Oxalates of the alkaloids deoxyvasicinone, 2,3-tetra-, and their seven-membered analog 2,3-pentamethylen-3,4-dihydroquinazol-4-one and the complex of 2,3-pentamethylen-3,4-dihydroquinazol-4-one hydrochloride with oxalic acid were synthesized. It was found that 2:1, 2:1, and 1:1 alkaloid:oxalic acid complexes, respectively, were formed. The last complex had 2,3-pentamethylen-3,4-dihydroquinazol-4-one, oxalic acid, and HCl in a 2:1:2 ratio, respectively. X-ray crystal structures of single crystals were performed. The oxalate of 2,3-pentamethylen-3,4-dihydroquinazol-4-one and its hydrochloride formed salts with a protonated N1 atom and involvement of only one hydroxyl. The other alkaloids formed a complex with oxalic acid through N1…H-O H-bonds involving both acid hydroxyls.