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Synthesis and Synthetic Applications of 1,2,4-Oxadiazole-4-Oxides
No full textThe first 1,2,4-oxadiazole-4-oxide was prepared by Wieland a century ago, but-this compound remained largely a chemical curiosity until very recently. 1,2,4-Oxadiazole-4-oxides are a family of heterocycles closely related to the chemistry of nitrile oxides which were actively studied in the past half century and provided the basic knowledge on 1,2,4-oxadiazole-4 -oxides. The dimerizations of nitrile oxides under acidic or basic conditions were thoroughly studied by different research groups, and offer the more common entries to the symmetrical substituted 1,2,4-oxadiazole-4-oxides. A more general route to symmetrical and unsymmetrical substituted 1,2,4-oxadiazole-4 -oxides is based on the nitrile oxide cycloadditions to amidoximes. A variety of other 1,2,4-oxadiazole-4-oxides forming reactions are also known in the literature. Many of these reactions were neither fully exploited nor mechanistically understood since they require unusual starting reagents, often difficult to prepare, or take place affording complex mixtures of products. Some of these methods still await for improved procedures and proper mechanistic attention to be of general use and will be reviewed shortly.
The chemistry of 1,2,4-oxadiazole-4-oxides is related to the fragility of the heterocyclic ring, which undergoes thermal or photochemical cycloreversion to nitriles and nitrosocarbonyl intermediates. Trapping of the nitrosocarbonyls takes place easily with dienes and enes, affording a variety of hetero Diels-Alder and ene adducts, which attract great interest because of their useful synthetic elaboration toward many natural products of potential pharmaceutical applications.
The high efficiency of the photochemical cleavage of 1,2,4-oxadiazole-4-oxides at room temperature or well below affords the softest entry to the nitrosocarbonyls and allows for the study of their chemistry under convenient and simple experimental conditions. The photochemical cleavage have been applied successfully to Solid Phase chemistry, allowing for a safe and environmental friendly methodology for the synthesis of important intermediates.
This report is comprehensive of the synthesis and synthetic applications of the 1,2,4-oxadiazole4-oxides
A photochemical generation of nitrosocarbonyl intermediates
No full textNitrosocarbonyl intermediates are photochemically generated from 1,2,4-oxadiazole-4-oxides and efficiently trapped with enes and dienes
Classical and non-classical secondary orbital interactions and Coulombic attraction in regiospecific dimerization of acrolein
No full textNon-classical (bridging) and classical (Woodward-Hoffmann) secondary orbital interactions as well as a favourable electrostatic interaction are involved in the stabilization of the two lowest transition passes of the acrolein dimerization. The heteroatoms provide anchimeric assistance to the hetero Diels-Alder reaction through a neighbouring-group mechanism
Nitrosocarbonyl Intermediates as Super-Enophiles: a Mild Method for Carbon-Nitrogen Bond Formation
No full textNitrosocarbonyl intermediates, generated at r.t. by the mild oxidation of nitrile oxides, undergo clean ene reactions with tetramethyl- and trimethyl-ethylene and with cyclohexene. With less substituted ethylenes the ene pathway is still active but the oxidation step of the nitrile oxides competes with the cycloadditions to the olefins
Crystal structure and DFT calculations of 3,8–Diphenyl–3a,4,5,5a,8a,8b–hexahydro–benzo[1,2–d: 3,4–d’]diisoxazole, C20H18N2O2 J. Mol. Struct., 2005, 741, 135.
No full textThe molecular and crystal structures of 3,8-diphenyl-3a,4,5,5a,8a,8b-hexahydro-benzo[1,2-d: 3,4-d']diisoxazole have been XRD determined. The compound crystallizes in the monoclinic system (space group P2(1)/c) with cell dimensions a=15.278(1), b = 9.839(1), c = 10.912(2) &ANGS;, β = 92.15(1)°. The structure was solved from 2193 reflections with I ≥ 2σ(I). The final R was 0.039 for 289 variables
An unexpected bispericyclic transition structure leading to 4+2 and 2+4 cycloadducts in the endo dimerization of cyclopentadiene
No full textThe stereospecific endo dimerization of cyclopentadiene takes place through an asynchronous and symmetrical bispericyclic transition structure, which shows a merging of the 4+2 and 2+4 cycloaddition paths. The shape of the transition structure testifies to the presence of attractive Salem/Houk secondary orbital interactions assisting the endo approach
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
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
A straightforward synthesis of isoxazoline-carbocyclic nucleosides from 1,3-cyclohexadiene through nitrosocarbonyl chemistry
No full text3-Benzoyl-2-oxa-3-azabicyclo[2.2.2]oct-5-ene undergoes cycloaddition with benzonitrile oxide to afford a mixture of syn and anti regioisomeric cycloadducts. The anti cycloadducts were easily elaborated to stereodefined isoxazoline-based carbocyclic aminols that serve as synthons for the linear construction of purine nucleosides
Intra- and Intermolecular Hydrogen Bonding Effects in Cycloadditions between Nitrile Oxides and 4-Benzoylamino-2-cyclopenten-1-ol and its Derivatives
No full textCycloadditions between nitrile oxides and cis-4-benzoylamino-2-cyclopenten-1-ol offer an example in which a strong intramolecular hydrogen bond completely offsets the syn-directing ability of the cyclopentene substituents. Solvents affected the conformational equilibrium of the cyclopentene dipolarophile but did not sizeably influence the cycloaddition selectivity, showing the absence of directing effects between the addends. Removal of the intramolecular hydrogen bond by OH protection or oxidation activated the syn-directing ability of the amido substituent and provided a convenient route to syn stereoselection
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