Cyclodehydrogenation

Cyclodehydrogenation of 1,4,5-triphenylpyrazole (174) to give 1-phenyl-l//-phenanthro[9,10-c]pyrazole (175) has been observed (Scheme 13), but in the presence of benzophenone ring opening to 3-anilino-2,3-diphenylacrylonitrile (176) was preferred (77JCS(PD2095). 

Diels-Alder reaction of 22 and cyclopentadienones 24. The second step is an oxidative cyclodehydrogenation to yield HBCs 27. 

A typical aryl-aryl coupling and subsequent cyclodehydrogenation have afforded higher homo-logues of 42, such as the terrylene-47 and quaterry-... 

An alternative route to pyranthrone, involving baking 1,6-dibenzoylpyrene (6.96) with aluminium chloride, was also devised by Scholl (Scheme 6.18). The Scholl reaction is a key step in the synthesis of several polycyclic quinones the cyclisation of 1-benzoylnaphthalene to give benzanthrone (6.73) has already been mentioned. The mechanism of this cyclodehydrogenation reaction may involve an initial protonation step, if traces of water are present, or complexation with aluminium chloride. Electrophilic substitution is thereby... 

For the quinone imine cyclization of iron complexes to carbazoles the arylamine is chemoselectively oxidized to a quinone imine before the cyclodehydrogenation [99]. The basic strategy of this approach is demonstrated for the total synthesis of the 3-oxygenated tricyclic carbazole alkaloids 4-deoxycarbazomycin B, hyellazole, carazostatin, and 0-methylcarazostatin (Scheme 17). 

Scheme 29). Cyclodehydrogenation of 82 with stoichiometric amounts of palladium(II) acetate in N,AT-dimethylformamide (DMF) under reflux provides murrayastine 83 [17]. The same sequence of reactions can be applied to the synthesis of murrayaline A [17]. 

Crospovidone is a cross-linked homopolymer of A-vinyl-2-pyrrolidone. Acetylene and formaldehyde react to form butynediol. Hydrogenation and subsequent cyclodehydrogenation gives butyrolactone. The reaction of butyrolactone with ammonia produces pyrrolidone, which is vinylated with acetylene under pressure. The linear polymerization of the vinylpyrrolidone yields polyvinylpyrrolidone, a soluble binder, whereas the popcorn (branched) polymerization yields crospovidone, an insoluble... 

We planarized the appropriate oligophenylene derivative via oxidative intramolecular cyclodehydrogenation, which has been introduced by Kovacic and... 

Pyrazolotropolones 81a,b and 83 are generated by cyclodehydrogenation of 4-acetyltropolone tosylhydrazone (80 66BCJ253) and 4-ethyl-5-(p-tolylazo)tropolone (82a 65BCJ358), respectively (Scheme 20). The dehydrogenation of 82a is facilitated by iodine or benzoquinone. In contrast. 

Menendez et al. reported the synthesis of murrayafoline A (7) by palladium(II)-mediated oxidative double C-H activation of a diarylamine assisted by microwave irradiation (585). The aniline derivative 598 was obtained by O-methylation of 5-methyl-2-nitrophenol (625) followed by catalytic hydrogenation. The required diarylamine 654 was obtained by N-arylation of the aniline derivative 598 with phenyllead triacetate (653) in the presence of copper(II) acetate. Under microwave-assisted conditions, in the presence of more than the stoichiometric amount of palladium(II) acetate and a trace of dimethylformamide, the diarylamine 654 was cyclodehydrogenated to murrayafoline A (7) (585) (Scheme 5.47). 

Gulacsi, K. et al., A short and facile synthetic route to prenylated flavones. Cyclodehydrogenation of prenylated 2 -hydroxychalcones by a hypervalent iodine reagent. Tetrahedron, 54, 13867, 1998. 

Another related photocyclodehydrogenation synthesis uses an N- styrylpyridinone (164 Scheme 92), which is generated by N- alkylation of pyridinone with a styryl bromide. Irradiation in acid solution in the presence of oxygen causes isomerization and cyclodehydrogenation to afford 4//-benzo[a]quinolizin-4-one (165) and its 7-phenyl derivative (166) in yields of 37% and 60% respectively (77JOC1122). 

