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III. Radical Reactions

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    24004
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    A. Simple Reduction

    1. Typical Reaction Conditions

    Simple reduction of a halogenated carbohydrate is a reaction in which the only change that occurs in the substrate is replacement of a halogen atom with a hydrogen atom. This change typic­ally is brought about by heating a solu­tion of a halogenated carbohydrate, tri-n-butyltin hydride, and a catalytic amount of 2,2'-azobis(isobutyronitrile) (AIBN) in benzene or toluene at 80-110 oC for a few hours.

    2. Thermodynamic Driving Force for Reaction

    There is a powerful thermodynamic driving force for simple reduction of halogenated compounds with tri-n-butyltin hydride.14 This driving force is apparent in the highly exothermic reaction of bromomethane with tri-n-butyltin hydride (eq 6).14 The exothermic nature of this reaction derives from a carbon–bromine bond being replaced by a stronger carbon–hydrogen bond and a tin–hydrogen bond being exchanged for a stronger tin–bromine bond. The data in eq 7 show that simple reduction of chloromethane is also quite exothermic.14

    II2(6).png

    II2(7).png

    3. Synthesis of Deoxygenated Sugars and Nucleosides

    Simple reduction is a common reaction for halogenated sugars and nucleosides. A typical example, one in which halogen replacement takes place at C-4 in a pyranoid ring, is shown in eq 8.23 Other reactions, ones where the halogen atom is located at C-1,24,25 C‑2,26,27 C-3,28 C‑4,29,30 C-5,31 or C-632,33 in a pyranoid ring, are common. There are fewer reports of sim­­ple reduction when the halogen atom is attached to a furanoid ring (C‑1,34,35 C‑5,36,37 or C-638 ), but many nucleoside reactions are known where the halogen atom is bonded to C‑2',39,40 C-3',41,42 or C-5' 43,44 in the substrate. (Each reference listed above refers to a simple-reduction reaction repre­sent­ative of those taking place at the indicated carbon atom.)

    II2(8).png

    When a halogen atom is replaced by a deuterium atom, simple reduc­tion can provide infor­mation about reaction stereoselectivity; thus, in the reac­tion shown in Scheme 7 the approach of Bu3SnD to the nucleoside radical 3 is from the less hindered face of the furanoid ring.45 Reaction at the 2'‑position in other halo­gen­ated nucleosides also is stereo­selective, but this selectivity, which is heavily influenced by the structure and positioning of ring sub­stituents, sometimes is modest.46–48

    II2s7.png

    4. Protecting Group Modification

    Simple reduction sometimes is used to mod­ify protecting group reac­tivity.49–57 In the reac­tion shown in eq 9, for example, replacement of the three chlorine atoms in the tri­chloro­eth­yl­idene group with three hydrogen atoms makes hydrolytic removal of this group much easier.49 In a related example, the trichloroacetamido group is converted to the more easily hydrolyzed acet­amido group by three, successive simple reduction reactions (eq 10).56 Reaction with tri-n-butyltin hydride as a means of replacing all the chlorine atoms in a protecting group with hydrogen atoms is not always successful because complete dechlorination sometimes is elusive (eq 1158).58–60

    II2(9).png

    II2(10).png

    II2(11).png

    5. Acyloxy Group Migration

    When an anomeric halide with a neighboring acyloxy group reacts with tri-n-butyltin hydride, acyloxy group migration leading to a 2-deoxy sugar competes with simple reduc­tion (Scheme 8).61 If 2-deoxy sugar synthesis (and not simple reduction) is the goal of the reaction,61–64 conditions that maximize group migration need to be selected. Generally, this means maintaining the tri-n-butyltin hydride concentration at a low enough level to allow time for migration to occur.

    II2s8.png

    B. Addition

    Halogenated compounds are common radical precursors for addition reactions involving carbo­hydrates, but other carbohydrate derivatives also function in this capa­city; consequently, radi­cal addition reactions are mentioned in most of the chapters in this book. They are discussed extensively not only in this chapter but also in Chapters 10, 12, 13, and 18. In Chapter 18 radical addition is considered from a different point of view, that is, with a focus on the compound to which addition is occurring rather than on the radical precursor.

