Ring expansion three-membered rings

The diaziridines arc somewhat more stable than the oxaziranes. The three-membered ring of the oxaziranes is decomposed in all of its reactions, but with the diaziridines substitution on the nitrogen atoms can be effected and reactions involving fission and expansion of the ring to a five-membered ring are i)ossiblc,... 

The first member of the three-membered ring sulfones was synthesized about 70 years ago5, and its unsaturated analogue has been known for only 20 years6. Since the midsixties, an explosive expansion in the chemistry of some of these small- to middle-sized sulfone and sulfoxide heterocycles has taken place. 

Expansions of cyclic sulfones to cyclic sulfinates are known124, and a similar mechanistic pathway of the expansion of the three-membered ring to a four-membered ring has been suggested for the photolytic fragmentation of the 2,3-diphenylthiirene oxide 18a22. 

Other 1,3-dipolar reagents show the same mode of reactivity towards cyclopropenones. Thus, the Munchnones 412 serving as potential azomethine ylides259-261 or the nitrile ylids 41 3262 effect expansion of the three-membered ring to the 4-pyridone systems 411/414 as a result of (2 + 3) cycloaddition to the C /C2 bond. 

O ) By formation of seven-from three-membered rings This is the most widely used synthetic route to monocyclic oxepins. The key step in the synthesis of oxepin-benzene oxide (7) is the dehydrohalogenation of a dibromoepoxide precursor (64AG(E)S10). Since the benzene oxide valence tautomer is formed initially the valence tautomerization of the latter to oxepin (equation 51) may be considered as a ring expansion reaction. 

This reaction has been applied to tricyclic oxaziridines to obtain monocyclic lactams (6) by a three-membered ring expansion of cyclic ketones 4 (equation II). 

One atom incorporation reactions are discussed in Chapter II subdivided into carbon, nitrogen, and oxygen incorporation. A few of these reactions are discussed in other sections. Because of their special reactivity most of the three-membered ring compounds used for expansion are combined in Chapter III. Reactions with four-membered intermediates are collected in Chapter IV Reactions of the type 1/9 — 1/10 will be found in Chapter V and those of 1/7 —> 1/8 (see Scheme 1/1) in Chapter VII. Bicyclic starting materials will be discussed in... 

From three-membered rings In pro tic media, l-acyl-2-cyclopropene derivatives undergo a ring expansion reaction to cyclobutenols [7]. - Ring expansion of cyclopropylmethanols to fluorinated cyclobutans [8]. 

Zwanenburg, B., ten Hoite, P. The synthetic potentiai of three-membered ring aza-heterocycies. Top. Curr. Chem. 2001, 216, 93-124. Bonini, B. F., Fochi, M., Comes-Franchini, M., Ricci, A., Thijs, L., Zwanenburg, B. Synthesis of ferrocenyi-oxazoiines by ring expansion of N-ferrocenoyi-aziridine-2-carboxyiic esters. Tetrahedron Asymmetry 2003, 14, 3321-3327. 

Similarly, cyclopropanation of cycloalkenes and regioselective opening of the three-membered ring can be used in ring-expansion procedures. In the presence of a catalytic amount of bis(acrylonitrile)nickel(0), bicyclo[2.1.0]pentane reacts under mild conditions (40-60 C) with various electron-deficient alkenes, such as methyl acrylate, acrylonitrile, dimethyl maleate and fumarate to give bicyclo[2.2.1]heptane derivatives. An analogous thermal reaction of bicyclo[2.1.0]pentane with, e.g. fumaronitrile, requires much higher temperatures (160°C). ... 

Breaking open one of the three-membered rings gets us off to a better start. This gives a hydroxy-ketone that can rearrange in a pinacol fashion with ring expansion of the remaining cyclopropane. 

Three membered Rings.—As has been mentioned earlier, oxaziridines are readily formed by photochemical isomerization of nitrones. A purely chemical method has been discovered by some Japanese workers during the course of their work with ethyl 3-hydroxypyrrole-2-carboxylates (240). They have found that the esters produce oxaziridines (241) upon reaction with nitrous acid rather than the expected nitroso derivatives. Ring expansion may occur via a push-pull mechanism and is also seen upon reaction of (240) with aryl diazonium salts, when diaziridines (242) are formed if the pH of the reaction is carefully controlled. 

In electrocyclizations that involve the participation of three-membered rings, the equilibrium favors the ring-opened species. The venerable Buchner ring expansion provides an apt example. A key step in Mander and co-workers har-ringtonolide synthesis involves a Buchner ring expansion of an aryl ring to a cycloheptatriene (Scheme 19.45). ... 

Cyclotrisilanes and related three-membered ring compounds containing one, two, or three silicon atoms have attracted considerable attention (1). The ring strain of these molecules allows them not only to be used for various efficient ring-expansion and ring-opening reactions (2), but also for the generation of low-coordinated and unsaturated silicon compounds, especitilly silylenes, silenes, and disilenes (1,3). 

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