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Indoloquinazolines functionalized at C-12, which are structural analogs of the natural alkaloid cephalanthrin B, are readily constructed via Ph3P/I2-mediated one-pot reactions of isatins with aromatic alcohols. In the presence of excess phenols, the C-12 aryloxy ester products are obtained in moderate to good yields under mild conditions. Moreover, fused bicyclic hydroxyaryl derivatives such as 8-hydroxyquinoline give rise to novel C-12 spiro-γ-lactone derivatives. A reactive iminium cation species derived from dehydration of the C-12 hydroxy ester precursor is proposed as the transient intermediate responsible for these transformations.

The development of chiral borane Lewis acid catalysts opened the door for transition-metal-free catalyzed asymmetric organic reactions. Herein, we have summarized our work on the preparation of two classes of novel chiral bicyclic bisborane Lewis acid catalysts derived from C 2-symmetric [3.3.0] dienes and [4.4] dienes, respectively. These catalysts not only form frustrated Lewis pairs with Lewis bases to catalyze asymmetric hydrogenation reactions but also activate Lewis basic functional groups in traditional Lewis acid catalyzed asymmetric reactions.1 Introduction2 Synthesis of C 2-Symmetric Fused Bicyclic Bisborane Catalysts and Their Use in Imine Hydrogenation3 Synthesis of Spiro Bicyclic Bisborane Catalysts and Their Use in ­N-Heteroarene Reduction4 Other Types of Asymmetric Reactions Promoted by Chiral ­Bicyclic Bisborane Catalysts5 Conclusion

Daphniphyllum alkaloids have complex molecular structures; consequently, their synthesis can be challenging. A new method for the construction of the [7-5-5] tricyclic core of Daphniphyllum alkaloids was developed. The bicyclo[5.3.0]decane skeleton was constructed through a divinylcyclopropane rearrangement of a cyclopentenone derivative with a vinylcyclopropyl group at the β-position. After introduction of a 2-iodoethyl group by a regioselective Michael addition with phenyl vinyl selenone, the [7-5-5] tricyclic system was formed by intramolecular alkylation of a cyclopentadienyl anion species.

Bicyclic lactams can be prepared from diethyl aminomalonate and substituted benzaldehydes by formation of a dimerised imidazolidine cycloadduct followed by a Dieckmann ring closure. The resulting N,N-heterocycles are metal-chelating but show no antibacterial activity.

Bicyclo[3.1.0]hex-1-ene is a highly strained bicyclic intermediate that generates trimethylenemethane (TMM) diradical through breaking the C–C bond of its methylene cyclopropane moiety. The reactivity of bicyclo[3.1.0]hex-1-enes and trimethylenemethane (TMM) diradicals depend on the reaction temperature and substitution patterns. This short review covers various strain-induced transformations of bicyclo[3.1.0]hex-1-enes and their formal [3+2] cycloadditions through TMM diradicals and presents synthetic applications to natural products containing triquinane and tropone structures.1 Introduction2 Early Reports on Bicyclo[3.1.0]hex-1-enes3 Approaches to Form Bicyclo[3.1.0]hex-1-enes4 Structure and Reactivity of Bicyclo[3.1.0]hex-1-enes4.1 Isomerization4.2 Dimerization4.3 [3+2] Cycloaddition4.4 [4+2] Cycloaddition5 Synthetic Applications to Natural Products6 Summary and Outlook

β-Seleno-α-amino acids, known as selenocysteine (Sec) derivatives, have emerged as important targets because of their role in chemical biology, not only as part of selenoproteins with important redox properties, but also because of their activity as antivirals or metabolites effective in inhibiting carcinogenesis. In addition, there is demand for this type of compounds due to their use in native chemical ligation to construct large peptides. Therefore, this review summarizes the various synthetic methods that have been published to construct Sec derivatives. Most of them involve the generation of the C–Se bond by nucleophilic substitution reactions, but other reactions such as radical or multicomponent strategies are also reported. Of particular importance is the Se-Michael addition of Se-nucleophiles to chiral bicyclic dehydroalanines, in which the stereogenic center is generated under complete stereocontrol.1 Introduction2 Previously Reviewed Synthesis of Sec3 Retrosynthesis of Sec Derivatives4 Sec Derivatives by Nucleophilic Substitutions5 Sec Derivatives by Radical Processes6 Sec Derivatives by 1,4-Conjugate Additions7 Conclusion

