General Syntheses and Rational Parameters for Structural Assignment of Isomeric Derivatives of [3,4]‐fused Pyrazoles4 isomeric 1‐ or 2‐methyl‐, and 1‐ or 2‐benzyl‐pyrazolo[3,4‐b]pyridones, i.e. the 4‐oxo‐types17a, bor11a, band the 6‐oxo‐types16a, bor10a, b, are synthesized unambiguously. Cyclisation of 1‐substituted 3‐ or 5‐(1‐methyl‐2‐ethoxycarbonyl‐vinylamino)‐pyrazoles9a, bor.15a, b, which were synthesized from 1‐substituted 3‐ or 5‐amino‐pyrazoles and ethyl acetoacetate yields11a, bor17a, bin downtherm, but10a, bor16a, bin presence of acidic catalysts. The acidic cyclisation is preceded by a new rearrangement of9or15into 1‐ substituted 3‐27or 5‐amino‐4‐(1‐methyl‐2‐ethoxycarbonyl‐vinyl)‐pyrazoles30; mechanism and concurring reactions are explained. Because of their higher electron densities at C‐4 it is easier to cyclise derivatives of 5‐amino‐pyrazoles compared to 3‐amino‐pyrazoles. All isomeric 1‐ or 2‐substituted 4(6)‐chloro‐6(4)‐methyl‐pyrazolo‐[3,4‐b]pyridines are formed with POCl3from the corresponding oxo‐compounds.The position of a substituent at N‐1 or N‐2 of [3,4]‐fused pyrazoles can be assigned using the significant1H‐n.m.r.‐parameter Δ HMPTCDCL 3= δ CDCl 3— − δHMPT(conc. HC—3). If solvent influences are considered, δ(C  O) is a useful13C‐n.m.r.‐parameter to distinguish the 4‐oxo‐types (11a, b; 17a, b) from the 6‐oxo‐types (10a, b;