1H-Azirine

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Alexander F. Khlebnikov - One of the best experts on this subject based on the ideXlab platform.

  • buchner reaction azirine modification approach toward cycloheptatriene containing nitrogen heterocyclic scaffolds
    Journal of Organic Chemistry, 2021
    Co-Authors: Ekaterina E. Galenko, Mikhail S. Novikov, Mariya A Kryukova, Vladimir A Bodunov, Alexander F. Khlebnikov
    Abstract:

    The reaction conditions for the Buchner reaction of 2-(diazoacetyl)-2H-azirines with benzene leading to 2-(cyclohepta-2,4,6-trien-1-yl)-2H-azirines have been found. The azirine fragment of these compounds was subsequently used to prepare 5-(cyclohepta-2,4,6-trien-1-yl)isoxazoles and α-(cyclohepta-2,4,6-triene-1-ylcarbonyl)-1H-pyrroles. NMR and DFT calculations showed that the cycloheptatriene-norcaradiene valence isomerization in the corresponding moieties of synthesized azirines, isoxazoles, and pyrroles is very fast at room temperature. Cycloheptatriene valence isomer predominates in azirines and pyrroles, while 5-(cycloheptatrienyl)isoxazoles exist in a pure cycloheptatriene form at 25 °C in chloroform solution. In accordance with the X-ray analysis, two of the synthesized pyrroles crystallized as norcaradiene isomers from a solution of equilibrium mixture of valence isomers. Reductive isoxazole ring opening using the Mo(CO)6/H2O reductive system afforded (Z)-3-amino-3-aryl-1-(cyclohepta-2,4,6-trien-1-yl)prop-2-en-1-ones, which are pure cycloheptatriene isomers. α-(Cycloheptatrieneylcarbonyl)-1H-pyrroles were transformed into the α-(benzylcarbonyl)-1H-pyrroles under cooperative TFA/Rh2(TFA)4 catalysis.

  • 1 2h azirine 2 carbonyl benzotriazoles building blocks for the synthesis of pyrrole containing heterocycles
    Organic and Biomolecular Chemistry, 2020
    Co-Authors: Ekaterina E. Galenko, Mikhail S. Novikov, Vladimir A Bodunov, Firuza M Shakirova, Alexander F. Khlebnikov
    Abstract:

    A one-pot method was developed for the preparation of 2H-azirine-2-carbonylbenzotriazoles, formed by the reaction of benzotriazole with 2H-azirine-2-carbonyl chlorides, which were generated by the Fe(ii)-catalyzed isomerization of 5-chloroisoxazoles. The Co(ii)-catalyzed reaction of 2H-azirine-2-carbonylbenzotriazoles with 1,3-diketones provides 2-((benzotriazol-1-yl)carbonyl)pyrroles in moderate to good yields. Base-promoted annulations of 2-((benzotriazol-1-yl)carbonyl)pyrroles with aldehydes, ketones, isocyanates and isothiocyanates afford various substituted pyrrolo[1,2-c]oxazole and 1H-pyrrolo[1,2-c]imidazole derivatives in moderate to high yields. The 6-acyl group of these adducts can be removed by triflic acid, giving further new pyrrolo-fused O- and N-heterocycles, such as 6-unsubstituted pyrrolo[1,2-c]oxazol-1(3H)-one and 1H-pyrrolo[1,2-c]imidazole-1,3(2H)-dione, while the 6-acetyl substituent of 1H-pyrrolo[1,2-c]imidazole-1,3(2H)-dione, when treated with POCl3/pyridine, is transformed into the 6-ethynyl substituent.

  • 2h azirine 2 carbonyl azides preparation and use as n heterocyclic building blocks
    Journal of Organic Chemistry, 2020
    Co-Authors: Liya D. Funt, Mikhail S. Novikov, Yulia V Krivolapova, Olesya V Khoroshilova, Alexander F. Khlebnikov
    Abstract:

