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have optimized the experimental conditions for the silver(I)-promoted oxidative
coupling of methyl p-coumarate (I) and methyl ferulate (II),
which is the most frequently used methodology to synthesize the bioactive dihydrobenzofuran
neolignans 1 ((±)-trans-dehydrodicoumarate dimethyl
ester) and 2 ((±)-trans-dehydrodiferulate
dimethyl ester). Most of the tested conditions affected the conversion (i.e.,
the consumption of I and II) and the selectivity (i.e., the
percentage of I and II that was converted into 1 and 2,
respectively), so the optimized conditions were the conditions that afforded
the best balance between conversion and selectivity. Silver(I) oxide (0.5 eq) is the most efficient
oxidant agent amongst the silver(I) reagents that were tested to convert methyl
esters I and II into compounds 1 and 2,
respectively. Acetonitrile, which has not yet been reported as a solvent for
this reaction, provided the best balance between conversion and selectivity,
besides being “greener” than other solvents that are more often employed (e.g.,
dichloromethane and benzene). Under the optimized conditions, the reaction time
decreased from 20 h to 4 h without significantly impacting the conversion and
selectivity. However, the relation between the results obtained by adding a radical
initiator (AIBN) or a radical inhibitor (isoquinoline) and the previously
reported involvement of radical intermediate species in the silver(I)-promoted
oxidative coupling of I and II is not clear and deserves further investigation.