3,4-Bis-O-propargyl-1,2:5,6-di-O-isopropylidene-D-mannitol: a study of multiple weak hydrogen bonds in the solid...

The title homochiral compound, C₁₈H₂₆O₆, 1, was examined by single-crystal X-ray crystallography in order to understand its potential as a synthetic building block, particularly in inter- and intramolecular cyclocondensation reactions. It has also proven to be an excellent model for understanding multiple weak donor-acceptor D-H...A interactions involving terminal acetylenes as donors and as acceptors. The asymmetric unit of 1 comprises three almost identical independent molecules, each with the mannitol 2R,3R,4R,5R configuration and different conformations. Like independent molecules align in strands through acetylenic donor C-H...O contacts with equivalent dioxolanyl acceptor groups. Two of the strands are aligned unidirectionally, in parallel, while the third strand aligns perpendicular to the first two, to give interwoven layers in the supramolecular structure. A detailed study of the interdigitation of the second propargyl group from each independent molecule between strands, and of other short interstrand C-H...O contacts, provides new insight into the application of weak hydrogen-bond theory within the context of a conformationally flexible symmetrical molecule. Analyses of the Cambridge Structural Database using Crystal Packing Features and ConQuest search motifs support the importance of the D...A distance parameter, demonstrate the different influences of donor and acceptor types, and reveal the interplay between H...A and D...A contributions in different contact types.