Ring Expansion Cascade Research Articles

Cascade ring expansion reactions for the synthesis of medium-sized rings and macrocycles.

This Feature Article discusses recent advances in the development of cascade ring expansion reactions for the synthesis of medium-sized rings and macrocycles. Cascade ring expansion reactions have much potential for use in the synthesis of biologically important medium-sized rings and macrocycles, most notably as they don't require high dilution conditions, which are commonly used in established end-to-end macrocyclisation methods. Operation by cascade ring expansion method can allow large ring products to be accessed via rearrangements that proceed exclusively by normal-sized ring cyclisation steps. Ensuring that there is adequate thermodynamic driving force for ring expansion is a key challenge when designing such methods, especially for the expansion of normal-sized rings into medium-sized rings. This Article is predominantly focused on methods developed in our own laboratory, with selected works by other groups also discussed. Thermodynamic considerations, mechanism, reaction design, route planning and future perspective for this field are all covered.

Single Step Upgradation of Isatin to Bioactive Fused Heterocycles via Ring Expansion Reactions

AbstractIsatin, also known as indoline‐2,3‐dione, is a fused, N‐heterocyclic aromatic compound first isolated and purified in 1841. Since then, isatin and its derivatives emerged as a hot research topic for the scientific community. The number of research publications on isatin justifies their popularity. Researchers explored isatins and their derivatives as potential candidates in multidisciplinary research areas, including medicinal chemistry, synthetic organic chemistry, and polymer science. Synthetic chemists’ perspective of tailoring isatin and its family members led to more efficient synthesis strategies in which ring‐opening reactions of isatins were particularly significant due to the product's importance. Our investigation aims to group the different types of cascade ring expansion reactions of isatin to synthesise fused heterocyclic compounds such as derivatives of quinolines, quinazolinones, acridines, acridones, and few spiro heterocyclic compounds, all of which highlight an inevitable role in the field of medicinal chemistry.

Divergent Cascade Ring-Expansion Reactions of Acryloyl Imides.

Macrocyclic and medium-sized ring ketones, lactones and lactams can all be made from common acryloyl imide starting materials through divergent, one-pot cascade ring-expansion reactions. Following either conjugate addition with an amine or nitromethane, or osmium(VIII)-catalysed dihydoxylation, rearrangement through a four-atom ring expansion takes place spontaneously to form the ring expanded products. A second ring expansion can also be performed following a second iteration of imide formation and alkene functionalisation/ring expansion. In the dihydroxylation series, three- or four-atom ring expansion can be performed selectively, depending on whether the reaction is under kinetic or thermodynamic control.

A Lewis Acid-Promoted Michael Addition and Ring-Expansion Cascade for the Construction of Nitrogen-Containing Medium-Sized Rings

A Lewis acid-promoted annulation of azadienes and cyclobutamines was developed. This reaction proceeded through Michael addition and ring-expansion cascade, affording the corresponding nitrogen-containing medium-sized rings with a broad scope in moderate to high yields. The catalytic asymmetric version of this reaction has also been explored using a chiral base.

An efficient approach to angular tricyclic molecular architecture via Nazarov-like cyclization and double ring-expansion cascade

A modular and efficient method for constructing angular tri-carbocyclic architectures containing quaternary carbon center(s) from 1,3-dicycloalkylidenyl ketones is established, which involves an unconventional synergistic cascade of a Nazarov cyclization and two ring expansions. It features high selectivity, mild conditions and convenient operation, wide scope and easy availability of substrate. Substitution with R1 and R2 at the 4πe-system with electron-donating group favors this reaction, while that with electron-withdrawing group or proton disfavors. The electron-donating group as R1 directs the initial ring expansion at its own site, while the p-π- or n-π- associated substituent as R2 favors selectively the later ring expansion near its location because of the beneficial maintenance of an original conjugated system. The stereoselectivity has proved to be governed by either the steric effect of R3 and R4 at the expanded rings, or the migration ability of the migrating atom. Density Functional Theory calculation suggests the initial Nazarov cyclization would be the rate-determining step. A racemic total synthesis of the natural (±)-waihoensene is realized in 18 steps by use of this methodology.

A Thiol-Mediated Three-Step Ring Expansion Cascade for the Conversion of Indoles into Functionalized Quinolines.

