Synthesis of heterocycles by radical cyclisation

Bowman, W. Russell; Cloonan, Martin O.; Krintel, Sussie L.

doi:10.1039/a909340k

Cited by 79 publications

(26 citation statements)

References 149 publications

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“…[3] Previously [4d] we had shown that the trichloroacetate 1 (Scheme 1) undergoes sequential 5-exo, 6-exo ATRC reactions to provide rapid access to the 2-oxabicyclo[4.3.0]nonane ring system. Alternate modes of cyclization (by 9-exo or 10-endo pathways [5] ) were not observed.…”

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confidence: 99%

A Remarkably Simple and Efficient Benzannulation Reaction

2007

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Dedicated to Professor E. J. Thomas on the occasion of his 60th BirthdayWhereas the application of atom-transfer radical cyclization reactions [1] (ATRC reactions) to the synthesis of g-butyrolactams and g-butyrolactones is now reasonably well established, [2] the same cannot be said of this approach to the synthesis of medium-sized rings.[3] Previously [4d] we had shown that the trichloroacetate 1 (Scheme 1) undergoes sequential 5-exo, 6-exo ATRC reactions to provide rapid access to the 2-oxabicyclo[4.3.0]nonane ring system. Alternate modes of cyclization (by 9-exo or 10-endo pathways [5] ) were not observed. As a continuation of our interest [4] in this area we investigated the cyclization of trichloroacetate 3 (Scheme 1) under ATRC conditions. The synthesis of benzoxocins by radical cyclization reactions has received scant attention, [6,7] and at the outset we wished to determine if ATRC of 3 would occur by a 7-exo or 8-endo pathway [8] and whether structural features could be incorporated into the substrate to modify the regiochemical outcome of the reaction.[9] Furthermore, we also believed that this potentially problematic cyclization [10] reaction would also serve as a suitable test case for the development and evaluation of more efficient catalysts for ATRC reactions. [11] Cyclization (CuCl, 5 mol %; 2, [12,13] 5 mol %; toluene; reflux) of the readily available ester 3 proved to be rather sluggish (95 % completion after 48 h) and afforded the labile [13] lactone 4 through an 8-endo-trig reaction. [7] This cyclization was also accompanied by the formation of a minor, wholly aromatic, by-product whose generation lagged behind the formation of lactone 4. Repeating the ATRC reaction of 3 for 120 h in refluxing toluene resulted in the total consumption of both ester 3 and the lactone 4, and the formation of 1-chloronaphthalene (6), which was isolated [14] in 23 % yield after column chromatography. Moreover we were delighted to observe that irradiation [15][16][17] of a solution of the ester 3 in 1,2-dichloroethane containing our standard catalyst system in a microwave reactor [18] afforded 6 in 84 % yield after a much reduced reaction time of two hours (Scheme 1).Wishing to establish the synthetic generality of this reaction, we subjected a variety of readily available aryl trichloroacetates (8, 10, 12 a-12 f; Scheme 2) to the same experimental protocol and found that, in each case, benzannulation proceeded smoothly over two hours at 200 8C to produce their respective naphthalene derivatives. The requisite ortho-allyl phenols used in this study are themselves readily available from an ortho-Claisen rearrangement [19] of their respective allyl aryl ethers, a process which in many cases can also be efficiently accomplished in a microwave reactor. [20] Of note is the observation that the benzannulation reaction proceeds with complete regiochemical control to afford an aryl chloride as a single regioisomer in each case. For example, subjecting the isomeric naphthyl esters 8 and 10 separately to our standard react...

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A Remarkably Simple and Efficient Benzannulation Reaction

2007

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“…Picking up from that point, the present compilation focuses on recent applications of α-amino radicals in intramolecular processes mainly leading to nitrogen heterocycles. 18 As expected based on their stability, one of the attractive features of the use of these radicals is their ease of generation, and several methods have been developed that perform this task efficiently. In the sections that follow applications are organized according to the method followed for radical generation, with special attention given to reactions of the less often utilized α-(dialkylamino)alkyl radicals.…”

Section: Methodsmentioning

confidence: 99%

Recent developments in cyclization reactions of α-aminoalkyl radicals

Aurrecoechea

Suero²

2004

Arkivoc

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The intramolecular additions of C-centered neutral α-aminoalkyl radicals onto suitably positioned C-C double bonds provide a ready access into functionalized carbocycles and heterocycles. This strategy offers considerable versatility in the selection of starting materials since a number of methods using different functional groups and reagents are available for α-aminoalkyl radical generation. This review covers the recent progress in this field.

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“…The combined organic extracts were dried, and concentrated to yield a yellow solid. The crude solid was subjected to flash column chromatography (DCM) to give 16b ( 2,110.5,112.6,128.9,129.7,129.8,129.9,131.6 (ArC), 135.2 (C-7a), 140.6 (C-2 0 ), 154.4 (COCH 3 ), 166.8 (CO); CI-MS m/z 387 ([MCH] C , 13%); HREI-MS m/z calcd for [M] C C 19 …”

Section: Genernal Procedures For the Preparation Of Iodoacetamides (15mentioning

confidence: 99%

“…1,2 The more successful intramolecular radical cyclizations proceed to form five-membered rings, [3][4][5] and these are more general than those forming six-membered 6 or seven-membered 7,8 rings. However, cyclization leading to indole fused eightmembered rings is less well investigated.…”

Section: Introductionmentioning

confidence: 99%