Transformations in bromo- and alkoxybenzotropones

I R Narayanan Namboothiri, S N Balasubrahmanyam


The ipso/cine ratios in the displacement of bromine by methoxide changes from 9:1 in the case of 5-bromo-2,3- benzotropone 1 to 2:7 in that of 2-bromo-4, 5-benzotropone 2. Under a possibility that an aryne-type mechanism operates, there regioselectivities show a considerable steric influence on the part of the pri hydrogen, implying a lateral or in-plane approach of methoxide torwards the benzylic position of benzodehydrotropone intermediates.

None ot the four deried alkoxybenzotropones furnished an oxonium salt on treatment with dimethyl sulphate but an unexpected transformation was mediated in the case of 3-methoxy-4,5-benzotropone 4b, the cine product from 2, into its carbonyl-ether transposition product, 6-methoxy- 2,3-benzotropone 9a. While a similar transformation cannot be apparent, for symmetry reasons, in the case of 4-methoxy- 2,3-benzotropone 3b, the mechanism proposed suggests why thc transformation does not take place in the case of the two remaining methoxybenzotropones 3a and 4a.


Regioselectivity; aryne mechanism; carbonyl transposition; bromo-; alkoxy-; dehydrobenzotropones.

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