An operationally simple, high yielding three-step cascade process is described for the direct conversion of indole-tethered ynones into functionalized quinolines. A single “multitasking” thiol reagent is used to promote a three-step dearomatizing spirocyclization, nucleophilic substitution, and one-atom ring expansion reaction cascade under remarkably mild conditions. In addition, a novel route to thio-oxindoles is described, which was discovered by serendipity.

Allenes in Manganese(I)-Catalyzed C–C Activation and a Strategy for Cascade Ring Expansion

Summary Transition metal-catalyzed C–C bond functionalization has recently been identified as an increasingly versatile tool for organic syntheses, as otherwise unreactive functionalities can be used as efficient handles for synthetic manipulations. This work describes site-selective manganese(I)-catalyzed C–C hydroarylations of heteroarenes with substituted allenes. The C–C activations are efficiently catalyzed by MnBn(CO)5 and afford 1,2,3-tri-substituted arenes with notable diastereoselectivity. Moreover, the ring-expansion reaction proceeds via a C–C allylation/Michael addition cascade, providing challenging 8-membered carbocycles, thus highlighting the unique characteristics of the MnBn(CO)5 complex. Mechanistic studies reveal that the organometallic C–C activation occurs faster than typical C–H activations, thus providing a complementary approach to well-established methodologies for late-stage diversification.

One-pot construction of functionalized aziridines and maleimides via a novel pseudo-Knoevenagel cascade reaction.

An Ugi, novel pseudo-Knoevenagel, ring expansion cascade reaction was discovered and utilized for the synthesis of aziridinyl succinimides in one-pot. Subsequently, densely functionalized aziridines and maleimides have been designed and synthesized through similar cascade reactions. The target compounds were prepared by means of a mild reaction and a simple operation procedure, which could be applicable to a broad scope of starting materials. This series of novel cascade reactions generates opportunities for the tailored synthesis of a wide range of biologically active scaffolds through tuneable Ugi inputs. Discovery of compound 8i with comparable potency to sorafenib in liver cancer cell lines could provide a new avenue for liver cancer drug discovery.

One-Pot Synthesis of Spirocyclopenta[a]indene Derivatives via a Cascade Ring Expansion and Intramolecular Friedel-Crafts-Type Cyclization.

A one-pot efficient synthetic approach for the rapid construction of spirocyclopenta[a]indene derivatives has been developed via an iodine-initiated cascade ring expansion and intramolecular Friedel-Crafts-type cyclization from propargyl alcohol-tethered alkylidenecyclobutanes under mild conditions with broad substrate scope. This cascade process can be elegantly conducted on a gram scale. A plausible reaction mechanism has been proposed on the basis of a series of deuterium labeling and control experiments.

Enantioselective Organocatalytic Four-Atom Ring Expansion of Cyclobutanones: Synthesis of Benzazocinones.

An enantioselective Michael addition- four-atom ring expansion cascade reaction involving cyclobutanones activated by a N-aryl secondary amide group and ortho-amino nitrostyrenes has been developed for the preparation of functionalized eight-membered benzolactams using bifunctional aminocatalysts. Taking advantage of the secondary amide activating group, the eight-membered cyclic products could be further rearranged into their six-membered isomers having a glutarimide core under base catalysis conditions without erosion of optical purity, featuring an overall ring expansion- ring contraction strategy.

Enantioselective Organocatalytic Four‐Atom Ring Expansion of Cyclobutanones: Synthesis of Benzazocinones

AbstractAn enantioselective Michael addition– four‐atom ring expansion cascade reaction involving cyclobutanones activated by a N‐aryl secondary amide group and ortho‐amino nitrostyrenes has been developed for the preparation of functionalized eight‐membered benzolactams using bifunctional aminocatalysts. Taking advantage of the secondary amide activating group, the eight‐membered cyclic products could be further rearranged into their six‐membered isomers having a glutarimide core under base catalysis conditions without erosion of optical purity, featuring an overall ring expansion– ring contraction strategy.

Copper-Catalyzed Ring-Expansion Cascade of Azirines with Alkynes: Synthesis of Multisubstituted Pyridines at Room Temperature.

