Cyclonona-l,4,7-triyne

In contrast to the series of hydrocarbons, heterocyclic analogues, even of cyclonona-l,4,7-triyne 2 (u = 3), are known. This is because heteroatom linkers more easily adopt smaller bonding angles and thereby provide some relief of overall angle strain. Since heteroatoms have different electronic properties to carbon atoms, the perceived homoconjugative and homoaromatic effects might be expressed more pronouncedly in heterocyclic [n]pericyclines. 

If cyclonona-l,4,7-triyne were subjected to cyclization, MNDO RHF calculations have suggested that its valence isomer, triscyclopropabenzene (1) (J//f 947 KJmol" ) might be formed via an allowed [ 2s -t- + 2j] cycloaddition process. 

Early attempts to synthesize the intriguing cyclonona-l,4,7-triyne molecule, 1 [1], grew from a recognition that the in-plane p-orbitals of the three acetylenes around its perimeter should encroach upon each other s space while the out-of-plane p-orbitals should consitute an essentially ideal trishomobenzene. The tantalizing prospect that a [2 + 2 + 2] cycloaddition requiring very little atomic motion might transform this monocylic triyne into tricyclopropabenzene (2, Fig. 9-1) added further incentive to prepare 1. 

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