A simple three-step synthesis of two essentially new classes of bicyclic heterocyclic compounds, the syn- and anti-1,5-diazabicyclo[3.3.0]octadienediones, from 5-keto esters via the pyrazolinone and halopyrazolinone is described. The syn compounds (-3,6-diene-2,8-diones) are usually strongly fluorescent; the anti derivatives (-3,7-diene-2,6-diones) are normally nonfluorescent and very phosphorescent. A short form name, 9,10-dioxabimanes, is introduced (bi, two, and manus, hand), with substituents adjacent to the carbonyl designated R, (or a) and substituents at the second carbon being labeled R2 (or β). “9,10-Dioxabimanes” may be described as syn-(ov anti-)(R2,R1)B. 9,10-Dioxabimanes are relatively high-melting, sublimable, stable molecules with simple NMR and fairly characteristic IR spectra. The (R2,Cl)B derivatives can be hydrogenated to (R2,H)B compounds. Bromination of (CH3R1) produces useful monobromo and dibromo compounds. A plausible mechanism for the formation of both classes of 9,10-dioxabimanes is presented. The formation of mixed 9,10-dioxabimanes (e.g., (R2,R1)(R2’,R1')B) from mixtures of halopyrazolinones is readily accounted for. The 9,10-dioxabimane rings are hydrogenated with some difficulty to a mixture of products, including some in which the N-N bond has been cleaved. syn-(CH3,CH3)B resists oxidation by a variety of agents but can be converted to an -acetoxy derivative with ceric ion.