Synthesis and DFT conformational analysis of trimethyl-functionalized [2.2]metacyclophanes and their Lewis-acid assisted reactions

Abstract

The synthesis of the strained trimethyled anti-[2.2]metacyclophanes 5a and 5b is reported. They were synthesized by ring contraction of the corresponding unstrained cyclic precursors 2,11-dithia[3.3]metacyclophane-2,2,11,11-tetraoxides, 4a and 4b by the vacuum-pyrolysis elimination of sulfur dioxide ("sulfone pyrolysis‘).Tetraoxides 4a and 4b were generated by the oxidation of their respective dithia[3.3]metacyclophane precursors 3a and 3b. The macrocyclic coupling reactions which formed 3a and 3b also produced both anti- and syn-conformational isomers which could be separated from their respective reaction mixtures and whose structures were confirmed by 1H-NMR spectroscopy. Single-crystal X-ray analyses confirmed the anti conformation of anti-3b in the solid state and that the corresponding oxidized tetraoxide, syn-4b adopts a syn conformation. The Lewis acid (TiCl4/AlCl3-MeNO2)-catalyzed reactions of anti-5b under various conditions led to transannular cyclization to afford tetrahydropyrene 7, pyrene derivative 8 and/or de-tert-butylated anti-5a. A DFT (Density Functional Theory) computational study was conducted to determine the stabilities of the different conformational isomers of the target compounds.