Efficient and regioselective synthesis of bridged ring systems by domino Knoevenagel – hetero-Diels–Alder reaction

The bridged heterocycles 10–12 were efficiently prepared by a domino Knoevenagel – hetero-Diels–Alder reaction of N,N'-dimethylbarbituric acid 2 and the aldehydes 6–8 which contain an allyl moiety as dienophile. In contrast, reaction of 2 and aldehyde 5 with a dimethylallyl moiety leads to the annulated product 9. For the formation of 10–12, an exo-Z-syn and for that of 9 an endo-E-syn transition structure is proposed.Key words: bridged compounds, domino reaction, Diels–Alder reaction, heterocycles, Knoevenagel reaction.