Regioselective synthesis of 2,3,4- or 2,3,5-trisubstituted pyrroles via [3,3] or [1,3] rearrangements of O-vinyl oximes

The regioselective synthesis of 2,3,4- or 2,3,5-trisubstituted pyrroles has been achieved via [3,3] and [1,3] sigmatropic rearrangements of O-vinyl oximes, respectively. Iridium-catalyzed isomerization of easily prepared O-allyl oximes enables rapid access to O-vinyl oximes. The regioselectivity of pyrrole formation can be controlled by either the identity of the α-substituent or through the addition of an amine base. When enolization is favored, a [3,3] rearrangement followed by a Paal-Knorr cyclization provides a 2,3,4-trisubstituted pyrrole; when enolization is disfavored, a [1,3] rearrangement occurs prior to enolization to produce a 2,3,5-trisubstituted pyrrole after cyclization. Optimization and scope of the O-allyl oxime isomerization and subsequent pyrrole formation are discussed and mechanistic pathways are proposed. Conditions are provided for selecting either the [3,3] rearrangement or the [1,3] rearrangement product with β-ester O-allyl oxime substrates.     

An unexpected base-induced [1,4]-phospho-Fries rearrangement

An unexpected [1,4]-phospho-Fries rearrangement that gives rise to the formation of a O,O,O,O-tetraethyl methylenebis(thiophosphonate) derivative is reported. The regioselectivity of the metallation with n-BuLi or t-BuLi is the key factor that explains either the [1,4] or [1,3] rearrangement observed.     

Selective [5,5]‐Sigmatropic Rearrangement by Assembly of Aryl Sulfoxides with Allyl Nitriles

Aromatic [5,5]‐sigmatropic rearrangement is an appealing protocol for accessing 1,4‐substituted arenes. However, such a protocol has not been well utilized in organic synthesis because of the difficulties in the synthesis of the substrates, selectivity issues, and limited substrate scope. Described herein is a new [5,5]‐sigmatropic reaction utilizing readily available aryl sulfoxides and allyl nitriles. This reaction features mild reaction conditions, high chemo‐ and regioselectivity, excellent functional‐group compatibility, and broad substrate scope. Computational studies suggest that the success of the reaction can be attributed to the selective electrophilic assembly of the rearrangement precursors, in which a linear ‐C=C=N‐ linkage favors [5,5]‐sigmatropic rearrangement over the competitive [3,3]‐sigmatropic rearrangement.     

二价钌催化的金属卡宾经由的硫叶立德[2,3]-σ重排反应研究

报道烯(炔)基硫醚与α-重氮羰基化合物, 在[RuCl₂(p-cymene)]₂催化下, 经由金属卡宾发生硫叶立德[2,3]-σ重排反应(Doyle-Kirmse反应). 在Ru(II)作用下, α-重氮羰基化合物与烯丙基硫醚的反应以较好收率生成相应的[2,3]-σ重排产物高烯丙基硫醚. 同样条件下与炔丙基硫醚的反应则生成[2,3]-σ重排产物联烯和呋喃衍生物, 后者是联烯进一步在Ru(II)作用下重排的产物.     

Distinct chemoselectivities in the platinum-catalyzed 1,2-carboalkoxylations of 5-alkoxypent-1-yn-3-ol derivatives

Two distinct Pt-catalyzed carboalkoxylations of alkynes are reported. The cycloisomerization of 5-alkoxypent-1-yn-3-ol derivatives 5 produces 2,6-dioxabicyclo[3.1.0]hexanes 6; the mechanism is postulated to involve a hydroxyl-triggered [3.3]-sigmatropic allyl rearrangement. As the same catalysis is extensible to their tertiary alcohol analogues 7, distinct dihydrofuranyl alcohols 8 were obtained through a [3.3]-allyl rearrangement that is not assisted by the hydroxyl group.     

