An Ionic Diels-Alder Reaction

In an interesting and potentially very useful extension of the so-called ionic Diels-Alder reaction it was shown that reaction of acetals of the type 1 with various 1,3-dienes 2 in the presence of an acid catalyst followed by hydrolysis with TsOH in methanol gave good yields of cycloadducts 3. In most cases HBF4.0Et2 was found to be the most efficient catalyst and diastereomeric ratios as high as 200 1 could be achieved. 

Give a mechanistic explanation for this ionic Diels-Alder reaction. 

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