Studies toward the Total Synthesis of Nogalamycin: Construction of the Complete ABCDEF-Ring System via a Convergent Hauser Annulation

An Unexpected Lewis Acid-Catalyzed Cascade during the Synthesis of the DEF-Benzoxocin Ring System of Nogalamycin and Menogaril: Mechanistic Elucidation by Intermediate Trapping Experiments and Density Functional Theory Studies

An unexpected Lewis acid-catalyzed carbohydrate rearrangement of a 1,5-bis-glycopyranoside to the product of a formal intramolecular C-aryl glycosylation reaction is reported. Mechanistic studies based mainly on intermediate trapping experiments and density functional theory (DFT) calculations reveal a cascade process involving three transient (a)cyclic oxocarbenium cations, the breaking of three single C(sp3)-O bonds, and the formation of three single bonds (i.e., exo-, endo-, and C-glycosidic bonds), leading to the 2,6-epoxybenzoxocine skeleton of bioactive natural glycoconjugates related to serjanione A and mimocaesalpin E. DFT calculations established that the generation of the pyran moiety embedded in the bridged benzoxocin ring system is likely to proceed through an unusual ring-closure of an ortho-quinone methide intermediate in which the attacking nucleophile is a carbonyl oxygen.

Progress toward the Total Synthesis of Nogalamycin Using a Benzyne Cycloaddition Strategy

Nogalamycin (NOG) is a member of the anthracycline glycoside natural products; no total syntheses have yet been reported, and there is minimal understanding of how the aglycone substitution pattern and identities of the A- and D-ring sugars impact the anticancer activity and toxicity. This paper reports progress toward a modular approach to NOG that could enable systematic structure-activity relationship studies. Key steps include a regioselective benzyne cycloaddition and reductive ring-opening to assemble a versatile AB core for analogue synthesis.

A Fries-Type Rearrangement Strategy for the Construction of Stereodefined Quaternary Pseudoanomeric Centers: An Entry into C-Naphthyl Ketosides

The stereodefined construction of quaternary pseudoanomeric centers by way of a BF3·Et2O-catalyzed, Fries-type rearrangement of O-ketosides is described. This method provides new access to C-naphthyl ketosides related to biologically relevant products with good to complete stereocontrol in favor of the β product.

Formal Glycosylation of Quinones with exo-Glycals Enabled by Iron-Mediated Oxidative Radical-Polar Crossover

The intermolecular C-O coupling reaction of 1,4-quinones with exo-glycals under iron hydride hydrogen atom transfer (HAT) conditions is described. This method provides a direct and regioselective access to a wide range of phenolic O-ketosides related to biologically relevant natural products in diastereomeric ratios up to >98:2 in the furanose and pyranose series. No trace of the corresponding C-glycosylated products that might have resulted from the radical alkylation of 1,4-quinones was observed. The results of mechanistic experiments suggest that the key C-O bond-forming event proceeds through an oxidative radical-polar crossover process involving a single-electron transfer between the HAT-generated glycosyl radical and the electron-acceptor quinone.

N-Heterocyclic carbene based catalytic platform for Hauser-Kraus annulations

The venerable Hauser–Kraus annulation is an effective and convergent method for generating oxygenated polycyclic aromatic compounds. Despite its application in complex molecule synthesis, the harsh and strongly basic conditions can limit its utility in more functionalized molecular settings. We have developed the first catalytic Hauser–Kraus annulation based on N-heterocyclic carbene catalysis that proceeds under milder conditions. We demonstrate the scope of the transformation in the presence of several functional groups. We also propose a concerted mechanism for the annulation that proceeds through a non-canonical Breslow intermediate.

The venerable Hauser–Kraus annulation is an effective and convergent method for generating oxygenated polycyclic aromatic compounds. A catalytic Hauser–Kraus annulation has been developed based on N-heterocyclic carbene catalysis.

Unraveling Plant Natural Chemical Diversity for Drug Discovery Purposes

The screening and testing of extracts against a variety of pharmacological targets in order to benefit from the immense natural chemical diversity is a concern in many laboratories worldwide. And several successes have been recorded in finding new actives in natural products, some of which have become new drugs or new sources of inspiration for drugs. But in view of the vast amount of research on the subject, it is surprising that not more drug candidates were found. In our view, it is fundamental to reflect upon the approaches of such drug discovery programs and the technical processes that are used, along with their inherent difficulties and biases. Based on an extensive survey of recent publications, we discuss the origin and the variety of natural chemical diversity as well as the strategies to having the potential to embrace this diversity. It seemed to us that some of the difficulties of the area could be related with the technical approaches that are used, so the present review begins with synthetizing some of the more used discovery strategies, exemplifying some key points, in order to address some of their limitations. It appears that one of the challenges of natural product-based drug discovery programs should be an easier access to renewable sources of plant-derived products. Maximizing the use of the data together with the exploration of chemical diversity while working on reasonable supply of natural product-based entities could be a way to answer this challenge. We suggested alternative ways to access and explore part of this chemical diversity with in vitro cultures. We also reinforced how important it was organizing and making available this worldwide knowledge in an "inventory" of natural products and their sources. And finally, we focused on strategies based on synthetic biology and syntheses that allow reaching industrial scale supply. Approaches based on the opportunities lying in untapped natural plant chemical diversity are also considered.