    Addition reactions can be divided into three groups based on what happens after formation of the adduct radical. The first group (addition-abstraction reactions) contains those reactions where the adduct radical abstracts a group or atom from a donor present in solution. Atom abstraction is more common that group abstraction and nearly always involves a hydrogen atom. The second type of reaction (addition-combination) is one in which the adduct radical combines with another radical or an organometallic reagent present in the reaction mixture. In the third reaction type (addition-elimination or addition-fragmentation) the adduct radical forms an un­sat­u­rated compound by a β-fragmentation process that eliminates a radical. Examples in which halo­­gen­ated carbohydrates serve as radical precursors for each of these reaction types are given in the following three sections.

    1. Addition-Abstraction Reactions

    Formation of a carbon-centered radical by reaction of a glycosyl halide with a tin- or sili­con-centered radical is the first step in many addition reac­tions. An addition-abstraction reaction takes place when a radical produced by halogen-atom abstraction adds to a multiple bond to generate an adduct radical that then abstracts a hydrogen atom. The hydrogen-atom transfer is nearly always a tin- or silicon hydride. An example of this type of reaction is given in eq 12, where a pyranos-1-yl radical generated from the D-manno­pyranosyl bromide 4 adds to the α,β-unsaturated ketone 5.65 Pyran­os-1-yl radicals generated from halogenated carbohydrates are known to add to α,β-unsa­tur­ated nitriles,66,67 esters,67,68 aldehydes,67 ketones,69,70 lac­tones,71,72 and phos­phon­ates.73,74 In all of these reactions forma­tion of the adduct radical is followed by hydrogen-atom abstraction. Pyranos-1-yl and other carbohydrate radicals formed from halogenated compounds also add to oximes75–77 and electron-deficient enol ethers.78,79 The reactions shown in equations 13 and 14 underscore a critical feature of radical addition reactions, namely, that unless a multiple bond is electron-deficient (eq 13), addition of a nucleophilic radical (which nearly all carbohydrate radicals are) will be too slow to compete effectively with simple reduction (eq 14).80

    II2(12).png

    II2(13).png

    II2(14).png

    Addition-abstraction reactions that do not involve pyranos-1-yl radi­cals are much less common than those that do. An example of a non-pyranos-1-yl radical reaction is shown in eq 15.81 The pyranos-5-yl radical formed in this reaction has reactivity similar to that of a pyranos-1-yl radical because both are nucleophilic species that are stabilized by a ring oxygen atom. This sim­ilarity in reactivity can be seen when comparing the reaction shown in eq 1581 with that in eq 16.82 Several addition-abstraction reac­tions of radicals centered on C-683–86 are known, but reaction of a radical located on an atom in or attached to a pyranoid ring (other than C-1or C-6) is rare.87 There are reports of addition-abstraction reactions where the radical is centered on a carbon atom that is part of or attached to a furanoid ring.88–90

    II2(15).png

    II2(16).png

    2. Addition-Combination Reactions

    Although radical formation from a glycosyl halide normally involves halogen-atom abstraction by a tin or silicon centered radical, a radical also can be gen­er­ated by electron transfer to the glycosyl halide from a metal ion, such as the samarium ion in SmI2 or the titanium ion in Cp2TiCl. An example of this type of reaction is shown in Scheme 9, where the pyranos-1-yl radical (R·) is formed by electron transfer from titanium to the glycosyl bromide (RBr).91 In the reaction shown in Scheme 9, radical addition to an electron-deficient multiple bond is faster than combination of the radical with a second mole­cule of Cp2TiCl. Once addition occurs, however, the situation changes. The reactivity of the adduct radical is so different from the far more nucleophilic pyranos-1-yl radical that the fastest reaction for the adduct radical is combin­ation with a molecule of Cp2TiCl. (Chapters 20-24 contain further discussion of addition-combination reactions brought about by electron-transfer from organometallic complexes to carbo­hydrate halides.)

