Diastereoselective Cyclization Research Articles

Stereoselective Approach to the Core Structure of (+)-Phainanoid A via Strategically Engineered Cascade Polyene Cyclization.

Stereoselective synthesis of 3b and its cascade polyene cyclization to 18b have been described. Acyclic polyene 3b was prepared from allyl bromide 4 and 1,3-dithiane 5, and intermediates 4 and 5 were synthesized from the commercially available geraniol (6) and cyclopenten-2-one (8), respectively, using enantioselective reduction of ketone, Johnson-Claisen rearrangement, and the Suzuki reaction as key steps. Au(I)-mediated diastereoselective polyene cyclization of 3b efficiently afforded tetracyclic compound 18b.

Stereoselective Synthesis of Baloxavir Marboxil Using Diastereoselective Cyclization and Photoredox Decarboxylation of l-Serine.

Baloxavir marboxil (1; BXM) is a potent drug used for treating influenza infections. The current synthetic route to BXM (1) is based on optical resolution; however, this method results in the loss of nearly 50% of the material. This study aimed to describe an efficient and simpler method for the synthesis of BXM. We achieved a stereoselective synthesis of BXM (1). The tricyclic triazinanone core possessing a chiral center was prepared via diastereoselective cyclization utilizing the readily available amino acid l-serine. The carboxyl moiety derived from l-serine was removed via photoredox decarboxylation under mild conditions to furnish the chiral tricyclic triazinanone core ((R)-14). The synthetic route demonstrated herein provides an efficient and atomically economical method for preparing this potent anti-influenza agent.

Study on Pyridine-Boryl Radical-Promoted, Ketyl Radical-Mediated Carbon-Carbon Bond-Forming Reactions.

Ketyl radicals are synthetically versatile reactive species, but their applications have been hampered by harsh generation conditions employing highly reducing metals. Recently, the pyridine-boryl radical received wide attention as a promising organic reductant because of its mildness as well as convenience in handling. While probing the utility of the pyridine-boryl radical, our group observed facile pinacol coupling reactivity that had not been known at that time. This serendipitous finding was successfully rendered into a practical synthesis of tetraaryl-1,2-diols in up to 99% yield within 1 h. Subsequently, upon examinations of various reaction manifolds, a diastereoselective ketyl-olefin cyclization was accomplished to produce cycloalkanols such as trans-2-alkyl-1-indanols. Compared to the previous methods, the stereocontrolling ability was considerably enhanced by taking advantage of the structurally modifiable boryl group that would be present near the bond-forming site. In this full account, our synthetic efforts with the O-boryl ketyl radicals are disclosed in detail, covering the discovery, optimization, scope expansion, and mechanistic analysis, including density functional theory (DFT) calculations.

Concise Total Synthesis of (-)-Bipolarolide D.

Here we report the first and concise total synthesis of a complex ophiobolin-derived sesterterpene, bipolarolide D, which hinges on two strategic applications of pentafulvene: (1) enantioselective pentafulvene-involved [6+2] cycloaddition; (2) regioselective and diastereoselective pentafulvene-involved Heck cyclization. Late-stage selective allylic addition to the ketone moiety facilitates the successful installation of the side chain. This strategy enabled the accomplishment of its first enantioselective total synthesis through a modular approach. This synthesis will facilitate the investigation of relevant biological activities and provide a synthetic blueprint for utilizing fulvenes as versatile synthons in other complex natural product synthesis.

