https://pubs.rsc.org/en/content/articlepdf/2019/NP/C8NP00092A

Marine natural products.

2.4. Tandem sp-sp Coupling/Cyclization 3418 3. Cycloaddition Reactions 3418 3.1. [3+2] Cycloaddition 3418 3.1.1. Triazole/Isoxazole-Forming Reactions 3418 3.1.2. Reactions of Azomethine Ylides 3419 3.1.3. Cycloaddition of Nitrones 3422 3.1.4. Other [3+2] Reaction 3422 3.2. [4+2] Heterocycloaddition 3422 3.3. [2+2] Cycloaddition 3424 4. Cycloisomerization of Enynes/Diynes 3425 5. Intramolecular Friedel-Crafts-Type Reactions 3428 6. Reactions of R-Diazocarbonyl Compounds 3429 7. Aziridination of Olefins 3431 8. N/O-Vinylation/Arylation 3431 9. Radical Cyclization of Haloalkenes and Haloalkynes 3433

Coinage Metal-Assisted Synthesis of Heterocycles

Allenes are highly valuable synthetic precursors in preparative organic chemistry because of their ability to undergo a variety of transformations. Among many different reaction modes, activation of the cumulated double bonds by treatment with a Brønsted or Lewis acid is particularly useful because it allows a nucleophilic attack, which leads to the formation of a new C-C or Cheteroatom bond in an interor intramolecular fashion. Because of their axial chirality, allenes can undergo this bond formation with chirality transfer, which renders the method attractive for stereoselective target-oriented synthesis. Because of their soft and carbophilic character, gold catalysts are particularly well suited for the selective activation of allenes in the presence of other reactive functionalities. Compared to intermolecular additions, gold-catalyzed cyclization reactions of allenes by intramolecular nucleophilic attack have received much more attention. Here, the gold catalyst can coordinate to either allenic double bond, and the regioselectivity of the subsequent nucleophilic attack depends on the structure of the substrate, in particular the length of the tether connecting allene and nucleophile (Scheme 1). Hence, four different endoor exo-cyclization products can be obtained, but the formation of fiveor sixmembered rings via σ-gold species A or D, i.e., by nucleophilic attack at a terminal allenic carbon atom, is favored in most cases, whereas products arising from nucleophilic attack at the central allenic carbon atom (via intermediates B and C) are rare. Normally, these cyclization products are chiral and can be accessed in a stereoselective manner either from chiral allenes by axis-to-center chirality transfer or from achiral allenes utilizing chiral gold catalysts. This account comprises gold-catalyzed cyclization reactions of allenes by attack of carbon or heteroatom nucleophiles. Intermolecular addition reactions to allenes, as well as gold-catalyzed transformation of nonallenic substrates in which an allene is generated in situ (e.g., by sigmatropic rearrangement) and converted without isolation, will not be covered.

Gold-catalyzed nucleophilic cyclization of functionalized allenes: a powerful access to carbo- and heterocycles.

Heterocycles constitute the largest and the most diverse family of organic compounds Among them, aromatic heterocycles represent structural motifs found in a great number of biologically active natural and synthetic compounds, drugs, and agrochemicals Moreover, aromatic heterocycles are widely used for synthesis of dyes and polymeric materials of high value 1 There are numerous reports on employment of aromatic heterocycles as intermediates in organic synthesis 2

Although, a variety of highly efficient methodologies for synthesis of aromatic heterocycles and their derivatives have been reported in the past, the development of novel methodologies is in cuntinious demand Particlularly, development of new synthetic approaches toward heterocycles, aiming at achieving greater levels of molecular complexity and better functional group compatibilities in a convergent and atom economical fashions from readily accessible starting materials and under mild reaction conditions, is one of a major research endeavor in modern synthetic organic chemistry Transition metal-catalyzed transformations, which often help to meet the above criteria, are among the most attractive synthetic tools

