Reactions of epoxides with neighbouring nucleophiles (1972)
AuthorsSwallow, W. H.show all
The syntheses and acid catalysed rearrangements of cis- and trans-3,4-epoxypentan-1-ols (28,22), 4,5-epoxyhexan-1-ols (30,24) and 5,6-epoxyheptan-1-ols (32,26) have been described. Isomerizations of these compounds gave cyclic ether derivatives as the major products. The 5,6-epoxyheptan-1-ols (32,26), which can form either a six- or a seven-membered oxide ring, were found to react to give six-membered ring products. With the 4,5-epoxyhexan-1-ols (30,24) the five-membered ring product was formed more readily than the six-membered one. Finally with the 3,4-epoxypentan-1-ols (28,22) the five membered ring product was formed in preference to the four-membered alternative. For the cis- and trans-4,5-epoxyhexan-1-ols (30,24) and 5,6-epoxyheptan-1-ols (32,26) where the hydroxyl group can readily approach the rear of at least one carbon atom of the epoxide ring the products were formed via an A2 mechanism with complete inversion of stereochemistry. Steric hindrance to nucleophilic attack by the hydroxyl group at one of the epoxide carbon atoms of cis-4,5-epoxyhexan-1-ol (30) was found to reinforce the preference to form a five-membered ring product. In the case of the cis- and trans-epoxypentan-1-ols (28,22) where approach of the hydroxyl group to the rear of the epoxide ring is more difficult, compounds involving retention as well as those of inversion of configuration were formed. These products have been rationalized in terms of an A1 mechanism. The acid catalysed reactions of cis- and trans-3,4- epoxypentan-1-ol acetates (29,23), 4,5-epoxyhexan-1-ol acetates (31,25) and 5,6-epoxyheptan-1-ol acetates (33,27) were also investigated. Isomerizations of cis- and trans-3,4-epoxypentan- 1-ol acetates (29,33) and 4,5-epoxyhexan-1-ol acetates (31,25) but not 5,6-epoxyhexan-1-ol acetates gave cyclic ether derivatives having complete retention of configuration. These compounds are considered to form via ortho ester intermediates.