Intramolecular cyclization electrophilic

A series of new pyrazoloindolizinium salts has been prepared for their interest as dyestuffs. The amide 278 undergoes an intramolecular cyclization to the tricycle 279 upon heating in an ethanol/piperidine mixture. The methylene group of this can then react with different electrophiles, for example, aldehydes, to extend the chromo-phore <2002JCCS571> (Scheme 76). 

Despite the fact that the electrochemical oxidation of most of the nonconjugated dienes generally does not give products which result from interaction of the double bonds with one another, the anodic oxidation l-acetoxy-l,6-heptadienes gives intramolecularly cyclized products, that is, the cyclohexenyl ketones (equation 15)13. The cyclization takes place through the electrophilic attack of the cation generated from enol ester moiety to the double bond. 

Other condensations were described using pyridin-2-ones instead of 2-chloropyridines, but they need to be rendered more electrophilic by iV-acylation. Thus, the reaction of iV-acetyl pyridin-2-one 200 with hydrazine led to 201 (Equation 22) <2003PS1129>. Similarly, the intramolecular cyclization of 202 afforded the pyridotriazolone 203 (Equation 23) <1995JIC735>. 

The present volume contains 13 chapters written by experts from 11 countries, and treats topics that were not covered, or that are complementary to topics covered in Volume 1. They include chapters on mass spectra and NMR, two chapters on photochemistry complementing an earlier chapter on synthetic application of the photochemistry of dienes and polyenes. Two chapters deal with intermolecular cyclization and with cycloadditions, and complement a chapter in Volume 1 on intramolecular cyclization, while the chapter on reactions of dienes in water and hydrogen-bonding environments deals partially with cycloaddition in unusual media and complements the earlier chapter on reactions under pressure. The chapters on nucleophiliic and electrophilic additions complements the earlier chapter on radical addition. The chapter on reduction complements the earlier ones on oxidation. Chapters on organometallic complexes, synthetic applications and rearrangement of dienes and polyenes are additional topics discussed. 

The third method involves the tandem formation of the Nj—C2 and C3—N4 bonds. This strategy combines some of the methods used in the intramolecular cyclization approaches outlined in the first and second methods and is accomplished by the reaction of a bivalent electrophile and an a-amino acid. ... 

Likewise, aryllithiums generated by lithium-iodine exchange undergo intramolecular cyclization to give pyrrolo-azepine 72. The best results were obtained when Weinreb (R = Me, = OMe) or morpholine amides were used as internal electrophiles, resulting in 66 and 70% yields, respectively (Equation (7) (2005T331D). 

In many of these cases, the nucleophile is a C=C double bond (usually an alkenic group and less frequently an aromatic group). Alkenic oxime mesylates enable intramolecular cyclization by an electrophihc addition of the double bond to the electrophilic intermediate. These reactions are terminated by a proton loss. 

The alkylating agents have in common that, through intramolecular cyclization to form an ethylenei-minium ion, they become strong electrophiles which may directly or via formation of a carbonium ion intermediate transfer of an alkyl group to cellular target molecules. These reactions result in the... 

A recent report on trifluoromethylsulfenylation of (3-keto acids and derivatives describes isolation of 29 in good yield from reaction of 27 with trifluoromethyl-sulfenyl chloride (Scheme 6.10). Mechanistically, this was rationalized via electrophilic attack of trifluoromethylsulfenyl chloride on the enamine tautomer 27a to generate 28 followed by intramolecular cyclization through the imide oxygen with concomitant loss of CF3SH to produce 29. The product was characterized spectroscopically. 

A very large number of these systems with ring junction heteroatoms exists, and this number is constantly increasing. Only illustrative examples of the preparation of such systems can be given here. The synthetic methods for the formation of this type of heterocycle can be usefully classified as follows (i) various cyclocondensations between the corresponding heterocyclic derivatives and bifunctional units, (ii) intramolecular cyclizations of electrophilic, nucleophilic or (still rare) radical type, (iii) cycloadditions, (iv) intramolecular oxidative coupling, (v) intramolecular insertions, (vi) cyclization of open-chained predecessors, (vii) various reactions (quite often unusual) which are specific for each type of system. Examples given below illustrate all these cases. 

The electrophiles isobutyraldehyde and its dimethyl acetal gave surprisingly high ratios of syn anti attack, around 10 90. Since the silicon centre is more removed from the reactive site, this could be taken to indicate that electronic effects predominate. However, the intramolecular cyclization of 141 (equation 98) was found to take place with an enantiomeric excess of only 20%. 

SnCLt-mediated intramolecular cyclization of vinylsilane 136 with the acetal moiety leads to the corresponding cyclization product 137 (equation 114). The electrophile apparently attacks at the /1-carbon to the silyl substituent205. It is noteworthy that desilylation has been observed in the similar case of 138 when BCI3 is employed as the Lewis acid catalyst (equation 115)206. 

The final example in this section features a rare instance where the electrophilic center is s -hybridized carbon, as most cyclative cleavages involve the attack of carbonyl derivatives. Oxazolidinones are formed cyclatively18 by the displacement of a sulfonate ester by an acylsulfonamide (Fig. 5). In a variant19 of this cyclization, a quasi-meso bis-sulfonate partitions into a pair of quasi-enantiomeric sulfonates, one resin bound and the other cleaved, depending on the direction of intramolecular cyclization. The resin-bound enantiomer can then be displaced by an external nucleophile. 

Method Cis based on intramolecular cyclization of 2-aminothiophenes containing a substituent with the electrophilic y-carbon atom at position 3. This class of compounds, for example, amines 50, can be prepared by multicomponent cascade heterocyclization of phenyl isothiocyanate, CH-acids, and alkylating agents (1991SL229, 1992G147, 1992M341, 2001PS215). Cyclization of compounds 50 affords 6-oxo(imino)-6,7-dihydrothieno[2,3-Z>]pyridines 51 as the final products. 

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