Unexpected reactions of α,β-unsaturated esters with hydrazine hydrate

The reactions of hydrazine hydrate with methyl acrylate and ethyl acrylate do not result in the formation of 1,5-diazabicyclo[3.3.0]octane-2,6-dione 1 at reaction temperatures below 200 °C; 1,2-bis(alkoxycarbonylethyl)pyrazolidin-3-one 8, 1-(alkoxycarbonylethyl)pyrazolidin-3-one 6, and 2-(alkoxycarbonylethyl)pyrazolidin-3-one 11 were isolated and identified. At 260–280 °C, however, 1 could be isolated in 20–28% yield; formation of 1 could also be observed by spectroscopic measurements when the reaction was carried out above 200 °C. It is concluded that the major pathway involves initial Michael addition to give a 1,1-disubstituted hydrazine that cannot yield 1 without undergoing unfavourable retro-Michael reactions. The apparently similar reaction of hydrazine with excess diethyl glutaconate to give 4,8-bis(ethoxycarbonylmethyl)-1,5-diazabicyclo[3.3.0]octane-2,6-dione 2 takes a quite different course. Initial Michael addition gives a 1,2-disubstituted adduct for steric reasons. Although double cyclisation of this adduct to 2 appears straightforward, retro-Michael reaction of the monocyclised adduct is very rapid and competes with the second cyclisation. The reaction of ethyl methacrylate and hydrazine hydrate at 80 °C results in a complex reaction mixture, which is transformed into 4-methylpyrazolidin-3-one 13 at 390 °C under reduced pressure.