Oxa bowls: synthesis and C-H···O mediated solid state structure of pentaoxa-[5]-peristylane

Abstract

We have conceptualized [n]-oxa-[n]-peristylanes as a new family of "oxa bowls" having a crownlike shape and potential C_nv symmetry. Recognizing their equivalence to the corresponding [n]-formyl-[n]-cycloalkanes, we have conceived of a general synthetic strategy in which "intramolecular cascade acetalizations" constitute the pivotal step. As our first synthetic effort, we describe the synthesis of pentaoxa-[5]-peristylane 3b from the readily available Diels-Alder adduct 10 of 5,5-dimethoxy-tetrachlorocyclopentadiene and maleic anhydride. The norbornyl framework of 10 serves as the rigid structural matrix to build the stereochemical features and latent aldehyde functionalities as in 9 through a series of functional group adjustments. Ozonation of 9 furnished 3b in a one-step transformation in which five tetrahydrofuran rings were generated. While 3b exhibited C_5v symmetry in solution, its symmetry in the solid state was observed to be C_s by X-ray crystallography. In the solid state, 3b is endowed with a multicolumnar architecture, whose distinctive features include C-H···O interactions involving the less acidic cyclopentane hydrogens. Indeed, all the 10 CH moieties and 5 oxygen atoms (through both the lone pairs) of 3b (C₁₀H₁₀O₅) are involved in short C-H···O contacts (strong and soft) and its crystal structure indicates marked proclivity toward maximal hydrogen bonding.