Three-membered ring systems epoxides

This review covers the chemical literature on epoxides and aziridines for the year 2007. As in previous years, this review is not comprehensive but rather covers a selection of synthetically useful and interesting reactions. Three-membered ring systems, epoxides and aziridines in particular, are excellent synthetic intermediates. This is largely due to their ability to be converted into other functional groups such as diols, diamines, and amino alcohols to name but a few. The chapter has been divided into two sections, one covering epoxides and the other covering aziridines. Each of these sections has been further divided into two additional sections, one on the synthesis of the heterocycle and one on the reactions of the heterocycle. There is some overlap between methods for the synthesis of epoxides and aziridines and any overlap has been noted in the text. 

Ethylene oxides, like other three-membered ring systems, possess many singular features that invite a basis in theory. To satisfy Hub demand, much effort has boon devoted to the task of determining with precision such fundamental properties of the molecule os bond lengths, bond angles, and bond energies- With the advent of modern instrumental methods it has been possible to develop a dependable physical basis fin- theoretical speculations on the electronic structure of ethylene oxide. The present section is concerned with this aspect of epoxide chemistry. 

One of the commonest three-membered ring systems is the three-membered cyclic ether, otherwise known as an epoxide, or oxirane. The first thing to... 

In this scheme electrophilic oxygen attacks the N=0 bond in an analogous way to the epoxidation of olefins the initially formed putative three-membered ring system then rearranges to the stable nitro-group. 

The construction of strained three-membered ring systems remains a mainstay in organic synthesis. Catalytic asymmetric epoxidation and cyclo-propanation reactions continue to attract attention due to the inherent value of epoxides and cyclopropanes in pharmaceuticals and as synthons towards... 

For our current discussion, we will focus on oxiranes, cychc ethers containing a three-membered ring system. This ring system is more reactive than other ethers because it has significant ring strain. Substituted oxiranes, which are also called epoxides, can have up to four R groups. The simplest epoxide (no R groups) is often called by its common name, ethylene oxide. 

The oxygen containing three-membered ring systems known either as oxiranes or epoxides are special ethers in the sense that they are both readily formed and highly reactive. These substances are commonly formed by one of three methods intramolecular nucleophilic displacement, methylenation of a carbonyl compound or epoxidation of an olefin. All three synthetic approaches are represented in the phase transfer literature. 

Although epoxides are always considered to have their oxygen atom as part of a three membered ring the prefix epoxy in the lUPAC system of nomenclature can be used to denote a cyclic ether of vanous sizes Thus... 

In the synthesis of cyclohexene oxide from cyclohexene shown, this does implicate the less favourable diaxial conformer in the epoxide-forming step. Cyclohexene oxide contains a c/s-fused ring system, the only arrangement possible, since the three-membered ring is necessarily planar (see Section 3.5.2). 

A carbene or nitrene transfer reaction to a carbon-carbon or carbon-heteroatom double bond system leads to the formation of three-membered rings, such as a cyclopropane, an aziridine or an epoxide. These processes can be catalyzed by applying iron catalysts and the different cyclic systems are discussed here. 

For the period 1995-2006, the formation of fused-ring thietanes from three-membered rings is exclusively confined to the transformation of oxiranes fused to complex ring systems. It was reported <1997PS389> that the 5- y-acetyl-2,3-epoxide derived from D-xylose 116 was easily converted to 2-oxa-6-thia-bicyclo[3.2.0]heptane 117 by the action of sodium acetate (Equation 34). 

The three-membered peroxide systems produced from ketones in the presence of peroxymonosulfate have been discussed separately above because of their importance. There is, however, another three-membered ring peroxide which needs noting the reaction of alkoxysulfuranes with anhydrous hydrogen peroxide. Such systems have been employed for the low temperature epoxidation of alkenes (Figure 2.50).156... 

The force field covers all alkenes, with the exception of those containing directing groups where additional double bonds or three-membered rings are at least three bonds (two saturated carbon atoms) and every other type of unsaturation or heteroatoms at least four bonds (three saturated carbon atoms) apart from the double bond to be epoxidized, and those where the double bond is not part of a cyclopropane or cyclobutane system. 

The effect of substitution is also evident in the epoxidation of verticillene (21)153 at the 7,8-double bond giving 22. Once again, the conformation of the transition state for the preferred mode of attack is, ideally, puckered. The additional three-carbon bridge prevents total ring inversion additional substitution of the double bonds limits conformational mobility. Thus, even for this 14-membered ring system high diastereoselectivity is predicted. Experimental evidence so far, however, indicates only that no second diastcrcomcr of the 7,8-cpoxidc 22 has been isolated from the reaction mixture. 

This chapter concerns the net conversion of a C =X ir-bond (X = O and N) to various three-membered ring heterocycles. The most common such process is the conversion of an aldehyde or ketone to a homologous epoxide. These reactions will be discussed along with the analogous process which takes place on imines and related compounds. Additionally, methods to effect the addition of elements other than carbon across either unsaturated system will considered. The synthesis of thiiranes by the addition of carbon across the C—S rr-bond is the subject of a recent comprehensive review and will not be covered here. 

Epoxides and other three-membered rings present special problems since they are in between tet and trig systems. Generally, three membered rings prefer the exo mode of attack (see 183) to give 184. 0 The endo... 

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