Stereodivergent and Protecting-Group-Free Synthesis of the Helicascolide Family: A Rhodium-Catalyzed Atom-Economical Lactonization Strategy

Rhodium-Catalyzed Allylic Addition as an Atom-Efficient Approach in Total Synthesis

ConspectusFinding efficient synthetic methods for the asymmetric synthesis of complex molecules has always been of interest to organic chemists. Creating and controlling the stereochemistry of stereogenic centers bearing branched allylic moieties in organic molecules using a catalytic process is an attractive and successful method for the synthesis of several natural products and medicinally important compounds. Remarkable progress toward their synthesis has been achieved via transition-metal catalysis, especially in the case of allylic substitution and allylic C-H oxidation chemistry. However, for allylic substitution the preinstallation of a leaving group is essential, and for allylic C-H oxidation, stoichiometric amounts of oxidant are required. Besides that, the control of regioselectivity with these methods is often problematic because the linear product can be produced as a major isomer. Our research group has developed a regioselective, enantioselective, and atom economic route toward the more valuable branched product via a Rh-catalyzed coupling of easily accessible alkynes or the double-bond isomeric allenes with pronucleophiles. It was demonstrated that, using this new approach, it is possible to add different pronucleophiles to alkynes or allenes to form branched allylic moieties through C-C and C-heteroatom bond formation. Since new organic reactions offer new opportunities in chemical synthesis and the benchmark for new synthetic methods is their application in target-oriented synthesis, we have demonstrated several successful syntheses of natural products and medicinally relevant targets. For example, in the total syntheses of Quercuslactones, Helicascolides A-C, Epothilone D, Homolargazole, and Thailandepsin B, the Rh-catalyzed hydro-oxycarbonylation of allenes was used as key step via C-O bond formation. Remarkably, the Rh-catalyzed C2-symmetric dimerization strategy was used to synthesize the complex molecules Clavosolide A and Vermiculine, leading to an extreme increase in structural complexity within a single step. For the total syntheses of Centrolobine, Pitavastatin, and Rosuvastatin, C-O bond formation was achieved through the addition of a hydroxy function to the allene moiety. The potential of the addition of nitrogen pronucleophiles to allenes was demonstrated in the total syntheses of Cusparein, Angusterein, Cermicin C, Senepodin G, Homoproline, Pipecolinol, Coniceine, Coniine, Ruxolitinib, Sitagliptin, Abacavir, Glucokinase activators, and Chaetominine. All of these examples testify to the wide applicability of the Rh-catalyzed addition of pronucleophiles to allenes or alkynes in target-oriented synthesis, and in this Account we summarize our contribution.

Discovery of (E)-2-Methoxyethene-1-sulfonyl Fluoride for the Construction of Enaminyl Sulfonyl Fluoride

A new sulfonyl fluoride reagent (E)-2-methoxyethene-1-sulfonyl fluoride (MeO-ESF) was developed and successfully applied for the construction of enaminyl sulfonyl fluoride (N-ESF). This protocol provides highly atom-economical access to diverse N-ESF and produces CH₃OH as the sole byproduct under mild and environmentally benign conditions.

Two new bioactive secondary metabolites from the endophytic fungus Talaromyces assiutensis JTY2

One new lactone derivative helicascolide F (1), one new pyrrolidine derivative talaromydine (2), along with six known compounds (3-8) were isolated from the fungus Talaromyces assiutensis JTY2. The structure of the new compounds 1 and 2 was determined by 1D and 2D NMR as well as by HRESIMS. The inhibitory activity of all compounds against six phytopathogenic fungi and three cancer cell lines was evaluated.

Rhodium-Catalyzed Cyclization of Terminal and Internal Allenols: An Atom Economic and Highly Stereoselective Access Towards Tetrahydropyrans

A comprehensive study of a diastereoselective Rh-catalyzed cyclization of terminal and internal allenols is reported. The methodology allows the atom economic and highly syn-selective access to synthetically important 2,4-disubstituted and 2,4,6-trisubstituted tetrahydropyrans (THP). Furthermore, its utility and versatility are demonstrated by a great functional-group compatibility and the enantioselective total synthesis of (-)-centrolobine.

Rhodium-Catalyzed Diastereo- and Enantioselective Tandem Spirocyclization/Reduction of 3-Allenylindoles: Access to Functionalized Vinylic Spiroindolines

A highly selective rhodium-catalyzed tandem spirocyclization/reduction of 3-allenylindoles is reported. By employing a Hantzsch ester as reductant, vinylic spiroindolines are obtained in excellent yields as well as diastereo- and enantioselectivity. In addition, the reaction's synthetic utility is highlighted by broad functional group compatibility and exemplified by a gram scale reaction with subsequent assorted transformations.

Rhodium-catalyzed asymmetric intramolecular hydroarylation of allenes: access to functionalized benzocycles

A rhodium-catalyzed regio- and enantioselective cyclization of tethered allenylbenzenes is reported. Employing a RhI/Josiphos catalytic system a diverse set of 6-membered, α-chiral benzocycles were obtained, scaffolds found in many bioactive compounds. Moreover, a gram scale reaction as well as the application of suitable transformations are demonstrated.

