Intramolecular Electrophilic Attack on

The key step in syntheses of if/-quebrachamine (122-127) and if/-dihydro-cleavamine (12S) is the oxidation of tertiary amines with mercuric acetate to cyclic imonium salts, which give rise to an intramolecular electrophilic attack on an indole. 

The reactivity of allenyl ketones is also manifested in the Hg(II)-catalyzed ipso substitution that converts 54 to spirodione 55 (Eq. 13.17) [19]. The reaction presumably involves activation of the allene by Hg(II), followed by intramolecular electrophilic attack on the aromatic ring. Hydrolytic cleavage of the metal from the intermediate product of the reaction, followed by rearrangement leads to the observed spirocyclic dione. 

Alkvnes do rot dimerize photochemically to give cyclobutadienes, but dimers are formed from arylalkynes under conditions of electron-transfer sensitization (2.105). These dimers arise from a reaction of the alkyne radical cation with ground-slate alkyne, followed by intramolecular electrophilic attack on the benzene ring. 

Treatment of the heptamethinium perchlorate (336) with dimethylchloro-methyleneammonium chloride and dimethylformamide in chloroform (80°, 2 h) leads after work-up, to 1,3,5-triformylbenzene (339) in 40% yield. The acyclic intermediate (337), initially formed by double formylation, undergoes a cyclization that might well be seen as an intramolecular, electrophilic attack on the enamine -position of (337). The alternative interpretation of this as an electrocyclic process in the mesomeric formula (337 ) to (338), however, also seems to be more convincing ... 

Both benzo[fc]furans and benzo[fc]thiophenes can be obtained from the phenol or thiophenol respectively, by O-Zi -alkylation with a bromoacetaldehyde acetal and then acid-catalysed ring closure involving intramolecular electrophilic attack on the ring. 

Cyclisation of 2-arylthio- or 2-aryloxy-aldehydes, -ketones or -acids via intramolecular electrophilic attack on the aromatic ring, with loss of water, creates the heterocyclic ring this route is a common method for the ring synthesis of benzothiophenes. 

Several other cyclizations have been achieved by intramolecular electrophilic attack on thiophene. Thus, acid-catalyzed cyclization of suitable substrates has been used to synthesize several thi-enomorphans <91H(32)107>. Interestingly, the product of cyclization of (69) depends on the acid catalyst employed. While polyphosphoric acid gives the expected (70), refluxing TFA leads to the rearranged product (71) (Scheme 15) <82CC793>. 

Electrophilic attack on the sulfur atom of thiiranes by alkyl halides does not give thiiranium salts but rather products derived from attack of the halide ion on the intermediate cyclic salt (B-81MI50602). Treatment of a s-2,3-dimethylthiirane with methyl iodide yields cis-2-butene by two possible mechanisms (Scheme 31). A stereoselective isomerization of alkenes is accomplished by conversion to a thiirane of opposite stereochemistry followed by desulfurization by methyl iodide (75TL2709). Treatment of thiiranes with alkyl chlorides and bromides gives 2-chloro- or 2-bromo-ethyl sulfides (Scheme 32). Intramolecular alkylation of the sulfur atom of a thiirane may occur if the geometry is favorable the intermediate sulfonium ions are unstable to nucleophilic attack and rearrangement may occur (Scheme 33). 

In the presence of an aluminum reagent, 2,3-butadienyltrimethylsilane can also accept the intramolecular electrophilic attack of the ketone-aluminum complex to afford bicyclic products via intermediate 60 [31]. The structures of the products depend on the aluminum reagent used [31]. 

A ring-interconversion approach has been used to access the functionalized 2-benzazepin-3-ones 331 from the bromo lactam 330 (Scheme 43). The critical ring expansion was initiated by lithium-bromine exchange in 330 followed by intramolecular carbanion attack on the lactam carbonyl group. Electrophilic capture of the ring-expanded lactam intermediate then afforded 331 in moderate to good yields <2003SL2025>. None of the isomeric 2-benzazepin-3-ones 332 expected from the lithium enolate intermediate were observed. 

In conclusion, it should be noted that the Barton-Zard condensation formally might be considered as a tandem SnH-Sn ipso process (see Section III.B.3). However, the corresponding crH-complcx is stabilized here not via elimination of any auxiliary leaving group or oxidation but by means of an intramolecular nucleophilic attack on an isonitrile fragment. This is due to the amphoteric nature of isocyanoacetate ion acting at first as a nucleophile and then as an electrophile. 

An intramolecular reaction, which could be viewed as an electrophilic attack on nitrogen, is mentioned under the reactions of the carbocyclic ring (see Section 7.21.6.1.3) <83CPB1518>. The dihydrobenzodipyrrole (45) was acylated on the pyrrolidine nitrogen atom (87CC1406). The oxidation of some benzodipyrroles, e.g. compound (6), and related tripyrroles, to their JV-oxides can be viewed as an electrophilic attack on nitrogen. 

It is often considered that such a cyclizative coupling involves intramolecular electrophilic attack of arylpalladium(II) moiety on another aromatic ring in the key intermediate (Scheme 3) [91,94,95]. 

The selenenylation reaction can be viewed as an electrophilic attack on the enolate, or as an Sn2 attack by the enolate on selenium. Selenenylation is useful because the phenyl selenide readily undergoes intramolecular elimination to yield the conjugated enone. 

A similar reaction of an arsenic derivative failed 234), It should be noted that many Ar—M bonds (including Si) are cleaved in the presence of AICI3 and, therefore, the reaction method is somewhat restrictive. Ring closure of the alcohols, Ph3Si(CH2) CH20H( = 2), in the presence of a Lewis acid in nitrobenzene was unsuccessful due to cleavage of Si—Ph bonds 361). Intramolecular cyclization involving electrophilic attack on a benzene nucleus has been used more successfully for formation of heterocycles of arsenic ... 

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