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At least 19 recordsLinked to original sources
PubMed2004

Mechanistic studies of the transfer dehydrogenation of cyclooctane catalyzed by iridium bis(phosphinite) p-XPCP pincer complexes.

Reaction of bis(phosphinite) PCP iridium pincer complexes (p-XPCP)IrHCl (5a-f) [X = MeO (5a), Me (5b), H (5c), F (5d), C(6)F(5) (5e), Ar(F)(= 3,5-bis(trifluoromethyl)phenyl) (5f)] with NaOtBu in neat cyclooctane (COA) generates 1:1 mixtures of the respective (p-XPCP)IrH(2) complexes 4a-f and the cyclooctene (COE) olefin complexes (p-XPCP)Ir(COE) (6a-f) at 23 degrees C. At higher temperatures, complexes 4 and 6 are equilibrated because of the degenerate transfer dehydrogenation of COA with free COE (6 + COA right harpoon over left harpoon 4 + 2COE), as was shown by temperature-dependent equilibrium constants and spin saturation transfer experiments at 80 degrees C. At this temperature, the COE complexes 6 exchange with free COE on the NMR time scale with the more electron-deficient complexes 6 exchanging COE faster. The exchange is dissociative and zero order in [COE]. Further analysis reveals that the stoichiometric hydrogenation of COE by complex 4f, and thus the separated back reaction 4f + 2COE --> 6f + COA proceeds at temperatures as low as -100 degrees C with the intermediacy of two isomeric complexes (p-Ar(F)PCP)Ir(H)(2)(COE) (8f, 8f'). COE deuteration with the perdeuterated complex 4f-d(38) at -100 degrees C results in hydrogen incorporation into the hydridic sites of complexes 8f,8f'-d(38) but not in the hydridic sites of complex 4f-d(38), thus rendering COE migratory insertion in complexes 8f,8f' reversible and COE coordination by complex 4f rate-determining for the overall COE deuteration.

Catalysis↗
PubMed2006

From common carbohydrates to enantiopure cyclooctane polyols and glycomimetics via deoxygenative zirconocene ring contraction.

D-Arabinose and D-glucose are transformed into the identical vinyl furanoside, whose role is to serve as the precursor to enantiopure cyclooctadienone 6. The key steps of this relay involve a zirconocene-promoted ring contraction and [3,3] sigmatropic rearrangement of an enynol. Subsequently defined are convenient synthetic routes to several cyclooctane-1,2,3-triols, 1,2,3,4,5-pentaols, and structurally related glycomimetics.

Alcohols↗
PubMed2000

A [4 + 4] 2-pyridone approach to taxol. 3. Stereocontrol during elaboration of the cyclooctane.

Intramolecular photocycloaddition of 2-pyridones connected through a four-carbon tether (6-[4-(1,2-dihydro-1-methyl-2-oxo-3-pyridinyl)-4-[[(1,1-dimethylethyl)++ +dimethylsilyl]oxy]butyl]-4-methoxy-1,3-dimethyl-2(1H)-pyridinone) yields a single tetracyclic product with four new stereogenic centers. The diastereoselectivity of this [4 + 4] reaction is fully controlled by a stereogenic carbon of the tether. Treatment of the photoproduct with osmium tetraoxide transforms the alkene to a diol and the enol ether to an alpha-hydroxy ketone, with stereocontrol dictated by nearby lactams that block one face of each alkene. Allylmagnesium bromide addition to the ketone also yields a single diastereomer, but unexpectedly this product results from approach of the nucleophile to the most-hindered face of the ketone. Study of this reaction in a model system has found the allylic nucleophile to be unique, with nonallylic reagents approaching along the expected, least-hindered path. This contrasteric addition likely results from coordination of the allylic nucleophile to the nearby amide. The amide can therefore act either as a steric shield or as a directing group. The three steps of photocycloaddition, cis-hydroxylation, and nucleophilic addition constructs both quaternary carbons of the cyclooctane and four of the five stereogenic centers found in the eight-membered ring of Taxol.

Antineoplastic Agents, Phytogenic↗
PubMed1996

Bridged to Fused Ring Interchange. Methodology for the Construction of Fused Cycloheptanes and Cyclooctanes. Total Syntheses of Ledol, Ledene, and Compressanolide.