The synthesis of original structures for the fragrance industry bearing bicyclic scaffolds is described. To the best of our knowledge, these structures are not found in the fragrance industry, neither from natural nor synthetic pathways. NHC–gold-type catalysts showed excellent activities leading to light bicyclic enol ethers. Several bicyclic adducts were prepared in good to excellent yields (18–99%). Evaluation of NHC–Au complexes allowed to reach a TOF of 300 h–1. The evaluation for the organoleptic properties of [4.1.0]-bicyclic ethers were compared with the unprecedented properties of the 1,6-enyne precursors. Evaluations of starting materials showed a great interest in these structures with various olfactory facets. In this study, we depicted the similarity and differences between starting ethers and their cycloisomerized bicyclic counterparts.

Aza-Diels–Alder reactions are straightforward processes for the construction of N-heterocycles, featuring inherent atom-economy and stereospecificity. Intramolecular strategies allow the formation of bicyclic core structures with up to three stereocenters within a single step. Herein, this concept is combined with the chemistry of chiral Brønsted acid bound ortho-quinone methide imines to generate a range of interesting fused tetrahydroquinolines in a diastereo- and enantioselective­ manner.

The construction of multiply functionalized bicyclo[1.1.1]pentanes (BCPs) is of high synthetic value, as they are frequently harnessed as bioisosteres of 1,3-disubstituted benzene rings, alkynes, and tert-butyl groups in medicinal chemistry. Herein, we disclose a practical radical-mediated carbosulfonylation of propellane for the synthesis of sulfonyl β-keto-substituted BCPs by using vinyl sulfonates as dual-function reagent. This protocol features broad functional group tolerance and excellent atom-economy, leading to a variety of valuable difunctionalized BCP derivatives under mild photochemical conditions.

We report the first synthetic entry to a steroid with an unusual bicyclo[4.3.1]dec-3-en-10-one A/B ring substructure as a close structural analogue of the anti-inflammatory monanchosterols. Under optimized conditions, regioselective cis-dihydroxylation of the Δ5-double bond of 7-dehydrocholesterol and subsequent Criegee oxidation yields the corresponding 5,6-seco-steroid as a pure Z-isomer which upon treatment with K2CO3 in MeOH diastereoselectively affords 24,25-dihydro-6-epi-monanchosterol A through intramolecular aldol addition (cyclization). The developed three-step sequence proceeds in 17% overall yield without the need of any protecting group. The title compound was characterized by X-ray crystallography.

A highly efficient [2+2] cycloaddition between maleimides and unsaturated moieties, utilizing a visible-light triplet sensitization mode, has been developed for the direct synthesis of multifunctional 3-azabicyclo[3.2.0]heptane derivatives. This reaction relies on selective activation of the maleimide functionality upon energy transfer from a new photosensitizer that outperforms diverse well-established photosensitizers. The strategy developed herein overcomes previous obstacles such as limited substrate scope and undesired reaction pathways under harsh UV irradiation.