    2H-Azirine-2-carbonyl azides, new reactive heterocyclic building blocks, were synthesized in high yield by the reaction of sodium azide with 2H-azirine-2-carbonyl chlorides, generated by the Fe(II)-catalyzed isomerization of 5-chloroisoxazoles. 2-(Azidocarbonyl)-1H-pyrroles, prepared by the Ni(II)-catalyzed reaction of 2-(azidocarbonyl)-2H-azirines with 1,3-diketones, easily undergo the Curtius rearrangement in boiling tBuOH to give Boc-protected α-aminopyrroles in high yield. Heating of 2-(azidocarbonyl)-1H-pyrroles for a short time in inert solvents leads to the high-yield formation of benzo- and hetero-fused 1H-pyrrolo[2,3-b]pyridin-6(7H)-ones, which are formed via a 6π electrocyclization involving the vicinal aryl or hetaryl substituent and the N═C bond of isocyanate, generated by the Curtius rearrangement of the azidocarbonyl group. The Pd-catalyzed cross-coupling reaction of 1-acetyl-2-methyl-3H-pyrrolo[2,3-c]isoquinolin-5-yl triflate, easily prepared from the corresponding pyrroloisoquinolone, leads to variously 5-substituted 3H-pyrrolo[2,3-c]isoquinolines in excellent yields.

  • an azirine strategy for the synthesis of alkyl 4 amino 5 trifluoromethyl 1h pyrrole 2 carboxylates
    Synthesis, 2018
    Co-Authors: Liya D. Funt, Mikhail S. Novikov, Olesya A. Tomashenko, Alexander F. Khlebnikov
    Abstract:

    1-(3,3,3-Trifluoro-2,2-dihydroxypropyl)pyridin-1-ium bromide serves as a trifluoromethyl-containing building block for the preparation of trifluoromethyl-substituted aminopyrroles based on the 2H-azirine ring expansion strategy. The primary products, 3-aryl-2-(methoxycarbonyl)-4-(pyridin-1-ium-1-yl)-5-(trifluoromethyl)pyrrol-1-ides, can be hydrogenated by H2/PtO2 to form alkyl 3-aryl-4-(piperidin-1-yl)-5-(trifluoromethyl)-1H-pyrrole-2-carboxylates and transformed into alkyl 4-amino-3-aryl-1-methyl-5-(trifluoromethyl)-1H-pyrrole-2-carboxylates via methylation/hydrazinolysis.

  • fe ii catalyzed isomerization of 5 chloroisoxazoles to 2h azirine 2 carbonyl chlorides as a key stage in the synthesis of pyrazole nitrogen heterocycle dyads
    Journal of Organic Chemistry, 2018
    Co-Authors: Kirill I Mikhailov, Ekaterina E. Galenko, Mikhail S. Novikov, Alexey V. Galenko, Alexander Yu. Ivanov, Galina L. Starova, Alexander F. Khlebnikov
    Abstract:

    2-(1H-Pyrazol-1-ylcarbonyl)-2H-azirines were synthesized by in situ trapping of 2H-azirine-2-carbonyl chlorides, generated by Fe(II)-catalyzed isomerization of 5-chloroisoxazoles, with pyrazoles. According to DFT calculations, the selectivity of nucleophilic substitution at the carbonyl group of 2H-azirine-2-carbonyl chloride by a pyrazole nucleophile, which is a mixture of two tautomers, is controlled by thermodynamic factors. 2-(1H-Pyrazol-1-ylcarbonyl)-2H-azirines are excellent precursors for the preparation of two other pyrazole–nitrogen heterocycle dyads: 5-(1H-pyrazol-1-yl)oxazoles by photolysis and 1-(1H-pyrrol-2-ylcarbonyl)-1H-pyrazoles by a Ni(II)-catalyzed reaction with 1,3-dicarbonyl compounds. 5-(1H-Pyrazol-1-yl)oxazoles show strong emission in acetonitrile at 360–410 nm with high quantum yields.

Mikhail S. Novikov - One of the best experts on this subject based on the ideXlab platform.