    II2s9.png

    3. Addition-Elimination Reactions

    When a carbon-centered radical adds to allyltri-n-butyltin, it begins a sequence of reactions that replaces a halogen atom with an allyl group (Scheme 1).1 This type of transformation (an addition-elimination reaction) often occurs when the halogen atom is attached to C‑11,92–94 but also takes place when such an atom is bonded elsewhere in a carbo­hydrate frame­work.95,96 For exam­ple, the reaction between the tri-n-butyltin acrylate 6 and the deoxyiodo sugar 7 involves a radical centered at C-6 (eq 17).97

    II2(17).png

    A reaction mechan­istically similar to that shown in eq 17, but one with a quite different out­come, occurs when a halogen atom is abstracted by Bu3Sn· in the presence of t-butyl iso­cyanide. When this happens, an addition-elimination reac­tion produces a nitrile (Scheme 10).98

    II2s10.png

    Most addition-elimination reactions transfer an allyl or substituted allyl group from a tin-con­taining compound to a carbon-centered radical; however, this transfer can be tin-free.99,100 In the reaction shown in eq 18 the allyltin reactant is replaced by allyl ethyl sulfone.99

    II2(18).png

    C. Cyclization

    New ring formation is pervasive in the radical reactions of carbo­hydrates; consequently, radi­cal cyclization, like radical addition, is mentioned in many of the chapters in this book. Sig­nif­icant discussion exists in this chapter because halogenated carbohydrates often are the precursors for radical-based formation of new ring systems. A large portion of Chapter 12 also is devoted to radical cyclization because O-thiocarbonyl compounds frequently are precursors for radicals involved in new ring formation. Chapter 19 is devoted entirely to cyclization reactions and is con­cerned less with radical formation and more with the internal radical addition that produces new rings.

    1. Substrate Reactivity

    As with simple reduction reactions, the abstracting radical that begins radical cyclization nearly always is derived from a tin or silicon hydride. The well-established order of reac­tivity for halogenated compounds with silicon and tin hydrides (RI > RBr > RCl >> RF), mentioned in Section II.B, accounts for the usual selection of a carbohydrate iodide or bromide as the sub­strate in a cyclization reaction.

    Halogen identity is especially critical to dehalo­genation with either SmI2 or Cp2TiCl. In the cyclization reaction shown in Scheme 11 the iodide gives a good yield of the substituted cyclo­pen­tane, but the bromide is enough less reactive that it produces only bromine-containing dimers arising from reduction of the double bond by SmI2.101 Reaction of a similar compound with Cp2TiCl results in an 82% yield of substituted cyclopentanes from the iodide but only an 18% yield from the corresponding bromide (Scheme 12).102 More forcing reaction conditions might have improved product yield from the bromide because its attempted cyclization returned pri­mar­ily unreacted starting material.

    II2s11.png

    II2s12.png

    Successful cyclization of halogenated carbohydrates by reaction with SmI2 or Cp2TiCl de­pends upon reaction conditions and additives. The pres­ence of hexamethylphosphoramide (HMPA) is so important to the reactivity of SmI2 that the cyclic product shown in Scheme 11 does not form unless HMPA is present in the reaction mixture.101 In a similar fashion, UV radiation is critical to the reaction shown in Scheme 12. If it is omitted, little reaction takes place.102

    2. Competition between Cyclization and Hydrogen-Atom Abstraction

    Whenever a cyclization reaction is conducted in the presence of a tin or silicon hydride, hydrogen-atom abstrac­tion by the initially formed radical, leading to simple reduction, com­petes with cyclization (Scheme 13).38 Other reagents that are not hydrogen-atom transfers but are capable of gen­er­ating radicals from halogenated carbo­hydrates (e.g., Bu3SnSnBu3,103 SmI2,101,104,105 and Cp2TiCl102) have the advantage that simple reduction is suppressed. Even though simple reduction is not a problem when using SmI2 and Cp2TiCl, each of these compounds can prevent cyclization by combining with a carbon-centered radical before ring forma­tion takes place; therefore, rapid ring closure remains an important criterion for successful reaction.


    II2s13.png

    3. Organization of Cyclization Reactions

    Successful radical cyclization requires a multiple bond and radical center that are suitably positioned with respect to each other. The radical center in such a reaction can be either on an atom that is part of the carbo­hy­drate frame­work (Figure 2) or on a substit­uent group. The same pos­si­bil­ities exist for the multiple bond. For purposes of discussion it is useful to divide cyclization reac­tions into the four basic types shown in Figure 3. This Figure also contains a short-hand termin­ology that has been proposed to identify each reaction type.105 Chapter 19 con­tains extensive tables of cyclization reactions in which radicals are formed from halogen­ated and nonhalo­genated carbohydrates.