Hydantoin and its analogues such as thiohydantoin and iminohydantoin have received substantial attention from both a chemical and a biological point of view. Several compounds of this class have shown useful pharmacological activities such as anticonvulsant, antitumor, antiarrhythmic, and herbicidal properties that have led, in some cases, to clinical applications. Because of these broad-spectrum activities, intensive research efforts have been dedicated in industry and academia to the synthesis and structural modifications of hydantoin and its derivatives. Realizing the importance of hydantoin in organic and medicinal chemistry, we also initiated a research program that successfully designed and developed new routes and methods for the formation of hydantoin, thiohydantoin, and iminohydantoin substituted at various positions, particularly at the N-1 position without following a protection–deprotection strategy. Because combinations of two or more pharmacophoric groups can lead to hybrid molecules that display a mixed mechanism of action on biological targets, we extended our developed strategy to the syntheses of new types of hydantoin-based hybrid molecules by combining hydantoin with a triazole, isoxazoline, or phosphate scaffold as a second pharmacophore to exploit their diverse biological functions.1 Introduction2 Chemistry and Properties2.1 Physical Properties2.2 Chemical Properties2.3 Biological Properties3 General Synthetic Methods4 Synthesis of Diversely Substituted Hydantoins5 Synthesis of Diversely Substituted Thiohydantoins6 Synthesis of Diversely Substituted Iminohydantoins7 Fused or Bicyclic (Thio)hydantoins8 Di- or Multivalent (Thio)hydantoins9 Hydantoin-Based Hybrid Molecules9.1 Hydantoin–Isooxazoline Hybrids9.2 Hydantoin–Triazole Hybrids9.3 Hydantoin–Phosphate Hybrids: Phosphorylated Hydantoins10 Summary and Outlook

Homoconjugation is a phenomenon discussed for various π-systems where classical conjugation is broken by e.g. methylene units but still a stabilization by electronic communication exists. In this respect, triptycene with its rigid C3 symmetric geometry is an ideal scaffold to study this phenomenon. Although several studies based on triptycene strengthen the hypothesis of homoconjugation, in all described cases the electronic communication through space relies on different π-blades. Here, two triptycenes are presented having the exact same π-extended chromophore, but differently annulated to the bicyclic core. Both compounds were investigated by spectroscopic as well as computational means and compared with the corresponding model compound, elucidating the influence of the attachment site to the triptycene core on potential homoconjugation.

An efficient method for the synthesis of highly substituted bicyclic pyridone derivatives containing the dithiocarbamate group is reported via a one-pot three-component reaction of 2-(alkylthio)thio­azlactones, diamines, and nitroketene dithioacetal in EtOH under catalyst-free conditions. The reaction proceeds via a domino amidation–­intramolecular 1,4-addition-type Friedel–Crafts alkylation reaction to afford the corresponding fused bicyclic pyridones with high yields and diastereoselectivity.

The stereoselective synthesis of a model compound containing the ABC ring system of cyclocitrinol was accomplished. After connecting a C ring allyltitanium segment with an A ring bi­cyclo[4.1.0]heptanone segment, the seven-membered B ring moiety was constructed by an intramolecular cyclization reaction of an epoxy nitrile. The enone moiety was introduced through an oxidative decyanation reaction, and the bicyclo[4.4.1]undecane skeleton with the highly strained olefin moiety was formed through a ring-opening reaction of the bicyclo[4.1.0]heptane substructure.

Efficient and reliable synthesis of substituted hydrazinylpyridines in thick-wall ACE tubes via nucleophilic substitution of a chlorine substituent in different chloropyridines is presented. Hydrazine hydrate and alkylhydrazines were used as nucleophiles and simple alcohols and diethyl ether were the only organic solvents necessary, making the process environmentally and user friendly, potentially reaching 100% atomic efficiency. In the next step, transformations of succinic anhydride moieties fused to the bicyclo[2.2.2]octene framework into succinimide moieties via nucleophilic substitution of oxygens were conducted. As nucleophiles two of the synthesized hydrazinylpyridines (2-hydrazinyl-3-nitropyridine and 2-hydrazinyl-5-nitropyridine) and also hydrazine hydrate, phenylhydrazine, and 4-nitrophenylhydrazine were used. Reactions were again carried out in ACE tubes and only simple alcohols, diethyl ether, and acetone were needed as solvents. One of the prepared bicyclo[2.2.2]octene adducts displayed water solubility thus being a promising candidate for future studies as a novel bidentate ligand for various metal cations in aqueous solutions or acting as an unprecedented halogen bond acceptor.

