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Structure Determination of 8-Benzyl-5-phenyl-3-oxa-4,8-diazatricyclo[5.2.1.02,6]dec-4-ene and 1-(9-Ethoxy-5-phenyl-3-oxa-4,8-diaza-tricyclo[5.2.1.02,6]dec-4-en-8-yl)-ethanone: Their Synthesis, Chemical Relationship and Comparison with Similar Compounds
No full textThe molecular and crystal structures of 8-benzyl-
5-phenyl-3-oxa-4,8-diaza-tricyclo[5.2.1.02,6]dec-4-ene 11a
and 1-(9-ethoxy-5-phenyl-3-oxa-4,8-diaza-tricyclo[5.2.1.02,6]
dec-4-en-8-yl)-ethanone 13a have been XRD determined.
The compounds crystallize in the monoclinic system with
space groups P21/n and P21/c, respectively. The synthesis
and chemical relationship between the two compounds are
reported and their structures are compared with others of the
aza-norbornene and isoxazoline families
PHOTOCHEMICAL GENERATION OF NITROSOCARBONYL INTERMEDIATES ON SOLID PHASE: SYNTHONS TOWARD HETERO-DIELS-ALDER AND ENE ADDUCTS THROUGH PHOTOCLEAVAGE
Full text linkThe synthesis of 1,2,4-oxadiazole-4-oxides on polystyrenic solid phase docked at the position 3 of the
heterocyclic ring has been performed through the cycloaddition of stable supported nitrile oxides to
amidoximes. The photochemical cycloreversion of these heterocycles afforded the free nitrosocarbonyl
intermediates that were trapped by suitable dienes or enes. The method is proposed as a clean and
environmental friendly approach to the fleeting nitrosocarbonyl intermediates, which afford valuable adducts
for various synthetic applications. The isomeric heterocycles docked at the position 5 of the ring have also
been obtained by cycloaddition of nitrile oxides to supported amidoximes. Their photolysis afforded resinbound
nitrosocarbonyls that were trapped with dienes affording valuable supported adducts suitable for
further elaboration on solid-phase chemistry
From 1,3-cyclohexadiene through nitrosocarbonyl chemistry, the synthesis of pyrimidine isoxazoline-carbocyclic nucleosides
No full textN-Benzoyl-2-oxa-3-azabicyclo[2.2.2]oct-5-ene undergoes cycloaddition with benzonitrile oxide affording a mixture of syn and anti regioisorneric cycloadducts. The anti cycloadclucts were easily elaborated to stereodefined isoxazoline-carbocyclic arninols that served as synthons for the linear construction of pyrimidine nucleosides, while the syn cycloadclucts do not enter the same synthetic route
Palladium(0)-catalyzed syntheses of cyclopentenyl-nucleoside analogues
Full text linkThe N-benzoylamino-2-cyclopenten-1-ol, derived from the nitrosocarbonyl hetero Diels-Alder cycloaddition reaction followed by the mild reductive N-O bond cleavage, allows for the preparation of cyclopentenyl-nucleoside analogues through the palladium(0)-catalyzed addition of both purine and pyrimidine heterobases to the allylic acetate derivative with retention of the starting material stereochemistry. A systematic study is performed on different heterobases to test the general application of the protocol
The Remarkable Cis Effect in the Ene Reactions of NitrosocarbonylIntermediates
Full text linkNitrosocarbonyls are fleeting and highly reactive intermediates that undergo ene reactions in a two-step
fashion. The addition steps are rate and product determining and lead to polarized diradicals that readily
enter the H-abstraction step yielding the ene products. The addition TSs are reached early, and the
stabilizing CH· · ·O contacts drive the reactions to the cis adducts. B3LYP calculations alone do not
describe the correct ordering of addition TSs in the ene reaction with trimethylethylene and (E)- and
(Z)-3-methyl-2-pentenes. Only at the MPWB1K level of treatment, medium-range noncovalent interactions
are successfully recovered, accounting satisfactorily for the experimental selectivities. The more stable
and isolable ArNOs exhibit late addition TSs, and distortion energies become dominant driving the reaction
exclusively to the Markovnikov adducts
From cyclopentadiene to isoxazoline-carbocyclic nucleosides: a rapid access to biological molecules through aza-Diels-Alder reactions
No full textA rapid access to carbocyclic nucleosides containing a fused isoxazoline ring is proposed through the Grieco cycloaddition of cyclopentadiene to iminium salts. The prolific elaboration of the isoxazoline cycloadducts allowed preparation of the target aminols through the unmasking of the hydroxymethylene group at the C3 level of the azanorbornene structure. The heterocyclic aminols are readily converted into nucleosides via the linear construction of purine heterocycles
Variable Markovnikov orientation and cis effect in ene reactions of nitrosocarbonyl intermediates
No full textNitrosocarbonyl intermediates, generated at room temperature
by the mild oxidation of nitrile oxides, undergo clean ene
reactions with trisubstituted olefins. Allylic hydrogens on
the more congested side of the alkene are exclusively
abstracted (the “cis effect”), thus resembling singlet oxygen
behavior. Nitrosocarbonyl benzene follows a Markovnikov
orientation and abstracts preferentially the twix hydrogens
over the lone ones. With the more sterically demanding
nitrosocarbonyl mesitylene, the Markovnikov directing effect
is relieved, and comparable twix and lone abstraction are
observed
Isoxazoline-carbocyclic aminols for nucleoside synthesis through aza-Diels–Alder reactions
No full textA novel approach to useful aminols for the synthesis of carbocyclic nucleosides is reported starting from a convenient source, the 2-azanorborn-5-enes. These are readily available through the Grieco cycloaddition of cyclopentadiene with iminium salts and are reactive dipolarophiles toward nitrile oxides. The prolific elaboration of the isoxazoline cycloadducts allowed preparation of the target aminols through the unmasking of the hydroxymethylene group at the C3 level of the azanorbornene structure
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
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