Synthesis of Diazatricyclic Common Structure of Madangamine Alkaloids

Palladium-Catalyzed Synthesis of Fused Carbo- and Heterocycles: Recent Advances

The use of palladium catalysts in fused ring synthesis has been increasingly noteworthy in recent years in organic synthesis. It has a lot of potential compared to other transition metal catalysts, because of its one-of-a-kind feature that makes them more widely applicable in a variety of disciplines application. Palladium is important in a variety of Heck processes, including intramolecular, intermolecular, and reductive Heck reactions, which produce diverse carbocycles and heterocycles of biological importance. Under optimal reaction conditions, carbocyclization or heterocyclization occurs, resulting in the production of numerous structural building blocks of naturally occurring compounds. Beside intramolecular Heck-type reactions, cycloaddition, cycloalkylation, oxidative coupling, C-H functionalization, cross-coupling reactions, and carboamidation reactions have also been employed extensively to access fused carbo- and heterocycles of immense biological importance. This review article provides a well-summarized discussion (since 2001) on fused carbo- and heterocycle ring synthesis using palladium catalysts, overviewing their applications, and mechanistic insights.

A New Organocatalytic Desymmetrization Reaction Enables the Enantioselective Total Synthesis of Madangamine E

The enantioselective total synthesis of madangamine E has been completed in 30 steps, enabled by a new catalytic and highly enantioselective desymmetrizing intramolecular Michael addition reaction of a prochiral ketone to a tethered β,β′-disubstituted nitroolefin. This key carbon–carbon bond forming reaction efficiently constructed a chiral bicyclic core in near-perfect enantio- and diastereo-selectivity, concurrently established three stereogenic centers, including a quaternary carbon, and proved highly scalable. Furthermore, the pathway and origins of enantioselectivity in this catalytic cyclization were probed using density functional theory (DFT) calculations, which revealed the crucial substrate/catalyst interactions in the enantio-determining step. Following construction of the bicyclic core, the total synthesis of madangamine E could be completed, with key steps including a mild one-pot oxidative lactamization of an amino alcohol, a two-step Z-selective olefination of a sterically hindered ketone, and ring-closing metatheses to install the two macrocyclic rings.

Stereocomplementary and Parallel Syntheses of Multi-Substituted (E)-, (Z)-Stereodefined α,β-Unsaturated Esters: Application to Drug Syntheses

Ubiquitous α,β-unsaturated esters are well recognized as key structural olefin scaffolds in organic chemistry. (E)- and (Z)-steroselectivity is the most critical issue in their synthesis, however, (E)- and (Z)- stereocomplementary synthetic methods remain quite limited. The present account discloses general (E)-, (Z)-stereocomplementary syntheses of a variety of α,β-unsaturated esters from highly accessible (E)-, (Z)-stereodefined enol tosylates derived from β-ketoesters and α-formyl esters. Step 1 toward the stereocomplementary preparation of (E)-, (Z)-stereodefined enol tosylates is implemented by using inexpensive reagents under mild reaction conditions. Step 2 toward the highly stereoretentive synthesis of (E)- and (Z)-stereodefined α,β-unsaturated esters involves Suzuki-Miyaura, Negishi, Sonogashira, Iron-catalyzed, Mizoroki-Heck, and Buchwald-Hartwig cross-coupling reactions. Notably, this strategy was successfully applied for parallel drug syntheses of (E)- and (Z)-zimelidine, (E)- and (Z)-tamoxifen, and Merck's cyclopropane pharmacophore. Representative successful utilizations by other groups are also introduced.

Recent progress on the total syntheses of macrocyclic diamine alkaloids

This review highlights the progress and challenges in the chemical synthesis of macrocyclic diamine alkaloids since 2006.

Access to Enantiopure Advanced Intermediates en Route to Madangamines

The synthesis of enantiopure ABCE and ABCD tetracyclic advanced intermediates en route to madangamine alkaloids and studies for the construction of the triunsaturated 15-membered D ring of madangamine B and the saturated 13-membered D ring of madangamine E are reported.

Enantioselective formal synthesis of (+)-madangamine A

A formal synthesis of (+)-madangamine A from phenylglycinol-derived lactam 1, involving the final closure of the peripheral rings from the skipped polyunsaturated diazatricyclic derivative 19, is reported.

Oxamidation of Unsaturated O-Alkyl Hydroxamates: Synthesis of the Madangamine Diazatricylic (ABC Rings) Skeleton

A novel approach to the diazatricyclic madangamine ABC ring system and the synthesis of an advanced, differentially protected intermediate for the synthesis of madangamine D is reported. Central to the success of this approach is the iodine(III)-mediated intramolecular oxamidation of an unsaturated O-methyl hydroxamate, a π-N⁺-type cyclization which proceeds in high yield and with complete regioselectivity to generate the 2-azabicyclo[3.3.1]nonane (morphan) system encompassing rings A and C.

Divergent Synthetic Access to E- and Z-Stereodefined All-Carbon-Substituted Olefin Scaffolds: Application to Parallel Synthesis of (E)- and (Z)-Tamoxifens

A highly substrate‐general synthesis of all‐carbon‐substituted E‐ and Z‐stereodefined olefins is performed. The method comprises two sets of parallel and stereocomplementary preparations of (E)‐ and (Z)‐α,β‐unsaturated esters involving two robust and distinctive reactions: 1) stereocomplementary enol tosylations using readily available TsCl/diamine/(LiCl) base reagents, and 2) stereoretentive Negishi cross‐coupling using the catalysts [Pd(dppe)Cl₂] (for E) and [Pd(dppb)Cl₂] (for Z). The present parallel approach is categorized as both type I (convergent approach: 16 examples, 56–87 % yield) and type II (divergent approach: 18 examples, 70–95 % yield). The obtained (E)‐ and (Z)‐α,β‐unsaturated ester scaffolds are successfully transformed into various E‐ and Z‐stereodefined known and novel olefins (8×2 derivatization arrays). As a demonstration, application to the parallel synthesis of both (E)‐ and (Z)‐tamoxifens, a representative motif of all‐carbon‐substituted olefins, is accomplished in a total of eight steps with an overall yield of 58 % (average 93 %) and 57 % (average 93 %), respectively.

Stereoselective strategies for the construction of polysubstituted piperidinic compounds and their applications in natural products’ synthesis

An abridged and far-reaching review communication on the construction of the polysubstituted piperidinic core using diverse methodologies for the benefit of organic chemists interested in the total synthesis of biologically active compounds.