    II2f2.png

    II2f3.png

    a. Addition of a Framework Radical to a Framework Multiple Bond

    The possibilities for a framework radical adding internally to a frame­work multiple bond are limited by the size of the rings that are easily pro­duced; thus, only five- and six-membered rings generally form rapidly enough to compete with other radical reactions. Since radical cyc­li­zation produces five-membered rings more quickly than six-membered ones, the typical cycli­zation reaction produces a pair of heavily substituted, cyclo­pentane derivatives (eq 19106).106–111 When five-membered ring formation is not possible but pro­ducing a six-membered ring is, cyclization gives a subs­ti­tuted, cyclo­hexane derivative (eq 20).112 Addition of a framework radical to a frame­­work multiple bond also can produce a bicyclic compound (Scheme 14).113

    II2(19).png

    II2(20).png

    II2s14.png

    Although forming either a five- or six-membered ring is the typical result of radical cyc­li­za­tion, larger rings are possible if the carbohydrate framework is held so that the radical center easily can approach the multiple bond. Striking examples of such a situation are found in the reactions shown in equations 21 and 22, where the O-isopropylidene groups cause the iodides 8 and 9 to adopt conformations that favor formation of seven- and eight-membered rings, respectively.114


    II2(21).png

    II2(22).png

    One way to increase the reactivity of a multiple bond in a radical cycli­zation reaction is to replace one of the carbon atoms with an electro­negative hetero­atom.115–121 The remaining carbon atom then will have electron density drawn from it and, as a result, have enhanced reactivity toward nucleo­philic radicals. The oxime 10 contains a double bond activated in this way (eq 23).115 Having reactive centers able easily to come within bonding distance translates into cyclic products being formed in reactions involving other carbon–nitrogen116–119 and even carbon–oxygen120,121 multiple bonds. In the reaction shown in Scheme 15, for example, ring formation occurs because the radical center easily comes into contact with the cyano group.118

    II2(23).png

    II2s15.png

    b. Addition of a Framework Radical to a Substituent Multiple Bond

    An example of a reaction in which a framework radical adds to a sub­stituent multiple bond to form a five-membered ring is shown in eq 24.122 If five-membered-ring formation introduces too much strain into a system, reaction to give a larger ring becomes a possibility; thus, the radical centered at C-5 in 11 adds to the substituent double bond to form a six-membered ring (Scheme 16) and, in so doing, avoids producing highly strained, trans-fused, five-membered rings.123

    II2(24).png

    II2s16.png

    Although bimolecular addition of a carbo­hydrate radical to a mul­ti­ple bond is fast enough to be observed only when the multiple bond is electron-deficient, radical cycli­za­tion is not limited in this way. Having a radical center, such as the one at C-5 in 11 (Scheme 16), in close prox­imity to a double bond makes cyclization competitive with other radical reac­tions (e.g., simple reduction) even though the multiple bond is not electron-deficient.

    Radical cyclization followed by ring opening that breaks the newly formed bond is a de­gen­erate addition-elimination reaction that is rarely useful or even detectable. Sometimes, how­ever, ring opening breaks a different bond from that produced during cyclization.124,125 The result of such a reaction is an addition-elimination process, such as that shown in Scheme17, where the effect of the reaction is migration of a part of a sub­stit­uent group.124

    II2s17.png

    A unique type of cyclization between a framework radical and a substituent multiple bond takes place in nucleosides that have properly placed dibromovinyl groups.103,126–131 Bromine-atom abstraction by Bu3Sn· produces a vinyl radical that begins a sequential reaction leading to two spiro compounds (Scheme 18).103 Using Bu3SnSnBu3 in this reaction (rather than Bu3SnH) improves the yield of the cycli­zation product because competing simple reduction involving a tin hydride is not an option.103

