SUBSTITUENT EFFECT ON THE DIASTEREOSELECTIVITY OF THE ACID HYDROLYSIS AND TRICHLOROACETOLYSIS OF 9,10-OXIDES DERIVED FROM trans-1,2,3,4,4a,10a-HEXAHYDROPHENANTHRENE. MECHANISM OF THE EPOXIDE RING OPENING

Article abstract of DOI:10.1016/S0040-4020(01)90343-6

The mechanism of the acidic ring opening reactions of 2-aryloxiranes which are simple models of arene oxides, is still under discussion.Two different mechanisms have been suggested to rationalise the product distributions of the acid hydrolysis of the two types of conformationally restricted 2-aryloxiranes (2 and 3, and 5 and 6) : it would appear to be difficult to reconcile the two rationales.In order to gain insight into the reactions of benzo-epoxides of type 5 and 6, the 6-methoxy (5c and 6c) and the 7-bromo derivatives (5a and 6a) were synthesised and their acid hydrolysis (1:1 dioxane/water) and trichloroacetolysis in benzene were studied and compared with those of the unsubstituted compounds (5b and 6b).Contrary to expectations based on the results obtained with the epoxides of type 2 and 3, the introduction of the substituent on the aromatic moiety, in particular the strong electron-donating 6-methoxy, does not modify the complete anti diastereoselectivity observed in the acid hydrolysis of the unsubstituted epoxide 6b.In the case of the epoxides 5, on the contrary, the percentage of syn adduct increases noticeably with the ability of the aromatic moiety to stabilise the benzylic carbocationic centre.As for the trichloroacetolysis reactions, significant amounts of syn adducts are observed for both the epoxides 5 and 6; the syn stereoselectivity increases for both the epoxides 5 and 6 with the ability of the aryl to stabilise a benzylic carbocationic centre.A Hammet-type linear correlation was found between the diastereoselectivity and the ?'constants for the acid hydrolysis of 5a-c and for the trichloroacetolysis reactions of 5a-c and 6a-c.The results obtained are difficult to explain on the basis of either of the mechanisms hypothesised for 2-aryloxiranes, at least as they were originally proposed.