Total Synthesis of (-)-Hymenosetin Ulrich Kauhl, † Lars Andernach, † Stefan Weck, † Louis P. Sandjo, † Stefan Jacob, ‡ Eckhard Thines, ‡,§ and Till Opatz* ,† † Institute of Organic Chemistry, University of Mainz, Duesbergweg 10-14, D-55128 Mainz, Germany ‡ Institut fü r Biotechnologie und Wirkstoff-Forschung gGmbH (IBWF), Erwin-Schrö dinger-Str. 56, D-67663 Kaiserslautern, Germany § Johannes Gutenberg-University Mainz, Institute of Biotechnology and Drug Research, Duesbergweg 10-14, D-55128 Mainz, Germany * S Supporting Information ABSTRACT: The 3-decalinoyltetramic acid (-)-hymenosetin and its N-methyl analogue were prepared in 11 and 8 steps, respectively, from (+)-citronellal using an intramolecular Diels-Alder reaction as the key step. This method represents the first example for the synthesis of a 3-decalinoyltetramic acid with a free NH moiety. The stereochemistry of the title compound, an unnatural diastereomer, and of a decalin building block was studied in detail using circular dichroism spectros- copy in the IR and UV/VIS freqeuncy range. This allowed to determine the absolute configuration of the natural product and to plan the synthetic route. ■ INTRODUCTION In 2014, Stadler and co-workers reported the isolation of the new fungal metabolite hymenosetin (1) from Hymenoscyphus pseudoalbidus (recently renamed to H. fraxineus), an invasive species causing severe dieback of the European ash. 1,2 Besides antifungal and moderate cytotoxic effects against the mouse fibroblast cell line L929, the compound was found to show promising biological activities against Gram-positive bacteria, including strong bacteriostatic inhibition of methicillin-resistant Staphylococcus aureus (MRSA), the growing resistance of which to most common antibiotics necessitates the discovery and development of new antibacterial agents. 1 Hymenosetin belongs to the 3-decalinoyltetramic acids, a class in which equisetin (2) is the first discovered and best investigated member (Figure 1). This class of natural products can be subdivided based on the presence or absence of an N-methyl group. 3 Further examples for non-N-methylated 3-decalinoyl- tetramic acids are paecilosetin, altersetin, coniosetin, and epi- trichosetin. 4-6 The absolute and relative configurations of 1 have been determined based on comparison of its ECD (electronic circular dichroism) spectra with those of equisetin, phomasetin, and epi- trichosetin as well as by HSQC-HECADE-experiments and is the same as in equisetin. 1,7-9 HECADE is a 2D-NMR experiment suitable for the determination of heteronuclear long-range coupling constants which can be used for stereochemical assignment of acyclic structures. 7,10 Both the quaternary center of the decalin part (C-2) and the stereocenter in the tetramic acid part (C-5′) are S-configurated. In contrast to equisetin, hymenosetin bears an additional methyl group in the decalin part, an L-threonine-derived instead of L-serine-derived side chain, and a nonmethylated nitrogen in the tetramic acid moiety. We aimed to develop an efficient synthetic strategy for the synthesis of 3-decalinoyltetramic acids with a free NH moiety to establish structure-activity relationships for this class of natural products and to prove the absolute configuration of hymenosetin through chemical synthesis. Furthermore, we report the isolation and structure elucidation of hymenosetin from the ascomycete IBWF-E99318 (Phoma sp.), which was used as a reference material in our subsequent studies. Here, we report the firm stereochemical assignment by CD spectroscopy at different wavelengths as well as the first total synthesis of (-)-hymeno- setin. Received: November 3, 2015 Published: December 4, 2015 Figure 1. Structures of hymenosetin and equisetin. Article pubs.acs.org/joc © 2015 American Chemical Society 215 DOI: 10.1021/acs.joc.5b02526 J. Org. Chem. 2016, 81, 215-228