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At least 1,099 records · Page 61Linked to original sources
PubMed1997

Synthesis of Linear and Angular Triquinane Skeletons by O-Stannyl Ketyl-Promoted Fragmentation-Cyclization Reactions of alpha-Keto Cyclopropanes.

Several investigations of rigid alpha-keto cyclopropane cleavage by O-stannyl ketyls are summarized herein. Tricyclo[3.3.0.0(2,8)]octan-3-one ring systems were treated with nBu(3)SnH, which produced different ring-cleavage products depending on the location and type of substituent present. An examination of both radical-stabilizing substituents and stereoelectronic factors was initiated to understand what factors bias bond cleavage in a configurationally restricted alpha-ketocyclopropane via O-stannyl ketyls. A preference for cleavage of the cyclopropane bond with the best orbital overlap with the ketyl radical sp(2)-orbital even in the presence of radical stabilizing groups is indicated by these results. An O-stannyl ketyl ring scission-cyclization resulted in the novel synthesis of either a linear or an angular triquinane skeleton depending on the length and location an alkene tether on the tricyclo[3.3.0.0(2,8)]octan-3-one precursor.

Journal Article↗
PubMed1998

Synthesis of the Pentacyclic Skeleton of the Aspidosperma Alkaloids Using Rhodium Carbenoids as Reactive Intermediates.

A series of diazo amido keto esters prepared from N-alkenyl-substituted 3-carbalkoxy-2-piperidone derivatives was treated with rhodium(II) acetate. Attack of the amido carbonyl oxygen at the resultant rhodium carbenoid center produced a transient push-pull carbonyl ylide dipole which underwent an intramolecular dipolar cycloaddition reaction. A related annulation sequence was used to prepare the pentacyclic skeleton of the aspidosperma family of alkaloids. Synthesis of the required diazo imide was carried out from 3-carboxy-3-ethyl-2-piperidone and N-methyl-3-indoleacetic acid. Treatment of the diazo imide with rhodium(II) acetate afforded a transient 1,3-dipole which subsequently underwent cycloaddition across the indole pi-bond. The resulting cycloadduct is the consequence of endo cycloaddition with respect to the dipole which is fully in accord with the lowest energy transition state. The cycloadduct was converted in three steps into desacetoxy-4-oxo-6,7-dihydrovindorosine. The stereochemistry of the final product was established by a X-ray crystallographic study.

Journal Article↗
PubMed1998

Unusual Terpenes with Novel Carbon Skeletons from the West Indian Sea Whip Pseudopterogorgia elisabethae (Octocorallia)(1).

From the hexane solubles of the West Indian gorgonian Pseudopterogorgia elisabethae collected near San Andrés Island, Colombia, were isolated a marine diterpenoid, two nor-diterpenoids, and a bisnor-diterpenoid, all of which possess most unusual carbocyclic skeletons. The structures and relative configurations of novel metabolites elisabethins A-C (1-3) and elisabanolide (4) were elucidated by interpretation of overall spectral data, which included 2D NMR correlation methods; IR, UV, and accurate mass measurements (HREIMS); chemical reactions; and X-ray diffraction analyses. One of these, elisabethin B (2), showed significant differential antitumor activity, and compounds 3 and 4 have weak in vitro antituberculosis activity.

Journal Article↗
PubMed1999

Nucleobase Recognition by Artificial Receptors Possessing a Ferrocene Skeleton as a Novel Modular Unit for Hydrogen Bonding and Stacking Interactions.

Novel ferrocene-modified artificial nucleobase receptors were designed and synthesized. The nucleobase receptors possess hydrogen-bonding and pi-stacking interaction sites that act simultaneously for binding to 1-butylthymine utilizing the pivot character of the ferrocene skeleton. Diamidopyridine was chosen for the hydrogen-bonding moiety, and various polynuclear aromatics were used for pi-stacking one. The two components were tethered to the cyclopentadienyl rings via ethynediyl and oxymethylene spacers. The binding affinity of the receptors to 1-butylthymine was found to be dependent on the aromatic structures. Thus, the association constants for perylene-linked receptors were approximately doubled compared to those of aromatic-free ones, an energy difference of approximately 0.5 kcal/mol. Detailed comparisons between the 10 receptors clarified the value of the pivot character of the ferrocene for construction of the intermolecular interaction site.

Journal Article↗
PubMed2006

Hybrid organic semiconductors including chalcogen atoms in pi-conjugated skeletons. Tuning of optical, redox, and vibrational properties by heavy atom conjugation.

In this work, the interactions between heteroatoms (S, Se, and Te) and conjugated skeletons are analyzed. The study is carried out by using electronic absorption and fluorescence spectroscopies, electrochemistry, vibrational Raman spectroscopy, and theoretical calculations in the framework of DFT and TD-DFT theories. Optical spectra are described in terms of one-electron promotions between orbitals around the energy gap. Electrochemistry, in the framework of the Koopman's approach, is also interpreted. The vibrational Raman spectra are assigned to molecular modes and the evolution changing the heteroatom is addressed and an effective tuning of these properties is found. Part of this modulation is associated with local electronic interactions depending on the relative S, Se, and Te electronegativities. Unconventional long-range heteroatom-heteroatom interactions have been proposed which arise from the existence of effective pi-conjugated channels. The molecular level understanding of structure-property relationships in these organic/inorganic semiconductors are of great interest in the interdisciplinary area of material science.

Chalcogens↗
PubMed2002

Jiquilpane hydrocarbon skeleton generated by two successive Wagner-Meerwein rearrangements of longipinane derivatives.

