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

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 (C(10)H(10)O(5)) are involved in short C-H...O contacts (strong and soft) and its crystal structure indicates marked proclivity toward maximal hydrogen bonding.