Unusual Structure (unusual + structure)

Distribution by Scientific Domains

Selected Abstracts

S3O2 , An Unusual Structure with ,*,,* Interaction

CHEMINFORM, Issue 23 2006
Ming Wah Wong
Abstract ChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 200 leading journals. To access a ChemInform Abstract, please click on HTML or PDF. [source]

Sodium Hydro(isothiocyanato)borates: Synthesis and Structures

Heinrich Nöth
Abstract Sodium thiocyanate reacts in THF solution with 18-crown-6 to give the molecular compound Na(18-crown-6)(THF)NCS (3) with the N atom of the NCS anion oriented towards Na+. The same reaction with 15-crown-5 yields the ion pair Na(15-crown-5)NCS (4). In contrast, Na(NCS)(py)4, obtained by treating a solution of Na(H3BNCS) in THF with pyridine, yields Na(py)4(NCS) (5), which has a chain structure with hexacoordinate Na atoms coordinated to five N atoms and an S atom. Na(NCS) in THF adds 1 equiv. of BH3 to give Na(H3BNCS)·nTHF. Addition of 18-crown-6 to this solution yields crystals of the salt [Na(18-crown-6)(THF)2][H3BNCS] (1), as shown by X-ray crystallography. Both the cation and the anion show site disorder. However, when 15-crown-5 is used for complexation, the salt [Na(15-crown-5)(THF)][H3BNCS] (2) can be isolated. Its anion shows an almost linear B,N,C,S unit. Only a mixture of (catecholato)(isothiocyanato)borates results on treating Na(NCS) in THF with catecholborane. However, the borate Na[catB(NCS)2] is readily formed by adding Na(NCS) to B -(isothiocyanato)catecholborane. Single crystals of this compound were obtained as the salt [Na(18-crown-6)(THF)2][catB(NCS)2] (6). On the other hand, the reaction of Na(NCS) with 9-borabicyclo[3.3.1]nonane (9-BBN) in THF yields Na[(9-BBN)NCS)]·nTHF, and, on addition of 18-crown-6, the complex [Na(18-crown-6)(THF)2][(9-BBN)NCS] was isolated. Suitable crystals for X-ray structure determination were, however, only obtained by crystallization from tetrahydropyran. This solvate has the rather unusual structure [Na(18-crown-6)(thp)2][{(9-BBN)NCS}2Na(thp)4] (8). The sodiate anion has an Na atom coordinated by two S and four O atoms. DFT calculations support these experimental results: The (isothiocyanato)borates are more stable than the thiocyanato isomers. For the latter a bent structure of the B,S,C,N unit with a B,S,C bond angle of 105.7° is predicted. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2004) [source]

Reactions of Silicon Atoms with Conjugated ,-Systems: A Matrix-Spectroscopic Study

Günther Maier
Abstract The reaction of silicon atoms with cyclopentadiene and butadiene in argon at 10 K has been studied. Addition to the conjugated ,-system of cyclopentadiene occurs in a [1,4]-fashion. Upon irradiation the ,-adduct 3 rearranges to the formal insertion product 2 which has an unusual structure. Remarkably, a photoequilibrium between the two exo,endo isomers 2 and 4 can be observed. In the case of butadiene the addition is less stereospecific. The planar [1,4]-adduct 11 is formed together with the [1,2]-adduct 14. Surprisingly, besides 11 a second, nonplanar [1,4]-adduct 10 can be detected. The isomers 11 and 14 are transformed into 10 upon irradiation. The final photoproduct is silole 9. The structural elucidation of the new species is based on isotopic labelling and a comparison of the experimental observations with the results from density functional calculations. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2003) [source]

The genome of a Gondwanan mammal

BIOESSAYS, Issue 11 2007
Marilyn B. Renfree
Australia is thought of as the home of marsupials, but South America has 60 or so species of these interesting mammals. The genome of one of these, the South American grey short-tailed opossum, Monodelphis domestica, has just been sequenced and published in June.1 The high quality 6× coverage is the first marsupial genome completed, pipping the 2× coverage of the Australian tammar wallaby at the post by half a year. The opossum genome has an unusual structure with fewer chromosomes than the human genome (9 pairs versus 23 pairs) but a longer total length (3.4 billion versus 3 billion bases). The opossum autosomes, like those of all marsupials, are extremely large but, in contrast, the X chromosome is only 76 Mb long. The opossum genome has turned up several surprises and provided critical new information on the evolution of mammalian genomes2,6. BioEssays 29:1073,1076, 2007. © 2007 Wiley Periodicals, Inc. [source]

Nonribosomal Peptide Synthesis in Schizosaccharomyces pombe and the Architectures of Ferrichrome-Type Siderophore Synthetases in Fungi

