EFFECT OF THE 9-METHYL SUBSTITUENT ON THE CHEMICAL BEHAVIOUR UNDER ACID CONDITIONS OF THE DIASTEREOISOMERIC 9,10-OXIDES DERIVED FROM trans-1,2,3,4,4a,10a-HEXAHYDROPHENANTHRENE

Chini, Marco,Crotti, Paolo,Macchia, Franco

, p. 827 - 836 (2007/10/02)

Significant differences are present in the chemical behaviour, under acidic ring-opening conditions, between the diastereoisomeric couples of 2-aryl substituted oxiranes 3a and 3e, and 4a and 4e.The results of the acid hydrolysis of the two types of epoxides (3 and 4) have been rationalized through two different mechanisms which cannot easily be reconciled.A substantial structural difference in the two systems is the presence of a tertiary benzylic oxirane carbon in epoxides 3 and of a secondary one in epoxides 4.Therefore, the 9-methyl substituted epoxides 5a and 5e, in which the benzylic oxirane carbon is tertiary, have been prepared and studied.The introduction of the methyl substituent in the 9-position does not modify, in the case of epoxide 5a, the complete anti diastereoselectivity originally observed in the hydrolysis of the unsubstituted epoxide 4a.By way of contrast, in the hydrolysis of epoxide 5e and in the trichloroacetolysis of both epoxides 5a and 5e, a considerable increase in the syn/anti ratio is observed when compared with the data obtained in the corresponding reactions of the non-methyl-substituted epoxides (4a and 4e).Furthermore, in the trichloroacetolysis reactions of both epoxides 5a and 5e, marked amounts of the corresponding non-addition products (ketone and unsaturated alcohol) are revealed.Apart from the increased syn diastereoselectivity in the hydrolysis of epoxide 5e and in the trichloroacetolysis of 5a and 5e, and from the incursion of non-addition products in the trichloroacetolysis of both 5a and 5e, the general trend of the reactions of epoxides 5a and 5e remains substantially unmodified with respect to the one observed with the parent compounds 4a and 4e.The formation of rearrangement products in the reactions of epoxides 5 was rationalized keeping into consideration either the steric or/and the elctronic effect of the 9-methyl substituent.The results obtained indicate that the secondary nature of the benzylic oxirane carbon of epoxides 4a,e is not responsible for the above-mentioned differences in chemical behaviour between epoxides of type 3 and 4.