pubs.acs.org/joc Stereoselective Total Synthesis of Etnangien and Etnangien Methyl Ester Pengfei Li,† Jun Li,‡,§ Fatih Arikan,‡, ) Wiebke Ahlbrecht,† Michael Dieckmann,† and Dirk Menche*,†,‡ †Institut fur€ Organische Chemie, Ruprecht-Karls-Universitat€ Heidelberg, Im Neuenheimer Feld 270, D-69120 Heidelberg, Germany, and ‡Helmholtz-Zentrum fur€ Infektionsforschung, Medizinische Chemie, Inhoffenstrasse 7, D-38124 Braunschweig, Germany. §Present address: Institut fur€ Pharmazeutische Wissenschaften, ETH Zurich..€ Present) address: ABX GmbH, Radeberg.
[email protected] Received February 5, 2010 A highly stereoselective joint total synthesis of the potent polyketide macrolide antibiotics etnangien and etnangien methyl ester was accomplished by a convergent strategy and proceeds in 23 steps (longest linear sequence). Notable synthetic features include a sequence of highly stereoselective substrate-controlled aldol reactions to set the characteristic assembly of methyl- and hydroxyl- bearing stereogenic centers of the propionate portions, an efficient diastereoselective Heck macro- cyclization of a deliberately conformationally biased precursor, and a late-stage introduction of the labile side chain by means of a high-yielding Stille coupling of protective-group-free precursors. Along the way, an improved, reliable protocol for a Z-selective Stork-Zhao-Wittig olefination of aldehydes was developed, and an effective protocol for a 1,3-syn reduction of sterically particularly hindered β-hydroxy ketones was devised. Within the synthetic campaign, a more detailed under- standing of the intrinsic isomerization pathways of these labile natural products was elaborated. The expedient and flexible strategy of the etnangiens should be amenable to designed analogues of these RNA-polymerase inhibitors, thus enabling further exploration of the promising biological potential of these macrolide antibiotics.