  • buchner reaction azirine modification approach toward cycloheptatriene containing nitrogen heterocyclic scaffolds
    Journal of Organic Chemistry, 2021
    Co-Authors: Ekaterina E. Galenko, Mikhail S. Novikov, Mariya A Kryukova, Vladimir A Bodunov, Alexander F. Khlebnikov
    Abstract:

    The reaction conditions for the Buchner reaction of 2-(diazoacetyl)-2H-azirines with benzene leading to 2-(cyclohepta-2,4,6-trien-1-yl)-2H-azirines have been found. The azirine fragment of these compounds was subsequently used to prepare 5-(cyclohepta-2,4,6-trien-1-yl)isoxazoles and α-(cyclohepta-2,4,6-triene-1-ylcarbonyl)-1H-pyrroles. NMR and DFT calculations showed that the cycloheptatriene-norcaradiene valence isomerization in the corresponding moieties of synthesized azirines, isoxazoles, and pyrroles is very fast at room temperature. Cycloheptatriene valence isomer predominates in azirines and pyrroles, while 5-(cycloheptatrienyl)isoxazoles exist in a pure cycloheptatriene form at 25 °C in chloroform solution. In accordance with the X-ray analysis, two of the synthesized pyrroles crystallized as norcaradiene isomers from a solution of equilibrium mixture of valence isomers. Reductive isoxazole ring opening using the Mo(CO)6/H2O reductive system afforded (Z)-3-amino-3-aryl-1-(cyclohepta-2,4,6-trien-1-yl)prop-2-en-1-ones, which are pure cycloheptatriene isomers. α-(Cycloheptatrieneylcarbonyl)-1H-pyrroles were transformed into the α-(benzylcarbonyl)-1H-pyrroles under cooperative TFA/Rh2(TFA)4 catalysis.

  • 1 2h azirine 2 carbonyl benzotriazoles building blocks for the synthesis of pyrrole containing heterocycles
    Organic and Biomolecular Chemistry, 2020
    Co-Authors: Ekaterina E. Galenko, Mikhail S. Novikov, Vladimir A Bodunov, Firuza M Shakirova, Alexander F. Khlebnikov
    Abstract:

    A one-pot method was developed for the preparation of 2H-azirine-2-carbonylbenzotriazoles, formed by the reaction of benzotriazole with 2H-azirine-2-carbonyl chlorides, which were generated by the Fe(ii)-catalyzed isomerization of 5-chloroisoxazoles. The Co(ii)-catalyzed reaction of 2H-azirine-2-carbonylbenzotriazoles with 1,3-diketones provides 2-((benzotriazol-1-yl)carbonyl)pyrroles in moderate to good yields. Base-promoted annulations of 2-((benzotriazol-1-yl)carbonyl)pyrroles with aldehydes, ketones, isocyanates and isothiocyanates afford various substituted pyrrolo[1,2-c]oxazole and 1H-pyrrolo[1,2-c]imidazole derivatives in moderate to high yields. The 6-acyl group of these adducts can be removed by triflic acid, giving further new pyrrolo-fused O- and N-heterocycles, such as 6-unsubstituted pyrrolo[1,2-c]oxazol-1(3H)-one and 1H-pyrrolo[1,2-c]imidazole-1,3(2H)-dione, while the 6-acetyl substituent of 1H-pyrrolo[1,2-c]imidazole-1,3(2H)-dione, when treated with POCl3/pyridine, is transformed into the 6-ethynyl substituent.

  • 2h azirine 2 carbonyl azides preparation and use as n heterocyclic building blocks
    Journal of Organic Chemistry, 2020
    Co-Authors: Liya D. Funt, Mikhail S. Novikov, Yulia V Krivolapova, Olesya V Khoroshilova, Alexander F. Khlebnikov
    Abstract:

    2H-Azirine-2-carbonyl azides, new reactive heterocyclic building blocks, were synthesized in high yield by the reaction of sodium azide with 2H-azirine-2-carbonyl chlorides, generated by the Fe(II)-catalyzed isomerization of 5-chloroisoxazoles. 2-(Azidocarbonyl)-1H-pyrroles, prepared by the Ni(II)-catalyzed reaction of 2-(azidocarbonyl)-2H-azirines with 1,3-diketones, easily undergo the Curtius rearrangement in boiling tBuOH to give Boc-protected α-aminopyrroles in high yield. Heating of 2-(azidocarbonyl)-1H-pyrroles for a short time in inert solvents leads to the high-yield formation of benzo- and hetero-fused 1H-pyrrolo[2,3-b]pyridin-6(7H)-ones, which are formed via a 6π electrocyclization involving the vicinal aryl or hetaryl substituent and the N═C bond of isocyanate, generated by the Curtius rearrangement of the azidocarbonyl group. The Pd-catalyzed cross-coupling reaction of 1-acetyl-2-methyl-3H-pyrrolo[2,3-c]isoquinolin-5-yl triflate, easily prepared from the corresponding pyrroloisoquinolone, leads to variously 5-substituted 3H-pyrrolo[2,3-c]isoquinolines in excellent yields.

