Four N-Lewis base-(1,2,5-oxadiboroles) rac-I [X = O, Do = ammonia (NH3), quinuclidine (Q), pyridine (Py), g-picoline (g-Pic)] are prepd. by reaction of cis-Et2BC(Et):C(Et)BEt2 with KNH2 via the tetraethyl-substituted heterocycles. I (X = NSiMe3, Do = -) reacts with water with elimination of Me3SiOH to give rac-I (X = O, Do = NH3). The latter undergoes exchange reactions with various N-bases (Py, g-Pic, Q) quant. forming the 1:1 addn. compds. rac-I [X = O, Do = g-Pic (x-ray structure anal.)]. Rac-I (Do = NH3, X = O) reacts with Ph3B to yield H3N-BPh3 and the unstable compd., which dimerizes and/or reacts with (EtBO) elimination to afford the boroxin (EtBO)3 and, presumably, the nido-carborane Et4C4B2Et2. The potassium salt II is transformed with ClPbMe3 to EtPbMe3 and/or to the rearranged heterocycle III.