Debenzylation boron trichloride

It is of interest to note in passing that In more recent work (ref.72), use has been made in synthesis of the transformation of chalcones to isoflavones with thallium(lll) nitrate. Thus, 6-acetyl-2,2-dimethyl-7-hydroxy-5-methoxychromanone was converted to the chalcone with 2,4-dibenzyloxybenzaldehyde. The O-acetyl derivative by treatment with the thallium reagent followed by acidic cyclisation gave a bischromanone structure. Selective reduction of the least hindered carbonyl group in the bischromanone, acidic dehydration of the resultant alcohol and final debenzylation with boron trichloride gave the linear isofiavone. 

A similar ring contraction was obtained under Lewis acid catalysis, when the elongated bicycles type 67, with 6R or 65 configuration, were debenzylated in the presence of boron trichloride (Scheme 4.10). The cleavage of the endocyclic O—Cl bond of the pyranose ring results in the oxonium ion formation that suffers then the nucleophilic attach of OH to generate the p-furanoside 70 in a good yield [21]. 

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