| 摘要 |
A process for preparing a compound of the formula: <IMAGE> wherein R is hydrogen, lower alkyl of 1 to 6 carbon atoms, lower alkenyl of 2 to 6 carbon atoms, phenyl or benzyl; and R' is lower alkyl of 1 to 6 carbon atoms which comprises contacting a compound of the formula <IMAGE> wherein R and R' are defined as above at -10 DEG to 50 DEG C. with an essentially anhydrous acid selected from the group consisting of formic acid, trifluoroacetic acid, p-toluenesulfonic acid, methanesulfonic acid, citric acid, oxalic acid, chloroacetic acid, sulfuric acid, hydrochloric acid, phosphoric acid and hydrofluoric acid.
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