Nickel-catalyzed switchable arylative/endo-cyclization of 1,6-enynes

Carbo- and heterocycles are frequently used as crucial scaffolds in natural products, fine chemicals, and biologically and pharmaceutically active compounds. Transition-metal-catalyzed cyclization of 1,6-enynes has emerged as a powerful strategy for constructing functionalized carbo- and heterocycles. Despite significant progress, the regioselectivity of alkyne functionalization is entirely substrate-dependent. And only exo-cyclization/cross-coupling products can be obtained, while endo-selective cyclization/cross-coupling remains elusive and still poses a formidable challenge. In this study, we disclose a nickel-catalyzed switchable arylation/cyclization of 1,6-enynes in which the nature of the ligand dictates the regioselectivity of alkyne arylation, while the electrophilic trapping reagents determine the selectivity of the cyclization mode. Specifically, using a commercially available 1,10-phenanthroline as a ligand facilitates trans-arylation/cyclization to obtain seven-membered ring products, while a 2-naphthyl-substituted bisbox ligand promotes cis-arylation/cyclization to access six-membered ring products. Diastereoselective cyclizations have also been developed for the synthesis of enantioenriched piperidines and azepanes, which are core structural elements of pharmaceuticals and natural products possessing important biological activities. Furthermore, experimental and density functional theory studies reveal that the regioselectivity of the alkyne arylation process is entirely controlled by the steric hindrance of the ligand; the reaction mechanism involves exo-cyclization followed by Dowd-Beckwith-type ring expansion to form endo-cyclization products.

Total Synthesis of Dragocins A-C through Electrochemical Cyclization.

The first total synthesis of dragocins A-C, remarkable natural products containing an unusual C4' oxidized ribose architecture bridged by a polyhydroxylated pyrrolidine, is presented through a route featuring a number of uncommon maneuvers. Several generations towards the target molecules are presented, including the spectacular failure of a key C-H oxidation on a late-stage intermediate. The final route features rapid, stereocontrolled access to a densely functionalized pyrrolidine and an unprecedented diastereoselective oxidative electrochemical cyclization to forge the hallmark 9-membered ring. Preliminary studies suggest this electrochemical oxidation protocol is generally useful.

Total Synthesis of Dragocins A−C through Electrochemical Cyclization

AbstractThe first total synthesis of dragocins A−C, remarkable natural products containing an unusual C4’ oxidized ribose architecture bridged by a polyhydroxylated pyrrolidine, is presented through a route featuring a number of uncommon maneuvers. Several generations towards the target molecules are presented, including the spectacular failure of a key C−H oxidation on a late‐stage intermediate. The final route features rapid, stereocontrolled access to a densely functionalized pyrrolidine and an unprecedented diastereoselective oxidative electrochemical cyclization to forge the hallmark 9‐membered ring. Preliminary studies suggest this electrochemical oxidation protocol is generally useful.

Catalyst-free light-mediated polyene cyclization

Polyene cyclization reactions are highly efficient and unique biomimetic transformations that yield complex polycyclic molecules from their acyclic precursors. Various cation-mediated and radical-mediated synthetic methods have been developed to obtain highly diastereoselective cyclizations, along with the use of visible-light in the recent years. In this paper, we report a visible-light-mediated, catalyst-free cascade cyclization of geranyl phenols containing bromo- and chloro-groups at different positions of the phenolic ring. The desired cyclization product was obtained with high stereoselectivity using a blue LED light in hexafluoro-2-propanol without any photocatalyst. The presence of a halogen, and particularly chlorine or bromine is essential for the success of this cyclization, as is the use of blue instead of green LED light. Although the exact reason for this success is unclear, some possible explanations are discussed and further control studies of photoreactions should be a part of any efforts in this area.

Forging structural complexity: diastereoselective synthesis of densely substituted β-lactams with dual functional handles for enhanced core modifications.

The preparation of the β-lactam motif containing both C-Br and N-O bonds as functional handles remains an unmet synthetic challenge. Described herein is a novel and highly diastereoselective NBS-mediated cyclization of N-alkoxy α,β-unsaturated silyl imino ethers to furnish nearly three dozen α-bromo N-alkoxy β-lactams. The reaction gives rapid and convenient access to structurally diverse monocyclic, spirocyclic and fused β-lactams in moderate to good yields. The two functional handles were shown to be useful for the further elaboration of the β-lactam core.

Organocatalyzed diastereoselective cyclization of β-alkyl nitroolefins with alkylidene malononitriles: new approach to azetidine nitrones and isoxazoles

An organocatalytic domino reaction of β-alkyl nitroolefins with alkylidene malononitriles to give azetidine nitrones with good diastereoselectivities is reported. The in situ-generated nitrones were transmuted into isoxazoles catalyzed by acid.

