Bromination of benzhomobarrelene derivatives .10. High temperature bromination

The electrophilic addition of bromine to benzhomobarrelene derivatives 18a and 18b in CCl4 at low temperatures (0 degrees C) followed by repeated chromatography combined with fractional crystallization allowed us to isolate seven (19a-25a) and five products (19b-23b), respectively. Structural determinations of these compounds revealed that the barrelene skeleton was mainly rearranged. Alcohol 24a and ketone 25a are secondary products and arise from 21a and 22a. However, high temperature (77 OC) bromination of compounds 18a and 18b gave only nonrearranged products 19 and 20 whose barrelene skeletons were preserved. It is our conclusion that the low temperature reaction is ionic, where in the high temperature reaction radical intermediates are involved. The mechanism for the formation of the products is discussed and is supported by computational calculations. The structures of 21, 22 and 23 revealed that bromine approachs the double bond from the endo-face of the pi-system. The reaction of 18a with m-chloroperbenzoic acid gave only endo-epoxide 40, which in turn supports also the endoselectivity of the double bond in 18a and 18b. (C) 1997 Elsevier Science Ltd.