Ring expansion aziridine rings, with

Rhodium-complexed dendrimers, supported on a resin, have been reported to show high activity for the carbonylative ring expansion of aziridines with carbon monoxide to give p-lactams (Scheme 59), [150]. 

Scheme 59 Carbonylative ring expansion of aziridines with rhodium-complexed dendrimers...

Rearomatization is much more common in bi- and tricyclic azides than ring expansion although ring expansion may be achieved employing sodium methoxide in methanol-dioxan as solvent. This dichotomy again may be resolved by invoking an aziridine intermediate vide supra). The formation of aminoketals from 2-azidoanthracene and 48 is of particular interest as similar products are commonly formed from aliphatic azirines on treatment with mildly basic methanol. At the moment, the intermediacy of benzazirine in phenyl azide photolysis at room temperature cannot be ruled out. In the case of bi- and tricyclic aromatic azides and azidouracil decompositions the nature of the products strongly supports azirine involvement. Only further experimental work will resolve this mechanistic dilemma. With this in mind, benzazirine intermediacy will be assumed for the purposes of this discussion. 

The carbonylative ring expansion of aziridines with carbon monoxide is accomplished in the presence of rhodium complexed dendrimers to afford j8-lactames 63 in quantitative yields . ... 

The most useful reactions combine carbanion nucleophiles with activated aziridines. For example, the ring expansion which occurs on treatment of aziridines (219) with malonate salts typifies the heterocyclic synthesis possible. The conversion is quite general since many analogous transformations have been observed in which different carbanion stabilizing substituents were employed (73S546). 

Ring expansion of activated aziridines (43) with sulfur ylides also provides a synthesis of azetidines (75JOC2990, 58BSF345, 81CC417). The highly reactive sulfonium methylide (44 R = R = H) undergoes further reaction with the azetidines (46), but the reaction is satisfactory for substituted methylides. The less reactive sulfoxonium methylide (45 R = R = H)... 

A number of 2-acylazetidines have been prepared by reaction of 1,3-dihaloacyl compounds with amino derivatives (Section 5.09.2.3.l(m)). This is illustrated for azetidine 2-carboxylic acid (56), the only known naturally occurring azetidine. Ring expansion of activated aziridines (43) and contraction of 4-oxazolines (55) has also found limited use (Section 5.09.2.3.2(f) and Hi)). 

N-Acylaziridine-2-carboxylates readily rearrange to oxazolines under thennal, acidic, or nucleophilic conditions [91, 123-127]. Treatment of trans-aziridine-2-car-boxylate 176 (Scheme 3.63) with Nal in acetonitrile, for example, resulted in ring-expansion product 177 through the so-called Heine reaction. The reaction involves initial opening of the aziridine ring by iodide and subsequent oxazoline ring-closure by Sn2 displacement of the resultant iodide intermediate [127]. 

Treatment of aziridine 137 (Scheme 3.64) with benzoyl chloride in the presence of pyridine and DMAP afforded ring-expansion product 179 in 90% yield [45], The reaction was believed to proceed through the intermediate N-acylaziridine 178. 

When aziridine 194 (Scheme 3.71) was treated with a catalytic amount of NaOEt in ethanol it underwent an intramolecular ring-expansion to pyrrolidinone 195 in 88% yield [130]. The ring-opening took place via an internal SN2 reaction, which was confirmed by an X-ray analysis of the product 195. It is interesting to note that under similar reaction conditions 196 (Scheme 3.72) afforded P-lactam product 197 [130]. 

The highly strained and reactive 2iT-azirines have been extensively studied for various synthetic purposes, such as ring expansion reactions, cycloaddition reactions, preparation of functionalized amines and substituted aziridines. The older literature on azirines in synthesis has extensively been reviewed [69]. Concerning azirines with defined chirality only scarce information is available. Practically all reactions of azirines take place at the activated imine bond. Reduction with sodium borohydride leads to cz5-substituted aziridines as is shown in Scheme 48 [26,28]. 

These findings can be interpreted in terms of a normal ring-opening mechanism of intermediate 325 with proton transfer favored by protic solvent, whilst in aprotic solvent cycloreversion of the unstable aziridinium grouping in 325 followed by ring expansion prevails. Likewise, 2,3-disubstituted aziridines follow this reaction pattern, while N-substituted aziridines do not225. ... 

Several groups have reported reactions giving thiazoloquinazoline compounds, probably the most interesting of which is the cyclization-ring expansion of the aziridine thiourea derivative 225 to the dihydro-thiazolo[2,3-6]quinazoline 226 in hydrochloric acid of Howard and Klein.395 Other preparations of this ring system involve the treatment of l-(2-carbethoxyphenyl)-2-thioureas with a-chloro 396-398 or a-bromo396,397 ketones or with vicinal dibromides.398 Tetrahydro salt... 

Routes to dihydro-1,4-thiazines are more diverse (B-78MI22701). For example, ring expansion of the thiazolidine (273) by heating with elemental sulfur and n-butylamine affords a 3 1 mixture of the isomers (276) and (277) and it appears likely that these products are derived from their tautomers (274) and (275) respectively which are the initial reaction products (Scheme 118) (70LA(739)32). Thiazolidines also accompany dihydrothiazines as products when aldehydes or ketones are reacted with aziridines in the presence of sulfur and DMF or potassium carbonate and it seems certain that a similar mechanistic sequence is involved (79M425). 

Substituent effects on aziridine formation from N-(2-halogenoethy 1) arylsulphonamides by cyclization in basic media have been studied,425 and Hammett equation studies have also been of utility in determination of mechanisms of thermal decomposition of 3-chloro-3-aryldiazirines,426 427 the acid-catalyzed methanolysis of arylepoxides,428 and the ring expansion reactions of 2-aryl-l,l-dimethylaziridinium salts with benzaldehyde to form 5-aryl-3,3-dimethyl-2-phenyloxazolidinium salts.429... 

The ring expansion of aziridines has been reported in 2001 as a well established protocol [144] for preparing p-lactams in a regioselective manner [145]. A variety of aziridines with different substituents and stereochemistry was subjected to cobalt carbonyl-catalyzed carbonylation to give p-lactams. The ring expansion to... 

Rh-complexes in carbonylative ring expansion reactions of a variety of aziridines with carbon monoxide, which resulted in the formation of (3-lactams in good yields. It was reported that the catalytic system could be easily recovered by simple filtration and recycled without significant loss of activity. 

A nitrene generated from the reaction of A-aminophthalimide (101) and PhI(OAc)2 was key to the metal-free ring expansion of alkylidenecyclopropanes (102) and an alkylidenecyclobutane.85 The authors propose two plausible mechanisms for these ring-expansion reactions either an aziridine is formed which undergoes facile rearrangement to form the final 2,2-disubstituted cyclobutylidene hydrazine product (103), or reaction of the alkylidenecyclopropane with the nitrene generates an ionic or diradical species which rearranges. 

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