Oxirane ring cleavage

Recently, considerable interest has been shown in the photochemistry of a,/9-epoxyketones.16 Although the photochemistry of this system is undoubtedly the result of an n->ir excitation in the carbonyl function, the orbital overlap with the bent bonds of the three-membered ring, for which there is considerable evidence,16 is also implicated in the process. The major product of irradiation of an a,/8-epoxyketone is the corresponding /8-diketone, the result of oxirane ring cleavage and migration of a /9-substituent to the a-position [Eq. (3)]. Other photoproducts arise mainly from the /9-diketone. 

Reaction of epoxides with nitrogen nucleophiles affords a variety of P-hydroxyamines and their equivalents. Comprehensive review articles have been published. In conjunction with the development of diastereoselective and enantioselective syntheses of epoxy alcohols, selective oxirane ring cleavage of 2,3-epoxy alcohols and carboxylates by nitrogen nucleophiles has become a versatile route to various amines and their equivalents. For details, the reader is referred to refs. lOSc and 103d. 

In contrast to the dianhydrohexopyranoses, their deoxy derivatives 30, 131, 132, and 133 have an oxirane ring that is quite unsymmet-rically substituted384,742 with regard to polar effects. Consequently, in reactions with nucleophiles, polar effects may operate more effectively than steric effects, and this may result in oxirane-ring cleavage at the most electron-deficient position (as indicated by the arrows). Depending on the structure of the dianhydro derivative, the steric... 

With ethanol, the best results were obtained with PTSA. The oxirane ring cleavage was eomplete at 90°C, after 12 h, under 1 bar. Eflier formation was confirmed by different determinations, particularly the saponification value and GC analysis of the reaetion products. The main component of the reaction mixture was the methyl ethoxyhydroxystearate (>50%, by GC). 

Chapter 19. It was found that in aqueous base 3,4-anhydro-2,6-dideoj -D,L-ni6o- and -j fo-hofose dimetl I acetals isomerise much more quickly to the corresponding 4,5-epoxides than they tmdergo oxirane ring cleavage. ... 

L-ascorbic acid. 2,6-Dideoxy-3- and S-O-methyl-DL-hoKue isomers have been synthesized from 2,3-anhydro-4-deo] -DL-riho-hatose dimethyl acetal which rearranges to the corresponding 3,4-anhydro-Jty/o-hexose derivative faster than it undergoes oxirane ring cleavage. ... 

Acidic cleavage of the oxirane ring in (210) results in formation of the alkynic aldehyde (211) (68TL4905). 

A -1,3,4-Oxadiazoline, fluoromethyl-rearrangement, 6, 437 Oxadiazolines alkylation, 6, 431 irradiation, 6, 437 mass spectra, 6, 380 oxirane synthesis from, 7, 117-118 ring cleavage... 

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. 

When the unsaturated Y-benzoyl a-amino ester 443 was treated with MCPBA, the oxirane 444 formed first immediately underwent an intramolecular rearrangement with cleavage of the oxirane ring by attack of the amide oxygen to give a 3 1 mixture of two cis- and /ra r-isomers of 5,6-dihydro-47/-l,3-oxazine derivatives 445 and 446, the configurations of which were determined via their 0-3,5-dinitrobenzoyl derivatives (Scheme 85) <2003M69>. 

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