Synthesis of 5 H -chromeno[3,4- b ]pyridines via DABCO-catalyzed [3 + 3] annulation of 3-nitro-2 H -chromenes and allenoates.

The DABCO-catalyzed [3 + 3] annulation between 3-nitro-2 H -chromenes and benzyl 2,3-butadienoate has been developed as a route to 5 H -chromeno[3,4- b ]pyridine derivatives. Under optimal reaction conditions, 5 H -chromeno[3,4- b ]pyridines incorporating two allenoate units were obtained in moderate to good yields (30-76%). The same type of transformation could be carried out using butynoates as allene surrogates. Mechanistic studies by mass spectrometry allowed the identification of the key intermediates involved in the reaction mechanism. The reported synthetic methodology represents an entirely new approach for the synthesis of the 5 H -chromeno[3,4- b ]pyridine core structure based on allene chemistry.