The 3-(carboalkoxy)pyrazolin-5-ones derived from dialkyl oxaloacetates or diethyl α-methyloxaloacetate through reaction with hydrazine can be converted into the strongly fluorescent 4,6-bis(carboalkoxy)-1,5-diazabicyclo-[3.3.0]octa-3,6-diene-2,8-diones [4,6-bis(carboalkoxy)-9,10-dioxa-syn-bimanes, syn-(COOR,R1)B (6), R = CH3 or CH3CH2, R1 = CH3, Cl, Br] by base treatment of the corresponding chloro or bromo derivative. The structure of one bis ester, 4,6-bis(carbomethoxy)-3,7-dimethyl-1,5-diazabicyclo[3.3.0]octa-3,6-diene-2,8-dione [6f, syn-(COOCH3, CH3)B], has been determined by X-ray crystallography. Lithium bromide and the esters in CH3CN or DMF yield via dealkylation and decarboxylation the corresponding syn-(H,R1)B (11), (R1 = H, CH3, Cl, Br, I) or the “mixed” bimanes syn-(EtOOC,R1)(H,R1)B (10, R1 = Cl or CH3). A dicarboxylic acid (R1 = CH3; LiBr/CH3CN/60 °C; two COOCH3's) readily decarboxylates. Hydrogenation of halogenated bimanes over Pd/C(AcOH) replaces one or both halogens, the 2H product from syn-(H,Cl)B being the parent syn-bimane, syn-(H,H)B. syn-(COOR,H)B and ICI yield syn-(COOR,I)B, which gives syn-(H,I)B on dealkylation-decar-boxylation. Replacement of Cl in syn-(COOCH2CH3, Cl)B by C6H6S− yields syn-(COOCH2CH3, C6H6S)B. Both ester groups and halogens shift absorption and fluorescence maxima to longer wavelengths than those recorded for syn-(CH3, CH3)B. In 1H NMR spectra, the β-hydrogens of the syn-bimane appear at considerably lower fields (7.52–8.21 ppm) than the α-hydrogens (5.42–6.13 ppm).