Thiol Mediated Free Radical Cyclization of Alkenyl and Alkynyl Isocyanides (original) (raw)
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Organic Letters, 2003
But-3-enylthioimidoyl radicals were shown by EPR spectroscopy and end product analysis to ring-close predominantly in the 5-exo mode with a rate constant of 2.4 × 10 4 s-1 at 300 K to afford substituted dihydrothiophenylmethyl radicals. This ring closure was in competition with dissociation to but-3-enyl radicals and an isothiocyanate. The dissociation predominated at temperatures above ca. 300 K. Interest in imidoyl radicals (R 1-NdC • X-R 2) as synthetic intermediates has been stimulated by the discovery of their mediation of several useful annulations 1,2 and also as a consequence of their formation from sulfanyl radical additions to alkenyl isonitriles and subsequent cyclizations. 3,4 Recently, arylthioimidoyl radicals, made by addition of either arylsulfanyl radicals to aryl isonitriles or of aryl radicals to isothiocyanates, have been exploited in cascade syntheses of benzothieno-quinoxalines 5 and-quinolines 6 and thio-chromenoindoles. 7 Alkanethioimidoyl radicals (X) S, R 2) alkyl) undergo rapid scission of the S-R 2 bond to produce isothiocyanates, together with the corresponding alkyl radicals (R 2•). 8 Despite competition from this dissociation, Bachi and co-workers successfully exploited ring closures of thioimidoyls, onto unsaturated N-substituents (R 1) alkenyl), to afford a variety of N-heterocycles, including kainic acid. 3b-d The rate of dissociation of alkanesulfanylimidoyl radicals is high when the released C-centered radical is tertiary or resonance stabilized but moderate for primary radicals. 9 We reasoned, therefore, that ring closure onto the alkenesulfanyl moiety should be observable in some circumstances leading
Tetrahedron, 1992
Key Wordr~ sequential radical reactions; 3 +2 cycloadditions; S-phenylthio-3-pentenyl radicals, thiohydmxamic esters of 6-phenylthio4hexenoic acids. Abstmct: Sequential radical a&iition/5-exo-cyclization&elimination reactions, accomplished by therm&y or photolyticalty induced decompositions of 0-acyl derivatives of N-hydroxypyridine-2-thione or other thiohydroxamic esters of 6-phenylthio-4-herenoic acids II in the presence of an excess of radicophilic ole~ns, afforded the 2vinylcyclopentanc den'vatives 13 in 5090% yields, while decompositions of thiohydroxamic esters or 6-phenylthio-4-hexynoic acids 24, under the same experimental conditions and in the presence of electron deficient oleflts, affordedthecorre.vponding2-vinylidenecyclopentane (2-o-alleniccyclopentane)derivatiws25 @I-72%). However, when &compositions of thiohydroxamic esters 39 were carried out in boiling toluene solution without rodicophilic olefins. 3-e-co-cyclization took place and the corresponding 2-vinylcyclopropane der&tives 40 were obtained in 43-60% yields. Tandem sequence of radical addition/cyclization reaction represents a simple and very useful methodology for annulation of polysubstituted cyclopentane rings. te3 Starting from two appropriate unsaturated synthons 1 and 2, by this sequence of reactions, 3 + 2 cycloaddition takes place involving two carbon-carbon bonds formations. 134*5 In addition to the tandem sequences4*6-'6. higher sequential radical transformations were Scheme 1. systematically investigated by designing the appropriate substrates, selection of reaction conditions and reagents, and following sequences such as additionlcyclizationladdition, 1*2 addition/cyclization/atom transfer17 and fragmentation/addition/cyclization'8 were successfully applied in the synthesis of complex polycyclic compounds. t6 In all of these sequential radical reactions several radical intermediates were involved in the propagation step of these radical chain reactions. The reactivities of the intermediary radicals involved in the key sequence, e.g. addition/cyclization, have to be in accordance with the reactivity of the corresponding unsaturated bonds,"
Radical annulation: a method for preparation of carbocycles
Journal of the Chemical Society, 1985
By thermally or photolytically induced decomposition of 0-acyl derivatives of N-hydroxypyridine-2-thione or other thiohydroxamic esters, in the presence of an excess of electron deficient olefins, 2-vinylcyclopentane derivatives were obtained. This sequence of addition/cyclization/elimination reaction is mediated by a phenylthio radical. Cyclopentane ring annulation by a 3 + 2 radical cycloaddition reaction of 3-butenyl type radical 2 to the electron deficient olefins 1 represents a useful methodology for the construction of polysubstituted cyclic molecules 4. 1-6 On the other hand a method for the 7 free radical allylation by allyltributylstannanes or ally1 t-butylsulphide8, involving Reaction products were characterized by 2, nmr and mass spectra and e.g. 2-vinyl-4-benz
Manganese(III)-promoted free radical cyclizations of enamides leading to β-Lactams
Tetrahedron, 1997
Reaction of variously substituted enamides with Mn(OAc)y2H20 afforded 13lactamic products in modest to good yields, depending mostly on the solvent and reaction conditions. The substitution pattern of the enamidic double bond was found of primary importance for the outcome of the reaction. © 1997 Published by Elsevier Science Ltd.
C-C Bond Formationby Radical Cyclization: Synthesis of Pyrimidine-Annulated Heterocycles
Synthesis, 2003
A number of pyrimidino[3,2-c]tetrahydroisoquinolin-2,4-diones are synthesized in excellent yields from 1,3-dialkyl-5-(N-2¢-bromobenzyl,N-methyl)aminopyrimidine-2,4-diones by intramolecular addition of aryl radical to the uracil ring bearing the amino nitrogen atom. Usual aerial oxidation in this type of cyclization with Bu 3 SnH is not observed in the present instance.
Cyclizations of N-Stannylaminyl Radicals onto Nitriles
Organic Letters, 2004
Stannylaminyl radicals derived from radical reactions of Bu 3 SnH with azidoalkylmalononitriles exhibit highly efficient 5-and 6-exo cyclization onto either nitrile group to give aminoiminyl radicals that in turn are reduced to amidines or undergo successive 5-exo cyclization onto an internal alkene. Radical cyclization reactions have become a powerful tool for the construction of carbocyclic and heterocyclic systems, even those occurring in natural products. 1 Radical cyclizations most often involve carbon-carbon bond formation, whereas those leading to carbon-nitrogen bonds are much less documented. The reported methods rely on additions of aminyl 1e,2 and iminyl 1e,3 radicals to CdC and CdO double bonds or additions of carbon radicals to nitrogen atoms of imines. 1e,4 The synthetic potential of azide radical reactions has so far been rather poorly investigated, though the reported studies have revealed that the azido moiety can act as a valuable radical acceptor toward carbon-and heteroatomcentered species to yield an aminyl radical after loss of molecular nitrogen by the initial triazenyl adduct. 2e,f,5 Indeed, intramolecular additions of aryl, 6 thiocarbonyl, 7 alkyl, 8 vinyl, 9