Functionalized hydropentalenes (i.e., bicyclo[3.3.0]octanones) constitute important building blocks for natural products and for ligands for asymmetric catalysis. The assembly and tailored functionalization of this convex roof-shaped scaffold is challenging and has motivated a variety of synthetic approaches including our own contributions, which will be presented in this account.1 Introduction2 Biosynthesis of Hydropentalenes3 Hydropentalenes through the Pauson–Khand Reaction4 Hydropentalenes through Transannular Oxidative Cyclization of Cycloocta-1,4-diene5 Functionalization of Bicyclo[3.3.0]octan-1,4-dione to Dodecahydrocyclopenta[a]indenes6 Functionalization of Bicyclo[3.3.0]octan-1,4-diones to Crown Ether Hybrids7 Functionalization of Bicyclo[3.3.0]octan-1,4-dione to Cylindramide8 Tandem Ring-Opening Metathesis/Ring-Closing Metathesis/Cross-Metathesis of Bicyclo[2.2.1]heptanes9 Functionalization of Bicyclo[3.3.0]octan-1,4-dione to Geodin A10 Hydropentalenes through Enantioselective Desymmetrization of Weiss Diketones11 Functionalization of Weiss Diketones by Carbonyl Ene Reactions12 Functionalization of the Weiss Diketone to Cylindramide and Geodin A Core Units13 Biological Properties of Bicyclo[3.3.0]octanes14 Hydropentalenes through Vinylcyclopropane Cyclopentene Rearrangement15 Functionalization of Bicyclo[3.3.0]octanes toward Chiral Dienes16 Miscellaneous Syntheses of Hydropentalenes17 Conclusion and Outlook

The metal-free preparation of diazoalkanes through the ring rearrangement of bicyclic triazolines is reported here. Their use in 1,3-dipolar cycloaddition reactions with electron-withdrawing alkenes was investigated. This synthetic procedure allows differently substituted pyrazolines to be obtained in few steps and with high atom economy.

The remarkable structural and electronic properties of rigid non-conjugated hydrocarbons afford attractive opportunities to design molecular building blocks for both medicinal and material applications. The bridgehead positions provide the possibility to append diverse functional groups at specific angles and in specific orientations. The current review summarizes the synthetic development in CH functionalization of three rigid scaffolds namely: (a) cubane, (b) bicyclo[1.1.1]pentane (BCP), (c) adamantane.1 Introduction2 Cubane2.1 Cubane Synthesis2.2 Cubane Functionalization3 Bicyclo[1.1.1]pentane (BCP)3.1 BCP Synthesis3.2 BCP Functionalization4 Adamantane4.1 Adamantane Synthesis4.2 Adamantane Functionalization5 Conclusions and Outlook

The samarium diiodide promoted reductive cyclization of a series of γ-aryl ketones with acetoxy, alkoxy, and siloxy groups in ortho-, meta-, and para-positions was investigated. Only precursors with p-acetoxy, p-tert-butoxy, or p-siloxy substituents furnished decent yields of the desired 7-oxy-1,2,3,4,6,8a-hexahydronaphthalene derivatives. The products were formed without contamination with the regio­isomeric bicyclic products containing conjugated double bonds. Typical reactions exploiting the silyl enol ether moiety of the 7-(tert-butyl­dimethylsiloxy)-1,2,3,4,6,8a-hexahydronaphthalene derivative were performed, allowing stereoselective access to highly substituted hexahydro­-, octahydro-, or decahydronaphthalene derivatives.

Cyclopropenone 1,3-propanediol ketal is an increasingly versatile building block for the introduction of various C3 and dioxaspiro units due to the emergence of numerous methodologies for the application of strained hydrocarbons. This Practical Synthetic Procedure gives an updated synthesis for a basic ketal of cyclopropenone with lowered solvent toxicity and a detailed step-by-step guide for the safe production of potassium amide in liquid ammonia and for its application in the synthesis of the cyclopropene ring.

