ORIGINAL RESEARCH Synthesis, NMR and X-ray structure analysis of macrolide aglycons Irena C ´ aleta • Ana C ˇ ikos ˇ • Dinko Z ˇ iher • Ivica Ðilovic ´ • Marko Duks ˇi • Dubravka Gembarovski • Ivan Grgic ˇevic ´ • Mirjana Bukvic ´ Krajac ˇic ´ • Darko Filic ´ • Dubravka Matkovic ´-C ˇ alogovic ´ • Ivica Malnar • Sulejman Alihodz ˇic ´ Received: 23 December 2011 / Accepted: 22 February 2012 / Published online: 15 March 2012 Ó Springer Science+Business Media, LLC 2012 Abstract Macrolide aglycons (E)-9-hydroxyimino-6-O- methylerythronolide A (4), 9a-aza-9-deoxo-9,9-dihydro-9a, 11-O-dimethyl-9a-homoerythronolide A (5) and 9a-aza-9- deoxo-9,9-dihydro-9a-homoerythronolide A (6) were pre- pared by multistep syntheses. A conformational study of these new macrolide aglycons was performed using single crystal X-ray crystallography to gain information about the solid state, while a combination of NMR spectroscopy and molecular modelling was employed to study the solution structures. The crystal structures were found to be stabilised by a complex network of hydrogen bonds and van der Waals interactions. To some extent, the same building motif of infinite molecular chains held together by O–HÁÁÁO hydrogen bonds was present in the crystal structure of all three com- pounds. Thorough analysis and comparison of the obtained solid state structures with their solution counterparts showed no significant differences between them, confirming the constrained flexibility of the macrocyclic ring. Moreover, in all three compounds, in both solution and solid state, the macrolactone ring adopts energetically more favoured fol- ded-out conformations. Keywords Macrolide Á Aglycon Á NMR Á MS Á Single crystal X-ray analysis Á Structure analysis Introduction Macrolides containing 14- and 15-membered polyketide rings are a very well established class of antimicrobial compounds widely used in treatment of patients since 1952, when erythromycin A was isolated from the actino- mycete Saccharopolyspora erythraea [1]. These com- pounds have shown a broad spectrum of activity against Gram-positive and some Gram-negative bacteria [2–6], their antibacterial activity being related to their selective binding to the bacterial 50S ribosomal subunit. Upon binding to the bacterial ribosome they inhibit peptide elongation in the early stage of translation [7], stimulate dissociation of peptidyl-tRNA from ribosomes [8] and/or inhibit 50S ribosomal subunit assembly [9]. Recent crys- tallographic data on macrolide–ribosome complexes [10–15] have shown the importance of the macrolide shape for positioning within the active site and, hence, its bio- logical activity. Therefore, conformational analysis can be a source of valuable information in the rational design of new compounds with improved antibacterial activity. In spite of their overall success as a therapeutic class, some natural macrolides (e.g. erythromycin A, josamycin [16]) have shown various adverse properties (e.g. instability in acidic media [17], gastrointestinal side-effects, low bio- availability, short half-life [18]), resulting in much effort Electronic supplementary material The online version of this article (doi:10.1007/s11224-012-9984-3) contains supplementary material, which is available to authorized users. I. C ´ aleta (&) Á A. C ˇ ikos ˇ Á D. Z ˇ iher Á M. Duks ˇi Á D. Gembarovski Á I. Grgic ˇevic ´ Á M. B. Krajac ˇic ´ Á D. Filic ´ Á I. Malnar Á S. Alihodz ˇic ´ GlaxoSmithKline Research Centre Zagreb Ltd, Prilaz baruna Filipovic ´a 29, 10000 Zagreb, Croatia e-mail: irena.caleta@glpg.com I. C ´ aleta Á A. C ˇ ikos ˇ Á D. Z ˇ iher Á M. Duks ˇi Á D. Gembarovski Á I. Grgic ˇevic ´ Á M. B. Krajac ˇic ´ Á D. Filic ´ Á I. Malnar Á S. Alihodz ˇic ´ Galapagos Research Center Ltd, Prilaz baruna Filipovic ´a 29, 10000 Zagreb, Croatia I. Ðilovic ´ Á D. Matkovic ´-C ˇ alogovic ´ Department of Chemistry, Faculty of Science, University of Zagreb, Horvatovac 102A, 10000 Zagreb, Croatia 123 Struct Chem (2012) 23:1785–1796 DOI 10.1007/s11224-012-9984-3