Diaxial cleavage

In the estrone series diaxial cleavage of the 6a,7a-epoxide (10) with methylmagnesium bromide affords (11) in 60% yield. ... 

Contra-trans-diaxial cleavage of epoxides.2 The reaction of the a, p-epoxy alcohol 2 with NaSeC6H5 provides the 1,3-diol 3 in 90% yield. Use of 1 results in the opposite regioselectivity to provide 4 as the major product. 

A characteristic feature of the dianhydrohexopyranoses is their steric rigidity. Consequently, they usually react with high and predictable regio-and stereoselectivity, mainly by diaxial cleavage of the oxirane ring. Thus, they became versatile starting materials for synthesis of various carbohydrates as well as noncarbohydrate structures. The most important compounds are l,6 3,4-dianhydro-2-0-tosyl-/l-D-galactopyranose (146), the manno epoxide 143, the altro epoxide 159, and the 2,3- and 3,4-anhydro-a//o epoxides 138 and 139. The tosyl epoxide 146 (for its crystal structure,... 

Reductive cyclization by lithium aluminum hydride as the key step has been employed for the preparation of O-benzylated l,6-anhydro-2,3-dideoxy-2,3-epi-mino- and l,6-anhydro-3,4-dideoxy-3,4-epimino-p-D-hexopyranoses having the alio-, manno-, galacto-, and to/o-configurations from suitable 1,6 2,3- and l,6 3,4-dianhydro-p-D-hexopyranoses. Stereoselective trara-diaxial cleavage of the oxirane ring was effected by treatment with sodium azide and ammonium chloride in a 2-methoxyethanol-water mixture at 110-120 °C. Synthesis of 1,6-anhydro-4-(9-benzyl-2,3-dideoxy-2,3-epimino-p-D-marmopyranose (7) from di-anhydro derivative 4 illustrates this methodology. An alternative reduction by sodium borohydride in THF was also tested in the preparation of the 2,3-d-u//o-and D-ma u-epimines in this series. It provided a better yield (73%) than... 

In the reaction of A -benzylepimine 160 with azide anion, tra i-diaxial cleavage is favored since the azide anion is a poor leaving group, so that cyclization is not possible. 

Diaxial cleavage. Cholesterol 5,6j -epoxide allowed to react 7 hrs. at room temp, with HCN and triethylaluminum (3 5) in tetrahydrofuran 3j, 6j -dihydroxy-5-cyano-5a-cholestane. Y 97%. Also with diethylaluminum cyanide in toluene, and f, e. s. W. Nagata, M. Yoshioka, and T. Okumura, Tetrah. Let. 1966, 847. 

The cleavage of steroidal epoxides with Grignard reagents leads exclusively to alcohols with the tra 5-diaxial orientation of the hydroxyl group and the newly introduced alkyl group. °... 

Similarly, diethylaluminum azide gives (3-azido alcohols. The epoxide of 1-methylcyclohexene gives the tertiary azide, indicating that the regiochemistry is controlled by bond cleavage, but with diaxial stereoselectivity. 

One of the methods by which replacement of secondary hydroxyl groups by fluorine in pentofuranoses and pento- and hexo-pyranoses has been achieved is by cleavage of epoxide rings with fluoride ion. The epoxide rings are always cleaved trans-diaxially, according to the Fiirst-Plattner rule. 

---