Electrophilic ring-closure reactions

Electrophilic Ring-closure Reactions. - In a series of interesting papers, several thienospirans have been described, prepared by cyclization onto thiophen. Thus (50) and (51) were converted into (52) and (53), respectively, 

Semenovskii and M. M. Emel yanov,/zv. Akad. Nauk SSSR, Ser. Khim., 1980, 2578. 

Electrophilic Ring-closure Reactions.— The intramolecular acylation of acid chlorides (115) and (116) proceeds at the free 5-position to give (117) and (118). A solid-phase effect (silica gel) gives a 100% increase in yields and the mechanism 

It was shown that this system can be formed only by using strong Lewis acids. Treatment of (126), obtained through the reaction of the Grignard reagent of 3-thenyl bromide with a pyridinium compound, followed by sodium borohydride reduction and subsequent cyclization with 48% hydrobromic acid, led to the 

The acid chlorides of to-(2-thienyl) substituted butyric (122) and valeric acids (123), as well as the corresponding 5-alkyl-2-thienyl-substituted compounds, undergo internal Friedel-Crafts reaction (SnCl4,CS2) at the 3-position in 70-80% yield, to give the corresponding cyclohexenones (124) and cycloheptenones (125).  

From e-(5-ethyl-2-thienyl)caproic acid (126), however, only 8% of (127) is obtained. The acid chlorides of y-(2,5-diethyl-3-thienyl) 

The Friedel-Crafts cyclization of w-(2-thienyl) substituted longer fatty acids (130, n = 5,8,9) under high dilution conditions has been investigated by Goldfarb et They used boiling ethereal AlClg, 

Raney nickel desulfurization of the macrocyclic ketones or their reduction products confirmed the structures assigned to these compounds and simultaneously offers new methods for the synthesis of macrocyclic alicyclic compounds. =° 

The cyclization of thienylthioacetic acids and thienylthioacet-aldehyde acetals has been studied in connection with work on the chemistry of the thiophthenes. The cyclizations are achieved with concentrated or anhydrous usually in 

Cagniant, G. Merle, and D. Cagniant, Bull. Soc. chim. France, 1970, 316. P. Cagniant, G. Merle, and D. Cagniant, Bull. Soc. chim. France, 1970, 322. 

A useful route to cyclized 2-thienyl ketones has been found by Gol dfarb and co-workers. Starting from 2,5-dichlorothiophen, they obtained the keto-acids (78) by conventional Friedel-Crafts reaction with ester chlorides of succinic and glutaric acid followed by hydrolysis. By treatment with copper in refluxing propionic acid the 2-chlorine could selectively be reduced and Wolff-Kishner reduction then yielded (78a), which in turn could be ring-closed to (78b) by conventional means. Some other dechlorination methods have also been used.  

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A versatile approach to the 5-ary ldibenzo[7>,/]oxcpin system is the electrophilic ring-closure reaction of 2-phenoxy-substituted diphenylacetylenes or 1-bromostilbenes. Cyclization of the alkynes 4 occurs after addition of a suitable electrophile such as H +, PhS+, Br+, or I+ to give products 5.100-101 Addition across the triple bond gives stilbenes as byproducts. 

In some instances, the electrophilic ring closure reaction can be replaced by a nucleophilic reaction in an alkaline medium, such as used for polyfluorinated derivatives the compound obtained by condensing 2,3-dichloro-a-naphthoquinone with ethyl acetoacetate thus undergoes ring closure344 (Scheme 1). 

An electrophilic substitution reaction has been used for the key ladderforming step in the synthesis of soluble ladder-type poly(phenylene)s [51-53]. These aromatic polymers have a ribbon-like rigid, planar structure. They are of interest because of their optical and electronic properties [51,54,55]. The preparation of these polymers was accomplished by two basic steps. The first step was the construction of a substituted poly(p-phenylene) backbone. The ladder structure was obtained by a subsequent intramolecular electrophilic ring closure reaction. For example, the syn-... 

Oxidative Ring Closure Reactions 4.03.4.1.1 C—N bond formation N—N bond formation C—S bond formation N—S bond formation O—C bond formation O—N bond formation S—S, S—Se and Se—Se bond formation Electrophilic Ring Closures via Acylium Ions and Related Intermediates Ring Closures via Intramolecular Alkylations... 

Thermal reactions of N-aryl cyclopropenone imines 268 are differentiated by the nature of the N-aryl substituent. Imines 268 (Ar = phenyl, p-nitro-phenyl) undergo isomerization to N-aryl-2-phenyl-indenone imines 271 when heated in aprotic solvents202. Since in protic solvents, e.g. ethanol, only the iminoester 272 is isolated, evidence seems to be given for the intermediacy of 269 implying carbene and ketene imine functionality, which may either cause electrophilic ring closure with a phenyl group to form 271 or may add to the hydroxylic solvent (272). 

As we have seen (Section 4, p. 191) the range of effective molarities associated with ring-closure reactions is very much greater than that characteristic of intramolecular general acid-base catalysis the main classification is therefore in terms of mechanism. By far the largest section (I, Tables A-D) gives EM s for intramolecular nucleophilic reactions. These can be concerted displacements (mostly at tetrahedral carbon), stepwise displacements (mostly addition-elimination reactions at trigonal carbon), or additions, and they have been classified in terms of the nucleophilic and electrophilic centres. 

Carbon-Oxygen Bond Formation Hydroxyl or carboxylate groups can participate in a ring-closure reaction by an intramolecular nucleophilic attack to a generated electrophilic center as already described in Schemes 1 and 3. 

Further attention is given to the reaction of aryl- and heteryldiamines with S2CI2. In contrast with the complex reaction of 1,5-diaminonaphthalene, the condensation of 2,6-diaminonaphthalene with S2GI2 in the presence of pyridine proceeds via electrophilic ring closure at both pm-positions to afford naphtho[2,l-radical cation 154, which is reduced by PhjSb to neutral compound 24 (Scheme 29) <1998CC1939>. Chlorination of the remaining C-H positions does not occur. 

Ring-closure reaction of perfluoroalkenyl radicals, which are strong electrophilic sp3 carbon-centered radicals, is also extremely fast [25-27]. For example, ring-closure of 1,1,2,2-tetrafluoro- and l,l,2,2,3,3-hexafluoro-5-hexen-l-yl radicals is much enhanced relative to the parent radicals, as shown in Table 1.4. 

The third possibility for a selective monoalkylation is provided by intramolecular Friedel-Crafts alkylations. There are no multiple alkylations simply because all electrophilic centers react most rapidly intramolecularly (i.e., only once). Friedel-Crafts alkylations of this type are ring closure reactions. 

Electrophilic 5-endo-trig cyclizations continue to be the subject of considerable research efforts by several groups. Interest has been focused on the stereoselectivity of the ring closure reactions of homoallylic alcohols to substituted... 

A completely new cyclization reaction is the PhSeBr promoted conversion of alkenyl nitrones into 1,2-oxazines, which was recently reported by Tiecco [79]. This are the first examples to use alkenyl nitrones to effect ring closure reactions induced by electrophilic reagents. As indicated in Scheme 20, the nitrones 130 and 134 react with PhSeBr to give the cyclic iminium salts 131 and 135, respectively. These intermediates suffer attack by nucleophilic reagents at the partially positive carbon atom. The nature of the final products depends on the structure of the starting nitrone and of the nucleophile employed. Thus, the intermediate... 

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