Reactions between 3-nitrochromone and diazoalkanes; Michael additions catalysed by diazoalkanes as nitrogen bases

Abstract

3-Nitrochromone with diazomethane rapidly gives a derivative (4) of cyclopropa[b][1]benzopyran-7-one which reacts readily with water or alcohols to give 3,4-dihydro-2-hydroxy-4-nitro-1-benzoxepin-5(2H)-one or its ethers. The alcohol is maintained in a particular conformation (9) by hydrogen bonding between the hydroxy- and nitrogroups that makes enolisation impossible, but the ethyl ether (8c) enolises and is easily methylated. With 2-diazopropane the reaction of 3-nitrochromone is extremely vigorous; a derivative (13) of cyclopropa[b][1]benzopyran-7-one is again formed but reacts slowly with ethanol and the ring fission occurs in a different sense so that 2-(1-ethoxy-1-methylethyl)-3-nitrochroman-4-one (14) is formed under what is thought to be stereoelectronic control. With diazoethane the reaction of 3-nitrochromone is more complex and only 2-ethyl-3-nitrochromone (3b) was characterised. If 3-nitrochromone is treated with a diazoalkane in trichloromethane containing ethanol, one of the products (up to 17%) is formed by the Michael addition of the ethanol to the chromone followed by etherification of the resulting enol (17a). In these additions the diazoalkane is regarded as acting as a catalytic nitrogen base.