[3,3]-Sigmatropic rearrangement of allyl and 2-butenyl 1-naphthyl sulfides

The [3,3]-sigmatropic rearrangement of alkenyl 1-naphthyl sulfides in solutions with various polarities was investigated at 138–190 °C. The reaction proceeds through the formation of 2-alkenyl-1-naphthalene thiols, which subsequently undergo cyclization to compounds of the 2,3-dihydronaphtho[1,2-b]thiophene and naphtho[1,2-b]dihydrothiopyran series. 2-Butenyl 1-naphthyl sulfide, in addition to its passing directly through a [3,3]-sigmatropic rearrangement, to a considerable extent undergoes a prior [1,3]-sigmatropic rearrangement, which ultimately leads to the formation of four cyclic products. The kinetic parameters of the rearrangement of the sulfides were determined. The more negative entropies of activation constitute evidence for the high symmetry of the transition state.Translated from Khimiya Geterotskilicheskikh Soedinenii, No. 5, pp. 611–614, May, 1981.     

Neue 1,6-Methano[10]annulen-Derivate durch Umlagerungsreaktionen der Trimethylsilyl-Gruppe

New Derivatives of 1,6-Methano[10]annulene by Rearrangement of Trimethylsilyl Groups Oxidation of derivatives of trimethylsilyl-substituted bicyclo[4.4.1]undeca-1,3,5,7-tetraenes 1–3 leads to 1,6-methano[10]annulenes. In the case of 2 and 3a , rearrangement of the trimethylsilyl group takes place. This rearrangement can be used to prepare 2,9-disubstituted 1,6-methano[10]annulenes.     

Competing Rearrangements of Ammonium Ylides: A Quantum Theoretical Study

The competition between the Stevens [1,2] and Sommelet-Hauser [2,3] rearrangements for a prototype ylide, N-methyl-3-propenylammonium methylide, has been investigated using ab initio and semiempirical molecular orbital methods. The activation energies for the two processes are remarkably close, separated by only 2 kJ mol(-)(1) at ROMP/6-311+G(d,p). Increasing the size of the basis set leads to a relative stabilization of the Sommelet-Hauser transition geometry, while higher levels of electron correlation (such as CCSD(T)) favor the Stevens rearrangement. Incorporation of solvent effects via the SCRF polarizable continuum model leads to a lowering of the energy barrier of the concerted [2,3] rearrangement, but has little effect on the dissociative [1,2] pathway. The activation energies of both pathways have been calculated for ylides bearing substituents on the ammonium nitrogen and the double bond. Substituents at nitrogen lead to an ylide which is sterically unstable and hence a preference for the dissociative [1,2] rearrangement. Electron-withdrawing substituents on the double bond show a preference for the [2,3] rearrangement, while mildly electron-donating alkyl substituents have very little effect on activation energies.     

1,6-methano[10]annulene-stabilized radicals

The rate of the methylenecyclopropane rearrangement is remarkably enhanced by the 1,6-methano[10]annulene group, which greatly stabilizes the biradical intermediate. The diastereomeric products argue against a concerted rearrangement mechanism and support a stabilized biradical intermediate that lives long enough to undergo rotation before ring closure. [structure: see text]     

Rearrangements of the [2+2]-cycloadducts of DDQ and 2-ethynylpyrroles

The [2+2]-cycloadducts of DDQ and 2-ethynylpyrroles, upon ethanolysis (reflux, 15 min or room temperature, 24 h), rearrange from bicyclo[4.2.0]octadienediones to bicyclo[3.2.0]heptadienone- and cyclobutenyl-dihydrofuranone moieties in 55-83% yields, the former rearrangement being the major direction. Benzoquinone ring cleavage is regioselective to afford mostly bicyclo[3.2.0]heptadienone-pyrrole ensembles (85-90% selectivity) in 39-78% yields. The only exception is when the starting compounds contain an ethoxycarbonyl substituent and the pyrrole counterpart is a 4,5,6,7-tetrahydroindole fragment. In this case, the ratio of the rearrangement products is 1:1.2 in a total yield of 83%. An important feature of the dihydrofuranone pathway rearrangement is its 100% diastereoselectivity.     