    II2s18.png

    c. Addition of a Substituent Radical to a Framework Multiple Bond

    Cyclization involving halogenated carbo­hydrates often occurs when the halogen atom is part of a substituent group and the multiple bond is located in the carbohydrate framework. The sub­strate in many of these reac­tions is a silyl ether with a bromine atom incor­por­ated into the silicon-containing group.132–139 An example is shown in Scheme 19, which describes a reaction that stereoselectively creates a new C–C bond at C-3 in the product.132 Nonradical reaction of the resulting product transforms it into a diol (Scheme 19) that can be readily converted into other compounds. An extension of this type of reaction to a pair of sac­cha­ride units connected by a silaketal tether leads to formation of a pro­tected C-disaccharide (12) from which the tether is easily removed (Scheme 20).140

    II2s19.png

    II2s20.png

    Cyclization of an unsaturated carbohydrate in which an acetal substit­uent contains a hal­ogen atom is similar to cyclization of brominated silyl ethers such as that pictured in Scheme 19. High stereoselectivity is the norm in these reactions.141–147 In the process shown in Scheme 21, for example, the stereo­chem­istry at C-3 is controlled by the kinetically and ther­mo­dynamically favored formation of a cis-fused ring system.141 [There is a second chiral center (C-4) created during this reaction. The stereochemistry at this center is deter­mined by the least-hindered approach of tri-n-butyltin hydride to the reacting radical.] The reaction shown in eq 25 provides an example of new ring formation when a radical formed from a halogen-containing, acetal sub­stituent adds internally to a triple bond.142

    II2s21.png

    II2(25).png

    d. Addition of a Substituent Radical to a Substituent Multiple Bond

    Sometimes in a radical cyclization reaction neither the carbon atom bearing the radical cen­ter nor the carbon atoms of the multiple bond are part of the carbohydrate framework.148,149 When this occurs, the carbohydrate portion of the molecule has only an indirect influence on the reaction; that is, it can affect reaction stereoselectivity by acting as a chiral auxiliary, or its cyclic structure can bring reactive centers into bonding distance. It is the latter role that assists eleven-membered ring formation in the reaction shown in eq 26.148

    II2(26).png

    D. Elimination

    Free-radical elimination takes place when a vicinal dihalide reacts with tri-n-butyltin hy­dride (eq 27).150 A similar reaction occurs if one of the vicinal substituents is a halogen atom and the other contains an O-thio­car­bonyl group.151,152 In the reaction shown in eq 28151 there is little doubt that the initial interaction between Bu3Sn· and compound 13 in most instances is with the substit­uent at C-3' because the rate con­stant for reaction of Bu3Sn· with a compound containing an O-thiocarbonyl group is much larger than that for reaction with a chlorinated com­pound.

    II2(27).png

    II2(28).png

    Electrochemical reduction of glycosyl halides (and their sulfur-containing analogs154a) begins with electron transfer to the halide to produce a radical anion that reacts further to give the corresponding glycal (Scheme 22153). Reaction of glycosyl bromides with zinc dust also leads to glycal formation by electron-transfer to a glycosyl halide (eq 29).154b The mechanism for this reaction may parallel that shown in Scheme 22, but the possi­bility also exists that an organozinc intermediate forms following the initial electron transfer.

    II2s22.png

    II2(29).png

    E. Ring-Opening

    If halogen-atom abstraction produces a cyclopropylcarbinyl radical, cyclo­propane ring opening takes place. The radical remaining after ring opening then can undergo hydrogen-atom abstraction,155 addition,156 or, as shown in eq 30, cycli­za­tion.157

    II2(30).png

    F. Internal Hydrogen-Atom Abstraction

    Reaction of a halogenated carbohydrate to form a carbon-centered radical rarely results in this radical abstracting a hydrogen atom from a carbon–hydrogen bond in another molecule because such abstraction is, in most instances, too slow to compete with other radical reactions. If, however, the abstraction is internal and the radical is quite reactive (primary, vinyl or aryl), hydrogen-atom abstraction can take place.158–160 Dehalogenation of the bromide 14 begins such a reaction by producing a primary radical that abstracts a hydrogen atom from C‑1, a process that leads to epimerization at this carbon atom (Scheme 23).158 Internal hydrogen-atom abstraction by a vinyl radical takes place in the reaction pictured in Scheme 18.103