The first intermolecular ring-expansion cascade of azirines with alkynes for the synthesis of pyridines, enabled by a copper/triethylamine catalytic system via simultaneous generation and utilization of yne-enamine and skipped-yne-imine intermediates, is reported. Experimental as well as computational mechanistic studies revealed that the role of triethylamine is crucial in deciding the reaction pathway toward the pyridine products. This process offers a novel, one-step, direct, and practical strategy for the rapid construction of highly substituted pyridines under exceedingly mild conditions, and an installed alkyne functionality.

ChemInform Abstract: An Arylative Ring Expansion Cascade of Fused Cyclobutenes via Short‐Lived Intermediates with Planar Chirality.

Abstract4π‐Electrocyclic ring opening of fused cyclobutanes gives cyclic cis‐trans diene intermediates.

A Biomimetic Synthetic Approach to the Frondosins

The frondosins are a family of five marine sponge-derived meroterpenoids. We propose that the 6–7 ring system common to each of the frondosins is biosynthesized via ring expansion of a 6–6 ring system. Compelling evidence in favour of this proposal was obtained in the form of a biomimetic synthesis of the frondosin 6–7 ring system, which features a highly stereo- and regio-selective ring expansion cascade reaction as the key step.

Harnessing the Intrinsic Reactivity within the Aplysinopsin Series for the Synthesis of Intricate Dimers: Natural from Start to Finish

Aplysinopsin monomers are considered as plausible biosynthetic precursors of the wider aplysinopsin family of marine alkaloids. The idea of harnessing their intrinsic reactivity to undertake the synthesis of dictazoles or cycloaplysinopsins logically emerged from this status. These biosynthetic considerations led us to the first total syntheses of dictazole B and other valuable cyclobutanes. When further exploiting pre-encoded reactivity, our first total synthesis of tubastrindole B originated from the ring-expansion cascade of its dictazole-type precursor. Moreover, the isolation of a transient biosynthetic intermediate combined with dimerization outcomes of a hydantoin-containing monomer allowed us to explain the formation of cycloaplysinopsins A and B. 1 Introduction 2 Easy Access to Dictazole Cyclobutanes 3 Synthesis of Cycloaplysinopsins by Ring Expansion 4 Conclusion and Future Prospects

An Arylative Ring Expansion Cascade of Fused Cyclobutenes via Short-Lived Intermediates with Planar Chirality.

An arylative ring expansion cascade has been developed for the synthesis of medium-sized carbocycles from fused cyclobutenes. This reaction proceeds through a short-lived cis,trans-cycloalkadiene intermediate that is formed by thermal 4π electrocyclic ring opening. Chirality transfer experiments provide direct evidence for the transient generation of the intermediate.

ChemInform Abstract: Diastereoselective Synthesis of Functionalized Pyrrolidines Through N‐Bromosuccinimide‐Induced Aziridine Ring Expansion Cascade of Cinnamylaziridine.

AbstractThe bromonium ion‐induced aziridine ring expansion cascade affords a mixture of functionalized pyrrolidines (III) and piperidines (IV), with the former being the major products.

ChemInform Abstract: N‐Bromosuccinimide‐Induced Aminocyclization—Aziridine Ring‐Expansion Cascade: An Asymmetric and Highly Stereoselective Approach Toward the Synthesis of Azepane.

A novel N-bromosuccinimide induced aminocyclization–aziridine ring expansion cascade is reported. Substituted azepanes were isolated exclusively in good yields. The azepane products could be transformed into a number of functional molecules including piperidines, a bicyclic amine, and a bridgehead amide.

A unified bioinspired "aplysinopsin cascade": total synthesis of (±)-tubastrindole B and related biosynthetic congeners.

Applying a biomimetic approach, the first total synthesis of (±)-tubastrindole B is reported herein. This work features a ring-expansion cascade of a dictazole-type precursor into cycloaplysinopsin-type congeners. Moreover, the isolation of a transient biogenetic intermediate represents a milestone in the biosynthetic understanding of this family of marine alkaloids.

Diastereoselective synthesis of functionalized pyrrolidines through N-bromosuccinimide-induced aziridine ring expansion cascade of cinnamylaziridine.

An efficient aziridine ring expansion cascade of cinnamylaziridine has been developed. N-Bromosuccinimide was used as the promoter. The resulting functionalized pyrrolidines are the fundamental units of many useful molecules.