Divergent Transformations of 2-Nitro-1 H -benzo[ f ]chromenes in Reactions with Alkylidenemalononitriles: Access to Naphtho[2,1- b ]furans via Base-Mediated Pyran Ring Contraction.

The action of 2-(1-arylethylidene)malononitriles on 2-nitro-1 H -benzo[ f ]chromenes in the presence of Et3 N and MoO3 ·2H2 O results in naphtho[2,1- b ]furans containing an allylidenemalononitrile unit in the α-position. The reaction proceeds with contraction of the pyran ring via a cascade carba-Michael addition/retro-oxa-Michael reaction/tautomerization/SN 2/oxidation process. In contrast, the reaction of 2-nitro-1 H -benzo[ f ]chromenes with the cyclic Knoevenagel adduct derived from 1-indanone and malononitrile leads to dihydroindeno[1,2- c ]xanthenes. The possibility of further transformations of naphtho[2,1- b ]furan derivatives as useful precursors and their optical properties were also investigated.