Nikolai V. Rostovskii

Recent advances in isoxazole chemistry

The preparation methods and reactions of isoxazoles are described and systematized on the basis of analysis of the literature published from 2005 to present. In the discussion of synthesis, major attention is focused on the most efficient approaches: condensation of hydroxylamine with 1,3-dielectrophiles and reactions of nitrile oxides with alkenes and alkynes. Five-membered ring opening reactions leading to acyclic functionalized or other heterocyclic compounds are considered. The transformations of isoxazole derivatives that occur without ring cleavage to form fused heterocyclic systems, as well as reactions that lead to the introduction of C-substituents into isoxazoles, are considered. Data on the biological activity of some isoxazole derivatives are reported. The bibliography includes 439 references.

Pseudopericyclic 1,5- versus Pericyclic 1,4- and 1,6-Electrocyclization in Electron-Poor 4-Aryl-2-azabuta-1,3-dienes: Indole Synthesis from 2H-Azirines and Diazo Compounds

Transformations of 2-azabuta-1,3-dienes, formed in Rh2(OAc)4-catalyzed reactions of diazo carbonyl compounds with 2H-azirines, dramatically depend on the nature of substituents. 4,4-Diphenyl-2-azabuta-1,3-dienes with two electron-acceptor substituents at C(1) undergo thermal 1,5-cyclization to give indoles in good yields. The increase in electron-withdrawing ability of C(1)-substituents facilitates the reaction that proceeds via pseudopericyclic 1,5-electrocyclization of 2-azabutadiene into 7aH-indolium ylide followed by prototropic shift. 3,4-Diphenyl-2-azabuta-1,3-dienes, resulting from reaction of 2,3-diphenyl-2H-azirine and diazo compounds, do not produce indoles via 1,5-cyclization due to the trans-configuration of the 4-Ph-group and the nitrogen, but undergo 1,4-cyclization to 2,3-dihydroazetes. 1,6-Cyclization into 2H-1,4-oxazines with participation of the oxygen of ester or amide group at C(1) of corresponding 2-azabuta-1,3-dienes does not take place due to kinetic and thermodynamic reasons. Instead of this, 1,6-electrocyclization with participation of phenyl substituent at C(4) of the 2-azabuta-1,3-dienes, providing isoquinoline derivatives, can occur at elevated temperatures. The DFT-calculations (mPWB1K/6-31+G(d,p)) confirm the dependence of 2-azabuta-1,3-diene transformation type on the nature of substituents.

Cu(I)–NHC-Catalyzed (2 + 3)-Annulation of Tetramic Acids with 2H-Azirines: Stereoselective Synthesis of Functionalized Hexahydropyrrolo[3,4-b]pyrroles

A stereoselective and high-yield synthesis of hexahydropyrrolo[3,4-b]pyrroles from tetramic acids and 2H-azirines under Cu(I)-NHC catalysis is developed. An unusual N-C2 azirine bond cleavage, initiated by a copper enolate, was rationalized in terms of a free radical reaction mechanism.

Switchable Synthesis of Pyrroles and Pyrazines via Rh(II)-Catalyzed Reaction of 1,2,3-Triazoles with Isoxazoles: Experimental and DFT Evidence for the 1,4-Diazahexatriene Intermediate

4-Aminopyrrole-3-carboxylates and pyrazine-2-carboxylates were synthesized from 5-alkoxyisoxazoles and 1-sulfonyl-1,2,3-triazoles by tuning the Rh(II) catalyst and the reaction conditions. The reaction in chloroform at 100 °C under Rh2(OAc)4 catalysis provides 4-aminopyrrole-3-carboxylates in good yields. The use of Rh2(Piv)4 in refluxing toluene results in the formation of 1,2-dihydropyrazine-2-carboxylates as the main products, which can be converted by a one-pot procedure to pyrazine-2-carboxylates by heating with catalytic amounts of TsOH. According to the NMR and DFT investigations of the reaction mechanism, pyrroles and dihydropyrazines are formed, respectively, via 1,5- and 1,6-cyclization of common (5Z)-1,4-diazahexa-1,3,5-triene intermediates. The influence of the nature of the catalyst on the product distribution is rationalized in terms of the Rh-catalyzed isomerization of a pyrrolin-2-ylium-3-aminide zwitterion, the primary product of 1,4-diazahexatriene 1,5-cyclization.

Azirinium ylides from α-diazoketones and 2H-azirines on the route to 2H-1,4-oxazines: three-membered ring opening vs 1,5-cyclization

Summary Strained azirinium ylides derived from 2H-azirines and α-diazoketones under Rh(II)-catalysis can undergo either irreversible ring opening across the N–C2 bond to 2-azabuta-1,3-dienes that further cyclize to 2H-1,4-oxazines or reversibly undergo a 1,5-cyclization to dihydroazireno[2,1-b]oxazoles. Dihydroazireno[2,1-b]oxazoles derived from 3-aryl-2H-azirines and 3-diazoacetylacetone or ethyl diazoacetoacetate are able to cycloadd to acetyl(methyl)ketene generated from 3-diazoacetylacetone under Rh(II) catalysis to give 4,6-dioxa-1-azabicyclo[3.2.1]oct-2-ene and/or 5,7-dioxa-1-azabicyclo[4.3.1]deca-3,8-diene-2-one derivatives. According to DFT calculations (B3LYP/6-31+G(d,p)), the cycloaddition can involve two modes of nucleophilic attack of the dihydroazireno[2,1-b]oxazole intermediate on acetyl(methyl)ketene followed by aziridine ring opening into atropoisomeric oxazolium betaines and cyclization. Azirinium ylides generated from 2,3-di- and 2,2,3-triaryl-substituted azirines give rise to only 2-azabuta-1,3-dienes and/or 2H-1,4-oxazines.

