Total Synthesis, Configuration Assignment, and Cytotoxic Activity Evaluation of Protulactone A Martin Markovic ̌ , † Peter Koó s ̌ , ‡ Toma ́ s ̌ C ̌ arny ́ , † Saskia Sokoliova ́ , † Nikola Boha ́ c ̌ ikova ́ , † Ja ́ n Moncol′, § and Tibor Gracza* ,† † Department of Organic Chemistry, Institute of Organic Chemistry, Catalysis and Petrochemistry, and § Department of Inorganic Chemistry, Institute of Inorganic Chemistry, Technology and Materials, Slovak University of Technology, Radlinske ́ ho 9, SK-812 37 Bratislava, Slovakia ‡ Georganics Ltd., Korenič ova 1, SK-811 03 Bratislava, Slovakia * S Supporting Information ABSTRACT: The first total synthesis and absolute config- uration assignment of protulactone A (1) has been achieved. Four stereoisomers, 1a, ent-1a, 1b, and ent-1b, of this natural polyketide were prepared by chiral pool synthesis starting from L- and D-arabinose, respectively. The absolute and relative configurations of all isomers were assigned by single-crystal X- ray analysis. Target compounds were screened for their in vitro cytotoxicity toward certain human tumor cells (NCI 60 cancer cell line panel). P rotulactones A (1) and B ((-)-2) were isolated in 2010 from the marine-derived fungus Aspergillus sp. SF-5044. 1 The substructure unit of these two polyketide-derived fungal metabolites, a bicyclic lactone, is not a distinctive structure for the natural compounds isolated from fungi of the Aspergillus genus. However, similar compounds occur in plants of various families (e.g., Goniothalamus giganteus Hook f., Thomas (Annonaceae) and Polyalthia crassa). 2 The structures and relative configurations of protulactones A and B were determined by extensive NMR analysis. The absolute configuration of the rigid bicyclic skeleton of protulactone B ((-)-2) was assigned by Mosher’s MTPA ester method. However, the relative configuration of the C-7 center and the absolute configuration of 1 have not been described. 1 The biological activities of these two natural compounds have not been evaluated presumably due to the limited supply from natural sources (only 3.7 mg of 1 was isolated from 2.0 g of the EtOAc extract). Naturally occurring and structurally similar lactones, goniofufurone ((+)-3), 7-epi-goniofufurone ((+)-4), goniopypyrone ((+)-6), 2a,b and crassalactone C ((+)-5), 2c have exhibited significant cytotoxic activity in tests with several human tumor cell lines. 2,3 The incomplete configuration assignment and promising biological activity of 1 led us to develop a total synthesis of protulactone A with the aim to establish its absolute structure and evaluate its antiproliferative activities against the NCI 60 cancer cell line panel. ■ RESULTS AND DISCUSSION Because the absolute and C-7 configuration of the isolated compound 1 are unknown, we have targeted four different stereoisomers. We herein report the synthesis of all of these isomers of 1 (1a, 1b, ent-1a, and ent-1b) containing the bicyclic skeleton with a relative gluco configuration and different configurations at the C-7 center. The proposed retrosynthetic strategy of protulactone A (1) is shown in Scheme 1. In both selected routes, the methyl moiety is introduced at the end of Received: March 10, 2017 Published: April 18, 2017 Scheme 1. Retrosynthetic Analysis of the Protulactone A Article pubs.acs.org/jnp © 2017 American Chemical Society and American Society of Pharmacognosy 1631 DOI: 10.1021/acs.jnatprod.7b00212 J. Nat. Prod. 2017, 80, 1631-1638