Electrophilic Ring-closure

Electrophilic Ring-closure.—Cagniant and co-workers have extended their studies on the synthesis and properties of thiophens with annelated saturated rings to benzo[/ ]thiophen. The 3-benzo[6]thienylthio-acid (356), 

Cagniant, P. Cagniant, and J. Trierweiler, Bull. Soc. chim. France, 1969, 601. P. Cagniant, D. Cagniant, and J. Trierweiler, Bull. Soc. chim. France, 1969, 607. P. Cagniant and G. Kirsch, Compt. rend, 1971, 272, C, 406. 

Electrophilic Rii -closure.— Acids of the type (83), in toluene and polyphosphoric acid, ring-close to position 4 and eliminate a t-butyl group when n = 2 (84), and ring-close to position 2, when n = 3 or 4, yielding 

Ring-closure of the 2,5-dichloro-3-thienyl-substituted acids (86) to (87) is best achieved by using the acid chlorides and AICI3 as catalyst. Many different methods for the dechlorination of (87) were tried. Total dechlorination was achieved in only one case. The chlorine ortho to the carbonyl group could, however, be removed in various ways. A mixture of isomers corresponding to ring-closure at the 3- (20%) and 4-position (80%) of the 

Muraro, D. Cagniant, and P. Cagniant, Bull. Soc. chim. France, 1973, 343. 

Radical Reactkms.— The reaction of a-chloro ethers with zinc in the presence of thiophen yields di-2-thienyl alkanes (106). Photolysis of 3-iodopyridine in the presence of thiophen yields a mixture of 3-(2-thienyl)-pyridine and 3-(3-thienyl)pyridine in a 7 1 ratio. The synthesis and rate of decomposition of 2- and 3-thenoyl peroxides and of mixed thenoylfuroyl peroxides have been described. Aryl 2-thenoates were obtained in high yields by the reaction of substituted benzenes with 2-thenoyl peroxide in the 

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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... 

Electrophilic Ring Closures via Acylium Ions and Related Intermediates... 

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. 

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). 

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). 

Electrophilic ring closure of aryl-substituted compounds such as alkenes, halides, alcohols, and carbonyl compounds called cyclialkylation can be induced by conventional Friedel-Crafts catalysts309 and by superacids. Examples are also known in which an intermolecular alkylation step is followed by intramolecular alkylation of the intermediate to furnish a cyclic product. 

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. 

Treatment of symmetrically substituted bis(arylmethylthio)alkynes with either IC1 or Br2 and in the strict absence of water promotes cyclization to 1//-2-benzothiopyrans (Scheme 129). Initial formation of a cyclic halonium species induces the electrophilic ring closure. In the presence of nucleophiles, ipso attack is favored and a spirocyclohexa-dienone results. The alkynes are accessible from arylmethyl thiocyanates and sodium acetylide <1998JOC4626, 2003EJ047>. 

C-3 of the naphthalene nucleus that controls the electrophilic ring-closure of the vinyl ketene intermediate moreover, the degree of aromaticity of the angular rings in the phenanthrene skeleton exceeds that in the anthracene analogues. This regioselectivity observed in the benzannulation of chromium carbenes is paralleled by results observed for 2-naphthyl cyclo-butenones [77] and for the palladium-catalyzed cyclocarbonylation of 2-naphthyl allyl acetates [78]. 

The betaine anhydro-2-hydroxy-8-methyl-4-oxo-3 -phenyl-4//- imidazo[2,1 -b ][ 1,3]thi-azinium hydroxide undergoes thermal rearrangement to 2,3-dihydro-7-hydroxy-2-methyl-5-oxo-6-phenyl-3(2H),5H-pyrrolo[l,2-c]imidazolethione. A process involving an initial fission of the 1,2-bond and electrophilic ring closure of the resultant ketene intermediate into the 4-position of the imidazole nucleus is most likely involved in the rearrangement (equation 67) (80JOC2474). 

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-... 

A particularly valuable route to phenoxathiin utilizes the long-known reactions of diphenyl ether with sulfur in the presence of aluminum chloride. Thianthrene was similarly prepared from sulfur and benzene. These harsh conditions would not be suitable for the preparation of electron-rich alkoxylated dibenzo compounds (181). For the construction of compounds (181), a strategy involving electrophilic ring closure of preformed alkoxylated diaryl chalcogenides (180) was used. Thus, treatment of compounds (180) with sulfur dichloride in dry chloroform provided the derived phenoxathiins (181) in satisfactory yields <88JCS(Pl)2095>. 

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,... 

Electrophilic ring closure of alkylidenfuranones of type (76) (A -butenolides), which are available from furan-2-carbaldehyde and y-keto acids, with AICI3 yields 4-arylbenzofuran-6-carboxylic acids (Scheme 70) <77JPR689>. 

In order to study spiroconjugation by the u.v. technique, the spiro-fluorene derivatives (485)—(488) have been prepared. They were prepared by the reaction of fluorenone with 2-lithio-3,3 -bithienyl, 4,4 -dilithio-3,3 -bithienyl, 3-lithio-2,3 -bithienyl, and 3,3 -dilithio-2,2 -bithienyl, respectively, followed by electrophilic ring-closure of the intermediate triaryl-carbonium ions by the addition of a drop of hydrochloric acid." ... 

Three phenalenothiophens (521)—(523) have been synthesized by unambiguous methods from the corresponding phenalenothiophenones (524)— (526), which in turn were prepared by electrophilic ring-closure of the corresponding 3-(a-naphthyl)thiophen-2-carboxylic acid, 2-(a-naphthyl)-thiophen-3-carboxylic acid, and 3-(a-naphthyl)thiophen-4-carboxylic acid. The reaction of (527) with glycerol, sulphuric acid, and iron leads to the... 

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