the formation of mixed polymetallic macrocyclic complexes in water

The cationic cyclen based Eu(III)-phen conjugated 1.Eu was synthesised as a chemosensor for Cu(II), where the recognition in water at pH 7.4 gave rise to quenching of the Eu(III) luminescence and the formation of tetranuclear polymetallic Cu(II)-Eu(III) macrocyclic complexes in solution where Cu(II) was bound by three 1.Eu conjugates.     

Ruminant pregastric lipases: experimental evidence of their potential as industrial catalysts in food technology

The historical importance of pregastric enzymes in cheese-making is reviewed and the potential for extending their use is discussed in terms of requiring an understanding of their physicochemical parameters. Commericial extracts from the tongues and epiglotti of suckling lambs and calves and adult goats have been processed to yield partially purified samples of the primary pregastric lipase (PGL). The N-terminal sequence and molecular weight of lamb PGL have been determined.

The activity of lamb and goat PGLs against tributyrin has been determined over a range of pH and temperature values. Optimum conditions were pH 6.4, 43°C, and pH 6.0, 52°C, for lamb and goat PGL respectively. The possible influence of the development of a ruminant multi-chambered stomach on the difference in optimal temperature is discussed. A lengthening of the carboxylic acid chain of homoacid triglycerides causes a decrease in hydrolytic activity of lamb PGL but in all cases only a single free fatty acid was released. Against a series of 4-nitrophenylalkanoate esters, maximum activity was observed against the decanoate ester but, in contrast to hydrolysis of the acetate ester which exhibited full Michaelis-Menten kinetics with increasing substrate concentration, activity against the decanoate ester was restricted to the monomeric substrate. Taurocholate inhibits the activity of lamb PGL at concentrations >8 mM. Values of pK₂ equal to 6.69 and 7.92 respectively have been determined for lamb PGL.

Attempts to interesterify coconut oil and cocoa butter, and tributyrin and tricaprylin, catalysed by calf PGL were unsuccessful, although positive results obtained using Candida cylindracea encourage further investigation of alternative methods for immobilizing the PGL. Finally, anhydrous milk fat has been hydrolysed by calf, lamb and goat PGLs and the differences in relative amounts of released free fatty acids have been used to explain the differences in taste which arise when Parmesan cheese is produced using different sources of PGL.     

Unexpected stability of aryl beta-N-acetylneuraminides in neutral solution: biological implications for sialyl transfer reactions

A reagent panel comprised of seven aryl beta-D-N-acetylneuraminides was synthesized and then used to probe the mechanisms of nonenzymatic hydrolysis. These reactions proceeded via four independent pathways: (1) acid-catalyzed hydrolysis of the neutral molecule; (2) acid-catalyzed hydrolysis of the anionic form, or its kinetic equivalent spontaneous hydrolysis of the neutral form; (3) spontaneous hydrolysis of the anionic form; and (4) a base-promoted pathway. The pH-independent spontaneous hydrolysis of 4-nitrophenyl alpha-D-N-acetylneuraminide (5) occurs at a rate that is over 100 times faster than that of the corresponding reaction of 4-nitrophenyl beta-D-N-acetylneuraminide (4a). Spontaneous hydrolyses of four aryl beta-D-N-acetylneuraminides displayed a beta(lg) value of -1.24 +/- 0.16 (pH = 8.1, T = 100 degrees C), and at a pH value of 1.0 (50 degrees C), all seven panel members gave a beta(lg) value of 0.14 +/- 0.08. The aqueous ethanolyses of 4a and 5 gave similar products and displayed sensitivity parameters (m) in a standard Winstein-Grunwald analysis of -0.04 +/- 0.01 and +0.23 +/- 0.02, respectively. These results, plus the activation parameters calculated for the spontaneous hydrolyses of the anionic forms of 5 (DeltaH() = 116 +/- 2 kJ mol(-1) and DeltaS = 27 +/- 4 J mol(-1) K(-1)) and 4a (DeltaH = 138 +/- 3 kJ mol(-1) and DeltaS = 59 +/- 8 J mol(-1) K(-1)), are inconsistent with anomeric carboxylate assistance occurring during the hydrolysis reactions, and the likely cause for the enhanced reactivity of 5 in comparison to that of 4a is an increase in ground-state steric strain.