Mechanistic insights into the rearrangement of azetidine N-oxides to isoxazolidines

Various functionalized azetidines were oxidized with mCPBA or hydrogen peroxide, to produce the corresponding N-oxide and study its fate in the [1,2] Meisenheimer rearrangement. This ring expansion leading to isoxazolidines occurs readily, without trapping of the transient N-oxide. Starting with azetidines bearing a nitrile or an ester group at C-2, the rearrangement is regioselective. However, a varying amount of epimerization on the migrating radical is observed, which can also be observed with the related [1,2] Stevens rearrangement.     

Can relief of ring-strain in a cyclopropylmethyllithium drive the Brook rearrangement?

α-Cyclopropyl-α-trialkylsilyl alkoxides were formed either by addition of cyclopropyllithiums to acylsilanes or by addition of organolithiums to a cyclopropylformylsilane. [1,2]-Brook rearrangement led to α-silyloxy organolithiums which on warming underwent cyclopropane ring opening and [1,5]-retro-Brook rearrangement to yield γ-silyl ketones. Despite the favourability of the cyclopropane ring opening, the Brook rearrangement still required the presence of an anion stabilising group to proceed. β-Silylketones were similarly formed by Brook-retro-Brook rearrangement on warming acylsilanes with a vinyllithium.     

Diazabicycloalkanes with bridgehead nitrogen atoms. 28. Stevens rearrangement of benzodiazabicycloalkenes

Stevens rearrangement of the benzyl bromides of benzo[b]-1,4-diazabicyclo[2.2.2]octene and benzo[f]-1,5-diazabicyclo[3.2.2]-nonene occurs with expansion of the diazabicyclic fragments to form a mixture of stereoisomers. Both the ethylene and the trimethylene bridges participate in the rearrangement of the nonene.For Communication 27 see [1].Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 3, pp. 383–387, March, 1993.     

1,5]-Anion relay/[2,3]-Wittig rearrangement of 3,3-bis(silyl) allyl enol ethers: synthesis of useful vinyl bis(silane) species

The [1,5]-anion relay/[2,3]-Wittig rearrangement of 3,3-bis(silyl) enol allyl ethers has been developed. This reaction provides an efficient method to synthesize versatile vinyl bissilanes, which can be transformed into trisubstituted vinylsilanes through a [1,4]-Brook rearrangement/alkylation protocol using a wide range of electrophiles.     

The [1, 3] O-to-C rearrangement: opportunities for stereoselective synthesis

The relay of stereochemistry of a breaking C-O bond into a forming C-C bond is well-known in the context of [3, 3] sigmatropic shifts; however, this useful strategy is less well-known in other types of molecular rearrangements. Though the first successful example of a [1, 3] O-to-C rearrangement was reported more than 100 years ago, this class of reactions has received less attention than its [3, 3] counterpart. This perspective analyzes the various methods used for the activation and [1, 3] rearrangement of vinyl ethers with an emphasis on mechanism and applications to stereoselective synthesis. We also highlight our own contributions to this area.     

New spirocyclic oxindole synthesis based on a hetero Claisen rearrangement

[reaction: see text] A new method for preparing spirocyclic oxindoles is presented. Featuring a [3,3]-sigmatropic enolate rearrangement, the three-step process converts carboxylic acid starting materials to oxidnole products in overall yields of 52-76%. The enolate rearrangement step occurs at -78 degrees C and provides easy access to oxindole products that have previously been difficult to prepare.     

