The synthesis of 2,2-disubstituted 3-nitrochromenes from salicylaldehyde and 2,2-disubstituted 1-nitroalkenes

Reactions of salicylaldehyde (1) with β-nitrostyrenes (2), (4), and (6) in the presence of 1,4-diazabicyclo[2.2.2] octane (DABCO) without the use of solvent at 40 °C gave high yields of the corresponding 3-nitrochromenes, respectively. Other conjugated nitroalkenes were also used to react with salicylaldehyde under the similar conditions and the yields of substituted 3-nitrochromenes were moderate to high. 96% of cis-3-nitro-4-hydroxyflavane (11) was isolated as sole stereoisomer when 1 reacted with 2-nitromethyleneadamantane (10) under similar condition. When the reaction temperature was increased to 90 °C, 11 underwent dehydration to generate 74% of the corresponding 3-nitrochromene (12).