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Markovnikov reactions cyclization

Electronic factors also influenced the outcomes of these cyclization reactions cyclization of pyrrole 84 to bicyclic amine 85 is catalyzed by the sterically open complex 79a. In this reaction, initial insertion into the Y - H bond occurred in a Markovnikov fashion at the more hindered olefin (Scheme 19) [48]. The authors proposed that the Lewis basic aromatic ring stabilizes the electrophilic catalyst during the hydrometallation step, overriding steric factors. In the case of pyrroles and indenes, the less Lewis basic nitrogen contained in the aromatic systems allowed for the cyclization of 1,1-disubstituted alkenes. [Pg.234]

The addition, therefore, follows Markovnikov s rule. Primary alcohols give better results than secondary, and tertiary alcohols are very inactive. This is a convenient method for the preparation of tertiary ethers by the use of a suitable alkene such as Me2C=CH2. Alcohols add intramolecularly to alkenes to generate cyclic ethers, often bearing a hydroxyl unit as well. This addition can be promoted by a palladium catalyst, with migration of the double bond in the final product. Rhenium compounds also facilitate this cyclization reaction to form functionalized tetrahydrofurans. [Pg.996]

Stereoisomeric alcohols (93) and (94) yielded identical ring-expansion products [e.g. (97)] on formation of carbocations.168 This is evidence of a stepwise reaction in sterol biosynthesis, whereby a tertiary cation [e.g. the model (95)] rearranges to a secondary cation (96)-an anti-Markovnikov rearrangement . The synthetic aspects of biomimetic cyclizations of isoprenoid polyenes were reviewed.169 Included was a detailed discussion of carbenium ion-initiated cyclizations, with a discussion of the different mechanisms that have been proposed. A novel biomimetic carbocation polyene cyclization of a daurichromenic ester was reported an unusual 2 + 2-carbocation cyclization occurred as a side reaction.170... [Pg.223]

The mechanism of selenocyclization of yS,y-unsaturated acids and their derivatives has been studied. The reactions of ( )-4-phenylbut-3-enoic acid and its silyl and alkyl esters (15 R = H, SiMe3, alkyl) with benzeneselenenyl halide PhSeX (X = Cl, Br) have been examined by VT-NMR and in situ IR spectroscopic methods. Whereas the reactions of the acid in the presence of a base were irreproducible and complicated, reactions of the silyl esters were clean and spontaneously and quantitatively afforded the corresponding chloroselenylation adduct at -70 °C as a single (Markovnikov) isomer. This adduct underwent three processes as the temperature was raised (1) reversal to the starting materials, (2) isomerization to the anti-Markovnikov product, and (3) cyclization to the selenolactone (16). All of these processes are believed to proceed via a seleniranium ion, the intermediacy of which was established by independent synthesis and spectroscopic identification. The reversible formation of chloroselenide adducts was unambiguously established by crossover experiments. The reaction of (15) with PhSeBr was found to be rapid but thermodynamically unfavourable at room temperature.29... [Pg.321]

A Ni-catalyzed cyclization cross-coupling reaction of iodoalkenes with alkyl zinc halides has been employed for the synthesis of various tetrahydrofuran derivatives <2007AGE-ASAP>. The TiCU-catalyzed anti-Markovnikov hydration of alkynes has been applied to the synthesis of various benzo[3]furans <2007JOC6149>. [Pg.561]

Although Pt-alkene complexes are normally not as reactive as the corresponding Pd complexes toward nucleophilic addition, platinum can serve usefully as a catalyst in these reactions.59 The following cyclization was mediated by a Pt-PPP-pincer complex 22, demonstrating Markovnikov addition at each step of the cyclization (equation 8.42).60 The reaction is stereoselective and mimics the cyclization of squalene epoxide to lanosterol, a key step in the biosynthesis of cholesterol. [Pg.279]

A similar electrophilic activation of the vinylallene to 2-cyclopentenone cyclization is possible using either mercury(II) acetate or thallium acetate. Solvometallation of allenes is a well studied pro-cess following expected Markovnikov attack at the central carbon by the metal salt. The yield of cyclopentenones was generally higher with acetoxymercuration (49-79%) compared to acetoxythallation (25-68%). Compounds of similar substitution pattern as were investigated with epoxidation (Scheme 31) were prepared. Mechanistically the reaction is remarkable for the facility of solvodemercuration (Scheme 33). Intermediates such as (69) or (70) lose elemental mercury spontaneously in the acetic acid medium to give the 2-cyclopentenones. [Pg.774]

A transannular 6-endo reaction takes place in the boron trifluoride-diethyl ether complex induced cyclization of a bicyclic aldehyde. After the Markovnikov-orientated ring-closure step, a tertiary cation is generated, which is stabilized by //-proton loss and tetracycle 18, bearing a cyclopropyl unit, is formed in 67% yield24. In this case an ene reaction is impossible. [Pg.101]

