Dimethyl dispiro (tricyclo[3.3.1.13,7]decan-1,2'-bicyclo[1.1.0]butan-4',1"-tricyclo[3.3.1.13,7]decan-1',3'-dicarboxylate): a strain enhanced bicyclo[1.1.0]butane with a very short intramolecular C-H...H-C contact - Bats, Jan Willem; Gundl, Marc A.; Hashmi, A. Stephen K. (Acta crystallographica. Section C, Crystal structure communications, 2001)