The first example of exploiting the trifunctional character of alkyl 4-chloro-2-diazo-3-oxobutanoates in the reactions with vicinal N,S-bis-nucleophiles leading to the formation of bicyclic 6,7-dihydro-4H-[1,2,3]triazolo[5,1-c][1,4]thiazines is presented. The key to the successful realization of the atom-economical synthetic strategy is the initial S N 2 event, which facilitates the subsequent domino 1,2,3-triazole formation via the Wolff reaction. Equally important is the choice of sodium acetate as the base: acetic acid formed in the course of the initial nucleophilic substitution acts as an efficient catalyst for the Wolff reaction, which suppresses the competing, unwanted fragmentation path (observed when other bases are used).

Bicyclic lactams may be prepared from serine or cysteine and 2-methylpropanal; the resulting S,N-heterocycles are more stable than the corresponding O,N-heterocycles but both are synthetic intermediates capable of further elaboration.

A novel Sc(OTf)3-catalyzed Tiffeneau–Demjanov ring expansion of six-membered cyclic ketones has been developed. The resulting seven-membered cyclic β-keto carboxamides were found to be versatile precursors to rare bicyclic pyrazoles obtained in modest to good yields via condensation with aryl hydrazines in the presence of Lawesson’s reagent.

A stereoselective synthesis of the bicyclic unit constituting the A and E rings of calyciphylline B-type alkaloids is disclosed. The propionate ester of (1R)-cyclohex-2-en-1-ol, obtained by enzymatic resolution, is subjected to an Ireland–Claisen rearrangement. Subsequent reduction of the acid, Mitsunobu reaction to introduce a nitrogen functionality, oxidative cleavage to a dialdehyde, and intramolecular aldol and aza-Michael reactions afford the bicyclic subunit.

Bicyclic ketal skeletons are important structure fragments that are frequently contained in natural products. A novel tandem Friedel–Crafts reaction and subsequent stereoselective acetalization were developed with readily available aldehyde and ketone acid. The reaction proceeded smoothly in the presence of catalytic Brønsted acid and afforded the corresponding product with moderate yield and high stereoselectivity.

SmI2-catalyzed radical cascade cyclizations were used to generate complex carbocyclic products bearing quaternary stereocenters with high selectivity. Bicyclic scaffolds containing four contiguous stereocenters and one quaternary stereocenter were obtained in excellent yields (up to 99%) and with high diastereocontrol by using 5 mol% of SmI2 at ambient temperature in the absence of co-reductants or additives. Mechanistic studies support a radical relay mechanism.

The synthesis of a series of 1-azafagomine derivatives that are tethered with five- and six-membered 1,2-annulated ring systems is described. These compounds were used in order to explore whether a hydrogen-bond acceptor group on the carbon atom corresponding to the glycosidic oxygen is able to interact with the catalytic acidic residue of β-glucosidase. The hydrogen-bond acceptor group was installed at various positions on the annulated ring system making it possible to study the effect of altering the position of this group.

This Account describes studies carried out by my group during the past decade on both intra- and intermolecular conjugate additions of carbon nucleophiles to nitrosoalkenes. Using the Denmark protocol for the generation of nitrosoalkenes from α-chloro-O-silyloximes, a number of bridged and fused bicyclic ring systems can be prepared via the intramolecular version of this process. Intermolecular conjugate addition reactions of nitrosoalkenes with a wide variety of ester enolates as coupling partners can also be achieved efficiently using a similar procedure. Some stereochemical aspects of these nucleophilic additions have been studied with both acyclic and cyclic nitrosoalkenes. This methodology has been applied as key steps in synthetic approaches to some complex indole and Myrioneuron alkaloids.1 Introduction2 Conjugate Additions of Nitrosoalkenes2.1 Background2.2 Intramolecular Reactions2.3 Intermolecular Reactions2.4 Stereochemical Aspects3 Applications to Natural Product Synthesis3.1 Angustilodine and Related Alkaloids3.2 Approach to Apparicine-Type Alkaloids3.3 Myrioneurinol4 Summary and Outlook

The bicyclic core of the halimanic framework is synthesized in optically active form by an acid-induced rearrangement of a homo­drimanic epoxide. The substrate can follow two different pathways under acidic treatment. Using fluorosulfonic acid as a promoter at low temperature favors ring contraction to a perhydrindanic structure. In contrast, milder acids at higher temperatures bring about predominantly an angular methyl migration and formation of the halimanic bicyclic system. In particular, an acidic pillared clay selectively promoted this transformation.

