Synthesis of 2,5-piperidinediones. regioselectivity in the dieckmann cyclization

Dieckmann cyclization of ethyl N-benzyl-N-(ethoxycar-bonyl)methyl]succinamate ($\text{2}$) with sodium ethoxide gave regioselectively β-keto ester $\text{3}$, whereas when using potassium tert-butoxide or potassium hydride as a base the regioisomer $\text{4}$ was isolated as the main product. Transesterification of ethyl β-keto esters $\text{3}$ and $\text{4}$ with benzyl alcohol followed by hydrogenolysis and decarboxylation of the resulting benzyl esters $\text{5}$ and $\text{6}$ led to 1-benzyl- 2,5-piperidinedione ($\text{1}$). The preparation of some 4-alkyl- and 4-alkoxycarbonylalkyl derivatives was achieved by alkylation of $\text{5}$ with the appropriate halide and further hydrogenolysis.