p-Toluenesulfonic acid → 1,5-Dimethyl-8-oxa-bicyclo[3.2.1]oct-6-ene-2,4-dione

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Reaction Steps

1.61 g (7.59 mmol) of 4,4-dimethoxy-1,5-dimethyl-8-oxa-bicyclo[3.2.1]oct-6-en-2-one and 0.432 g (2.28 mmol) of p-toluenesulfonic acid are dissolved in a 2:1 mixture of acetone and water and heated for 50 minutes at a temperature of 70° C. (TLC monitoring: hexane/ethyl acetate 9:1). The acetone is then removed under reduced pressure. The aqueous phase is then adjusted to pH 9 with saturated sodium hydrogen carbonate solution and extracted three times with ethyl acetate to remove neutral components. The aqueous phase is then adjusted to pH 5 with dilute hydrochloric acid and extracted three times with fresh ethyl acetate. The organic phase is dried over sodium sulfate and concentrated by evaporation under reduced pressure, there being obtained 1.04 g (82.5%) of technically pure 1,5-dimethyl-8-oxa-bicyclo[3.2.1]oct-6-ene-2,4-dione in the form of a yellowish product, which can be used without further purification in the next reaction step to form compounds of formula I.

Provenance & Evidence