Chiral Auxiliary Approach for Gold(I)‐Catalyzed Cyclizations

AbstractTwo different classes of stereoselective cyclizations have been developed using a chiral auxiliary approach with commercially available [JohnPhosAu(MeCN)SbF6] as catalyst. First, a stereoselective cascade cyclization of 1,5‐enynes was achieved using the Oppolzer camphorsultam as chiral auxiliary. In this case, a one‐pot cyclization‐hydrolysis sequence was developed to directly afford enantioenriched spirocyclic ketones. Then, the stereoselective alkoxycyclization of 1,6‐enynes was mediated by an Evans‐type oxazolidinone. A reduction‐hydrolysis sequence was selected to remove the auxiliary to give enantioenriched β‐tetralones. DFT studies confirmed that the steric clash between the chiral auxiliary and alkene accounts for the experimentally observed diastereoselective cyclization through the Si face.

Chiral Auxiliary Approach for Gold(I)-Catalyzed Cyclizations.

Two different classes of stereoselective cyclizations have been developed using a chiral auxiliary approach with commercially available [JohnPhosAu(MeCN)SbF6 ] as catalyst. First, a stereoselective cascade cyclization of 1,5-enynes was achieved using the Oppolzer camphorsultam as chiral auxiliary. In this case, a one-pot cyclization-hydrolysis sequence was developed to directly afford enantioenriched spirocyclic ketones. Then, the stereoselective alkoxycyclization of 1,6-enynes was mediated by an Evans-type oxazolidinone. A reduction-hydrolysis sequence was selected to remove the auxiliary to give enantioenriched β-tetralones. DFT studies confirmed that the steric clash between the chiral auxiliary and alkene accounts for the experimentally observed diastereoselective cyclization through the Si face.

A Rapid Bioinspired N‐Acyliminium Ion Strategy for the ABC Core of the Stemona Alkaloids

AbstractA concise and highly diastereoselective bioinspired key cationic cyclization strategy for the asymmetric synthesis of the tricyclic core of the (−)‐stemoamide, together with 8,9‐bis‐epi‐stemoamide has been described. The key N‐acyliminium ion precursors were accessed from L‐tartaric acid and L‐malic acid respectively. The use of ethyl acetoacetate derived bifunctional allylidenetriphenylphosphorane reagent in the early stage of the synthetic strategy is advantageous for the rapid construction of highly functionalized key pyrrolo[1,2‐α]azepine frameworks.

Chromium-Catalyzed Diastereoselective Synthesis of Conformationally Constrained Spirotetrahydroquinolines.

We report the diastereoselective cyclization of anilines with cyclobutanones and congeners by chromium catalysis. This reaction can link two strained four-membered rings with tetrahydroquinolines by forming four bonds in a diastereocontrolled manner, forming medicinally interesting cyclobutane-fused and constrained spirotetrahydroquinolines (STHQs) and complex multiple spiro carbon-containing polyazacycles. The constrained STHQs have been used as versatile feedstocks to derive a range of oxygen-, nitrogen-, and thio-substituted spiro analogues, and dioxygen-incorporated spiroazacycles.

Photocatalytic deconstructive aliphatic carbon–carbon bond cleavage and functionalization of unactivated alcohols

Carbon–carbon (C–C) bonds are challenging to selectively cleave owing to their chemical inertness and omnipresence in organic molecules; however, they can be targeted during the late-stage modification of natural products and active pharmaceutical ingredients. Herein, we report a selective deconstructive C(sp3)–C(sp3) bond cleavage and difunctionalization of unactivated alcohols mediated by a vanadium visible-light photocatalyst under ambient temperatures and pressures. Our base metal photocatalyst operates via ligand-to-metal charge transfer followed by an inner-sphere C–C bond cleavage to generate alkyl radicals that are trapped with SOMOphiles. We achieved 10 different bond formations on 44 examples using a range of commercially sourced alcohols, including natural products and biomolecules. Our protocol can also be used for the regio- and diastereoselective cyclization to form an α-aminolactone, and for the late-stage functionalization of bioactive oligopeptides to access unnatural amino acid residues.

