Divergent Transformations of 2-Nitro-1H-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-1H-benzo[f]chromenes in the presence of Et3N and MoO3 center dot 2H(2)O results in naphtho[2,1-b]furans containing an allylidenemalononitrile unit in the alpha-position. The reaction proceeds with contraction of the pyran ring via a cascade carba-Michael addition/retro-oxa-Michael reaction/tautomerization/ S(N)2/oxidation process. In contrast, the reaction of 2-nitro-1Hbenzo[f]chromenes with the cyclic Knoevenagel adduct derived from 1-indanone and malononitrile leads to dihydroindeno[1,2c]xanthenes. The possibility of further transformations of naphtho[2,1-b]furan derivatives as useful precursors and their optical properties were also investigated.