Experimental and Theoretical Study on the Regioselective Radical Cyclization Reactions of 1-(o-Alkenylaryl)-2-amido-1-ketones for the Construction of Indeno[2,1-d][1,3]oxazin-9-ones

A simple and efficient two-step method for the construction of novel 2,4,9a-trisubstituted-4a,9a-dihydroindeno[2,1-d][1,3]oxazin-9-ones has been developed. The NHC-catalyzed aza-benzoin reaction of o-alkenyl benzaldehydes with N-acylarylimines afforded 1-(o-alkenylaryl)-2-amido-2-aryl-1-ethanones, which underwent regioselective 5-exo-trig radical cyclization to furnish the three-ring-fused heterocyclic products, generally in good yields. The synthetic method displayed good tolerance toward the nature of substituents, substitution pattern, and steric hindrance of o-alkenyl benzaldehydes. Based on this method, the synthesis of unprecedented dihydrobenzo[6,7]indeno[2,1-d][1,3]oxazin-7-ones and dihydropyrido[2',3':3,4]cyclopenta[1,2-d][1,3]oxazin-9-ones has been achieved by employing o-alkenylnaphthaldehyde and o-alkenylnicotinaldehyde as substrates. The regioselectivity between 5-exo-trig and 6-endo-trig radical cyclization reactions of different 1-(o-alkenylaryl)-2-amido-2-aryl-1-ethanones were elucidated with DFT calculations.

Solvent-Switched Manganese(I)-Catalyzed Regiodivergent Distal vs Proximal C-H Alkylation of Imidazopyridine with Maleimide

A sustainable Mn(I)-catalyzed exclusive solvent-dependent functionalization of imidazopyridine with maleimide via an electrophilic metalation at the distal (in 2,2,2-trifluoroethanol (TFE)) and chelation assisted at the proximal (in tetrahydrofuran (THF)) has been developed. The strategy was successfully applied to the drug Zolimidine and a broad range of substrates, thereby reflecting the method's versatility.

Construction of novel bridged aromatic ring-fused oxazocine frameworks via an N-heterocyclic carbene-catalyzed azabenzoin reaction and radical-initiated cascade cyclization

A novel method for the construction of 1,5-methanobenzo[f][1,3]oxazocin-6-ones and 6,10-methanopyrido[3,2-f][1,3]oxazocin-5-ones from o-vinyl aromatic aldehydes and N-acylarylimines was developed via NHC-catalyzed and radical-initiated reactions.

Catalytic Hydroxycyclopropanol Ring-Opening Carbonylative Lactonization to Fused Bicyclic Lactones

A novel palladium-catalyzed ring opening carbonylative lactonization of readily available hydroxycyclopropanols was developed to efficiently synthesize tetrahydrofuran (THF) or tetrahydropyran (THP)-fused bicyclic γ-lactones, two privileged scaffolds often found in natural products. The reaction features mild reaction conditions, good functional group tolerability, and scalability. Its application was demonstrated in a short total synthesis of (±)-paeonilide. The fused bicyclic γ-lactone products can be easily diversified to other medicinally important scaffolds, which further broadens the application of this new carbonylation method.

Total Syntheses of Norrisolide-Type Spongian Diterpenes Cheloviolene C, Seconorrisolide B, and Seconorrisolide C

The first total syntheses of three unusual norrisolide-type rearranged spongian diterpenes, cheloviolene C, seconorrisolide B, and seconorrisolide C, have been accomplished via a common intermediate through late-stage ring-scissoring. The synthesis features a Wolff ring contraction for the synthesis of the trans-hydrindane system, and a crucial retro Diels-Alder reaction/intramolecular ene cyclization for the rapid stereoselective construction of the furo[2,3-b]furan system, which is commonly seen in rearranged spongian diterpenes.

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.

C-H functionalization reactions under flow conditions

C-H functionalization technologies have progressed enormously in the last decade as testified by the great number of publications that have appeared in the literature, which are receiving great attention from researchers from different areas of expertise. While most of the protocols reported realize the C-H functionalization processes under batch conditions, there is a growing interest in the development of continuous-flow procedures aiming at increasing the performances of established methodologies or the definition of otherwise unfeasible transformations. This review summarizes the application of flow technologies for the realization of C-H functionalization reactions. According to the type of flow reactors necessary, two main general approaches are possible for the application of flow techniques, namely the use of homogeneous or heterogeneous conditions. Each example is discussed and accompanied by the description of the main features and benefits of the use of flow compared to batch conditions.