Model Studies Directed toward the Alkaloid Mersicarpine Utilizing a Rh(II)-Catalyzed Insertion/Cycloaddition Sequence

Cooperative Rh/Achiral Phosphoric Acid-Enabled [3+3] Cycloannulation of Carbonyl Ylides with Quinone Monoimines: Synthesis of Benzofused Dioxabicyclo[3.2.1]octane Scaffolds

A cooperative Rh/achiral phosphoric acid-enabled [3+3] cycloaddition of in situ-generated carbonyl ylides with quinone monoimines has been developed. With the ability to build up the molecular complexity rapidly and efficiently, this method furnishes highly functionalized oxa-bridged benzofused dioxabicyclo[3.2.1]octane scaffolds bearing two quaternary centers in good to excellent yields under mild conditions. Moreover, the utility of the current method was demonstrated by gram-scale synthesis and elaboration of the products into various functionalized oxa-bridged heterocycles.

Substrate-directed divergent synthesis of fused indole polycycles through Rh(II)-catalyzed cascade reactions of bis(diazo)indolin-2-ones

Herein, we report a substrate-directed diverse synthetic strategy toward two kinds of structurally intriguing fused indole polycycles through Rh(II)-catalyzed cascade reactions of bis(diazo)indolin-2-ones with enaminones. The subtle structural changes in enaminones lead to different reactivities. Cyclic enaminones produce indolo[2,3-b]indoles, whereas acyclic ones afford oxazolo[3,2-a]indoles.

Total synthesis of (±)-mersicarpine following a 6-exo-trig radical cyclization

Described is a total synthesis of racemic mersicarpine from diethyl 4-oxopimelate. The synthetic route takes advantage of a 2-indolyl radical cyclization to construct the pyrido[1,2-a]indole scaffold bearing the all-carbon quaternary stereocenter.

Construction of Oxo-Bridged Diazocines via Rhodium-Catalyzed (4+3) Cycloaddition of Carbonyl Ylides with Azoalkenes

Developing efficient strategies for synthesizing novel diazocine compounds is valuable because their use has been limited by their synthetic accessibility. This work describes the catalytic (4+3) cycloaddition reaction of carbonyl ylides with azoalkenes generated in situ. The rhodium-catalyzed cascade reaction features good atom and step economy, providing the first access to oxo-bridged diazocines. The product could be synthesized on a gram scale and converted into diversely substituted dihydroisobenzofurans.

Cooperative Catalysis for the Highly Diastereo- and Enantioselective [4+3]-Cycloannulation of ortho-Quinone Methides and Carbonyl Ylides

We describe herein a highly diastereo- and enantioselective [4+3]-cycloannulation of ortho-quinone methides and carbonyl ylides to furnish functionalized oxa-bridged dibenzooxacines with excellent yields and stereoselectivity in a single synthetic step. The combination of rhodium and chiral phosphoric acid catalysis working in concert to generate both transient intermediates in situ provides direct access to complex bicyclic products with two quaternary and one tertiary stereogenic centers. The products may be further functionalized into valuable and enantiomerically highly enriched building blocks.

Insights into C–O insertion in a carbene/alkyne metathesis cascade reaction catalyzed by Rh2(OAc)4: a DFT study

The mechanism and selectivities of C–O insertion in the carbene/alkyne metathesis of diazo compounds catalyzed by Rh₂(OAc)₄ are studied with DFT calculations.

Unified enantioselective total syntheses of (−)-scholarisine G, (+)-melodinine E, (−)-leuconoxine and (−)-mersicarpine

A unified strategy enabled the enantioselective syntheses of (−)-scholarisine G, (+)-melodinine E, (−)-leuconoxine and (−)-mersicarpine from a common 2-alkylated indole bearing an all-carbon quaternary stereogenic center.

Interception of Secondary Amide Ylide with Sulfonamides: Catalyst-Controlled Synthesis of N-Sulfonylamidine Derivatives

A novel, secondary amide activation strategy has been developed through the in situ generation of ylides from amides and diazoacetates. Under the developed reaction conditions, Mn-catalyzed ylide formation and interception reaction by sulfonamide delivered a variety of N-sulfonylamidines. Notably, when highly active Zn(OTf)₂ was used as the catalyst, further N-H insertion products were obtained. In contrast with traditional methods, our amide activation strategy is distinguished by accessible starting material, inexpensive catalyst, and broad substrate scope.

