Simultaneous Arming and SAR Studies of Natural Products
Natural products are essential tools for basic cellular studies leading to the identification of medically relevant protein targets and the discovery of potential therapeutic agents. We are developing mild, mono-functionalization strategies, which can be applied to a diverse set of natural products for simultaneous arming and SAR studies that also enable subsequent bioactive probe synthesis.
Our first strategy described further below involved Rh-catalyzed, O-H insertion reactions of natural products and we have developed 2nd generation reagents for this process. Strategies under study typically rely on metal catalysis, which can facilitate variation of site-selectivity based on catalyst/natural product interactions, an important feature for SAR studies.
1) "Simultaneous Arming and Structure/Activity Studies of Natural Products Employing O-H Insertions: An Expedient and Versatile Strategy for Natural Products-Based Chemical Genetics" Peddibhotla, S.; Dang, Y.; Liu, J. O.; Romo, D. J. Am. Chem. Soc., 2007, 129, 12222.
The development of selective methods that could simultaneously equip the natural product with a reactive group for subsequent conjugation to reporter tags and provide important structure-activity relationship (SAR) information, requiring only a knowledge of functional groups present in the natural product, could significantly decrease the time between bioactive natural product isolation and target identification. We report such a strategy that enables simultaneous arming and SAR studies of alcohol-containing natural products involving both chemo- and site-selective ("chemosite" selective) and site-nonselective O-H insertion reactions with rhodium carbenoids derived from alkynyl diazo acetates. This strategy was applied to a diverse set of natural products, and general guidelines for predicting chemosite selectivity were formulated. A subsequent Sharpless-Huisgen [3 + 2] cycloaddition reaction with the appended alkyne allows for attachment of a variety of reporter tags. Using this strategy, we synthesized a novel FK506-biotin conjugate that enabled pull-down of the entire "immunosuppressive complex" including FKBP12, calcineurins A and B, and calmodulin.
2) "Mild Arming and Derivatization of Natural Products via an In(OTf)3-Catalyzed Arene Iodination” Zhou, C.-Y.; Li, J.; Peddibhotla, S. ; Romo, D. Org. Lett., 2010, 12, 2104.
Iodination of arene-containing natural products employing N-iodosuccinimide catalyzed by In(OTf)3 at ambient temperature is reported as a versatile and mild method for natural product derivatization amenable to small scale. This process facilitates natural product derivatization of arene moieties for SAR studies, homo- and heterodimerization of natural products, and also conjugation with reporters such as biotin via subsequent metal-mediated coupling reactions.
3) "Diazo Reagents with Small Steric Footprints for Simultaneous Arming/SAR Studies of Alcohol-Containing Natural Products via O-H Insertion" Chamni, S.; He, Q.-L.; Dang, Y.; Bhat, S.; Liu, J. O.; Romo, D. ACS Chem. Biol., 2011, 6, 1175.
Natural products are essential tools for basic cellular studies leading to the identification of medically relevant protein targets and the discovery of potential therapeutic leads. The development of methods that enable mild and selective derivatization of natural products continues to be of significant interest for mining their information-rich content. We described novel diazo reagents for simultaneous arming and structure-activity relationship (SAR) studies of alcohol-containing natural products with a small steric footprint, namely, an α-trifluoroethyl (HTFB) substituted reagent. The Rh(II)-catalyzed O-H insertion reaction of several natural products, including the potent translation inhibitor lactimidomycin, was investigated, and useful reactivity and both chemo- and site (chemosite) selectivities were observed. Differential binding to the known protein targets of both FK506 and fumagillol was demonstrated, validating the advantage of the smaller steric footprint of O-trifluoroethyl derivatives. A p-azidophenyl diazo reagent is also described that will prove useful for photoaffinity labeling of low affinity small molecule protein receptors.
4) "Cyclopropanations of Olefinic Natural Products for Simultaneous Arming and Structure Activity Studies” Robles, O.; Serna-Saldívar, S.O.; Gutiérrez-Uribe, J. A.; Romo, D. Org. Lett., 2012, 14, 1394-1397.
Cyclopropanations of alkene-containing natural products that proceed under mild conditions are reported for simultaneous arming and structure-activity relationship studies. An alkynyl diazo ester under Rh(II) catalysis is employed for cyclopropanations of electron-rich olefins while an alkynyl sulfonium ylide is used for electron-poor olefins. This approach enables simultaneous natural product derivatization for SAR studies and arming (i.e., via alkyne attachment) for subsequent conjugation with reporter tags (e.g., biotin, fluorophores, photoaffinity labels) for mechanism of action studies including cellular target identification and proteome profiling experiments.