Attempted reduction of 2,3-diphenylquinoxaline by lithium results in cyclodehydrogenation to dibenzo[a,c]phenazine (77).93 2-Benzoyl-3-phenylquinoxaline is reduced by sodium amalgam to the red 2-benzoyl-3,4-dihydro-3-phenylquinoxaline (78a - - -78b).94... 

The cyclodehydrogenation of 2-(3,3-dimethylallyl)phenols with DDQ or chloranil (Scheme 7) has been envisaged77 on the basis of a biogenetic... 

These methods utilize molecules of type B (p. 362) (=104) a. Synthesis of Benzofurans by Thermal Ring Closure and Cyclodehydrogenation (Thermal, Catalytic or Photochemical) of o-Substituted Phenols. The Hansch Reaction. o-Alkylhydroxy-compounds on heating with a suitable catalyst give either a benzofuran or a 2,3-dihydrobenzo-furan with the same number of carbon atoms.285-292 Side products are formed by decomposition of the starting material. The initial phenol can be o-alkylated and ring-closed in one stage. 

The principal source of toluene is catalytic reforming of refinery streams. This source accounts for ca 79% of the total toluene produced. An additional 16% is separated from pyrolysis gasoline produced in steam crackers during the manufacture of ethylene and propylene. The reactions taking place in catalytic reforming to yield aromatics are dehydrogenation or aromatization of cyclohexanes, dehydroisomerization of substituted cyclopentanes, and the cyclodehydrogenation of paraffins. The formation of toluene by these reactions is shown. 

It has been known for some time that irradiation of azobenzene (324) in either 22 N sulfuric acid350 351 or acetic acid with added ferric chloride 352 yields benzo[c]cinnoline (325). This is accompanied by the formation of an almost equal quantity of benzidine (326), undoubtedly arising by rearrangement of hydrazobenzene (327). The mechanism of this reaction differs, therefore, from that of the stilbene cyclodehydrogenation, and azobenzene itself functions as the hydrogen acceptor. Yields of not more than 50% of benzo[c]cinnoline are generally observed. 

Certain aromatic imines undergo a similar cyclodehydrogenation to the phenanthridine. Benzylideneaniline (330) yields phenanthridine (331) only in sulfuric acid,355 whereas diphenylmethyleneaniline cyclizes in the presence of oxygen or iodine.356 Cyclizations have also been reported in Schiff s bases formed from a- and /J-naphthylamine357 and from 4-aminoquinoline,358 and in the mesomeric betaine 4,5-diphenyl-2-mercapto-l,3,4-thiadiazolium hydroxide.359 Irradiation of the anil of 2-aminonaphthalene (332) in ethanol, however, leads to incorporation of ethanol and the formation of 3-phenylbenzo[/]-quinoline (333).860 Additional photoproducts are obtained when the photolysis is carried out in w-hexanol.361... 

A number of related reactions are worthy of mention. Benzanilide (334) is converted into phenanthridone (335) by irradiation in benzene in the presence of iodine.362 Cyclodehydrogenation is also observed in the anilides of indole-2-carboxylic acid and indole-3-carboxylic acid on irradiation in acetone.363 Irradiation of diphenyl-amine304 and certain of its JV-substituted derivatives365 yields the corresponding carbazole. The mechanism of this reaction differs from... 

Catalytic cyclodehydrogenation of 5-amino-2-ethylthiophenol affords 6-aminobenzo[6]thiophene.239,241 6-Acetamido-2,3-di-bromo-,77 6-acetamido-2-bromo-3-methyl-,102 6-acetamido-3-bromo-2-methyl-,102 and 6-acetamido-3-bromobenzo[f>]thiophene107 may be obtained from the corresponding 6-acetyl compound by means of the Schmidt reaction. In some cases the 6-acetamido compound is accompanied by a smaller amount of the amine sulfate.102,107 6-Aminobenzo[6]thiophene may be converted into 6-chloro- or 6-cyanobenzo[6]thiophene by means of the Sandmeyer reaction.241... 

Total syntheses of chrysin (93) and luteolin (94), which are non-toxic potent inhibitors of reverse transcriptases from Rauscher murine leukemia (RLV) and the human immunodeficiency virus (HIV), were accomplished by Antus and coworkers via cyclodehydrogenation of the appropriately substituted 2 -hydroxy-chalcone (179) in the presence of PIDA/KOH in MeOH [136] (Scheme 45). 

---