Diaxial opening, oxirane

An oxirane formed by the direct epoxidation, which usually occurs from the sterically least hindered side of the molecule, can be converted into its stereoisomer by a reaction sequence which involves the diaxial opening (in acetic acid at 100° for 2 hr) of the epoxide to a diol mo noacetate. Subsequent mesylation followed by treatment with base gives the inverted oxirane, as shown for the sequence (69) (70) (71) (72). ... 

It was shown by monitoring the reaction by NMR spectroscopy that the overall process to involve the following steps firstly, a ring closure of 125 to the phenanthridone 126 (Scheme 17), secondly, the formation of 127 by thermal loss of the N-protecting group, thirdly, diaxial opening of the oxirane 127 to the triolbenzoate 128, fourthly, hydrolysis of 128 to the tetrol 129 and finally, a slow dealkylation of the O-benzyl ether 129 to (+)-pancratistatin (94). 

In the case of 1,6 2,3- (136) and l,6 3,4-dianhydro-/I-D-talopyranoses, the diaxial opening of the oxirane ring prevails, but a trend to diequatorial opening496 498 is apparent with l,6 2,3-dianhydro-4-deoxy- and 1,6 3,4-dianhydro-2-deoxy-/l-D-/yxo-hexopyranoses.169,432,493 Using 1,6 2,3- and l,6 3,4-dianhydrohexopyranoses as starting compounds allowed the preparation of a complete series of 12 isomeric 1,6-anhydro-monodeoxy-/Fn-hexopyranoses and 6 corresponding 1,6-anhydro-dideoxyhexoses,499 mainly by catalytic or complex hydride reductions.462,500... 

Reaction of the unconjugated intermediate 13-1 with monoperphthalic acid (MPHA) yields a single 5,10-epoxide. The /3 stereochemistry of this product (13-2) follows from the same factors as those for the chloro- and bromohydrins described above. Reaction of the epoxide with hydrofluoric acid leads to diaxial opening of the oxirane and formation... 

Construction of the simplest fused heterocyclic system that shows some anabolic activity starts by epoxidation of the olefin at C2 in 23-1 with peracetic acid buffered with sodium acetate (23-2) (Scheme 5.23). Reaction of this intermediate with potassium thiocyanate and phosphoric acid, essentially thiocyanic acid, leads to diaxial opening of the oxirane to form the 3a-hydroxy-2/8-cyanate 23-3. Treatment of the product with base leads to formation of the /3-thioepoxide 23-5. Formation of that product can be rationalized by assuming that the first step involves removal by base of a proton on the hydroxyl group. That alkoxide then proceeds to add to the cyano group at C2 to form a transient five-membered ring as in... 

A sequence of several steps, first discussed in connection with Scheme 6.13, installs chlorine at Cg. Reaction of 23-8 with basic hydrogen peroxide starts by selective conjugate addition of peroxide anion to the distal end of the diene. That transient intermediate goes on to form the a-epoxide. Diaxial opening of the oxirane with chlorine gives chlorohydrin... 

Nucleophilic attack on oxirane carbon usually proceeds with inversion of configuration (Scheme 44) as expected for Sn2 reactions, even under acid conditions (Scheme 45). Scheme 45 also illustrates the fact that cyclohexene oxides open in a fran5-diaxial manner this is known as the Fiirst-Plattner rule (49HCA275) and there are very few exceptions to it. 

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