Scalable total synthesis of aflaquinolone I and confirmation of the absolute configuration

The first total synthesis of aflaquinolone I (1), which represents the first example of natural occurring 3,4-dioxygenated 4-aryl-quinolin-2(1H)-one alkaloid with 3R, 4R configurations, was achieved in 9 and 4 steps by two parallel strategies, respectively. The key steps of the first approach included a base-catalyzed coupling reaction to forge α-hydroxyanilide and a diastereoselective aldol cyclization to construct the 3,4-dioxygenated-4-aryl-quinolin-2(1H)-one core. The second approach included a regioselective insertion of aryne into unsymmetric imide to build the N-glycolated 2-amino-benzophenone as a key step. In addition, the dehydrated product of aflaquinolone I, viridicatol (2), was successfully synthesized. We also achieved the chiral resolution of (±)-aflaquinolone I (1) and (±)-20, using chiral chromatography, and determined the absolute configuration of the enantiomers by electronic circular dichroism (ECD). The absolute configuration of aflaquinolone I was further confirmed. Interestingly, we found that the ECD spectra of aflaquinolone I and its related analogues were affected by the substituted groups on C-4 phenyl group and C-3 hydroxy group.

Expedient Synthesis of the Proposed Structure of Cryptoconcatone H Exploiting Hidden Symmetry

AbstractSynthesis of the originally assigned structure of the styryl tetrahydropyranol-dihydropyranone natural product cryptoconcatone H from C 2-symmetric (±)-1,8-nonadiene-4,6-diol is reported. Desymmetrization by Mitsunobu reaction with crotonic acid established the requisite inter-ring stereochemical relationship and was followed by a highly diastereoselective Re2O7-catalyzed Prins cyclization with cinnamaldehyde to construct the 2,4,6-cis-tetrahydropyranol ring. Ring-closing metathesis resulted in formation of the dihydropyranone ring and completed the synthesis in three steps and 32% overall yield. The brevity of the synthesis is the result of the recognition of hidden, inter-ring symmetry in the target and the ensuing choice of an appropriately symmetric diol as our starting material.

Total Syntheses of (+)-Ineleganolide and (-)-Sinulochmodin C.

Described herein are the first total syntheses of the nor-furanocembranoid natural products (+)-ineleganolide (1) and (-)-sinulochmodin C (2). The synthetic strategy is predicated on a transannular Michael reaction that provides both natural products from a common macrocyclic intermediate and leverages a diastereoselective radical cyclization to furnish a key bicyclic lactone. The latter is further advanced to a macrocyclic precursor via a Nozaki-Hiyama-Kishi cyclization and a one-pot furan oxidation/oxa-Michael cascade. Unexpected stereochemical nuances that guided the evolution and eventual completion of the total synthesis are discussed.

Multigram-scale total synthesis of (±)-ambreinolide by a diastereoselective polyene cyclization

Ambreinolide is a natural terpenoid with great value for perfume industry and natural product synthesis. Herein we report a novel total synthesis of ambreinolide on multigram-scale that employs a regio- and diastereoselective, high yielding, proton-initiated polyene cyclization using a catalyst easily generated in situ. Molecular structures were unambiguously confirmed by X-ray crystallography.

One-Pot Double Intramolecular Cyclization Approach to Tetralin-Based Cis-Fused Tetracyclic Compounds.

Presented herein is a BF3·OEt2-mediated, diastereoselective one-pot double cyclization of 4-aryl-2-[(arylthio)methyl]butanals leading to the formation of cis-tetrahydro-6H-naphtho[2,1-c]thiochromenes for the first time. Mechanistically, the formation of the title products involves the one-pot intramolecular Friedel-Crafts hydroxyalkylation/intramolecular Friedel-Crafts alkylation cascade. This synthetic methodology is featured by its high atom economy, broad substrate scope, mild transition-metal-free reaction conditions, capability to assemble two new rings in one pot, and moderate to high yields (up to 94% yield). It was then applied in the synthesis of a thia analogue of brazilane and a chromeno[3,4-c]chromene derivative. Moreover, the methodology was successfully extended to the synthesis of cis-hexahydrobenzo[c]phenanthrenes. Specifically, 1,5-diarylpentan-3-ones were first subjected to the Corey-Chaykovsky reaction, and the resulting epoxides, without being chromatographically isolated, were treated with BF3·OEt2 to afford the cyclized products in high yields (up to 84% yield over two steps).