  • an azirine strategy for the synthesis of alkyl 4 amino 5 trifluoromethyl 1h pyrrole 2 carboxylates
    Synthesis, 2018
    Co-Authors: Liya D. Funt, Mikhail S. Novikov, Olesya A. Tomashenko, Alexander F. Khlebnikov
    Abstract:

    1-(3,3,3-Trifluoro-2,2-dihydroxypropyl)pyridin-1-ium bromide serves as a trifluoromethyl-containing building block for the preparation of trifluoromethyl-substituted aminopyrroles based on the 2H-azirine ring expansion strategy. The primary products, 3-aryl-2-(methoxycarbonyl)-4-(pyridin-1-ium-1-yl)-5-(trifluoromethyl)pyrrol-1-ides, can be hydrogenated by H2/PtO2 to form alkyl 3-aryl-4-(piperidin-1-yl)-5-(trifluoromethyl)-1H-pyrrole-2-carboxylates and transformed into alkyl 4-amino-3-aryl-1-methyl-5-(trifluoromethyl)-1H-pyrrole-2-carboxylates via methylation/hydrazinolysis.

  • fe ii catalyzed isomerization of 5 chloroisoxazoles to 2h azirine 2 carbonyl chlorides as a key stage in the synthesis of pyrazole nitrogen heterocycle dyads
    Journal of Organic Chemistry, 2018
    Co-Authors: Kirill I Mikhailov, Ekaterina E. Galenko, Mikhail S. Novikov, Alexey V. Galenko, Alexander Yu. Ivanov, Galina L. Starova, Alexander F. Khlebnikov
    Abstract:

    2-(1H-Pyrazol-1-ylcarbonyl)-2H-azirines were synthesized by in situ trapping of 2H-azirine-2-carbonyl chlorides, generated by Fe(II)-catalyzed isomerization of 5-chloroisoxazoles, with pyrazoles. According to DFT calculations, the selectivity of nucleophilic substitution at the carbonyl group of 2H-azirine-2-carbonyl chloride by a pyrazole nucleophile, which is a mixture of two tautomers, is controlled by thermodynamic factors. 2-(1H-Pyrazol-1-ylcarbonyl)-2H-azirines are excellent precursors for the preparation of two other pyrazole–nitrogen heterocycle dyads: 5-(1H-pyrazol-1-yl)oxazoles by photolysis and 1-(1H-pyrrol-2-ylcarbonyl)-1H-pyrazoles by a Ni(II)-catalyzed reaction with 1,3-dicarbonyl compounds. 5-(1H-Pyrazol-1-yl)oxazoles show strong emission in acetonitrile at 360–410 nm with high quantum yields.

Jose M Alves - One of the best experts on this subject based on the ideXlab platform.

  • diels alder cycloaddition of 2 azadienes to methyl 2 2 6 dichlorophenyl 2h azirine 3 carboxylate in the synthesis of methyl 4 oxo 1 3 diazabicyclo 4 1 0 heptane 6 carboxylates
    ChemInform, 2004
    Co-Authors: Jose M Alves, Miguel M Duraes, Gil A Fortes
    Abstract:

    Abstract A number of fused 4-oxo-1,3-diazabicyclo[4.1.0]heptane-6-carboxylates, a new type of compound, have been obtained by Diels–Alder cycloaddition between nucleophilic 2-azadienes and an electrophilic 2 H -azirine. The reactions are completely endo- and regio selective, the azirine being added by its less hindered face to the diene. There are two isomers 7 and 8 formed from dienes 1 due either to isomerization of the cycloadducts 7 and 8 or by isomerization of the CN bond of the diene during the reaction. The isomer 10 is formed from diene 2e , and a single diastereoisomer structure 4a – i is formed from dienes 11 . Some pyrimidones 8a , 7c / 8c , 7e , 10 , 11d have been hydrolyzed leading to functionalised aziridines 12 , 13 and 15 .

