M. Perbost et al., UNUSUAL REACTION OF 1,8-DIAZABICYCLO-[5.4.0]UNDEC-7-ENE WITH DIETHYL MALEATE, Journal of heterocyclic chemistry, 30(3), 1993, pp. 627-629
Michael addition of DBU on diethyl maleate and subsequent cyclization
of the adduct afforded a tricyclic derivative which was susceptible to
react further with a second molecule of unsaturated diester.