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Iminium ions intermolecular reactions

CuBr/QUINAP System The CuBr/QUlNAP system was initially used in the enan-tioselective synthesis of proparyl amines via the reaction of alkynes and enamines (Scheme 5.5). It was rationalized that the enamines reacted with protons in terminal alkynes in the presence of copper catalyst to form zwitterionic intermediates in which both the generated iminiums and alkyne anions coordinate to the copper metal center. After an intermolecular transfer of the alkyne moiety to the iminium ion, the desired products were released and the catalyst was regenerated. The combination of CuBr as catalyst and the chiral ligand QUEMAP is crucial for the good reactivities and enantioselectivities seen in the reaction. Another potential... [Pg.132]

Application of an organocatalytic domino Michael addition/intramolecular aldol condensation to the preparation of a series of important heterocycles has recently received much attention [158] with methods being disclosed for the preparation of benzopyrans [159-161], thiochromenes [162-164] and dihydroquinolidines [165, 166]. The reports all use similar conditions and the independent discovery of each of these reactions shows the robust nature of the central concept. A generalised catalytic cycle which defines the principles of these reports is outlined in Fig. 10. Formation of iminium ion 102 is followed by an intermolecular Michael addition of an oxygen, sulfur or nitrogen based nucleophile (103) to give an intermediate... [Pg.314]

Also developed by Hill is a (diotochemical system (equations 41 to 48) based on a polyoxoacid, H3PWi2O40 (P)> The excited state of the acid probably oxidizes the alkane in the first step. The radical can then either attack the solvent to give an iminium radical, wiiich leads to ketone on hydrolysis, or it can be oxidized to the caifaonium ion, in wdiich case attack on the solvent leads instead to the -alkyl-acetamide. If the substrate has two adjacent tertiary C—bonds, then alkenes tend to be formed, llie Barton reaction, normally kmwn as an intramolecular C—activation, can give some intermolecular reaction in some examples. Thus, vdien n-octyl nitrite is rf)otolyzed in heptane, some nitrosoheptane is observed. ... [Pg.9]

Complex multi-ring heterocyclic molecules like 136 have been prepared in one pot and under mild conditions by combining a base-catalyzed intermolecular Michael addition reaction of an a,p-unsaturated carbonyl compound and a suitable p-ketoamide pronucleophile with an acid-catalyzed intramolecular N-acyl iminium ion cyclization of the resulting adduct (Scheme 3.40). [Pg.144]

IdU. See 5-Iodo-2 -deoxyuridine (IdU) Inline metathesis, DLs from, 31—34 Iminium ions, 134—135 Interannular cooperativity, 42—43, 56—59 binding of divalent ligand, 56f Interannular-chelate cooperative model, 62 Intermolecular process, 4f Internal symmetry number, 44—45 Intramolecular process, 3, 4f Intramolecular reactions, 78—81, 91 2 -Iodo-2 -deoxyuridine, 136, 138 5-Iodo-2 -deoxyuridine (IdU), 125, 125f, 136... [Pg.247]

Even though the use of (S)-proline (1) for the synthesis of the Wieland-Miescher ketone, a transformation now known as the Hajos-Parrish-Eder-Sauer-Wiechert reaetion, was reported in the early 1970s, aminocatalysis - namely the catalysis promoted by the use of chiral second-aiy amines - was rediscovered only thirty years later. The renaissance of aminocatalysis was prompted by two independent reports by List et al. on the asymmetric intermolecular aldol addition catalysed by (S)-proline (1) and by MacMillan et al. on the asymmetric Diels-Alder cycloaddition catalj ed by a phenylalanine-derived imidazolidinone 2. These two reactions represented the archetypical examples of asymmetric carbonyl compound activation, via enamine (Figure ll.lA) and iminium-ion (Figure 11.IB), respectively. [Pg.262]

Recently, a novel methodology has been developed to prepare densely functionalized heterocycles from the coupling of various Michael acceptors with readily available iminium ions (masked as N,0-acetals) via an intermolecular MBH-type reaction. An intramolecular MBH-type reaction was developed to construct a bicyclic pyrrolizidine ring. As shown Scheme 5.32, the total synthesis of ( + )-heliotridine, as this basic structure unit existed in many plants, was achieved. ... [Pg.517]

The term aminocatalysis has been coined [4] to designate reactions catalyzed by secondary and primary amines, taking place via enamine and iminium ion intermediates. The field of asymmetric aminocatalysis, initiated both by Hajos and Parrish [5] and by Eder, Sauer, and Wiechert [6] in 1971, has experienced a tremendous renaissance in the past decade [7], triggered by the simultaneous discovery of proline-catalyzed intermolecular aldol [8] and Mannich [9] reactions and of asymmetric Diels-Alder reactions catalyzed by chiral imidazolidinones [10]. Asymmetric enamine and iminium catalysis have been influential in creating the field of asymmetric organocatalysis [11], and probably for this reason aminocatalytic processes have been the object of the majority of mechanistic smdies in organocatalysis. [Pg.12]

Following the seminal report of MacMillan et al. in 2000 (27), several applications of iminium ion-activated Diels-Alder reactions in complex natural product syntheses have been reported, either carrying out the cyclization in an intra- or an intermolecular fashion. [Pg.52]