Skeletal Wagner-Meerwein rearrangement of perhydro-3a,6;4,5-diepoxyisoindoles

An investigation of a skeletal Wagner-Meerwein rearrangement of variously substituted or quinoline-annulated 3a,6;4,5-diepoxyisoindol-1-ones is reported. Optimum reaction conditions (Ac₂O, BF₃·OEt₂, rt) were discovered for the formation of the target 4,6-epoxycyclopenta[c]pyridines in 40-80% yields. It was shown that the direction of the sigmatropic rearrangement of 3a,6;4,5-diepoxyisoindol-1-ones depended dramatically on the carboxyl group position (exo-/endo-) in the oxabicyclo[2.2.1]heptane moiety. The spatial structure of previously unknown 7,9-epoxycyclopenta[4,5]pyrido[1,2-a]quinolines derived from Wagner-Meerwein rearrangement of 2,11b-epoxyoxireno[6,7]isoindolo[2,1-a]quinolines was established based on the X-ray analysis data. The skeletal rearrangement proceeded regio- and stereospecifically in all the cases examined due to the absence of the epimerization of the carbon atoms adjacent to the carbocation centres.     

Metal-free entry to phosphonylated isoindoles by a cascade of 5-exo-dig cyclization, a [1,3]-alkyl shift, and aromatization under microwave heating

[reaction: see text] When o-ethynylbenzyl alpha-aminophosphonates are heated under microwave conditions, a rearrangement occurs which results in the formation of phosphonylated isoindoles. The rearrangement consists of a 5-exo-dig cyclization followed by a [1,3]-alkyl shift and finally aromatization.     

Isomers and [2,3]-Sila-wittig Rearrangement of [(Allyloxy)sily]lithium Silylenoid in the Gas Phase and THF Solvent

Theoretical calculations of the [2,3]-sila-wittig rearrangement of isomers of [(allyl-oxy)silyl]lithium (C3H50)H2SiLi have been performed in the gas phase and THF solvent using theG3MP2B3 method.Seven isomers of silylenoid (C3H5O)H2SiLi, 1～7, are found.The [2,3]-sila-wittig rearrangement paths are followed using two isomers, 2 and 4, to yield the transition states as well as the products.In the transition state, the silicon center functions as a nucleophile and the allyl as an electrophile.The interaction between the silicon and allylic sites leads to the formation of Si-C[3] bond and the break of O-C[1] bond.Finally, the (allylsilyl)oxylithium (C3H5)H2SiOLi is obtained.The rearrangement paths are confirmed by the intrinsic reaction coordinate (IRC)calculations.The rearrangement mechanisms of reactions of 2 and 4 are similar, and the latter reaction is more favored in the gas phase and THF solvent.Also, the solvent effects are analyzed in this work.     

Interplay of orbital symmetry and nonstatistical dynamics in the thermal rearrangements of bicyclo[n.1.0]polyenes.

CASSCF and CASPT2 calculations have been carried out on some of the thermal rearrangements of bicyclo[2.1.0]pentene (BCP), bicyclo[4.1.0]hepta-2,4-diene (BCH), bicyclo[6.1.0]nona-2,4,6-triene (BCN), and 9,9-dicyanobicyclo[6.1.0]nona-2,4,6-triene (DCBCN). In addition, experiments have been conducted to determine the stereoselectivity and temperature dependence of the nondegenerate rearrangement of 9,9-dicyanobicyclo[6.1.0]nona-2,4,6-triene-exo-15N. The calculations and experiments allow a consistent picture to be drawn for these reactions. The principal conclusions are as follows. (1) The ring-walk rearrangements of BCP, BCN, and DCBCN are pericyclic reactions occurring with a strong preference for inversion of configuration at the migrating carbon. However, the ring-walk rearrangement of BCH is a nonpericyclic reaction. (2) The rearrangement of DCBCN to 9,9-dicyanobicyclo[4.2.1]nona-2,4,7-triene occurs with a preferred stereochemistry corresponding to a 1,3 migration with retention. However, this reaction is not a pericyclic process; the stereoselectivity is probably of dynamic origin. (3) Cyano substituents can significantly reduce the activation energy for a reaction occurring via a singlet biradical, but they do not necessarily cause the intermediate to sit in a deeper local minimum on the potential energy surface.