CHEMBIOCHEM, Issue 4 2006
Torsten Schwecke Dr.
Abstract A nonribosomal peptide synthetase (NRPS) in Schizosaccharomyces pombe, which possesses an unusual structure incorporating three adenylation domains, six thiolation domains and six condensation domains, has been shown to produce the cyclohexapeptide siderophore ferrichrome. One of the adenylation domains is truncated and contains a distorted key motif. Substrate-binding specificities of the remaining two domains were assigned by molecular modelling to glycine and to N -acetyl- N -hydroxy- L -ornithine. Hexapeptide siderophore synthetase genes of Magnaporthe grisea and Fusarium graminearum were both identified and analyzed with respect to substrate-binding sites, and the predicted product ferricrocin was identified in each. A comparative analysis of these synthetase systems, including those of the basidiomycete Ustilago maydis, the homobasidiomycete Omphalotus olearius and the ascomycetes Aspergillus nidulans, Aspergillus fumigatus, Fusarium graminearum, Cochliobolus heterostrophus, Neurospora crassa and Aureobasidium pullulans, revealed divergent domain compositions with respect to their number and positioning, although all produce similar products by iterative processes. A phylogenetic analysis of both NRPSs and associated L - N5 -ornithine monooxygenases revealed that ferrichrome-type siderophore biosynthesis has coevolved in fungi with varying in trans interactions of NRPS domains. [source]

Reductive Benzylation of Dimetallo Hexaaryl[70]Fullerenes on the Equatorial Region

Takeshi Fujita
Abstract Dianions of dimetallic hexa(organo)[70]fullerene [(C5R5)2Ru2C70Ar6]2, (R=H, Me; Ar=Ph, 4-MeC6H4, 4- tBuC6H4) react with benzylic bromide to yield the dibenzylated product dimetallic octa(organo)[70]fullerene (C5R5)2Ru2C70Ar6(CH2Ar,)2 (Ar,=Ph, 4-MeO2CC6H4), where the benzylic groups are attached to the equatorial belt region of [70]fullerene; this region is generally considered to be rather unreactive. This unusual structure was unambiguously determined by X-ray crystallography. Theoretical studies on the electronic properties of the monoanionic intermediate indicated that the highest spin density resides on the two carbon atoms in the belt region; one of them then couples with a benzylic radical to yield the octa(organo)fullerene product after ionic substitution of the fullerene anion with a benzylic bromide. Electrochemical analysis of the hexa(organo) and octa(organo) ruthenium complexes suggests that the modification of the belt region does not affect the electronic communication between the two metal centers. [source]

Woody Pretzels: Spirocycles from Vetiver to Patchouli and Georgywood®

Philip Kraft
Abstract This review, including new experimental results, is the summary of a talk at the RSC/SCI conference ,flavours & fragrances 2007' in London, Imperial College, 24,26 September, 2007. Though the third dimension of the receptor models of J.,E. Amoore rarely was exceeding 4,Å, the world of woody odorants such as (+)-cedrol (3; cedarwood), (,)-khusimone (4; vetiver), and (,)-patchoulol (5; patchouli) is anything but flat. Any tricyclic skeleton with a zero-bridge contains a spirocyclic ring system determining its 3D structure, so spirocycles (spira, Lat. pretzel) are the fastest access to the third dimension. In the vetiver family, a spirocyclic mimic 9 of (,)-khusimone (4) was first discovered by chance by Büchi in 1976, and also by chance, we obtained another system, 12, with a characteristic vetiver smell by tandem- Rupe,Nazarov reaction of alkyne diols. A 5-Å distance between a quaternary C-atom and a carbonyl group (or alternative HB acceptor) with an , -methyl or methylene branching is proposed to be the key to their vetiver odor. Upon scale-up of one of these odorants, 24, we discovered a very powerful (0.067,ng/l) impurity with a most typical patchouli scent: the spirocyclic, sterically crowded hydroxy ketone 33 , a most unusual structure for a patchouli odorant. Several spirocyclic hydroxy ketone analogs, also with inverted ring systems such as in 70 and 84, provided new insights into the structure,odor correlation of this family. A superposition analysis indicated the carbonyl function of the hydroxy ketone to overlay on the geminal dimethyl motive of (,)-patchoulol. And indeed, the corresponding hydroxy ketone of patchoulol, 59, synthesized in 13 steps from Cyclal C (63), also emanated a patchouli odor. Finally, the synthesis and olfactory properties of twelve rigid spirocyclic analogs, 95,97, 99,102, and 106,110, of Georgywood® (91) are presented that highlight stereochemical requirements for woody odorants and raise doubts about an , -helical binding motive postulated by Hong and Corey. [source]

Insect chemistry and chirality

CHIRALITY, Issue S1 2003
Patricia Y. Hayes
Abstract Examination of the chemistry of a number of Australian insect species provided examples of unusual structures and encouraged determinations of their absolute stereochemistry by stereocontrolled syntheses and chromatographic comparisons. Inter alia, studies with the fruit-spotting bug (Amblypelta nitida), certain parasitic wasps (Biosteres sp.), the aposematic shield bug (Cantao parentum), and various species of scarab grubs are summarized. The determination of enantiomeric excesses (ee's) for component epoxides, lactones, spiroacetals, and allenes are described. Stereochemical and related aspects of the biosynthesis of spiroacetals in certain fruit-fly species (Bactrocerae sp.) are also presented. Chirality 15:S116,S127, 2003. © 2003 Wiley-Liss, Inc. [source]