Electrophiles mediated cyclizations

C. Leteux, A. Veyrieres, and F. Robert, An electrophile-mediated cyclization on the 1,6-anhydro-D-glucopyranose framework, Carbohydr. Res., 242 (1993) 119-130. 

Electrophile-mediated cyclization reactions of alkynes tethered to pendant heteroatom nucleophiles is an emerging strategy for the synthesis of heterocycles. This methodology has now been applied to the synthesis of pyrroles. The iodocyclization of 3-aminoalkynes 1 led to the formation of dihydropyrrole 2 <07TL7906>. Treatment of the latter with mesyl chloride in the presence of triethylamine then gave (i-iodopyrrolcs 3. 

Freeman, F, Roharge, K D, Stereoselectivity in the electrophile-mediated cyclization of 2,3,5-tri-O-benzyl-L2-dideoxy-D-arafano-hex-l-enitol. A stereocontrolled s)mthesis of l-amino-2,5-anhydro-3, 4,6-tri-O-benzyl-l-deoxy-D-glucitol, Carbohydr. Res., 171, 1-H, 1987. 

Intramolecular cyclization of (76) in the presence of NaH gave exo (77) and endo (78) products (Equation (9)) <90SL584). Allene-based electrophile-mediated cyclization afforded iV-tosyl iodohexa-hydroazocines <94JCS(Pl)3549>. Photochemical intramolecular cycloaddition of phthalimide in acetone afforded the ring-opened product <82TL498l>. An unstable enaminone azocine derivative (81) was formed after the desulfurization of the bicyclic aminal (80), which in turn was synthesized from... 

Freeman F, Rabarge KD (1985) Electrophile-mediated cyclizations in carbohydrate chtanis-try synthesis of highly functionalized ribofuranose and ribopyranose compounds. Tetrahedron Lett 26 1943-1946... 

Allenes can also act as the -participant in electrophilic heteroatom cyclizations. Reviews of electrophilic additions to allenes discuss early examples of this type of cyclization.ld le-202 Numerous examples of cyclizations of a-functionalized allenes, including carboxylic acids, phosphonates, sulfinates and alcohols, to form five-membered heterocycles (equation 84) are cited in these reviews. The silver nitrate-mediated conversion of ot-allenic alcohols to 2,5-dihydrofurans203 has recently been applied to trimethylsilyl-substituted systems.204... 

Electrophile-mediated intramolecular cyclization of a 2-methylene oxetane functionalized with a pendant hydroxyl group, 80, gave l-iodomethyl-3,4-diphenyl-2,6-dioxobicyclo[2.2.0]hexane 81 (Equation 27) <1999TL7051>. This was particularly notable for being the first reported example of a [2.2.0]-fused ketal. 

The efficient catalytic cyclization (aminocarbonylation) of A-(3-hydroxy-4-pentenyl)amides and carbamates in acetic acid gave m-fused bicyclic pyrrolidine lactone compounds54,56 (Table 2), in agreement with the observed ra-directing capability of the hydroxy group in analogous electrophile mediated additions (Section 7.2.6). In tetrahydrofuran the reaction rate is unacceptably low. In methanol a competitive allylic substitution leads to 1,2,5,6-tetrahydropyridines. Furthermore, lower yields were obtained in the cyclization of the corresponding ureas. 

Substituted benzoMisothiazole d, d -dioxides 498 were synthesized through lithiation of 497 (X = H, F, Cl), aryne-mediated cyclization giving intermediates like 53a and subsequent quenching of aryllithium intermediates with various electrophiles (see Section 4.05.4.5) <1997T3615>. 

In all reactions discnssed in the two preceding paragraphs, an unsaturated organic palladium species was acting as the electrophilic mediator of the cyclization process. A Pd-mediated cyclization initiated by a palladium(II) hydride species was also developed on w-unsatnrated /3-dicarbonyl compounds. In this way, 5-acetylenic potassinm enolates, generated from the reaction of compounds of type 32 with potassium t-butoxide, smoothly underwent cyclization when treated with a palladium(O) complex in THF. This led to the formation of methylenecyclopentanes 54 or unsaturated esters 55, depending on the reaction conditions, and particularly on the amount (stoichiometric or catalytic) of potassium l-butoxide (Table... 

PPI63R M. Frederickson and R. Grigg, Org. Prep. Proc. Int., 1997, 29, 63-115. Electrophile Mediated Heteroatom Cyclization onto C—C rr-Bonds. 2. Metal Ion Mediated Cyclization. V... 

An interesting indole synthesis was realized by Zhao, who reported the PIFA-mediated cyclization of enamines 11 (Scheme 3) [8]. This reaction toward indoles 12 may rely on an electrophilic nitrogen atom to initiate cyclization or proceed through single electron transfer. Related pyrrole derivatives are also accessible by this method. 

While boron trihalide-mediated ether cleavage has been successfully applied in a number of organic syntheses, it was discovered that the BBr3-mediated demethylation of 2-methoxybiaryl systans could be problematic due to an intramolecular electrophilic aromatic cyclization (Schone 23.7). In these cases, 10-hydroxy-10,9-boroxarophenanthrenes are obtained as major products. 

In Chap. 2, the author disclosed zirconocene-mediated cyclization of bis(alky-nyl)silanes, nitriles, and unsaturated compounds. The reactive intermediates involving two or three molecules of nitriles were isolated and characterized. Thus, the author expects to further isolate the one molecule of nitrile involved intermediate and demonstrates the reaction mechanism toward the formation of Zr-/Si-containing three-ring fused intermediates (Scheme 3.1). The author tried to use only one equivalents of nitrile, lower the reaction temperature, and quench the reaction mixture generated in situ or trap with electrophiles. However, all these attempts failed, and the isolation and characterization of one-nitrile-involved intermediates... 

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