The type two intramolecular Diels-Alder reaction (T2IMDA) is an efficient method for the formation of medium rings. The methodology is particularly effective for the construction of seven- and eight-membered rings. A strategy for the synthesis of functionalized cycloheptanes and cyclooctanes has been developed that involves a bridged to fused ring interchange. The T2IMDA provides a synthesis for rigid bridged bicyclic molecules that can be stereoselectively elaborated before ozonolysis of the bridgehead double bond. Following oxidative cleavage, aldol condensation provides fused bicyclic ring systems that otherwise are difficult to synthesize. This methodology is amenable to the synthesis of terpene natural products. This is demonstrated here through total syntheses of (+/-)-ledol and (+/-)-ledene and a formal synthesis of (+/-)-compressanolide.

Journal Article↗
PubMed1999

Flexibility vs Rigidity of Singly and Doubly Tethered Biphenyls: Structure, Dynamic Stereochemistry, and Resolution of Tribenzo[a,c,f]cyclooctane, Tetrabenzo[a,de,h,kl]bicyclo[6.6.0]tetradecane, and Their Alkyl Derivatives.

The barrier for enantiomerization of tribenzo[a,c,f]cyclooctane (1a) as acquired from dynamic (1)H NMR experiments was found to be DeltaG() = 19.5-20.1 kcal mol(-)(1), and DeltaG() = 17.2-17.6 kcal mol(-)(1) for its alkylated derivatives (1b-e) at the coalescence temperatures. Tetrabenzo[a,de,h,kl]bicyclo[6.6.0]tetradecane (2a) and its tetraethyl derivative (2b) were separated to their enantiomers by chiral HPLC. The kinetic values for the enantiomerization were obtained by following the racemization rate of the pure enantiomer. Barriers of DeltaG()(596) = 49.6 kcal mol(-)(1) and DeltaG()(561) = 46.3 kcal mol(-)(1) for 2a and 2b, respectively, were obtained. Strong solvent and temperature effects on the (1)H NMR chemical shifts were observed. X-ray crystallography revealed torsional conformations for 1a, 1b, and 2a with dihedral angles of 57.8 degrees, 56.1 degrees /58.2 degrees (two conformations), and 57.7 degrees, respectively. Semiempirical calculations (AM1) reveal higher relative strain energies and smaller biphenyl dihedral angles for the bulky alkyl substituents, which correspond to their lower DeltaG() values.

Journal Article↗
PubMed2005

Enantioselective route from carbohydrates to cyclooctane polyols.

[reaction: see text] A synthetic route to select cyclooctane-1,2,3-triols and 1,2,3,4,5-pentaols has been defined. The starting materials are d-glucose or d-arabinose, and the key steps consist of a zirconocene-promoted ring contraction, a [3,3] sigmatropic rearrangement, and more extended functionalization of the resulting cyclooctadienone.

Arabinose↗
PubMed1983

Loss of hepatic monooxygenase activities, glutathione, and 'green pigment' formation after the administration of vinyl-cyclooctane to mice.

Vinylcyclooctane, when administered to mice at 500 mg/kg, produced reduction of microsomal cytochrome P-450, heme, aminopyrine-N-demethylase and ethoxycoumarin-O-deethylase activities with respect to control values; furthermore the hepatic reduced glutathione level was depleted suggesting that glutathione is involved in the vinylcyclooctane metabolism. The reduction of cytochrome P-450 and monooxygenase activities was accompanied by the formation of abnormal 'green pigments'.

Animals↗
PubMed2005

Exploratory studies aimed at a synthesis of vinigrol. 2. Attempts to exploit ring-closing metathesis for construction of the central cyclooctane belt.

A program directed to the possible adaptation of ring closing metathesis to a total synthesis of vinigrol is described. With a convenient route to intermediates of general type 3 available from a prior investigation, several candidate substrates were prepared. These included the epoxy dienes 10 and 22, the diacetoxy triene 42, and the heavily functionalized cyclohexane 48. The central issue of this approach was to convey a maximum degree of conformational flexibility to these functionalized intermediates, such that the olefinic termini of the side chains could enter into intramolecular carbon-carbon bond formation. In no example was ring closure observed to operate. Instead, the strategically placed pi-bonds were seen to migrate internally to the chain in select examples. Although the pivotal transformations failed, the deployment of a number of useful stereo-controlled reactions has ultimately resulted in the preparation of heavily substituted cis-decalins.

