Remarkable Rearrangement of a Camphor Derivative

Canadian workers have been exploring methods for the conversion of cheap and readily available camphor into enantiopure cyclopentane derivatives for use in terpene and steroid synthesis, e.g. 1 — 2. In a recent extension of these studies, em/o-3-bromo-4-methylcamphor 3 (0.1 mol) was 

Suggest a mechanism for the conversion of 3 into 4 which is consistent with the description given. 

Belgian workers needed to prepare the benzylidenethiophthalide 1 to test its potential use as a chain-stopper for a particular polymerisation process. 2-Thiophthalide 2 was therefore condensed with benzaldehyde in THF at 0°C using potassium t-butoxide as base. Work-up consisted simply of addition of water, acidification with HC1 and extraction with chloroform. None of the expected product 1 was obtained instead, the stilbene acid 3 was formed in 52% yield. 

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