Herein, we report a concise enantioselective synthesis of (–)-allosecurinine, a tetracyclic Securinega alkaloid featuring an α,β-unsaturated γ-lactone moiety. Starting from inexpensive and readily available trans-l-hydroxyproline, our strategy entails a rare late-stage [2+2+1]-hetero-Pauson–Khand cycloaddition between a ketone and an alkyne as the key complexity-generating step to rapidly install the CD-ring system. The reported W(CO)6-promoted intramolecular cyclization provides the first example of a tungsten-mediated hetero-Pauson–Khand reaction. This approach to the strained bicyclic CD motif present in ­allosecurinine provides some insights into the boundaries of this potentially powerful methodology that might be further extended to other butenolide-containing natural products.

This short review summarizes the examples to date of successful (4+3) cycloadditions, including formal (4+3) cycloadditions, where pyrrole derivatives reacted as the diene component, to provide aza-bridged bicyclic and polycyclic adducts.1 Introduction2 Unsubstituted Pyrroles as Dienes in (4+3) Cycloadditions3 C-Substituted Pyrroles as Dienes in (4+3) Cycloadditions4 Intramolecular Pyrrole (4+3) Cycloadditions5 Conclusions

Optically pure carboxylic acid segments, which are common key intermediates for the synthesis of naturally occurring bicyclic depsipeptide histone deacetylase inhibitors, have been produced efficiently. The method features the chromatographic separation of two diastereomers, which were formed by direct amide condensation of racemic (3RS,4E)-3-hydroxy-7-mercaptohept-4-enoic acid (rac-Hmh) with chiral amino acids. This approach offers a reliable and practical method for producing optically pure carboxylic acid segments, which can facilitate easier access to important anticancer agents derived from them.

The naturally occurring bicyclic depsipeptide histone deacetylase inhibitors thailandepsins D–F were efficiently synthesized for the first time in 49–61% overall yield over five steps, starting from known amine and carboxylic acid segments. The synthesis includes the condensation of the two known starting materials to directly assemble the corresponding seco-acids, which are the key precursors for macrolactonization. The seco-acids are then macrolactonized using the Shiina method to construct the requisite 15-member macrocycles.

A stereoselective synthetic route to embellished cis-hydrindanes from simple aromatic precursors is described. Oxidative dearomatization, ring expansion, and photochemical 1,3-acyl shift are the key features of our approach. Oxidative dearomatization of o-(hydroxymethyl)phenols followed by π4s + π2s cycloaddition furnishes bicyclo[2.2.2]octanes with contiguous keto epoxide groups, which upon ring expansion lead to bicyclo[3.2.2]nonanes endowed with a β,γ-enone chromophore. Unbridging of bicyclo[3.2.2]nonanes upon singlet excitation furnishes embellished cis-hydrindanes.

Utilization of three-dimensional cyclic scaffolds is important in modern drug discovery, both to provide greater opportunities for optimizing drug candidates and to expand the available chemical space of drugs. Among these scaffolds, bicyclo[1.1.1]pentane (BCP) is a high-value bioisostere for 1,4-disubstituted phenyl rings, internal alkynes, and the tert-butyl group, generally offering high passive permeability, high water solubility, and improved metabolic stability. However, the lack of methods for functionalizing BCP remains a significant challenge, and in particular, a versatile strategy for synthesizing a wide range of unsymmetrically 1,3-difunctionalized BCP derivatives has been lacking. In this account, we review recent advances in the synthetic chemistry of BCP, focusing especially on our recently developed radical multicomponent carboamination of [1.1.1]propellane.1 Introduction2 Overview of the Synthetic Chemistry of [1.1.1]Propellane, the Most Promising Precursor of Bicyclo[1.1.1]pentane3 Recent Advances in the Synthetic Chemistry of Unsymmetrically 1,3-Disubstituted Bicyclo[1.1.1]pentane Derivatives4 Radical Multicomponent Carboamination of [1.1.1]Propellane Permits Direct Synthesis of 3-Substituted Bicyclo[1.1.1]pent-1-ylamine Derivatives5 Conclusion

