Remote site bromination via a cascade rearrangement involving two bridging dioxonium species during oxidative cleavage of a benzylidene acetal

Attempted cleavage of the benzylidene group of 3,5-O-benzylidene-2,6,7-tri-O-trimethylacetyl-d-glycero-d-gulo-heptono-1,4-lactone with N-bromosuccinimide led to the formation of a 7-bromo derivative with a benzoyl group in the 3-position and trimethylacetyl groups in the 5 and 6 positions. Analysis of the coupling constants in the proton NMR spectra indicated that both trimethylacetyl groups had participated to shift the crowded electron-deficient center formed at the 5-position by the decomposition of the bromobenzylidene group to the much more accessible 7-position. The net result of this cascade rearrangement was retention at both the C-5 and C-6 positions. This is an interesting example of a cascade rearrangement under strict entropic and stereo control the synthetic utility of which is being investigated.