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[Google Scholar] 12. range of cellular processes.1 Such signaling is typically initiated by the binding of hormones to cell surface G protein-coupled receptors (GPCRs), which leads to recruitment of cellular guanine-nucleotide binding proteins (G-protein) and activation of adenylyl cyclases (AC), the enzymes responsible for converting ATP to cAMP. The elevated level of cAMP in turn regulates downstream cellular functions through effectors such as cAMP-dependent protein kinase (PKA) and the cAMP-GTP exchange factor Epac.2,3 Formation of cAMP by adenylyl cyclase and degradation by cAMP-specific phosphodiesterases (PDE) collectively determine cellular cAMP levels. Traditional pharmacological regulation of the cAMP signaling has been through GPCR agonists or antagonists and PDE inhibitors. The adenylyl cyclases have also been pharmacologically targeted by the diterpenoid forskolin, which binds to adenylyl cyclases and activates their enzymatic activity.4 Development of new modulators of the cAMP signaling have implications for treating heart failure, cancer, and neurodegenerative diseases.5 Thus, we were intrigued by a recent report from the Andersen lab describing isolation of alotaketal A (1) and B (2) from the marine sponge sp. collected in Papua New Guinea (Figure 1).6 These compounds were found to potently activate cAMP cell signaling in the absence of hormone binding in a cell-based pHTS-CRE luciferase reporter gene assay with EC50 values of 18 and 240 nM, respectively. In contrast, forskolin activates the cAMP signaling with an EC50 of 3 M. Alotaketals possess a sesterterpenoid carbon skeleton that cyclizes into a unique tricyclic spiroketal. In particular, simultaneous substitution of the spiroketal center by both allyl and vinyl groups is unprecedented in natural spiro-ketals. Contemporaneous to the Andersen report, the Rho lab described isolation of the closely related phorbaketals A-C (3C5) from the sponge sp.7 Their studies suggested that an unknown endosymbiotic microorganism might be the true producer of phorbaketals. We initiated our synthetic study of alotaketals/phorbaketals as part of a research program aimed at functionally characterizing natural products with useful biological properties. Herein we report the results of our efforts, ATN-161 which culminated in the first enantioselective total synthesis of (?)-alotaketal A and elucidation of the structure-activity relationship (SAR) of this potent agonist of cAMP signaling. Open in a separate window Figure 1 Alotaketals and phorbaketals Our convergent synthetic design to alotaketal A is depicted in Scheme 1. We planned to construct the tricyclic molecular skeleton by spiroketalization of the alcohol Hhex derived from silyl deprotection of 6. Unknown at the outset was the compatibility of the 11,23 alkene with the acidic reaction conditions that would be necessary to elaborate this unprecedented spiroketal ring system. Specifically, allylic activation of the C10 methylene by both the 11,23 alkene and the C9-oxocarbenium, transiently formed during spiroketalization, would cause the 11,23 alkene to be susceptible to undesired exo-to-endo isomerization. With the expectation that conditions could be identified to suppress such isomerization, we pursued this route given the efficiency gained by convergent coupling of bicyclic lac-tone 7 with allyl iodide 8 to afford the fully functionalized hemiketal 6. These two fragments would in turn be prepared from 5-hydroxycarvone 9 and ethyl acetoacetate 10, respectively. Open in a separate window Scheme 1 Synthetic design We developed a reductive allylation approach to the bicyclic lactone 7 as shown in Scheme 2. Regioselective allylic chlorination of 5-hydroxycarvone 9, readily prepared from R-(?)-carvone in 2 steps using the vinylogous em O /em -nitroso Mukaiyama aldol approach we recently developed,8 with hypochlorous acid gave allylic chloride 12.9 Mitsunobu reaction of 12 with formic acid went smoothly to give 13 in 70% yield in the presence of the electrophilic allyl chloride moiety.10 Diastereoselective Luche reduction of the enone of 13,11 protection of the hydroxyl group with TBSCl, and Finkelstein reaction gave iodide 14 as a single diastereomer.12 As expected, the powerful yet under-explored reductive allylation approach reported by Keck,13 achieved by treatment of 14 with excess SmI2 led to smooth cyclization to give lactol 15 as an inconsequential mixture of epimers through intramolecular Barbier-type allylation of the formate. Even though excess SmI2 was employed, further reduction of 15 was not ATN-161 observed. Oxidation of 15 with IBX furnished the hydrobenzopyranone 16. Open in a separate window Scheme 2 Synthesis of the bicyclic lactone 7 Reagents and conditions: a) HClO, CH2Cl2, 64%; b) HCO2H, DEAD, PPh3, THF, 70%; c) i. NaBH4, CeCl37H2O, MeOH; ii. TBSCl, imidazole, DMF, 88% for 2 steps; iii. NaI, acetone; d) SmI2, THF, 73% for 2 steps; e) IBX, DMSO, 72%; f) Hg(OAc)2, toluene; aq. KCl; g) I2, CH2Cl2, 81% for 2 steps; h) i. HCO2H, NaHCO3, DMF; MeOH-H2O, 86%; ii. PMBOC(NH)CCl3, pTSA, CH2Cl2, 92%. Further functionalization of lactone ATN-161 16 was complicated by its unexpected low reactivity toward common electrophilic reagents required to selectively functionalize the disubstituted 7,22 alkene in the presence of the trisubstituted 2,3 alkene. For example, no reaction occurred when 16 was treated with.