Synthesis of 2-halo-2 Н -azirine-2-carboxylic acid amides and esters by isomerization of 5-(dialkylamino/alkoxy)-substituted isoxazoles, catalyzed by iron(II) sulfate

N,N-Dialkylamides and esters of 2-(chloro/bromo/iodo)-2H-azirine-2-carboxylic acids were synthesized by isomerization of 4-halo-5-(dialkylamino/alkoxy)isoxazoles in the presence of catalytic amounts of FeSO4·7H2O. The use of iron(II) sulfate as catalyst, compared to its chloride, provides the advantage of avoiding halide exchange products in isomerization reactions of 4-bromo- and 4-iodoisoxazoles, as well as prevents catalyst deactivation by the 5-amino substituent of isoxazole, compared to rhodium(II) carboxylates.

(3Z)-2-azahexa-1,3,5-trienes: Generation and regioselectivity of 1,5- and 1,6-cyclizations

Unlike electron-rich analogs, electron-deficient azaand oxazapolyenes readily undergo thermal and catalytic 1,4-, 1,5-, and 1,6-cyclizations to produce various 4-, 5-, and 6-membered N-, N,N-, and N,O-heterocycles [1–10]. Such cyclizations have been reported for 2-aza-, 3-aza-, 1-oxa-4-aza-, 1-oxa-5-aza-, and 1,4diazahexa-1,3,5-trienes, some of which are fairly stable, while the others are reactive intermediates generated by either aza-Wittig reaction or carbenoidinitiated ring opening of 2H-azirines or isoxazoles. Only 1,6-cyclizations to pyridine derivatives have been reported for 2-azahexa-1,3,5-trienes generated by reaction of phosphazenes with carbonyl compounds [9, 10]. Herein we describe the first example of generation of electron-deficient (3Z)-2-azahexa-1,3,5-trienes (4) via carbenoid-initiated azirine ring expansion and their 1,5-cyclization; stereochemical factors determining the cyclization regioselectivity have also been determined. Initial azirines E-1 and Z-1 were synthesized according to the procedure reported in [11]. The reaction of E-1 with diazo ester 2 in boiling 1,2-dichloroISSN 1070-4280, Russian Journal of Organic Chemistry, 2016, Vol. 52, No. 12, pp. 1851–1853.

Rh 2 (OAc) 4 -catalyzed reaction of 2-(2-carbonylvinyl)-3-phenyl-2 H -azirines with diazo esters

Rh2(OAc)4-catalyzed reaction of 2-(2-carbonylvinyl)-3-phenyl-2H-azirines with diazo esters proceeds through an intermediate generation of azirinium ylide suffering a nonstereoselective ring opening to form (3Z)- and (3E)-2-azahexa-1,3,5-trienes. The former depending on configuration of the C5=C6 bond may undergo cyclization either in derivative of 2,3-dihydropyridine, or in pyrrolium ylide that isomerizes into a derivative of 1H-pyrrole. According to DFT calculation, the preferred formation of pyrroles at increasing volume of Z-substituent at the atom C6 and of substituents at the atom C1 of 2-azahexatriene occurs due to the destabilization of more sterically loaded transition states of 1,6-cyclization.

Two-atom azirine ring expansion reaction of methyl 2-diazo-3-(4-methoxyphenyl)-3-oxopropanoate via a dirhodium tetraacetate-catalyzed Wolff rearrangement

Rh2(OAc)4-catalyzed Wolff rearrangement of methyl 2-diazo-3-(4-methoxyphenyl)-3-oxopropanoate gave methoxycarbonyl(4-methoxyphenyl) ketene that was added to 2Н-azirines, resulting in opening of the three-membered ring at the N–C(2) bond and leading to the formation of 3,4-dihydro-2Н-pyrrol-2-one derivatives. Depending on the structure of the obtained products, they isomerized to the more stable 1Н-pyrrol-2(3H)-one derivatives or added a water molecule at the C=N bond.

Rh(II)-Catalyzed Transannulation of 1,2,4-Oxadiazole Derivatives with 1-Sulfonyl-1,2,3-triazoles: Regioselective Synthesis of 5-Sulfonamidoimidazoles

An effective method for the synthesis of fully substituted 5-sulfonamidoimidazoles by Rh(II)-catalyzed transannulation of 1,2,4-oxadiazole derivatives with N-sulfonyl-1,2,3-triazoles is reported. The reaction works well with both aromatic 1,2,4-oxadiazoles and 1,2,4-oxadiazol-5-ones providing a flexible approach to N-(alkoxy/amino)carbonyl- and N-alkyl-substituted imidazoles. Both the disclosed reactions are completely regioselective and provide the first examples of a carbenoid-mediated transformation of N,N,O-heterocycles.