William Pitt and co-workers have created a virtual exploratory heterocyclic library ‘VEHICLe’ containing over 200 unconquered bicyclic heteroaromatic rings, synthetically feasible with potential medicinal interest. Since the publication of the 22 ‘heteroaromatic rings of the future’ by Pitt in 2009, 15 of them have been successfully synthesized as bicyclic or polycyclic forms and evaluated for applications in both biology and material science. This short review presents the critical synthesis associated with innovative synthetic methodologies of the synthetically conquered ring scaffolds from the list of 22 with a spotlight on the scientific contribution of this fascinating article for the expansion of the chemical diversity.1 Introduction2 Heteroaromatic Rings of the Future: The Synthetic challenge?2.1 4-Pyrido[1,3]oxazin-4-one-P1 2.2 Pyrrolo[2,1-b][1,3]oxazin-4-one-P4 2.3 Furo[2,1-e]pyridazin-4(1H)-one-P5 2.4 Isoxazolo[3,4-c]pyridin-7-one-P6 2.5 5H,6H-[1,2]Thiazolo[5,4-c]pyridin-5-one-P7 2.6 4H,5H-Furo[3,2-b]pyridin-5-one-P8 2.7 1H,5H,6H-Pyrazolo[3,4-c]pyridin-5-one-P9 2.8 Thieno[3,4-c]pyridazine-P10 2.9 Pyrrolo[1,2-c][1,2,3]triazine-P11 2.10 Thieno[3,4-a]oxazole-P12 2.11 2,4-Dihydropyrrolo[3,2-c]pyrazole-P13 2.12 Pyrazolo[1,5-b]isoxazole-P14 2.13 Imidazo[1,5-c]pyrimidin-3(2H)-one-P16 2.14 Cyclopenta[b][1,4]oxazin-5(4H)-one-P17 2.15 2,3-Dihydro-2,6-naphthyridin-3-one-P18 3 Conclusion

The efficient Pd-catalyzed coupling of unstable bicyclic pyrimidines and pyrazoles under basic conditions was established for the first time. The reactions proceeded with excellent yields without hydrolysis using Pd2(dba)3, t BuXPhos, and K3PO4.

Herein, we portray a synthetic route to a bicyclic lactone containing a bridgehead hydroxy group, a structure that is present in many natural products of biological and medicinal relevance. Ethyl (E)-3-(dimethylphenylsilyl)-7,8-epoxyoct-2-enoate underwent radical-mediated­ reductive epoxide opening with concomitant intramolecular cyclization using Cp2Ti(III)Cl to give cis-6-(dimethylphenylsilyl)-3-oxabicyclo[4.3.0]nonan-4-one, a bicyclic lactone with a bridgehead silyl group serving as a masked hydroxy group. Furthermore, the bridgehead C–Si bond underwent stereoretentive oxidative cleavage to give cis-6-hydroxy-3-oxabicyclo[4.3.0]nonan-4-one in high yield under Tamao–Fleming oxidation conditions; this demonstrates the potential utility of this strategy in the synthesis of many natural products bearing similar hydroxylated bridgehead chiral center embedded in a bicyclic lactone framework.

The unexpected formation of a 4-hydroxytetrahydroquinolin-2(1H)-one from a bicyclo[3.3.1]nonanetrione system and an amino alcohol in the presence of TsOH is reported. The mechanism of this transformation was studied by DFT calculations. The reaction provides an entry to the synthesis of highly functionalized 4-hydroxytetrahydroquinolin-2(1H)-ones.

