Neighboring group participation in hypobromous acid addition to 19-substituted 5α-cholest-1-enes and in acid cleavage of their epoxy analogs

1982 ◽  
Vol 47 (11) ◽  
pp. 3062-3076 ◽  
Author(s):  
Václav Černý ◽  
Pavel Kočovský
Keyword(s):  
Neighboring Group ◽  
Group Participation ◽  
Acid Addition ◽  
Acid Cleavage ◽  
Hypobromous Acid ◽  
Neighboring Group Participation

Reactions of the title compounds (bearing an OH, OCH3 or OCOCH3 group at C(19)) involve 5(O)n, 7(O)π,n-participation by the 19-substituent or attack by an external nucleophile. The 6(O)π,n-participation does not occur. The behavior of 1,2-unsaturated (or epoxidated) compounds has been compared with the earlier described 2,3-unsaturated or epoxidated analogs. The 1,2-type is genarally less prone to participation. The reasons for this behavior are discussed.

Neighboring group participation in electrophilic additions: The role of Markovnikov and Fürst-Plattner rule in hypobromous acid addition to 5,6-unsaturated cholestane derivatives

1983 ◽  
Vol 48 (12) ◽  
pp. 3618-3628 ◽  
Author(s):  
Pavel Kočovský
Keyword(s):  
Methyl Ether ◽  
Hydroxyl Group ◽  
Unsaturated Alcohol ◽  
Neighboring Group ◽  
Group Participation ◽  
Acid Addition ◽  
Hypobromous Acid ◽  
Neighboring Group Participation ◽  
Electrophilic Additions

On reaction with hypobromous acid, the unsaturated alcohol IIIa yields the diequatorial bromo epoxide XIX arising from the 5α,6α-bromonium ion XVIIIa on cleavage at C(5) by 19b-hydroxyl group with 6(O)n participation. By contrast, the bromonium ion XVIIIb generated from the unsaturated methyl ether IIIb is cleaved by water as external nucleophile to yield the unstable diaxial bromohydrin XX which undergoes cyclization to the oxirane derivative XXI. A comparison with the reaction course in homologs of the type I and II permits the conclusion that the change in regioselectivity, generally possible outcome of the 5(O)n participation, is only possible for the 6(O)n process if the participating group is a hydroxyl.

Mechanism of hypobromous acid addition to unsaturated steroids. Competition between neighboring group participation and external nucleophile attack

1982 ◽  
Vol 47 (1) ◽  
pp. 117-123 ◽  
Author(s):  
Pavel Kočovský ◽  
František Tureček ◽  
Václav Černý
Keyword(s):  
Isotopic Labeling ◽  
Acetoxy Group ◽  
Carbonyl Groups ◽  
Neighboring Group ◽  
Group Participation ◽  
Ester Carbonyl ◽  
Acid Addition ◽  
Hypobromous Acid ◽  
Neighboring Group Participation ◽  
External Attack

Mechnism of hydrobromous acid addition to 2,3- and 5,6-unsaturated steroids bearing a 19-acetoxy group were established by means of isotopic labeling. In the cleavage of 2α,3α-bromonium ion III participation by the 19-acetoxyl predominates over external attack, 5(O)n process being the major (88%) and 7(O)π,n process the minor (10%) reaction. The external attack by water contributes only 2%. The 5α,6α-bromonium ion XI is cleaved by participation of the 19-acetoxyl at C(5) in a 6(O)π,n process (78%) via XII - XIII. Cleveage at C(6) leads to XIV and occurs partially with 7(O)π,n participation by the 19-acetoxyl (4%) and partially by external attack of water (8%). These results bring evidence for the formation of a seven-membered acetoxonium ion ring as intermediate in participation processes involving ester carbonyl groups.

Neighboring group participation and rearrangement in hypobromous acid addition to 10β-vinyl cholestane derivatives

1981 ◽  
Vol 46 (11) ◽  
pp. 2877-2891 ◽  
Author(s):  
Pavel Kočovský ◽  
František Tureček
Keyword(s):  
Main Product ◽  
Bond Formation ◽  
Cyclic Ethers ◽  
Double Bonds ◽  
Neighboring Group ◽  
Group Participation ◽  
Acid Addition ◽  
Hypobromous Acid ◽  
Neighboring Group Participation

Hypobromous acid addition to the diene VII yields two epimeric dibromo 6β,19a-epoxides, XVI (main product) and XVII, via the intermediary bromohydrin XIII. Products due to C-C bond formation were not observed. This fact is attributed to the mutual perpendicular arrangement of π-orbitals of the two double bonds in the diene VII. Hypobromous acid action upon the 5α,6β-diol IX and the dimethoxy derivative X afforded analogous products of 6(O)n participation by the 6β-substituent, i.e. the cyclic ethers XXII, XXIII and XXIV, XXV. Under the same conditions the 6β-acetoxy derivative XI gave a mixture of the cyclic ethers XXII and XXIII as products of 6(O)n participation, and the rearranged ketone XXVI. Mechanism of this rearrangement is discussed.

Mechanism of acid cleavage of some steroid epoxides. Competition between neighboring group participation and external nucleophile attack

1982 ◽  
Vol 47 (1) ◽  
pp. 124-129 ◽  
Author(s):  
Pavel Kočovský ◽  
František Tureček ◽  
Václav Černý
Keyword(s):  
Perchloric Acid ◽  
Cyclic Ether ◽  
Oxirane Ring ◽  
Neighboring Group ◽  
Group Participation ◽  
Acid Cleavage ◽  
Neighboring Group Participation

The mechanism of perchloric acid cleavage of epoxides I and II was established on the basis of experiments using H2 18O. The 2α,3α-epoxide I gave two products: the cyclic ether V (60%) arising by 5(O)n participation of the 19-acetoxyl and the diol VI (40%). The latter compound is formed by two mechanisms: 1) By direct cleavage of the oxirane ring with H2 18O as external nucleophile and 2) by 7(O)π,n participation via the ion III. Under the same conditions the 5α,6α-epoxide II yielded two diols: The diequatorial diol VIII (96%) arising by 6(O)π,n participation and the diaxial diol IX which is again formed by both direct cleavage of the oxirane ring with H2 18O and by 7(O)π,n participation via the intermediate ion X. The competition of several mechanisms is discussed.

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