    II2s23.png

    G. Radical Combination

    1. Dimerization

    Radical combination is not a common synthetic reaction for carbo­hydrates, but it does take place when conditions are adjusted so that reactions such as hydrogen-atom abstraction are to slow to compete.161–163 Having the source of the chain-carrying radical be Me3SnSnMe3 (as opposed to a tin hydride) reduces the rate of hydrogen-atom abstraction by the carbohydrate radical to the point that the three dimers shown in eq 31 are formed. {Similar dimer formation takes place during reaction of glycosyl phenyl sulfones [Chapter 3, Section VII.B.1.c] and glycosyl phenyl selenides [Chapter 4, Section II.B.6]} Electrochemical reactions of glycosyl halides also produce radical dimers.163

    II2(31).png

    2. Reaction with Molecular Oxygen

    Reaction of halogenated carbohydrates with tri-n-butyltin hydride in the presence of oxygen replaces the halogen atom with a hydroxyl group.164–168 Since combination of a carbon-centered radical with molecular oxygen is either diffusion-controlled or nearly so (k \(\cong\) 2 x 109 M‑1s-1),169 any other reaction of the radical taking place in the presence of oxygen must be rapid enough to happen before O2 reaches the radical center. An example of a cyclization reaction that is fast enough to meet this criterion is shown in eq 32.164 Since the oxygen concen­tration in the reaction mixture is approx­imately 1 x 10-3 M, the rate constant for cyclization needs to be at least 1 x 107 s-1 in order for an acceptable yield of a cyclic product to be obtained.166b A proposed mechanism for re­place­ment of a halogen atom with a hydroxyl group is given in Scheme 24.170

    II2(32).png

    II2s24.png

    Other reagents and reaction conditions also cause replacement of halogen atoms with hydroxyl groups. These include Et3B–O2 initiated reac­tion of a deoxyiodo sugar with molecular oxygen171,172 and adding O2 to a reaction mixture in which AIBN is both initiator and reactant.166 Another set of conditions leading to replacement of a halogen atom with a hydroxyl group consists of reacting a deoxyiodo sugar with NaBH4 and O2 in the presence of a catalytic amount of Co(salen) (eq 33173).162,173–175 A comparative study found Co(salen)-cata­lyzed oxidation reactions to be experimentally more convenient than those with a tin hydride.162

    II2(33).png

    H. Water-Soluble Halides

    Most halogenated carbohydrates used in synthesis are rendered soluble in organic solvents by the introduction of various hydroxyl protecting groups, but sometimes it is useful to be able to conduct dehalogenation reac­tions in aqueous solution on unprotected or partially protected carbo­hydrates. Such a situation requires a water-soluble replacement for the tin and silicon hydrides typically used. Water-soluble hydrogen-atom transfers can be formed by replacing the alkyl substituents normally attached to tin or silicon with more polar ones. This replacement produces hydrides that are suffici­ently soluble in water to allow simple reduction to take place in aqueous solution (eq 34176 and eq 35177).

    II2(34).png

    II2(35).png

    There are potential advantages to conducting reactions in water, advantages that extend beyond substrate solubility.176,177 One of these is that reactions conducted in aqueous solution may exhibit new reactivity because, rather than taking place in an essentially nonpolar liquid, these reactions occur in a polar, heavily hydrogen-bonded solvent. Since water generally does not participate in radical reactions, it is effectively an inert solvent. Also, using water as the reaction medium can reduce or even eliminate the need for recycling or dis­posal of organic solvents.

    I. Hypohalites

    Hypohalites are intermediates in the formation of alkoxy radicals. Reactions of hypohalites and the alkoxy radicals they produce are discussed in Chapter 6.

    J. Organotin Hydrides

    The majority of reactions of halogenated carbohydrates use an organo­tin hydride (nearly always Bu3SnH) as a hydrogen-atom transfer and as a source of a chain-carrying-radical; however, there are serious problems asso­ciated with the toxicity of tin-containing compounds and the difficulty in removing their residues from reaction products. A variety of solutions to these problems have been proposed. Since most of these solutions apply not only to reactions of halogenated carbohydrates but also to those of a broad range of carbohydrate derivatives, the solutions will not be discussed here (and then repeated in later chapters); rather, they are gathered together in Appendix 1, which is found at the end of this book.


    This page titled III. Radical Reactions is shared under a All Rights Reserved (used with permission) license and was authored, remixed, and/or curated by Roger W. Binkley and Edith R. Binkley.

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