The observed excellent stereoselectivities (dr=91 9 to >95 5, 94 to >99% ee) could be ascribed to the steric hindrance created by the employed catalyst in each step of the catalytic cycle reported below (Scheme 2.56). Once the chiral amine (S)-70 activates the acrolein 131 as electrophile by generating the vinylogous iminium ion A, the indole 171 performs an intermolecular Friedel-Crafts-type reaction. The resulting enamine B acts as nucleophile in the Michael addition of the nitroalkene 140 leading to the iminium ion D, which upon hydrolysis liberates the catalyst and yields the intermediate aldehyde 173. The latter compound enters in the second cycle by reacting with the iminium ion A, previously formed by the free catalyst. The subsequent intramolecular enamine-mediated aldol reaction of E completes the ring closure generating the intermediate F, which after dehydration and hydrolysis is transformed in the desired indole 172. [Pg.47]

AppUcations of the aqueous iminium-ion-based Diels-Alder reaction to more demanding systems have also been explored. One area of investigation centered on the reaction between iminium ions and stabilized diene systems. Initial attempts to effect intermolecular iminium-ion-based [4+2] cycloadditions with styrenes under the standard conditions failed, yielding only by-... [Pg.60]

Monocyclic piperidine or pyrrolidine alkaloids are also accessible in a Robinson-Schbpf-type condensation if substituted acetone monocarboxylic acids are used as reactive acetone equivalent. For example, the reaction of glutardial-dehyde, methylamine, and two equivalents 3-keto-3-phenyl-propionic acid (194) under slightly acidic conditions (pH=4) afforded ( )-lobelanine (53) in 90% yield (Scheme 11.44). Here, the reaction presumably proceeds via intermediate 195, which is formed in an initial intermolecular Mannich reaction between glutardialdehyde, methylamine, and one equivalent 194. Under the applied reaction conditions, water is eliminated. The formed iminium ion 196 is prone to nucleophilic attack by a second equivalent 194 to furnish the piperidine alkaloid ( )-lobelanine [132,134]. [Pg.417]

Scheme 42.10 Organocatalytic domino sulfa-Michael/amination process by way of an iminium ion-enamine sequence both catalytic steps are intermolecular reactions. Protocol point compound 34 was added after 30 min. Scheme 42.10 Organocatalytic domino sulfa-Michael/amination process by way of an iminium ion-enamine sequence both catalytic steps are intermolecular reactions. Protocol point compound 34 was added after 30 min.
Design of Enomine-Cyclization Cascade Reactions The nucleophilic Y in intermediate 6 can react with other electrophiles intermolecularly (Scheme 1.34a) or intramolecularly (Scheme 1.34b) as well as with the iminium ion. Moreover, the carbonyl group of 6 can also undergo intramolecular aldol reaction with nucleophilic X (Scheme 1.34c). These nucleophilic addition reactions after enamine catalysis induce cyclization reactions to produce versatile five- or six-membered ring structures. [Pg.19]

Structure (Scheme 3.9). A mechanism that has been proposed is shown in Scheme 3.10. Intermediate 60 was formed upon activation of phytal 56 by dien-amine catalysis. Subsequently, an intermolecular aldol reaction between phytal 56 and aldehyde 55 occurred in the presence of catalyst 57. Finally, the tricyclic core structure 58 was built after the adjacent phenol group captured the newly formed iminium ion 61 via an oxa-Michael reaction. The cascade product desired was obtained in 58% yield with 97% diastereomeric excess. The total synthesis of natural product a-tocopherol was achieved through a four-step chemical synthesis. [Pg.133]

Sargeson and his coworkers have developed an area of cobalt(III) coordination chemistry which has enabled the synthesis of complicated multidentate ligands directly around the metal. The basis for all of this chemistry is the high stability of cobalt(III) ammine complexes towards dissociation. Consequently, a coordinated ammonia molecule can be deprotonated with base to produce a coordinated amine anion (or amide anion) which functions as a powerful nucleophile. Such a species can attack carbonyl groups, either in intramolecular or intermolecular processes. Similar reactions can be performed by coordinated primary or secondary amines after deprotonation. The resulting imines coordinated to cobalt(III) show unusually high stability towards hydrolysis, but are reactive towards carbon nucleophiles. While the cobalt(III) ion produces some iminium character, it occupies the normal site of protonation and is attached to the nitrogen atom by a kinetically inert bond, and thus resists hydrolysis. [Pg.185]

Intermolecular addition of carbon nucleophiles to the ri2-pyrrolium complexes has shown limited success because of the decreased reactivity of the iminium moiety coupled with the acidity (pKa 18-20) of the ammine ligands on the osmium, the latter of which prohibits the use of robust nucleophiles. Addition of cyanide ion to the l-methyl-2//-pyr-rolium complex 32 occurs to give the 2-cyano-substituted 3-pyrroline complex 75 as one diastereomer (Figure 15). In contrast, the 1-methyl-3//-pyrrolium species 28, which possesses an acidic C-3-proton in an anti orientation, results in a significant (-30%) amount of deprotonation in addition to the 2-pyrroline complex 78 under the same reaction conditions. Uncharacteristically, 78 is isolated as a 3 2 ratio of isomers, presumably via epimerization at C-2.17 Other potential nucleophiles such as the conjugate base of malononitrile, potassium acetoacetate, and the silyl ketene acetal 2-methoxy-l-methyl-2-(trimethylsiloxy)-l-propene either do not react or result in deprotonation under ambient conditions. [Pg.18]


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See also in sourсe #XX -- [ Pg.2 ]

See also in sourсe #XX -- [ Pg.1057 , Pg.1066 ]

See also in sourсe #XX -- [ Pg.1057 , Pg.1066 ]

See also in sourсe #XX -- [ Pg.2 ]

See also in sourсe #XX -- [ Pg.1057 , Pg.1066 ]




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Iminium ion

Iminium ions reaction

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