An efficient chiral phosphoric acid catalyzed Biginelli reaction for the synthesis of optically pure 4-alkyl-3,4-dihydropyrimidin-2-(1H)-ones has been developed, with a wide range of the desired products being obtained in moderate to high yields with good to excellent enantioselectivities (up to >99% ee after one recrystallization). The synthetic utility of this reaction is illustrated in various aliphatic bicyclic guanidine compounds, which is applied in the formal synthesis of the bicyclic guanidine core of crambescin A and batzelladine A.

An approach to 2-azabicyclo[n.2.0]alkane derivatives (n = 1, 2), which relies on a tandem Strecker reaction–intramolecular nucleophilic cyclization (STRINC) sequence of the corresponding 2-(ω-chloroalkyl)cyclobutanones (in turn prepared by [2+2] cycloaddition of keteniminium salts and ethylene) is described. The utility of the method is demonstrated by multigram syntheses of bicyclic proline analogues, monoprotected diamines, as well as parent 2-azabicyclo[4.2.0]octane.

A redox annulation reaction of cyclic amines and α-ketoamides was developed. A variety of 1,2-fused bicyclic imidazolidin-4-ones were synthesized in moderate to good yields from cyclic amines by ­redox-neutral α-C–H functionalization.

An enantioselective and catalytic method for the synthesis of bridgehead quaternary carbon containing bicyclo[3.3.1]nonanes is described. The intramolecular desymmetric arylation of α-disubstituted 1,3-diketones was achieved using a chiral Pd/PHOX complex. A variety of bridged cyclic molecules, common skeleton for polyprenylated acylphloroglucinols, were obtained in up to 84% yield with 86% ee.

An intramolecular [3+2] cycloaddition of trans-2-allene-­vinylcyclopropanes for the synthesis of bicyclo[3.3.0]octane derivatives is developed.

A conceptually new and scalable strategy (ten linear steps, 13.9% overall yield) has been developed to synthesize (+)-7-deoxy­pancratistatin from optically pure 7-azabicyclo[2.2.1]heptanone scaffold. The crucial trans-B–C ring junction was fixed at an early stage of the synthesis by exploiting the rigid bicyclic structural framework of the starting precursor. Stereoselective installation of hydroxyl groups around the perimeter of the cyclohexenyl C-ring involved sequential epoxidation–phenylselenylation–oxydeselenylation sequence followed by dihydroxylation. The most attractive feature of this synthesis is the use of a protection–deprotection step only at the penultimate step making the protocol very efficient and atom economical.

A highly diastereo- and enantiostereoselective synthesis of bicyclic dihydropyranothiazoles combining a thiazole and δ-lactone skeleton via NHC-catalyzed [4+2] annulation of 5-alkenylthiazolones and α-chloroaldehydes has been developed. The heterocyclic products are formed via azolium enolate intermediates in good yields with high diastereo- and enantistereoselectivities.

DBN (1,5-diazabicyclo[4.3.0]non-5-ene) and DBU (1,8-diaza­bicyclo[5.4.0]undec-7-ene) react with methyl esters of acetylenic acids and excess water (the reactant molar ratio 1:1, in aqueous MeCN, at 20–25 °C, for 48 h) to afford acetylenic amides with pyrrolidone and caprolactam moieties in 89–100% yields. The synthesis involves amines formed in situ from the cyclic amidines, which further react with acetylenic esters.

Multiple bond-forming cascade transformations and one-pot procedures are valuable tools in organic synthesis and drug discovery. These atom-economical processes provide rapid access to natural product-like scaffolds from simple precursors. Herein, we report on ruthenium-catalyzed one-pot conversions of simple 1-alkenyl propargyl alcohols with cyclic 3-ketolactones and dienophiles. Thereby, structurally diverse fused polycycles and functionalized bicyclic structures are accessible from a common precursor with high selectivity. Some of the new drug-like molecules exhibit cytotoxic activity against KB cells.

A palladium(0)-catalyzed ring-opening cross-coupling reaction between tert-cyclobutenols and aryl halides produces γ-arylated β,γ-unsaturated ketones. In the case of aryl halides bearing functional groups at the ortho position, the resulting ring-opened ketones undergo intramolecular condensation to afford bicyclic aromatic compounds.