MacGillivray and colleagues have demonstrated that a supramolecular approach to covalent synthesis in the organized, solvent-free environment of the solid state can provide a solution to the problem of organizing two polyenes for an intramolecular reaction to give a ladderane. Specifically, by taking an approach to control reactivity in solids by using molecules that serve as linear templates, they have demonstrated the utility of cocrystallization of resorcinol (1,3-benzenediol), or a derivative, with an all-trans-bis(4-pyridyl)poly-m-ene (4-pyr-poly-m-ene) produces a four-component molecular assembly, 2(resorcinol)·2(4-pyr-poly-m-ene), in which each resorcinol preorganizes, through two O—H···N hydrogen-bonding interactions, two poly-m-enes for [2+2] photoaddition. The two polyenes are positioned by the templates such that the C=C bonds of the olefins lie parallel and separated by < 4.2 Å, a position suitable for the photoreaction. UV irradiation of the solid produces the targeted [n]ladderane, with the C=C bonds reacting to form the fused cyclobutane framework. Broadband UV-irradiation of two such hydrogen-bonded, four-component supramolecular assemblies furnishes the corresponding ladderanes stereospecifically and in quantitative yield in gram quantities.