  • stereoselective cycloaddition of 1 glucosyl 1 3 butadienes with tert butyl 2h azirine 3 carboxylate glyoxylates and imines
    Tetrahedron Letters, 2003
    Co-Authors: Jose M Alves, Gil A Fortes, I G Almeida, A P Freitas
    Abstract:

    Glucosyl dienes 1 have been reacted with the achiral 2H-azirine 4 and with glyoxylates, forming fused structures of type 5 and disaccharide-like compounds 7 with good to excellent selectivity. Glucosyl dienes 1 participated as dienophiles in reactions with Schiff bases derived from anilines forming isoquinolines 10 and 11. The diastereoselectivity of this reaction is poor.

  • cycloaddition of methyl 2 2 6 dichorophenyl 2h azirine 3 carboxylate to electron rich 2 azadienes
    Tetrahedron Letters, 2003
    Co-Authors: Jose M Alves, Miguel M Duraes, Gil A Fortes
    Abstract:

    Abstract tert -Butyldimethylsililoxy-2-aza-1,3-butadienes react with 2 H -azirine 3 leading to Diels–Alder cycloadducts in moderate yields. The reactions are endo - and regioselective with the azirine being added by its less hindered face. There is only one product in the case of 1b , 4b . There are two isomers ( 4 and 5 ) from 1a , 1c and 1d . A different result was obtained with the diene 1e . Diene 1e formed products 4e and 8 . Some of compounds 4 and 5 have been hydrolysed leading to functionalised aziridines 7 . Compound 8 gave aziridine 9 .

  • diastereoselective synthesis of aziridines from 1r 10 n n dialkylsulfamoyl isobornyl 2h azirine 3 carboxylates
    ChemInform, 2003
    Co-Authors: Yolanda S P Alvares, Jose M Alves, Nuno G Azoia, Jamie F Bickley, Thomas L Gilchrist
    Abstract:

    Reaction of the chiral azirine 1a with nucleophiles and dienes is described. Thiols, heteroaromatic nitrogen compounds and phenylmagnesium bromide add to the azirine 1a to give functionalised aziridines 2/3a–h. X-Ray crystal structures for the products 3e and 3g have been obtained. Diastereodifferentiation of the two faces of the azirine 1a is observed in most cases, but only thiophenol gives a single diastereomer (2a). Most mixtures of diastereomers were separated by dry flash chromatography. Benzylamine produced a dimer of the original azirine 1a, compound 9. Representative conjugated dienes (cyclopentadiene, furan and open chain dienes) were added to the chiral azirine 1a. Only poor selectivities were observed. The selectivity was not significantly enhanced in the cycloaddition of cyclopentadiene to the more bulky azirine 1b.

  • diels alder reactions of alkyl 2h azirine 3 carboxylates with furans
    Journal of The Chemical Society-perkin Transactions 1, 2001
    Co-Authors: Jose M Alves, Gil A Fortes, Nuno G Azoia, Jamie F Bickley, Thomas L Gilchrist, Ricardo Mendonca
    Abstract:

    Methyl 2-(2,6-dichlorophenyl)-2H-azirine-3-carboxylate 1 and furan give the aziridine 2 by a Diels–Alder cycloaddition reaction. The hydrolysis of compound 2 leads to a dihydrofuranol 11 by cleavage of a C–N bond. X-Ray crystal structures of compounds 2 and 11 have been determined. Compound 2 reacts with alcohols in a similar way to give 2-alkoxy-2,5-dihydrofurans as mixtures of cis and trans isomers. The structures of these compounds have been determined from an X-ray crystal structure of one of the methyl ethers, the trans isomer 13. The reaction of the azirine 1 with 1,3-diphenylisobenzofuran leads to the formation of two isomeric 1 ∶ 1 adducts that have been identified as the products of endo and exo cycloaddition, 3 and 4. The endo isomer 3 is converted into the exo isomer 4 by heat. Similar Diels–Alder reactions have been carried out between furans and benzyl 2H-azirine-3-carboxylate 6. Hydrolysis of the adduct 7 formed with furan again produces a dihydrofuranol 25 as the major product together with three minor products, two of which are 1-azabicyclo[4.1.0]hept-3-ene-2,5-diols 27 and 28 that result from C–O bond cleavage. Protection of the mixture of alcohols with TBS triflate gives the bis(TBS) ether 31 of the trans-1-azabicyclo[4.1.0]hept-3-ene-2,5-diol as the major product, showing that this ring system can be produced from the dihydrofuranol 25. The bis(TBS) ether 30 of the cis-2,5-diol is a minor product and its structure has been established by independent synthesis through a Diels–Alder reaction between the azirine 6 and 1,4-bis(tert-butyldimethylsilyloxy)butadiene 32.