Marine Natural Products from Indonesian Waters

Natural products are primal and have been a driver in the evolution of organic chemistry and ultimately in science. The chemical structures obtained from marine organisms are diverse, reflecting biodiversity of genes, species and ecosystems. Biodiversity is an extraordinary feature of life and provides benefits to humanity while promoting the importance of environment conservation. This review covers the literature on marine natural products (MNPs) discovered in Indonesian waters published from January 1970 to December 2017, and includes 732 original MNPs, 4 structures isolated for the first time but known to be synthetic entities, 34 structural revisions, 9 artifacts, and 4 proposed MNPs. Indonesian MNPs were found in 270 papers from 94 species, 106 genera, 64 families, 32 orders, 14 classes, 10 phyla, and 5 kingdoms. The emphasis is placed on the structures of organic molecules (original and revised), relevant biological activities, structure elucidation, chemical ecology aspects, biosynthesis, and bioorganic studies. Through the synthesis of past and future data, huge and partly undescribed biodiversity of marine tropical invertebrates and their importance for crucial societal benefits should greatly be appreciated.

Rhodium-Catalyzed Enantioselective Cyclization of 3-Allenyl-indoles: Access to Functionalized Tetrahydrocarbazoles

A highly selective rhodium-catalyzed cyclization of tethered 3-allenylindoles is reported. In a smooth reaction, 1-vinyltetrahydrocarbazoles are obtained in excellent yields and enantioselectivities. Aside from a great functional group tolerance, this method requires neither the Schlenk technique nor the use of anhydrous solvents. Preliminary mechanistic investigations proved that the reaction proceeds via an intermediary formed spiroindolenine which rapidly undergoes an acid-catalyzed stereospecific migration.

Transition metal catalyzed stereodivergent synthesis of syn- and anti-δ-vinyl-lactams: formal total synthesis of (-)-cermizine C and (-)-senepodine G

A stereodivergent and diastereoselective transition-metal-catalyzed intramolecular hydroamidation of allenes and alkynes furnishing δ-vinyl-lactams is reported. Employing a rhodium catalyst allowed for the selective synthesis of the syn-δ-lactam. Conversely, a palladium catalyst led to the formation of the anti-δ-lactam in high selectivity. The new method shows high functional group compatibility and assorted synthetic transformations were demonstrated as well as its utility for the enantioselective formal total syntheses of (-)-cermizine C and (-)-senepodine G.

Palladium-Catalyzed Methylation of Aryl, Heteroaryl, and Vinyl Boronate Esters

A method for the direct methylation of aryl, heteroaryl, and vinyl boronate esters is reported, involving the reaction of iodomethane with aryl-, heteroaryl-, and vinylboronate esters catalyzed by palladium and PtBu₂Me. This transformation occurs with a remarkably broad scope and is suitable for late-stage derivatization of biologically active compounds via the boronate esters. The unique capabilities of this method are demonstrated by combining carbon-boron bond-forming reactions with palladium-catalyzed methylation in a tandem transformation.

Rhodium-Catalyzed Diastereoselective Cyclization of Allenyl-Sulfonylcarbamates: A Stereodivergent Approach to 1,3-Aminoalcohol Derivatives

A diastereoselective and stereodivergent rhodium-catalyzed intramolecular coupling of sulfonylcarbamates with terminal allenes is described and it provides selective access to 1,3-aminoalcohol derivatives, scaffolds found in bioactive compounds. The reaction is compatible with a large range of different functional groups, thus furnishing products with high diastereoselectivities and yields. Moreover, multigram scale reactions, as well as the application of suitable product transformations were demonstrated.

Branching Out: Rhodium-Catalyzed Allylation with Alkynes and Allenes

We present a new and efficient strategy for the atom-economic transformation of both alkynes and allenes to allylic functionalized structures via a Rh-catalyzed isomerization/addition reaction which has been developed in our working group. Our methodology thus grants access to an important structural class valued in modern organic chemistry for both its versatility for further functionalization and the potential for asymmetric synthesis with the construction of a new stereogenic center. This new methodology, inspired by mechanistic investigations by Werner in the late 1980s and based on preliminary work by Yamamoto and Trost, offers an attractive alternative to other established methods for allylic functionalization such as allylic substitution or allylic oxidation. The main advantage of our methodology consists of the inherent atom economy in comparison to allylic oxidation or substitution, which both produce stoichiometric amounts of waste and, in case of the substitution reaction, require prefunctionalization of the starting material. Starting out with the discovery of a highly branched-selective coupling reaction of carboxylic acids with terminal alkynes using a Rh(I)/DPEphos complex as the catalyst system, over the past 5 years we were able to continuously expand upon this chemistry, introducing various (pro)nucleophiles for the selective C-O, C-S, C-N, and C-C functionalization of both alkynes and the double-bond isomeric allenes by choosing the appropriate rhodium/bidentate phosphine catalyst. Thus, valuable compounds such as branched allylic ethers, sulfones, amines, or γ,δ-unsaturated ketones were successfully synthesized in high yields and with a broad substrate scope. Beyond the branched selectivity inherent to rhodium, many of the presented methodologies display additional degrees of selectivity in regard to regio-, diastereo-, and enantioselective transformations, with one example even proceeding via a dynamic kinetic resolution. Many advances presented in this account were driven by detailed mechanistic investigations including DFT-calculations, ESI-MS and in situ IR experiments and enabled the application of our chemistry for target-oriented syntheses demonstrated by several examples shown herein. In general, this research topic has matured over the past years into a viable option when synthesizing chiral compounds, from small molecules such as quercus lactones to complex target structures such as Homolargazole or Clavosolide A. This demonstrates the importance and utility of these coupling reactions, especially considering the ease with which carbon-heteroatom bonds can be built stereoselectively, with many of the product classes displaying motifs common in modern APIs.