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A new protocol for the synthesis of thieno[3,2- c ]coumarins is disclosed. In this method, a 3-formyl-2-oxo-2 H -chromene-4-thiolate anion is generated in situ by treatment of 4-chloro-3-formylcoumarin with sodium sulfide. This chromene-4-thiolate undergoes an l -proline-catalyzed substitution/Knoevenagel cascade with various α-halo ketones to afford the desired thienocoumarins in moderate to good isolated yields. This protocol eliminates the need for stoichiometric amounts of inorganic bases and the use of foul-smelling thiols. The reaction conditions tolerate a variety of α-halo ketones. [ABSTRACT FROM AUTHOR]