Aryl mediated cyclization

Scheme 9 Iron(III) bromide-mediated cyclization of aryl glycidyl ethers to 3-chromanols...

Alkyl- and 5-aryl-2-amino-1,3,4-oxadiazoles were prepared by tosyl chloride/pyridine-mediated cyclization of thiosemicarbazides in good yields (79-99%). Interestingly, thiosemicarbazides exhibited a higher rate of cyclization than the corresponding semicarbazides. For example, 171 (X-S) was converted to oxadiazole 172 within 5 h <06JOC9548>. 

Buchwald parlayed the powerful Buchwald-Hartwig aryl amination technology [439-447] into a simple and versatile indoline synthesis [448-452], For example, indole 368, which has been employed in total syntheses of the marine alkaloids makaluvamine C and damirones A and B, was readily forged via the Pd-mediated cyclization shown below [448], This intramolecular amination is applicable to the synthesis of -substituted optically active indolines [450], and o-bromobenzylic bromides can be utilized in this methodology, as illustrated for the preparation of 369 [451]. Furthermore, this Pd-catalyzed amination reaction has been applied to the synthesis of arylhydrazones, which are substrates for the Fischer indole synthesis [453,454],... 

Improved methods for preparing isoquinolines through ring closure at the C4-aryl bond were reported (Scheme 9). The Pummerer reaction induces cyclization of the iV-formylbenzylamine sulfoxide 40 by treatment with TFAA or BFj OEt/TFAA <99H(51)119>. The unactivated titanium-mediated cyclization of the Schiff base 41 to 6-methoxyisoquinoline is effected through intermediacy of the Ai"-ethylcarbamate-aminophosphate derivative 42 <99SC1617>. 

Aryl-l,2-dihydro-3-nitro[l,8]naphthyridines have been obtained by the 6jt-thermal electrocyclization of l-(2-arylide-neamino-3-pyridyl)-2-nitroethylenes, obtained in situ from aromatic aldehydes and l-(2-amino-3-pyridyl)-2-nitroethylene in xylene <2002SC747>. 2-Chlorotetrahydro[l,8]naphthyridines have also been obtained from 2,6-dichloropyridines using a free radical xanthate-mediated cyclization <20040L3671>. 

Intramolecular addition of vinyl radicals to olefins as a method for heterocycle synthesis has been examined. The vinyl radicals can be conveniently generated from vinyl bromides and samarium(II) diiodide [95JOC7424], The intermediate radical after cyclization undergoes a further electron transfer from samarium to furnish a carbanion which is quenched at the end of the reaction. A samarium(II) diiodide mediated aryl radical cyclization onto a dihydrofuran has been reported [95T8555],... 

Reduction of the reaction time was achieved by the use of microwave dielectric heating in synthesis of 2-cyanobenzothiazoles from anilines and 4,5-dichloro-l,2,3-dithiazolium chloride [84,85]. Mn(OAc)3 promoted the radical-mediated cyclization of aryl- and benzoyl-thioformanilides. The re-... 

Hydrogenolysis of the racemic isoxazolyl alcohol 955 followed by an acid-mediated cyclization of the resulting diketone provides the dihydropyran-4-one 956, an intermediate during a synthesis of C-aryl glycosides (Equation 373) <20020L977>. 

Marco-Contelles and de Opazo utilised 8-ewd< -trig and 1-exo-Aig cyclizations, mediated by B113S11H, in their construction of chiral, polyfunctionalised medium-sized carbo-cycles670. For example, treatment of iodide (104) with BujSnll under standard radical conditions afforded 7-membered carbocycles (105) in moderate yield (equation 79). Prado and coworkers reported the synthesis of benzolactams by 11 -endo aryl radical cyclization, an example of which is highlighted in equation 80675. 

Stoichiometric palladium-mediated cyclization was used in natural product synthesis by Boger a number of years ago, as was noted in the introduction. More recently, an intramolecular palladium-catalyzed amination of a heteroaromatic halide has been used as a step in the synthesis of an a-carboline natural product analog [146]. As discussed above, the diphenylhydrazone arylation can also be used for nitrogen heterocycle synthesis [140]. 

An improved procedure for the rapid synthesis of aryl dihydrobenzofurans with a boron tribromide-mediated cyclization was discussed and is shown below <03HCA343>. In the second total synthesis of diazonamide A, the late key step accounting for the dihydrobenzofuran formation was a DIBAL-H mediated lactam reduction and cyclization <03AG(E)1753>. 

A mixture of a Grignard reagent and C0CI2 has also been used to initiate aryl radical cyclizations. Titanium(III)-mediated radical cyclizations are known, and Sml2-mediate reactions are possible in the presence of a nickel catalyst. Organoborane-mediated radical cyclizations are known.Electrochemically generated radicals also cyclize. The influence of the halogen atom on radical cyclization has been studied.Both phenylthio and phenylseleno... 

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