Gil A Fortes - One of the best experts on this subject based on the ideXlab platform.

  • diels alder cycloaddition of 2 azadienes to methyl 2 2 6 dichlorophenyl 2h azirine 3 carboxylate in the synthesis of methyl 4 oxo 1 3 diazabicyclo 4 1 0 heptane 6 carboxylates
    ChemInform, 2004
    Co-Authors: Jose M Alves, Miguel M Duraes, Gil A Fortes
    Abstract:

    Abstract A number of fused 4-oxo-1,3-diazabicyclo[4.1.0]heptane-6-carboxylates, a new type of compound, have been obtained by Diels–Alder cycloaddition between nucleophilic 2-azadienes and an electrophilic 2 H -azirine. The reactions are completely endo- and regio selective, the azirine being added by its less hindered face to the diene. There are two isomers 7 and 8 formed from dienes 1 due either to isomerization of the cycloadducts 7 and 8 or by isomerization of the CN bond of the diene during the reaction. The isomer 10 is formed from diene 2e , and a single diastereoisomer structure 4a – i is formed from dienes 11 . Some pyrimidones 8a , 7c / 8c , 7e , 10 , 11d have been hydrolyzed leading to functionalised aziridines 12 , 13 and 15 .

  • stereoselective cycloaddition of 1 glucosyl 1 3 butadienes with tert butyl 2h azirine 3 carboxylate glyoxylates and imines
    Tetrahedron Letters, 2003
    Co-Authors: Jose M Alves, Gil A Fortes, I G Almeida, A P Freitas
    Abstract:

    Glucosyl dienes 1 have been reacted with the achiral 2H-azirine 4 and with glyoxylates, forming fused structures of type 5 and disaccharide-like compounds 7 with good to excellent selectivity. Glucosyl dienes 1 participated as dienophiles in reactions with Schiff bases derived from anilines forming isoquinolines 10 and 11. The diastereoselectivity of this reaction is poor.

  • cycloaddition of methyl 2 2 6 dichorophenyl 2h azirine 3 carboxylate to electron rich 2 azadienes
    Tetrahedron Letters, 2003
    Co-Authors: Jose M Alves, Miguel M Duraes, Gil A Fortes
    Abstract:

    Abstract tert -Butyldimethylsililoxy-2-aza-1,3-butadienes react with 2 H -azirine 3 leading to Diels–Alder cycloadducts in moderate yields. The reactions are endo - and regioselective with the azirine being added by its less hindered face. There is only one product in the case of 1b , 4b . There are two isomers ( 4 and 5 ) from 1a , 1c and 1d . A different result was obtained with the diene 1e . Diene 1e formed products 4e and 8 . Some of compounds 4 and 5 have been hydrolysed leading to functionalised aziridines 7 . Compound 8 gave aziridine 9 .

  • diels alder reactions of alkyl 2h azirine 3 carboxylates with furans
    Journal of The Chemical Society-perkin Transactions 1, 2001
    Co-Authors: Jose M Alves, Gil A Fortes, Nuno G Azoia, Jamie F Bickley, Thomas L Gilchrist, Ricardo Mendonca
    Abstract:

    Methyl 2-(2,6-dichlorophenyl)-2H-azirine-3-carboxylate 1 and furan give the aziridine 2 by a Diels–Alder cycloaddition reaction. The hydrolysis of compound 2 leads to a dihydrofuranol 11 by cleavage of a C–N bond. X-Ray crystal structures of compounds 2 and 11 have been determined. Compound 2 reacts with alcohols in a similar way to give 2-alkoxy-2,5-dihydrofurans as mixtures of cis and trans isomers. The structures of these compounds have been determined from an X-ray crystal structure of one of the methyl ethers, the trans isomer 13. The reaction of the azirine 1 with 1,3-diphenylisobenzofuran leads to the formation of two isomeric 1 ∶ 1 adducts that have been identified as the products of endo and exo cycloaddition, 3 and 4. The endo isomer 3 is converted into the exo isomer 4 by heat. Similar Diels–Alder reactions have been carried out between furans and benzyl 2H-azirine-3-carboxylate 6. Hydrolysis of the adduct 7 formed with furan again produces a dihydrofuranol 25 as the major product together with three minor products, two of which are 1-azabicyclo[4.1.0]hept-3-ene-2,5-diols 27 and 28 that result from C–O bond cleavage. Protection of the mixture of alcohols with TBS triflate gives the bis(TBS) ether 31 of the trans-1-azabicyclo[4.1.0]hept-3-ene-2,5-diol as the major product, showing that this ring system can be produced from the dihydrofuranol 25. The bis(TBS) ether 30 of the cis-2,5-diol is a minor product and its structure has been established by independent synthesis through a Diels–Alder reaction between the azirine 6 and 1,4-bis(tert-butyldimethylsilyloxy)butadiene 32.

Rui Fausto - One of the best experts on this subject based on the ideXlab platform.

  • Methyl 3-Methyl-2H-azirine-2-carboxylate Photochemistry Studied by Matrix-isolation FTIR and DFT Calculations
    2016
    Co-Authors: Agnieszka Kaczor, Ana L. Cardoso, Rui Fausto
    Abstract:

    The aliphatic 2H-azirine, methyl 3-methyl-2H-azirine-2-carboxylate (MMAC), has been synthesized and its monomeric form investigated by IR spectroscopy in an argon matrix, at 10 K, as well as theoretically (DFT/ B3LYP/6-311++G(d,p)). Two low-energy conformers of MMAC (Ct and Cc) were found in the matrix, both exhibiting the cis conformation around the C-O bond but differing in the arrangement around the C-CR bond. The two conformers were photoreactive upon in situ broadband UV excitation (ì> 235 nm), yielding nitrile ylide (P1) and ketene imine (P2) type products, which resulted from cleavage of the C-C or C-N bond, respectively. The kinetics of the reactions leading to the formation of P1 and P2 are of first order, with the processes being favored when the reactant is in the Cc conformation. Very interestingly, the C-N bond photocleavage, which is unusual for aliphatic 2H-azirines, was found to be preferred over the generally favored in 2H-azirines C-C bond breakage. This behavior is attributed to the presence in the molecule of the electron-withdrawing methoxycarbonyl substituent, which accelerates the intersystem crossing toward the T1 triplet state and, in this way, favors the C-N bond cleavage. In addition to the primary photoprocesses leading to formation of P1 and P2, secondary photoprocesses leading to the decarboxylation and decarbonylation of P2 have been also observed

  • on the photochemistry of 1 2 benzisoxazole capture of elusive spiro 2h azirine and ketenimine intermediates
    European Journal of Organic Chemistry, 2016
    Co-Authors: Claudio M Nunes, Sandra Pinto, Igor Reva, Rui Fausto
    Abstract:

    The photochemistry of 1,2-benzisoxazole (1) was studied using low-temperature matrix isolation coupled with infrared spectroscopy and quantum chemistry calculations. We identified, for the first time, spiro-2H-azirine 2 and ketenimine 3 as intermediates in the photoisomerization of 1 to 2-cyanophenol (4). These results constitute indirect evidence for the existence of vinylnitrene intermediates in the photochemistry of 1,2-benzisoxazoles. The potential energy surface (PES) resulting from the N–O bond cleavage of 1 was compared with the respective PES of the parent isoxazole. Calculations at the CBS-QB3 level show that no stabilization is gained for the triplet vinylnitrene upon introduction of a benzene ring fused with isoxazole. However, the energies of 2 and 3 are higher by 13–15 kcal/mol comparing with the 2H-azirine and ketenimine analogs resulting from isoxazole, which explains why they had not been observed before. Our general mechanistic proposal also predicts well the photoisomerizations of 2 and 3 to 4.