Cyclization↗
PubMed2004

Peptide helices with pendant cycloalkane rings. Characterization of conformations of 1-aminocyclooctane-1-carboxylic acid (Ac8c) residues in peptides.

A pentapeptide, Boc-Leu-Ac8c-Ala-Leu-Ac8c-OMe 1, an octapeptide, Boc-Leu-Ac8c-Ala-Leu-Ac8c-Ala-Leu-Ac8c-OMe 2 and a tripeptide, Boc-Aib-Ac8c-Aib-OMe 3 containing the 1-aminocyclooctane-1-carboxylic acid residue (Ac8c) were synthesized and conformationally characterized by x-ray diffraction studies in the crystal state. Peptides 1 and 2 were also studied by NMR in CDC13 solution. Peptide 1 adopts a purely 3(10)-helical conformation in crystals, stabilized by three intramolecular 1 <-- 4 hydrogen bonds. Peptide 2 in crystals is largely 3(10)-helical with distortion in the backbone at the N-terminus by the insertion of a water molecule between Ac8c (2) CO and Ala (6) NH groups. Peptide 3 forms a C10-ring structure, i.e. a type III (III') beta- turn conformation stabilized by an intramolecular 1 <-- 4 hydrogen bond. Five cyclooctane rings assume boat-chair conformations, whereas the sixth [Ac8c(8) in 2] is appreciably distorted, resembling a chiral intermediate in the pseudorotational pathway from the boat-chair to the twisted boat-chair conformation. Internal bond angles of the cyclooctane rings are appreciably distorted from the tetrahedral value, a characteristic feature of the cyclooctane ring. Peptide 1 crystallized in the space group P212121 with a = 11.900(4) A, b = 18.728(6) A, c = 20.471(3) A and Z = 4. The final R1 and wR2 values are 0.0753 and 0.2107, respectively, for 3901 observed reflections [Fo > or = 3 sigma (Fo)]. Peptide 2 crystallized in space group P21 with a = 12.961(5) A, b = 17.710(10) A, c = 15.101(7) A, beta = 108.45(4) degrees and Z = 2. The final R1 and wR2 values are 0.0906 and 0.1832, respectively, for 2743 observed reflections [Fo > or = 3sigma (Fo)]. 1H-NMR studies on both the peptides strongly suggest the persistence of 3(10)-helical conformations in solution. Peptide 3 crystallized in the space group P21/n, with a = 10.018(1) A, b = 20.725(1) A, c = 12.915(1) A and Z = 4. The final R1 and wR2 values are 0.0411 and 0.1105, respectively, for 3634 observed reflections [Fo > or = 4sigma (Fo)].

Amino Acids, Cyclic↗
PubMed1996

Anesthetic and convulsant properties of aromatic compounds and cycloalkanes: implications for mechanisms of narcosis.

We examined the anesthetic and convulsant properties of 16 unfluorinated to completely fluorinated aromatic compounds, having six to nine carbon atoms (e.g., benzene to 1,3,5-tris(trifluoromethyl)benzene), and four cycloalkanes (cyclopentane to cyclooctane). Benzene, fluorobenzene, toluene, p-xylene, ethylbenzene, and cyclopentane caused excitation (twitching, jerking, and hyperactivity), and three aromatic compounds (perfluorotoluene, p-difluorotoluene and 1,3,5-tris(trifluoromethyl)benzene) and three cycloalkanes (cyclohexane, cycloheptane, and cyclooctane) produced convulsions. Cyclooctane and 1,3,5-tris(trifluoromethyl)benzene were nonanesthetics. Except for nonanesthetics and perfluorotoluene (too toxic to test for anesthetic potency), all compounds produced anesthesia or decreased the minimum alveolar anesthetic concentration of desflurane. Aromatic compounds were more potent and lipid-soluble than n-alkanes (data from previous report) and cycloalkanes. All three series increasingly disobeyed the Meyer-Overton hypothesis as molecular size increased. For a particular number of carbons (e.g., cyclohexane, n-hexane, and benzene), the deviation was cycloalkanes > or = normal alkanes > aromatic compounds. These results suggest that molecular shape (including "bulkiness") and size provide limited clues to the structure of the anesthetic site of action.

