tert-Butylphosphaethyne (2) undergoes a Diels-Alder in 4, complexation occurs exclusively at its phosphaalkene phosphorus atom. The reactivity of 2 towards the four reaction with the 1,3,2-diazaphosphinine 1 at room temperature to furnish the diazadiphosphabarrelene 3. In the functionally substituted 1,2-azaphosphinines 8a-d has also been examined. At room temperature smooth [4 + 2] presence of an excess of 2 in toluene under reflux compound 3 experiences elimination of a molecule of pivalonitrile to cycloaddition reactions proceed to yield the azadiphosphabarrelenes 9a-d. While thermolyses of the afford the 1-aza-2,4-diphosphinine 5 which, in turn, reacts with a further equivalent of 2 to form the transient barrelenes 9a-c in toluene under reflux lead exclusively to the starting compounds 8a-c by a [4 + 2] cycloreversion azatriphosphabarrelene 6. Formation of the final product, the stable tetraphosphatetracyclic compound 4, is the result of a process with concomitant elimination of 2, pyrolysis of 9d under the same conditions furnishes the 1,3-diphosphinine homo Diels-Alder reaction between 6 and yet another equivalent of 2. The structure of 4 has been confirmed by an 10, also unambiguously identified by analysis of its tpentacarbonyltungsten complex 11. X-ray crystallographic analysis of its pentacarbonyltungsten complex 7. As a consequence of the high steric overcrowding