Oxirane ring migration

These results combined with the total suppression of copolymerization in the presence of hydroquinone as inhibitor indicate that hydrostannylation takes place upon the polyaddition of diorganostannane to the epoxyolefine by a radical mechanism accompanied by hydrogen atom migration in each chain propagation, No addition of organostannanes to the oxirane ring was observed 98>. 

A further type of photoisomerization mechanism for oxepin (7) was found by using material specifically labelled with 2H at the 3- and 6-positions (74JA5578). The location of the 2H atoms in the finally isolated sample of (7) was consistent with the migration of the oxirane ring to a neighboring position. The latter process (Scheme 13) has been described... 

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. 

The photochemical reaction of non-K-region oxides like dideuterated benzene oxide (303) and naphthalene 1,2-oxide has been studied at room temperature and at 77 K in acetone. When D-(303) is irradiated in acetone at room temperature, products corresponding to migration of the oxirane ring to the next position are obtained. 

K. Capek and T. Vydra, Oxirane-oxetane-l,4-dioxane anhydro-ring migration in sucrose derivatives, Carbohydr. Res., 168 (1987) C1-C4. 

The aziridine ring is more stable than the oxirane ring in alkaline solution, as demonstrated by the low reactivity in attempts to accomplish isomerization of the hydroxyepimines to amino epoxides in alkaline media at room temperature, which contrasts with the rapid epoxide migration (see Sect. V,2). Isomerization of hydroxyepimines occurs only at high temperatures, and leads finally to the formation of amino derivatives of 1,6-anhydrohexoses.379,740 For example, when 166 is heated in 5% potassium hydroxide, 2-amino-l,6-anhydro-2-deoxy-/3-D-mannopyranose (168) is formed as the main product this can be explained by transient formation of 2-amino-l,6 3,4-dianhydro-2-deoxy-/3-D-altropyranose (167), and its subsequent, diaxial hydrolysis.379 Compound 167 is probably in equilibrium with epimine 166. Acid hydrolysis of the aziridine ring in 153 also follows a diaxial mechanism, without scission of the 1,6-anhydride bond, to give 4-amino-l,6-anhydro-4-deoxy-)8-D-mannopyranose756 (177). 

BF3 Et20, smoothly passes into (218) it may be noted that isomerization of the oxirane ring to the carbonyl function under the influence of BF3 Et20, the usual reaction, is blocked in the present case as the migration of the eut/o-hydrogen from C-1 to C-6 required by such an isomerization will result in inversion at C-6, which in the present case is sterically prohibitive 139). 

More remote substituents may also participate in epoxide opening/rearrangement. Christol and coworkers examined several S,(6)-substituted-2,3-norbomene ejco-oxides under acidic conditions, and found that certain 5-endo substituents played a part in oxirane opening. For example, oxa rings were formed in reactions of substrates bearing 5-entfo-methoxycarbonyl or -hydroxymethyl groups. A novel 1,4-migration of chloride was also detected (equation 28). ... 

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