A discrete tetragonal tube‐shaped complex (MT‐1) has been synthesised by coordination‐driven self‐assembly of a carbazole‐based tetraimidazole donor L and a Pd(II) 90° acceptor, that is, [cis‐(dppf)Pd(OTf)2] (dppf=diphenylphosphinoferrocene, OTf=CF3SO3−). Complex MT‐1 was characterised by multinuclear NMR, ESI‐MS and single‐crystal X‐ray diffraction analysis (SCXRD), which showed its symmetrical tetrafacial tube‐shaped architecture possessing a large cavity described by four aromatic walls. This coordination cage was successfully utilised as a molecular vessel to perform intramolecular cycloaddition reactions of O‐allylated benzylidinebarbituric acid derivatives inside its confined nanospace. The presence of a catalytic amount of MT‐1 promoted [4+2] cycloaddition reactions in a regio‐ and stereoselective manner, yielding the corresponding penta/tetracyclouracil derivatives in good yields under mild reaction conditions. This protocol is interesting compared with the literature reports for the synthesis of similar chromenopyran pyrimidinedione derivatives under high‐temperature reflux conditions or solid‐state melt reactions (SSMRs).