Several excellent reviews dealing with transition metal-catalyzed synthesis of heterocyclic compounds have been published in literature during recent years Many of them highlighted the use of a particular transition metal, such as gold,3 silver,4 palladium,5 copper,6 cobalt,7 ruthenium,8 iron,9 mercury,10 rare-earth metals,11 and others Another array of reviews described the use of a specific kind of transformation, for instance, intramolecular nucleophilic attack of heteroatom at multiple C–C bonds,12 Sonogashira reaction,13 cycloaddition reactions,14 cycloisomerization reactions,15 C–H bond activation processes,16 metathesis reactions,17 etc Reviews devoted to an application of a particular type of starting materials have also been published Thus, for example, applications of isocyanides,18 diazocompounds,19 or azides20 have been discussed In addition, a significant attention was given to transition metal-catalyzed multicomponent syntheses of heterocycles21 Finally, syntheses of heterocycles featuring formation of intermediates, such as nitrenes,22 vinylidenes,23 carbenes, and carbenoids24 have also been reviewed

The main focus of the present review is a transition metal-catalyzed synthesis of aromatic monocyclic heterocycles The organization of the review is rather classical and is based on a heterocycle, categorized in the following order: (a) ring size of heterocycle, (b) number of heteroatoms, (c) type of heterocycle, and (d) a class of transformation involved A brief mechanistic discussion is given to provide information about a possible reaction pathway when necessary The review mostly discusses recent literature, starting from 200425 until the end of 2011, however, some earlier parent transformations are discussed when needed

Transition Metal-Mediated Synthesis of Monocyclic Aromatic Heterocycles

2.1. Oxygen Nucleophiles 3174 2.2. Nitrogen Nucleophiles 3177 3. Ag(I)-Mediated Addition of Nucleophiles to Allenes 3178 3.1. Oxygen Nucleophiles 3178 3.1.1. Allenic Alcohols 3178 3.1.2. Allenic Aldehydes and Ketones 3179 3.1.3. Allenic Acids 3180 3.2. Nitrogen Nucleophiles 3180 3.2.1. Allenic Amines 3180 3.2.2. Allenic Sulfonamides 3182 3.2.3. Allenic Carbamates 3182 3.2.4. Allenic Amides 3182 3.2.5. Allenic Oximes 3183 4. Ag(I)-Mediated Additions to Alkenes 3183 5. Ag(I)-Mediated Cycloaddition Reactions 3185 5.1. 1,3-Dipolar Cycloaddition Reactions of Azomethine Ylides 3185

Silver-mediated synthesis of heterocycles.

Observations Regarding the Ag(I)-Catalyzed Conversion of Allenones to Furans

A total synthesis of the enantiomer 35 of kallolide B was achieved starting from (S)-(−)-perillyl alcohol (8). Oxidative cleavage to the ester aldehyde 11 was effected by treatment of the epoxide 9 with H5IO6 followed by CH2N2. The allenyl ketone 13, obtained by SnCl2-promoted addition of 1-bromo-2-butyne to aldehyde 11 and subsequent Swern oxidation, cyclized to the furan 14 in the presence of catalytic AgNO3. Homologation of the derived aldehyde 15 with CBr4−Ph3P followed by n-BuLi and CH2O led to the propargylic alcohol 17. Formylation of the furan 17 (s-BuLi, DMF) and then Still−Horner−Emmons homologation yielded the (Z)-conjugated ester 22. Conversion of the propargylic alcohol function to the chloride 23 and ester reduction (DIBAL-H) furnished the chloro alcohol 24, which formed the cyclic ether 25 upon treatment with NaH. Ether 25 underwent a highly diastereoselective [2,3]Wittig ring contraction to the propargylic alcohol 26. The derived mesylate 36 was converted to the allenic ester 37 with CO an...

Gary S. Bartley

Papers

Observations Regarding the Ag(I)-Catalyzed Conversion of Allenones to Furans

Total Synthesis of the Pseudopterane (−)-Kallolide B, the Enantiomer of Natural (+)-Kallolide B

Observations Regarding the Ag(I)-Catalyzed Conversion of Allenones to Furans.

Total Synthesis of the Pseudopterane (-)-Kallolide B, the Enantiomer of Natural (+)-Kallolide B.