Structure of epoxides

Fig. 3 Representative structures of epoxide inhibitors (Cbz-GluAla(aza)Leu-EP), AOMK inhibitors (Hpa-SerLeu-AOMK), fluorescent probe (AWP19, containing Cy5) and broad-spectrum cathepsin inhibitor JPM-OEt...

Fig. 6 Structures of epoxide-based inhibitors and activity-based probes used in the study of falcipain 1...

Epoxide polymers exhibit stereoisomerism originating from the chirality of tertiary carbon atoms present in the polymer main chain. The stereoisomers of epoxide polymers are therefore tactic polymers their tacticity is connected with the structure of epoxide monomers undergoing polymerisation. Epoxide... 

Figure 9.1 Structure of epoxide polymers of various tacticity obtained from mono- and disubstituted epoxides...

Several optically active, /i-unsaturated esters 1, bearing a chiral auxiliary as the ester group, were epoxidized with tor-butyl hydroperoxide in the presence of butyllithium with yields highly dependent on the structure of epoxidized compound and moderate to good diastereose-lectivity 28. (—)-(S )-S -Phenyl-.V-2-phenylvinyl-A, -tosylsulfoximide (3) was epoxidized under the same conditions to give almost quantitatively the crude epoxide 4 (which decomposes on attempted chromatography, isolated yield 20%) with complete diastereoselectivity28. 

The structure of epoxidized oligomers depends on the microstructure of the initial products, on their degree of epoxidation, and on the working conditions employed. In terms of these factors, in the epoxidized polydienes besides oxiranic groups there have been identified unreacted double bonds, too, as well as other structural motifs like acyloxy, hydroxyl, ketonic, and aldehydic groups and etheric links. 

Fig. 11.3 Chemical structure of epoxidized soy bean oil, a plasticizer used in the polyvinyl chloride (PVC) gaskets of glass jar closures intended for heat sterilization...

M. Nardini, I.S. Bidder, H.J. Rozeboom, K.H. Kalk, R. Rink, D.B. Janssen, B.W. Dijkstra, The X-ray structure of epoxide hydrolase from Agrobacterium radiobacter ADI - an enzyme to detoxify harmful epoxides, J. Biol. Chem. 274 (1999) 14579-14586. 

A catalytic enantio- and diastereoselective dihydroxylation procedure without the assistance of a directing functional group (like the allylic alcohol group in the Sharpless epox-idation) has also been developed by K.B. Sharpless (E.N. Jacobsen, 1988 H.-L. Kwong, 1990 B.M. Kim, 1990 H. Waldmann, 1992). It uses osmium tetroxide as a catalytic oxidant (as little as 20 ppm to date) and two readily available cinchona alkaloid diastereomeis, namely the 4-chlorobenzoate esters or bulky aryl ethers of dihydroquinine and dihydroquinidine (cf. p. 290% as stereosteering reagents (structures of the Os complexes see R.M. Pearlstein, 1990). The transformation lacks the high asymmetric inductions of the Sharpless epoxidation, but it is broadly applicable and insensitive to air and water. Further improvements are to be expected. 

The so-called phenoxy resins were a development of epoxide resin technology which had hitherto been used exclusively in the thermosetting resin field (see Chapter 26). As with the most important epoxide resins they are prepared by reacting bis-phenol A with epichlorohydrin to give the following structure (Figure 21.9) ... 

The first, and still the most important, commercial epoxide resins are reaction products of bis-phenol A and epichlorhydrin. Other types of epoxide resins were introduced in the late 1950s and early 1960s, prepared by epoxidising unsaturated structures. These materials will be dealt with in Section 26.4. The bis-phenol A is prepared by reaction of the acetone and phenol (Figure 26.1). 

Treatment of the following epoxide with aqueous acid produces a carbocation intermediate that reacts with water to give a diol product. Show the structure of the carbocation, and propose a mechanism for the second step. 

YVe saw in Section 17.4 that keLones react with NaBH4 to yield alcohols. We ll also see in Section 22.3 that ketones react writh Br2 to yield a-bromo ketones. Perhaps surprisingly, treatment with NaBH4 of the a-bromo ketone from acetophenone yields an epoxide rather than a bromo alcohol. Show the structure of the epoxide, and explain its formation. 

Finally, metalated epoxides undergo isomerization processes characteristic of traditional carbenoids (Scheme 5.2, Path C). The structure of a metalated epoxide is intermediate in nature between the structures 2a and 2b (Scheme 5.2). The existence of this intermediacy is supported by computational studies, which have shown that the a-C-O bond of oxirane elongates by -12% on a-lithiation [2], Furthermore, experimentally, the a-lithiooxycarbene 4a (Scheme 5.3) returned cydo-pentene oxide 7 among its decomposition products indeed, computational studies of singlet 4a suggest it possesses a structure in the gas phase that is intennediate in nature between an a-lithiocarbene and the lithiated epoxide 4b [3],... 

The structure of glabrescol was subsequently revised, and the new structure was synthesized enantioselectively through sequential hydroxy-directed anti-oxidative cyclization of acyclic y-alkenols with VO(acac)2/TBHP to construct the adjacent THF rings via epoxides under acid conditions [35b],... 

Epoxides are found in thousands of biological molecules and constitute vital functional entities. They can impart localized structural rigidity, confer cytotoxicity through their role as alkylating agents, or act as reactive intermediates in complex synthetic sequences. The widespread occurrence of epoxides is contrasted by only a handful of aziridines that are known to date. In this chapter we would like to introduce the different mechanisms by which enzymes produce epoxides. 

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