| prov:value
| - The compounds of formula (I) can be obtained by condensing a compound of formula ##STR4## wherein R2, R3 and L are as defined hereinabove, with a C1 -C4 alkyl ester of formic acid in the presence of suitable base such as, for example, an alkali metal alkoxide or hydride, e.g. sodium methoxide, potassium ethoxide, sodium hydride, lithium hydride, and the like, in a reaction-inert solvent; and treat
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