  • Photochemistry of 3-amino-1,2-benzisoxazole: unexpected photoisomerization of an amino-spiro-2H-azirine to a 1H-diazirine
    Tetrahedron Letters, 2016
    Co-Authors: Claudio M Nunes, Sandra Pinto, Igor Reva, Rui Fausto
    Abstract:

    Abstract UV irradiation of 3-amino-1,2-benzisoxazole isolated in an argon matrix leads to the formation of an amino-spiro-2 H -azirine. The amino-spiro-2 H -azirine was found to photoisomerize back to 3-amino-1,2-benzisoxazole and also to a 1 H -diazirine, which isomerizes to a carbodiimide. All the reported species were characterized experimentally by IR spectroscopy and confirmed by comparison with theoretical IR spectra. The discovery of the transformation of an amino-spiro-2 H -azirine into a 1 H -diazirine is unprecedented in the chemistry of reactive intermediates.

  • The Pyrolysis of Isoxazole Revisited: A New Primary Product and the Pivotal Role of the Vinylnitrene. A Low-Temperature Matrix Isolation and Computational Study
    2016
    Co-Authors: Cláudio M. Nunes, Igor Reva, Rui Fausto, Teresa Pinho M. V. D. E Melo, Tomáš Šolomek, Thomas Bally
    Abstract:

    This paper describes the pyrolysis of parent isoxazole and of its 5-methyl and 3,5-dimethyl derivatives by the high-pressure pulsed pyrolysis method, where activation of the precursor molecules occurs predominantly by collisions with the host gas (Ar in our case), rather than with the walls of the pyrolysis tube, where catalyzed processes may occur. The products were trapped at 15 K in Ar matrices and were characterized by vibrational spectroscopy. Thereby, hitherto unobserved primary products of pyrolysis of isoxazole and of its 5-methyl derivative, 3-hydroxypropenenitrile or 3-hydroxybutenenitrile, respectively, were observed. E–Z photoisomerization could be induced in the above hydroxynitriles. On pyrolysis of isoxazole, ketenimine and CO were observed as decomposition products, but this process did not occur when the 5-methyl derivative was pyrolyzed. Instead, the corresponding ketonitrile was formed. In the case of 3,5-dimethylisoxazole, 2-acetyl-3-methyl-2H-azirine was detected at moderate pyrolysis temperatures, whereas at higher temperatures, 2,5-dimethyloxazole was the only observed rearrangement product (next to products of dissociation). These findings are rationalized on the basis of quantum chemical calculations. Thereby it becomes evident that carbonyl-vinylnitrenes play a pivotal role in the observed rearrangements, a role that had not been recognized in previous theoretical studies because it had been assumed that vinylnitrenes are closed-shell singlet species, whereas they are in fact open-shell singlet biradicaloids. Thus, the primary processes had to be modeled by the multiconfigurational CASSCF method, followed by single-point MR-CISD calculations. The picture that emerges from these calculations is in excellent accord with the experimental findings; that is, they explain why some possible products are observed while others are not

  • the pyrolysis of isoxazole revisited a new primary product and the pivotal role of the vinylnitrene a low temperature matrix isolation and computational study
    Journal of the American Chemical Society, 2011
    Co-Authors: Claudio M Nunes, Igor Reva, Rui Fausto, Teresa Pinho M. V. D. E Melo, Tomas Solomek, Thomas Bally
    Abstract:

    This paper describes the pyrolysis of parent isoxazole and of its 5-methyl and 3,5-dimethyl derivatives by the high-pressure pulsed pyrolysis method, where activation of the precursor molecules occurs predominantly by collisions with the host gas (Ar in our case), rather than with the walls of the pyrolysis tube, where catalyzed processes may occur. The products were trapped at 15 K in Ar matrices and were characterized by vibrational spectroscopy. Thereby, hitherto unobserved primary products of pyrolysis of isoxazole and of its 5-methyl derivative, 3-hydroxypropenenitrile or 3-hydroxybutenenitrile, respectively, were observed. E–Z photoisomerization could be induced in the above hydroxynitriles. On pyrolysis of isoxazole, ketenimine and CO were observed as decomposition products, but this process did not occur when the 5-methyl derivative was pyrolyzed. Instead, the corresponding ketonitrile was formed. In the case of 3,5-dimethylisoxazole, 2-acetyl-3-methyl-2H-azirine was detected at moderate pyrolysis...