Ring expansion, of aryl

Early efforts to effect the photoinduced ring expansion of aryl azides to 3H-azepines in the presence of other nucleophiles met with only limited success. For example, irradiation of phenyl azide in hydrogen sulfide-diethyl ether, or in methanol, gave 17/-azepine-2(3//)-thione35 (5% mp 106—107 " O and 2-methoxy-3//-azepine (11 %),2 3 respectively. Later workers194 failed to reproduce this latter result, but found that in strongly basic media (3 M potassium hydroxide in methanol/dioxane) and in the presence of 18-crown-6, 17/-azepin-2(3//)-one was produced in 48% yield. In the absence of the crown ether the yield of azepinone falls to 35%. 

Lamara, K. and Smalley, R.K. (1991) 3H-Azepines and related systems. Part 4. Preparation of 3 H-azepin-2-ones and 6 H-azepino[2,l-b]quinazolin-12-ones by photo-induced ring expansion of aryl azides. Tetrahedron, 47 (12/13), 2277-2290. 

Scheme 4-286. Ring expansion of aryl epoxides with conjugated alkenes catalyzed by iron(II).

Ring expansion of the benzene ring of a calix[6]arene to a 1 //-azepine in 14% yield by photolysis of an aryl azide confined in the calix structure has been reported.294... 

In contrast to the acyl- and sulfonylnitrenes described in this section, arylnitrenes produced thermally or photolytically from aryl azides, including those bearing strongly electron-withdrawing substituents (e.g., CN, N02, CF3), fail to promote ring expansion of arenes to 1H-azepines, although intermolecular substitution of electron-rich substrates, e.g. mesitylene and A.TV-dimethylaniline, have been noted.167... 

Likewise, diamides, e. g. 90, formed by bis(2-azidobenzoylation) of diamines, suffer ring expansion of both aryl rings to yield the bis(aminocarbonyl) derivatives, e. g. 91. 

Ring expansions of 3-aryl-7-azido-2-chloroquinolines, e.g. 21, in potassium methoxide-meth-anol/dioxane yield mixtures of the expected 3-aryl-2-chloro-7-methoxy-9//-pyrido[2,3-f]pyrid-ines, e.g. 22, and the 2,7-dimethoxy derivatives, e.g. 23, formed by nucleophilic displacement of the 2-chloro group.154 ... 

Simiraly, alkynones undergo arylative cyclization with arylboronic acids in the presence of a rhodium catalyst (Equation (49)).400 When acetylenic /3-keto esters are employed as shown in Equation (50), arylative cyclization (formation of cyclobutanols) and subsequent, facile acid-catalyzed bond cleavage take place to give <5-keto esters.401 Ring expansions of cyclic [3-keto esters are also possible according to this reaction. 

N. V. Ganesh and N. Jayaraman, Synthesis of aryl, glycosyl, and azido septanosides through ring expansion of 1,2-cyclopropanated sugars, J. Org. Chem., 74 (2009) 739—746. 

The [Rh(COD)Cl]2- and Kl-mediated carbonylation of substituted A -alkylisothiazolidines 257 occurs regiospeci-fically at the S-N bond (Equation 23) <2004OL3489>. The most favorable solvent for this ring expansion of the A -alkylisothiazolidines to give tetrahydro-2/7-l,3-thiazin-2-ones 258 is toluene. The yields were dependent on the nature of the aryl group. A range of alternative metal catalysts were also screened, however carbonylation was observed with Co(CO)8, and Pd(OAc)2 only. 

Other synthetic routes to benzazepines involving ring expansion of six-membered heterocycles include the action of diazomethane (77CPB321), sulfonium ylides (77H(7)37> or acyl halides (75T1991) on quaternary 3,4-dihydroisoquinolines that of sulfoxonium ylides on quaternary quinolines (74IJC(B)1238) and the zinc-acetic acid reduction of quaternary 1-acyltetrahydroisoquinolines (77BSF893). Photoaddition of acyl- or aryl-nitrenes to the exocyclic alkene bond of 2-methylene-1,2-dihydroquinolines results in ring expansion to... 

Thermal ring expansion of cyclopropyl azides provides a general route (95 — 96) to alkyl- and aryl-azetines (79CB3914). 

A Lewis-acid-mediated ring expansion of isobenzofuranones 1154 provides the final step during the total synthesis of naturally occurring ( )-3-aryl-8-hydroxydihydroisocoumarins 1155 (Equation 446) <2001J(P1)3017>. 

The ring expansion of aziridines and azirines corresponds to this type of ring closure leading to A2-thiazolines. iV-Thioacylaziridines (366) rearrange under acid conditions to 2-aryl-A2-thiazolines (368) either by an S l or S 2 mechanism, depending on the nature of the acid catalyst and the solvent used (69JA5835,69JA5841). 

The 5,7-diaryl-2-fluoro-47/-l,3-diazepines 391 have been synthesized from 3-aryl substituted 2/7-azirines 390 and difluorocarbene generated from CF2Br2 (Scheme 184). This reaction involves isomerization of the initial azirinium ylide 392 into a 2-aza-l,3-diene 393, which undergoes [4 + 2] cycloaddition with the starting azirine, followed by ring expansion of cycloadduct 394 and dehydrofluorination <2006TL639>. 

Several [ 1,2,4]triazolobenzothiadiazocin-l 1-ones 174 were prepared via ring expansion of [l,2,4]triazolo[3,2-A -[2,4]benzothiazepin-10(5//)-ones in presence of sodium azide. The intermediate aryl isocyanate 173, formed as a result of Curtius rearrangement, was isolated and characterized by elemental analysis, IR, H NMR, and mass spectroscopies (Scheme 45 <2002PS2303>). 

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