Type-Ill cyclizations are initiated by conversion of an acetal, ketal or enol ether into an oxonium ion. Such an activated carbonyl group attacks an olefin intramolecularly in an exo reaction. Mono- and trialkylated double bonds form new C-C bonds regioselectively at the least hindered terminus according to Markovnikov s rule, and in this case endo cyclization products can be produced25. It is worth noting that acetals are also used as initiators in cation olefin cyclizations (see Section 1.5.5.3.)26. Use of activated acetals in Lewis acid catalyzed C-glycoside synthesis is also known 12c 12e. [Pg.102]

Tricyclizations including seven-membered ring closure are rarely described. Initiation of the reaction by a tetramethylallylic cation leads to a six-six-seven-membered ring system via a b-endo-b-endo-l-endo cyclization according to Markovnikov s rule. Two diastereomers are formed due to the lack of remote stereocontrol by the initiating cation66. [Pg.133]

Here catalysis involves the formation of a ruthenium vinylidene, an anti-Markovnikov addition of water (368), and cyclization of an acylmetal species onto the alkene. Although cyclization may occur via hydroacylation (Scheme 52, path A) (460-462) or the Michael addition reaction (Scheme 52, path B) (463,464), the requirement for an electron-withdrawing substiment on the alkene and the absence of aldehyde formation suggest path B to be the more likely mechanism (465,466). Trost discovered that the use of the cationic mthenium catalyst CpRu(MeCN)3+PFg is tolerant of 1,2-di-and trisubstimted alkenes and promotes cyclization of 1,6- and 1,7-enynes to five- and six-membered ring products (467). In a number of examples, the mthenium reaction is complementary to the Pd-catalyzed cyclization described above, selectively forming the 1,4-diene over the traditional 1,... [Pg.417]

S)-hydroxylunacrine (cf. 333) found in L. amara. The biosynthesis of (-)-(R)-lunasine is more likely to occur by anti-Markovnikov hydration of a prenylquinolone to an (S)-hydroxy derivative followed by cyclization to lunasine with inversion at the chiral center and thence conversion into lunacrine (335) (Scheme 29) direct cyclization of a prenylquinolone is less probable because at least in vitro acid-catalyzed reactions lead exclusively to pyranoquinolines. The (R)-hydroxy derivative lunacridine (cf. 337) has been obtained from L. amara but is probably an artifact arising from hydrolysis of (-)-(R)-lunasine during isolation, rather than a biosynthetic intermediate. [Pg.188]

In a series of fascinating studies, Dixneuf s group has demonstrated the capability of arene-ruthenium complexes for promoting the intramolecular cyclization of (Z)-3-methylpent-2-en-4-yn-l-ols, such as 116 and 118, to furan derivatives 117 and 119 (Scheme 32) [53,55-57]. This reaction corresponded to the intramolecular Markovnikov addition of a hydroxyl group to a non-activated terminal triple bond. It was found that the czs-geometry of double bond is crucial for the... [Pg.200]

Anti-Markovnikov cyclizations For anti-Markovnrkov cyclizations of 4-alkynols to dihydropyrans such as glycals, the catalysts or reagents introduced by McDonald [150] remain synthetically important [151,152]. Another catalyst based on ruthenium was presented by the Trost group. A rather finely tuned catalyst based on a CpRuCl (PAr3)2 complex (Ar = aryl) achieved similar reactions like the McDonald catalyst (Scheme 25c) [153]. [Pg.146]

The bulk of the studies on intramolecular hydroamination of alkenes catalyzed by lanthanide complexes have been conducted using lanthanocene complexes or half-sandwich lanthanide complexes. The prototypical cyclizations of aminoalkenes to form five- and six-membered rings are shown in Equation 16.61. These reactions occur with exclusive Markovnikov selectivity. These reactions have also been conducted using arylamines, as shown in Equation 16.62. The intramolecular reactions of amines catalyzed by certain lanthanide complexes occur with 1,1- and 1,2-disubstituted olefins (Equation 16.63), although such reactions require high temperatures. [Pg.702]


See other pages where Markovnikov reactions cyclization is mentioned: [Pg.619]    [Pg.717]    [Pg.730]    [Pg.304]    [Pg.377]    [Pg.378]    [Pg.326]    [Pg.337]    [Pg.469]    [Pg.66]    [Pg.271]    [Pg.308]    [Pg.605]    [Pg.608]    [Pg.177]    [Pg.287]    [Pg.210]    [Pg.37]    [Pg.774]    [Pg.1041]    [Pg.199]    [Pg.324]    [Pg.110]    [Pg.161]    [Pg.162]    [Pg.379]    [Pg.146]    [Pg.37]    [Pg.443]    [Pg.1464]    [Pg.389]    [Pg.285]    [Pg.377]   
See also in sourсe #XX -- [ Pg.337 ]




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Markovnikov reactions

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