Toward a total synthesis of the stemofoline alkaloids: Advancement of a 1,3-dipolar cycloaddition strategy

Strategies for the synthesis of Stemona alkaloids: an update

Covering: 2009 to 2022The Stemona alkaloids, which are found in plant species from the family Stemonaceae, represent a tremendously large and structurally-diverse family of natural products. This review presents and discusses a selection of case studies, grouped by alkaloid class, that showcase the key strategies and overall progress that has been made in the synthesis of Stemona alkaloids and related compounds since 2009. Structural reassignments that have been reported over this period are also identified where necessary.

Cycloaddition/annulation strategies for the construction of multisubstituted pyrrolidines and their applications in natural product synthesis

Pyrrolidines are privileged substructures of numerous bioactive natural products and drugs.

Natural Products and Their Mimics as Targets of Opportunity for Discovery

Diverse structural types of natural products and their mimics have served as targets of opportunity in our laboratory to inspire the discovery and development of new methods and strategies to assemble polyfunctional and polycyclic molecular architectures. Furthermore, our efforts toward identifying novel compounds having useful biological properties led to the creation of new targets, many of which posed synthetic challenges that required the invention of new methodology. In this Perspective, selected examples of how we have exploited a diverse range of natural products and their mimics to create, explore, and solve a variety of problems in chemistry and biology will be discussed. The journey was not without its twists and turns, but the unexpected often led to new revelations and insights. Indeed, in our recent excursion into applications of synthetic organic chemistry to neuroscience, avoiding the more-traveled paths was richly rewarding.

Synthesis of the Common Core Structure of the Stemofoline Alkaloids

A novel synthetic route to the common core structural motif of the stemofoline alkaloids has been developed. The key transformations include (1) an intramolecular 1,3-dipolar cycloaddition reaction of a highly functionalized nitrone, (2) the subsequent formation of a caged structure via lithiated allylic sulfoxide, and (3) the concomitant sila-Pummerer reaction of α-silylalkenyl sulfoxide to prepare a thioester precursor. A series of stereochemistries on the highly caged core structure characteristic of the stemofoline alkaloids was successfully assembled.

Diastereoselective addition of monoorganocuprates to a chiral fumarate: reaction development and synthesis of (-)-dihydroprotolichesterinic acid

Recent studies of diastereoselective conjugate additions of monoorganocuprates, Li[RCuI], to chiral γ-alkoxycrotonates and fumarates are disclosed. This methodology was applied to the shortest total synthesis of (-)-dihydroprotolichesterinic acid to date, but several attempts to prepare other succinate-derived natural products, such as pilocarpine and antrodin E, were unsuccessful.

Asymmetric disulfonimide-catalyzed synthesis of δ-amino-β-ketoester derivatives by vinylogous Mukaiyama-Mannich reactions

An organocatalytic asymmetric synthesis of δ-amino-β-ketoester derivatives has been developed. A chiral disulfonimide (DSI) serves as a highly efficient precatalyst for a vinylogous Mukaiyama-Mannich reaction of readily available dioxinone-derived silyloxydienes with N-Boc-protected imines, delivering products in excellent yields and enantioselectivities. The synthetic utility of this reaction is illustrated in various transformations, including a new CC bond-forming reaction, which provide useful enantioenriched building blocks. The methodology is applied in a formal synthesis of (-)-lasubin.

Asymmetric Formal Total Synthesis of the Stemofoline Alkaloids: The Evolution, Development and Application of a Catalytic Dipolar Cycloaddition Cascade

A formal synthesis of didehydrostemofoline and isodidehydrostemofoline has been accomplished by preparing an intermediate in the Overman synthesis of these alkaloids from commercially available 2-deoxy-D-ribose. The work presented in this account chronicles the evolution of our explorations to identify the optimal steric and electronic control elements necessary to generate the tricyclic core structure of these alkaloids in a single operation from an acyclic precursor. The key step in the synthesis is a novel dipolar cycloaddition cascade sequence that is initiated by cyclization of a rhodium-derived carbene onto the nitrogen atom of a proximal imine group to generate an azomethine ylide that then undergoes spontaneous cyclization via dipolar cycloaddition. The synthesis features several other interesting reactions, including a Boord elimination to prepare a chiral allylic alcohol, a highly diastereoselective Hirama-Itô cyclization, and a useful modification of the Barton decarboxylation protocol.

Total synthesis of (-)-dihydroprotolichesterenic acid via diastereoselective conjugate addition to chiral fumarates

A diastereoselective conjugate addition of a variety of monoorganocuprates, Li[RCuI], to chiral fumarates to provide funtionalized succinates has been developed. The utility of this reaction is demonstrated in a concise total synthesis of (-)-dihydroprotolichesterenic acid that required only four steps and proceeded in an overall 31% yield.

Enantioselective formal total syntheses of didehydrostemofoline and isodidehydrostemofoline through a catalytic dipolar cycloaddition cascade

Sweet to the core: Enantioselective formal total syntheses of the title compounds were accomplished in 24 steps from 2-deoxy-D-ribose. The synthesis features a novel cascade of reactions culminating in an intramolecular dipolar cycloaddition to form the tricyclic core of the stemofoline alkaloids from an acyclic diazo imine intermediate.