Oxirane ring reactions involving

It is also possible to convert carbonyl groups into oxirane rings with cenain carbenoid synthons. The classical Darzens reaction, which involves addition of anions of a-chloroacetic esters, has been replaced by the addition of sulfonium ylides (R. Sowada, 1971 C.R. Johnson, 1979). 

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. 

Since spontaneous polymerization is only observed in contact with air or in undesiccated benzene used as a reaction medium, it is natural to attribute this phenomenon to the catalytic effect of traces of water on the opening of the oxirane ring involved in the coordination interaction with the tin atom. 

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. 

The above-mentioned facts have important consequences on the stereochemical outcome of the kinetic resolution of asymmetrically substituted epoxides. In the majority of kinetic resolutions of esters (e.g. by ester hydrolysis and synthesis using lipases, esterases and proteases) the absolute configuration at the stereogenic centre(s) always remains the same throughout the reaction. In contrast, the enzymatic hydrolysis of epoxides may take place via attack on either carbon of the oxirane ring (Scheme 7) and it is the structure of the substrate and of the enzyme involved which determine the regioselec-tivity of the attack [53, 58-611. As a consequence, the absolute configuration of both the product and substrate from a kinetic resolution of a racemic... 

General Comments. The formation of deoxy sugars by hydrogenation over Raney nickel often leads to the abnormal isomer (namely, that formed by diequatorial opening of the oxirane ring) as the major product, in contrast to the product afforded by lithium aluminum hydride this suggests that a different mechanism is involved in the nickel-catalyzed reaction. 

Selective tosylation of 1,6-anhydro-a-D-galactofuranose (167) is favored for position C-3 because of steric accessibility and affords tosylate 51. Treatment of 51 with base results in intramolecular displacement of the tosyloxy group by 5-hydroxyl group with the formation of 1,6 3,5-dianhydro-a-D-gulofuranose (168) the reaction involves closure of the oxetane and not the oxirane ring.246... 

Reaction pathway for epoxyaikyi /3-giycoside substrate analogue with a jS-glucan hydrolase involving protonation of the oxirane ring by an acid group and the formation of a covalent bond with a nucleophile at the active site... 

Transannular reactions involving oxiranes are more common in acid-catalyzed ring openings. A representative example of these types of reactions is shown in Scheme 33 <87CB705>. Similar... 

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