The molecular rearrangement of (1R,3S,4S,5S,7S,8R,9S,10R,11R)-7,8,9-triacetyloxylongipinan-1-ol (4) under acidic conditions afforded (1S,4R,5R,7S,8R,9S,10S)-7,8,9-triacetyloxyuruap-3(12)-ene (5), while 6, the C(3)-stereoisomer of 4, after two consecutive Wagner-Meerwein rearrangements followed by two 1,2-hydride migrations, afforded (4R,5R,7S,8S,9S,10S,11S)-7,8,9-triacetyloxyjiquilp-3(12)-ene (7), which possesses a new hydrocarbon skeleton. The structures of the new substances were elucidated by 1D and 2D NMR data in combination with X-ray diffraction analyses of the uruapane, longipinane, and jiquilpane derivatives 5, 6, and 14, respectively. Molecular modeling at the ab initio level was used to study the reaction mechanisms, while deuterium labeling was employed to confirm the C-C bond migrations and the hydride shifts.

Models, Molecular↗
PubMed2002

Quirogane, prenopsane, and patzcuarane skeletons obtained by photochemically induced molecular rearrangements of longipinene derivatives.

Ultraviolet irradiation of (1R,3S,4S,5S,10R,11R)-1-acetyloxy-7-oxolongipin-8-ene (6), prepared from longipinene diesters isolated from Stevia salicifolia, afforded the new quirogane (7) and prenopsane (8) derivatives, as the major products, together with the minor secondary photoproduct (1R,3R,5R,8S,11S)-1-acetyloxy-7-oxopatzcuar-9-ene (9), which possesses a novel tricyclic sesquiterpene skeleton. The stereostructures of the new compounds 7-9 were mainly determined by NMR techniques including COSY, HSQC, HMBC, and NOESY in combination with molecular modeling obtained by density functional theory calculations. A reaction mechanism accounting for the observed transformations is proposed.

Magnetic Resonance Spectroscopy↗
PubMed1994

Spiroquinazoline, a novel substance P inhibitor with a new carbon skeleton, isolated from Aspergillus flavipes.

A novel substance P inhibitor, spiroquinazoline [1], was isolated from the fungus Aspergillus flavipes, which was originally obtained from soil. The structure of 1 was determined by analysis of spectroscopic data and 1 was shown to contain a new carbon skeleton containing a spiro-carbon center. Also isolated from the same culture extract were the new natural product, benzodiazepiedione [3], and the known compounds, acyl aszonalenin [4], N-benzoyl-L-phenylalaninol, and seven diketopiperazines.

Antineoplastic Agents↗
PubMed1994

Wallifoliol, a taxol congener with a novel carbon skeleton, from Himalayan Taxus wallichiana.

A new taxoid, wallifoliol [3], has been isolated, along with five known taxoids (taxol, cephalomannine, 10-deacetylbaccatin III, brevifoliol, 2-acetoxy brevifoliol = taxchinin A) from extracts of the needles of Himalayan Taxus wallichiana. The structure of wallifoliol has been assigned primarily from nmr studies. Wallifoliol [3] is assigned a structure in which rings A and B of the taxane system have undergone putative rearrangements producing a novel skeleton. Wallifoliol is the first diterpene to be found in nature with this particular 5/6/6/6/4 ring system.

India↗
PubMed1995

Malyngamide H, an ichthyotoxic amide possessing a new carbon skeleton from the Caribbean cyanobacterium Lyngbya majuscula.

Guided by ichthyotoxic activity against goldfish, a new lipopeptide, malyngamide H [1], and its corresponding free acid, 7-methoxytetradec-4(E)-enoic acid [2], have been isolated from the tropical marine cyanobacterium Lyngbya majuscula. The structure of the new carbon skeleton borne by malyngamide H was elucidated on the basis of spectroscopic analysis, mainly 2D nmr. The absolute stereochemistry of the cyclohexenone moiety of malyngamide H [1] was deduced by a combination of 2D NOESY and exciton chirality circular dichroism spectroscopy.

Animals↗
PubMed2001

Formal total synthesis of (+/-)-alpha- and beta-cedrene by preparation of cedrone. Construction of the tricyclic carbon skeleton by the use of a highly efficient intramolecular Khand annulation.

The cedrene carbon skeleton was rapidly assembled from a simple monocyclic precursor by the strategic use of a high yielding intramolecular Khand cyclization reaction. Further synthetic manipulations provided a concise formal total synthesis of alpha- and beta-cedrene. Reaction: see text.

Heterocyclic Compounds, 3-Ring↗
PubMed2001

Formal intramolecular [5 + 2] photocycloaddition reactions of maleimides: a novel approach to the CDE ring skeleton of (-)-cephalotaxine.

A concise approach to the cephalotaxine CDE ring skeleton based on the intramolecular formal [5 + 2] photocycloaddition of cyclopentenyl-substituted maleimides is described. An investigation of the diastereoselectivity afforded by various protected alkoxy groups demonstrated that the best selectivity (3.5:1) was afforded by the free hydroxyl group, strongly suggesting a hydrogen-bonded excited state. Reaction: see text.

Antineoplastic Agents, Phytogenic↗
PubMed2001

Highly stereoselective asymmetric construction of an acyclic carbon skeleton having two adjacent alkyl substituents by Michael addition of optically active allenyltitaniums to alkylidenemalonates.

[reaction: see text]. Enantio-enriched allenyltitaniums prepared in situ by the reaction of optically active secondary propargyl phosphates with a divalent titanium reagent Ti(O-i-Pr)4/2i-PrMgCl react readily with alkylidenemalonates with excellent regio- and diastereoselectivities to afford the Michael addition products with a high optical purity, thus opening up a new asymmetric method for construction of an acyclic carbon skeleton bearing two adjacent alkyl substituents.

Journal Article↗