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The synthesis of some new 'complex' pentaheterocyclic systems, pyrido[3′,2′:4,5]thieno(furo)[2,3‐e][1,2,4]triazolopyrimidines, containing a 1,2,4‐triazolo ring fused at [a] and [b] sides of the pyrimidine ring is described. Interestingly reacting the S,N‐dimethylated compounds 5(c‐h) (containing a furane ring) with hydrazine hydrate, an unusual rearrangement deriving by the breaking of an amide group (HN‐C=O) of the pyrimidine ring has been observed. As a result of the rearrangement 8(9)‐amino‐9(10)‐(methylamino)pyrano[4′′,3′′:4′,5′]pyrido[3′,2′:4,5]furo[3,2‐d]pyrimidin‐7(8)‐ones 9 were obtained: their cyclization led to the formation of new heterocyclic systems, 4(5)‐alkyl‐11(12)‐methylpyrido[3′,2′:4,5]furo[3,2‐d][1,2,4]triazolo[1,5‐a]pyrimidin‐7(8)‐ones 11. The structures of the obtained compounds were confirmed by different physico‐chemical methods. Looking at the syntheses of new 'complex' heterocyclic systems we have prepared some new pentaheterocyclic systems, pyrido[3′,2′:4,5]thieno(furo)[2,3‐e][1,2,4]triazolopyrimidines, containing a 1,2,4‐triazolo ring fused at [a] and [b] sides of the pyrimidine ring. Interestingly in the synthesis of the systems containing the furane ring we have observed an unusual and new rearrangement. The structures of the obtained compounds were confirmed by different physico‐chemical methods. [ABSTRACT FROM AUTHOR]