Polycyclic Ring Formation Using Bis-diazolactams for Cascade Stitching

The chemoselective reaction of donor/acceptor (D/A) and acceptor/acceptor (A/A) diazo moieties in the same molecule was examined using 3-diazo-1-(ethyl 2-diazomalonyl)indolin-2-one under rhodium(II) catalysis. The metallo carbenoid derived from the D/A diazo group is preferentially formed and undergoes selective CH, NH, and OH insertion reactions, cyclopropanation, cyclopropenation, sulfur ylide formation/2,3-sigmatropic rearrangement, as well as nitrogen ylide formation followed by azetidine ring expansion. The initial reaction can be paired with a subsequent tandem cascade sequence involving dipole formation/cycloaddition in either an intra- or intermolecular sense to generate polycyclic N-heterocycles in one pot, with the formation up to three new rings in a single operation. Excellent diastereoselectivity was observed in the intramolecular cycloaddition reaction producing 5 to 7-membered rings.

Rhodium-catalyzed pyridannulation of indoles with diazoenals: a direct approach to pyrido[1,2-a]indoles

A new rhodium-catalyzed [4 + 2]-pyridannulation of 3-substituted indoles with diazoenals resulted in the biologically important pyrido[1,2-a]indoles.

General Approach to Nitrogen-Bridged Bicyclic Frameworks by Rh-Catalyzed Formal Carbenoid Insertion into an Amide C-N Bond

Various nitrogen-bridged bicyclic skeletons are found in bioactive natural products and pharmaceuticals. The development of a new reaction to construct these molecular frameworks has attracted considerable attention in synthetic organic chemistry. We developed a novel synthetic method for obtaining a wide variety of nitrogen-bridged bicyclic compounds with a catalytic process, Rh-catalyzed formal carbenoid insertion into an amide C-N bond. Using 0.1-0.4 mol % Rh2(NHCO(t)Bu)4 catalyst, various azabicyclo[X.Y.Z]alkane derivatives were obtained in good to excellent yield, successfully demonstrating the broad substrate scope of the developed process. Experimental and computational studies to elucidate the reaction mechanism revealed that the formal insertion reaction of a carbenoid into an amide C-N bond proceeded via the formation of Rh-associated N-ylides, followed by an acyl group-selective Stevens [1,2]-shift through a concerted addition/elimination process on the sp(2)-hybridized carbon.

Total Syntheses of (-)-Mersicarpine, (-)-Scholarisine G, (+)-Melodinine E, (-)-Leuconoxine, (-)-Leuconolam, (-)-Leuconodine A, (+)-Leuconodine F, and (-)-Leuconodine C: Self-Induced Diastereomeric Anisochronism (SIDA) Phenomenon for Scholarisine G and Leuconodines A and C

Enantioselective total syntheses of title natural products from a common cyclohexenone derivative (S)-18 were reported. Ozonolysis of (S)-18 afforded a stable diketo ester (R)-17 that was subsequently converted to two skeletally different natural products, i.e., (-)-mersicarpine (8) with a [6.5.6.7] fused tetracyclic ring system and (-)-scholarisine G (9) with a [6.5.6.6.5] fused pentacyclic skeleton, respectively. The postcyclization diversification was realized by taking advantage of the facile conversion of (+)-melodinine E (6) to N-acyliminium ion 7, from which a hydroxy group was selectively introduced to the C6, C7, C10 and the central C21 position of diazafenestrane system, leading to (-)-leuconodine A (11), (+)-leuconodine F (12), (-)-scholarisine G (9), (-)-leuconodine C (13), and skeletally different (-)-leuconolam (5). Furthermore, an unprecedented non-natural oxabridged oxadiazafenestrane 68 was formed by oxidation of (+)-melodinine E (6). During the course of this study, a strong self-induced diastereomeric anisochronism (SIDA) phenomenon was observed for scholarisine G (9), leuconodines A (11) and C (13). X-ray structures of both the racemic and the enantiopure natural products 9, 11, and 13 were obtained. The different crystal packing of these two forms nicely explained the chemical shift differences observed in the (1)H NMR spectra of the racemic and the enantio-enriched compounds in an achiral environment.