Six-membered ring systems: with O and/or S atoms

A large variety of publications involving O- and S-6-membered ring systems have appeared in 2017. The importance of these heterocyclic compounds is highlighted by the huge number of publications on the total synthesis of natural oxygen derivatives and of other communications dedicated to synthetic derivatives. Reviews on stereoselective organocatalytic synthesis of tetrahydropyrans (17EJO4666), of tetrahydropyrans and their application in total synthesis of natural products (17CSR1661), on the synthesis of the less thermodynamically stable 2,6-trans-tetrahydropyrans (17S4899), on enantioselective synthesis of polyfunctionalized pyran and chromene derivatives (17TA1462), and on enantioselective and racemic total synthesis of camptothecins, including the formation of their pyran-2-one ring (17SL1134), have appeared. Advances in the transition metal-catalyzed synthesis of pyran-2/4-ones (17TL263), N-heterocyclic carbene (NHC)-catalyzed achiral synthesis of pyran-2-one, coumarin and (thio)chromone derivatives (17OBC4731), on the synthesis and transformation of 2H-pyran-2-ones (17T2529) and 2-styrylchromones (17EJO3115) into other heterocyclic compounds, have been surveyed. The strategies to build up the tetrahydropyranyl core of brevisamide (17H(95)81) and the reactions of ketyl radicals, generated from carbonyl derivatives under transition-metal photoredox-catalyzed conditions, leading to isochromen- and chroman-type compounds (17CC13093) were disclosed. Developments in the synthesis of pentafluorosulfanyl(chromene and coumarin) derivatives (17TL4803), photoswitchable D9-tetrahydrocannabinol derivatives (17JA18206), and aminobenzopyranoxanthenes with nitrogen-containing rings (17JOC13626) have been studied.info:eu-repo/semantics/publishedVersio

Oxidative Transformations of Allenes (Part I) and Enyne Ring-Closing Metathesis (Part II)

Exploration of new reactivity of silyl-substituted allenes along with the corresponding alkyl-substituted allenes under conditions with various oxidants and/or enophiles to induce nitration, Alder ene reaction, copper-catalyzed oxidative dimerization, and iron-catalyzed oxidative transformations. More specifically, in nitration chemistry; nitration of silylallenes with nitrogen dioxide radical, generated from NaNO2 and AcOH, to form α-nitro-α,β-unsaturated silyl oximes and also using Fe(NO3)3·9H2O and FeCl3·6H2O, to form regioisomeric chloride-trapped products depending on the steric bulk of the silyl group. Another novel class of compound isooxazolidinones were obtained upon treating the initially formed α-nitro-α,β-unsaturated silyl oximes with TBAF. In dimerization chemistry; silylallenes with a catalytic system of copper(I) chloride and N-hydroxyphthalamide (N-Hpth) along with a stoichiometric amount of a terminal oxidant diacetoxyiodobenzene were favored mainly for the formation of dimer products with or without 1,3-enynes, and N-Hpth adducts. In Alder ene reaction, activation of allenic C(sp2)–H bond over an allylic C(sp3)–H bond was described. In this ene reaction linear silylallenes preferentially engage an allenic C(sp2)–H bond with high selectivity but cycloalkyl-substituted silylallenes showed low or reversed selectivity. On the other hand, non-silylated allenes engage allylic C(sp3)–H bonds favorably regardless of their structural feature. DFT calculations were provided for further insight into the selectivity trend. In iron-catalyzed oxidative transformations substituent and oxidant-dependent transformations of allenes are described using alkyl- and silyl-substituted allenes with DDQ (2,3-dichloro-5,6-dicyano-p-benzoquinone) and TBHP (tert-butyl hydroperoxide). The reactions of non-silylated allenes involve allylic cation intermediate via forming C–O bond at the sp-hybridized C2 carbon and the reactions of silylated allenes favor the formation of propargylic cation intermediates via transferring the allenic hydride to the oxidant, generating 1,3-enynes or propargylic TBHP ethers. In addition, structure and reactivity of sulfonamide-, acetate-, alcohol, and alkene-chelated ruthenium alkylidene complexes derived from enyne RCM were investigated. All 5-membered sulfonamide chelates, hydroxy/ether chelates and 6-membered acetate chelates have the chelated oxygen and the NHC ligand in cis relationship. On the other hand, alkene-chelated ruthenium alkylidenes have N-heterocyclic carbene and chelated alkene in trans relationship. These newly generated variety of chelated complexes were further tested their metathesis activity as well as in some cases their behavior in presence of carbon monoxide were described. Also, the unexpected thermal bicyclization of metathesis substrates ynamide tethered triynes to generate novel class of tetrahydropyranopyridines were explored. Where the ynamide tethered triynes smoothly underwent 6-exo-mode ring-closure reaction by heating at 85 oC. The reaction has been explored by varying anion- and cation-stabilizing groups, including the ether side chain, and the alkyne substituent to provide broad range of tetrahydropyranopyridines

Nitration of Silyl Allenes To Form Functionalized Nitroalkenes

An efficient nitration of silyl allenes with nitrogen dioxide radical, generated from NaNO₂ and AcOH, to form α-nitro-α,β-unsaturated silyl oximes has been developed. A similar nitration could be achieved by using Fe(NO₃)₃·9H₂O and FeCl₃·6H₂O, but different from the regioselective oxime formation, two regioisomeric chloride-trapped products were isolated with varying ratios depending on the steric bulk of the silyl group. A novel ring-closure reaction of α-nitro-α,β-unsaturated silyl oximes upon treating with TBAF to form isooxazolidinone derivatives was also developed

Oxidative Dimerization of Silylallenes via Activation of the Allenic C(<i>sp</i>²)–H Bond Catalyzed by Copper(I) Chloride and <i>N</i>‑Hydroxyphthalimide

Novel oxidative dimerization of silylallenes is described. Treatment of silylallenes with a catalytic amount of copper­(I) chloride, a substoichiometric amount of <i>N</i>-hydroxyphthalamide, and a stoichiometric amount of a terminal oxidant diacetoxyiodobenzene afforded head-to-head dimers as the main products. Silyallenes containing a small ring afforded only dimers, whereas as the ring size increased 1,3-enynes became more favorable products. For silylallenes containing an acyclic substituent, dimer formation is a norm with exceptions where <i>N</i>-hydroxyphthalimide reacts at the propargylic center to generate the corresponding aminoxy ethers

Subtle Electronic Effects in Metal-Free Rearrangement of Allenic Alcohols

A general and stereoselective rearrangement of allenic alcohols to (<i>E,E</i>)-1,3-dien-2-yl triflates and chlorides was developed under metal-free conditions. Subtle electronic effects of the alkyl and aryl substituents on the carbon bearing the hydroxyl group has a profound impact on the reaction rate and efficiency such that vinyl triflates were obtained from electron-deficient substrates and trimethylsilyl triflate whereas vinyl chlorides were generated with an electron-rich substrate and trimethylsilyl chloride