OXIRANES REACTIVITY

As with other sections in this chapter, the discussion of oxirane reactivity is limited to cases where the bicyclic nature of the ring-fused epoxide plays an important role in the outcome of the reaction. The reader is directed to Chapter 1.03 for more general discussion of oxirane reactivity. 

Organoboranes are reactive compounds for cross-coupling[277]. The synthesis of humulene (83) by the intramolecular cross-coupling of allylic bromide with alkenylborane is an example[278]. The reaction of vinyiborane with vinyl-oxirane (425) affords the homoallylic alcohol 426 by 1,2-addition as main products and the allylic alcohol 427 by 1,4-addition as a minor product[279]. Two phenyl groups in sodium tetraphenylborate (428) are used for the coupling with allylic acetate[280] or allyl chloride[33,28l]. 

Preformed polymers containing reactive functionalities other than vinyl unsaturation (oxirane, hydroxyl, carboxyl, etc.)... 

The reactivity of phosphites described in Scheme 27 has been applied to various oxiranes to afford the expected alkenes in low (15%) to excellent (95%) yields (Fig. 6) [64]. 

The principle of active-site-directed inactivation of glycosidases by gly-con-related epoxides can be extended to compounds having an exocyclic oxirane ring, either directly attached to the six-membered ring (32) or at some distance (33,34). Studies with -o-glucosidase from sweet almonds and intestinal sucrase-isomaltase revealed that, in spite of the higher intrinsic reactivity of these epoxides, this shift of the position of the epoxide function causes a 10- to 30-fold decrease of kj(max)/Ki, an effect which probably reflects the limited flexibility of the catalytic groups involved in the epoxide reaction. 

Despite a higher intrinsic reactivity, epoxides of type 35 and 36 show a lower inactivation rate kj(max), as seen in Table XI, than the conduritol epoxides. This is probably caused by the greater flexibility of the epoxyalkyl chain in the active-site cleft, and by non-productive binding in positions where the oxirane is not within reach of the catalytic groups of the active site. For epoxypropyl oligosaccharides, this would hold even when the inhibitor occupies the correct subsites. 

Bonse G, Urban T, Reichert D, et al. 1975. Chemical reactivity, metabolic oxirane formation, and biological reactivity of chlorinated ethylenes in the isolated perfused rat liver preparation. Biochem Pharmacol 24 1829-1834. 

Epoxyketone 245 is readily available from 16-dehydropregnenolone via several steps, including a crucial microbiological 11a-hydroxylation. Dehydration of 245 gives the 9,11-olefin 246. The alcohol at C-21 is then converted to the mesylate (247), and this is reduced to give the methyl ketone (248). The olefin is then converted to the 9a-fluoro-llp-hydroxy array (250) by the standard sequence [addition of HOBr, closure to the oxirane (249), opening with HF]. Note that the reactivity of the epoxides in 249 is... 

Organotin epoxide monomers 30,100 104) containing fairly reactive oxirane rings and both C=C and Sn—C bonds can be used as starting components for the synthesis of new polymers, chemically active stabilizers and biocides for polymeric materials, e.g. for PVC. 

Polymeric molecules contain reactive hydrogen sites in the form of alcohols and carboxylic acids. The reaction sequence, alcohol plus anhydride and acid plus oxirane, results in an increase of one in the degree of polymerization if the oxirane is DGEBA. Polymeric species also supply oxiranes via the pendant R. These reactions are generalized by the notation ... 

The insertion of the oxiranes into a P-O-C- bond of the mixed ester takes place in the reaction of the O-alkyl O-silyl phosphonates 3 with oxiranes in contrast to the reaction above. The formed 0-(2-siloxyethyl) O-alkyl phosphonates 11 show the typical PH-reactivity with the protected HO-group for further reactions. 

Diepoxy-p-menthane, 6, rearranges when heated with alumina in toluene 73). What is the product of this reaction An organic chemist would predict that acid treatment of the diepoxide 6 (Figs. 3 and 32) would induce one or other of the oxirane rings to open. But which of the two will be the more reactive, and would overall reaction necessarily involve such an initial step Furthermore, for each oxirane there are two possible C—O cleavages. 

We owe to Kato and his colleagues a considerable advance in furan copper reagents. They have demonstrated the formation of the lithium di(3-furyl) cuprate species 87 which is highly reactive and possesses hard properties that suit it to reaction at hard centers, mainly carbonyl carbon.223 The reagent is easily prepared in situ from 3-furyllithium and Cu2I2. Simple copper derivatives do not react with ketones, but this cuprate reacts well and quantitatively with acid chlorides. It also reacts well with some epoxides (oxirans). Moreover, there is another form prepared in the presence of... 

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