Anesthesia, Inhalation↗
PubMed2006

Transition-metal (Ti,V,Cr,Mn,Fe,Co,Cu) containing ordered nanoporous materials: novel heterogeneous catalysts for selective oxidation reactions.

Transition-metal (Ti,V,Cr,Mn,Fe,Co,Cu) containing periodic nanoporous catalysts, were synthesized hydrothermally and characterized using various analytical and spectroscopic techniques. The catalytic performance of the different catalysts was systematically evaluated with a detailed study on TiMCM-41. All the catalysts showed promise for the selective oxidation of cycloalkanes, viz., cyclohexane, cyclooctane, and cyclododecane, under mild reaction conditions. Furthermore, these mesoporous molecular sieves were also active for all the chosen reactions even after several recycling and/or washing treatments. Among the various materials under investigation, the catalysts TiMCM-41 and VMCM-41 showed much higher substrate conversion and excellent product selectivity in conjunction with a minimal leaching of the active species. More importantly, the influence of pore size on the catalytic activity of the bulkier substrates such as cyclooctane and cyclododecane is demonstrated.

Catalysis↗
PubMed2005

Conformational investigations on analogs of inflammation response inducing chemotactic tripeptide fMLP.

Conformations of three analogs of for-L-Met-L-Leu-L-Phe-OH (fMLP), which initiates inflammatory response by interaction with the formyl peptide receptor (FPR), have been investigated by the application of the X-ray crystallographic technique. The investigated analogs of fMLP peptides are as follows: for-L-Met-1-amino-1-cyclooctane-carbonyl(Ac8c)-L-Phe-OMe; for-L-Met-L-Leu-L-p-iodo-Phe-OH; and for-L-Met-di-n-propylglycyl(Dpg)-L-Phe-OMe. The peptide backbone in and is constrained at position of fMLP by the introduction of Calpha,alpha-disubstituted glycines. In peptide, Phe-OMe is substituted by p-iodo-Phe-OH. Crystal structures reveal an overall folded conformation adopted by and. The former is folded in the type II beta-turn, which is stabilized by an intramolecular 1<--4 (formyl) C==O...H--N (Phe) hydrogen bond, whereas the latter is folded in an open turn without any intramolecular hydrogen bond. On the other hand, peptide has an extended conformation, and two different molecules in a crystallographic asymmetric unit form an antiparallel beta-sheet-like structure. In and, residues Ac8c and Dpg adopt left-handed helical and fully extended (C5) conformations, respectively. The cyclooctane ring in Ac8c acquires a boat-chair conformation. Crystal packing of is characterized by the association of aliphatic-aromatic rings via a C--H...pi interaction. In the crystal of, contrary to the usual observations, peptides are interlinked via networks of head-to-tail hydrogen bond and pi...pi interactions, which are generally observed to be mutually exclusive. The structure-function mechanism of the ligand-receptor interaction is discussed.

Chemotactic Factors↗
PubMed2004

Aromatic vs aliphatic C-H cleavage of alkyl-substituted pyridines by (PNPiPr)Re compounds.

Both (PNP)Re(H)(4) and (PNP)ReH(cyclooctyne) (PNP(i)(Pr) = ((i)Pr(2)PCH(2)SiMe(2))(2)N) react with alkylpyridines NC(5)H(4)R to give first (PNP)ReH(2)(eta(2)-pyridyl) and cyclooctene and then, when not sterically blocked, (PNP)Re(eta(2)-pyridyl)(2) and cyclooctane. The latter are shown by NMR, X-ray diffraction, and DFT calculations to have several energetically competitive isomeric structures and pyridyl N donation in preference to PNP amide pi-donation. DFT studies support NMR solution evidence that the most stable bis pyridyl structure is one that is doubly eta(2)- with the pyridyl N donating to the metal center. When both ortho positions carry methyl substituents, cyclooctane and the carbyne complex (PNP)ReH(tbd1;C-pyridyl) are produced. Excess 2-vinyl pyridine reacts with (PNP)Re(H)(4) preferentially at the vinyl group, to give 2-ethyl pyridine and the sigma-vinyl complex (PNP)ReH[eta(2)-CH=CH(2-py)]. The DFT and X-ray structures show, by various comparisons, the ability of the PNP amide nitrogen to pi-donate to an otherwise unsaturated d(4) Re(III) center, showing short Re-N distances consistent with the presence of pi-donation.

Journal Article↗