RESUMEN
A photoredox-mediated radical amidation ring-expansion sequence that enables the generation of all-carbon quaternary centers bearing a protected aminomethyl substituent is described. The methodology can be applied to both styrene and unactivated alkene substrates generating structurally diverse sp3 -rich amine derivatives in a concise manner.
RESUMEN
A highly enantioselective halogenation/semipinacol rearrangement of isatin-derived allylic alcohols has been developed with a chiral N,N'-dioxide/ScIII complex as catalyst. This strategy involved a pivotal stereodivergent kinetic resolution process and provided a facile and efficient entry to optically active halo-substituted quinolone derivatives and quinoline alkaloids with a quaternary stereocenter simultaneously under mild reaction conditions. Based on the control experiments together with kinetic studies and DFT calculations, a possible catalytic cycle was proposed to illustrate the reaction process and enantiocontrol.
RESUMEN
Seiridiasteriscane A is an asteriscane-type sesquiterpenoid bearing a trans-fused bicyclo[6.3.0]undecane skeleton. Although its biosynthesis has been proposed to involve a semipinacol rearrangement of a putative intermediary acetate bearing a bicyclo[7.2.0]undecane ring system (presumably derived from coisolated pestalotiopsin M) followed by epimerization of the resulting cis-fused seiridiasteriscane B, such a type of semipinacol rearrangement has never been reported so far. Our model study revealed that a 1-hydroxybicyclo[7.2.0]undecan-2-yl acetate underwent a smooth and stereospecific semipinacol rearrangement with the assistance of Et2AlCl to give the corresponding bicyclo[6.3.0]undecane-9-one. In addition, the resulting cis-fused 5,8-bicyclic ketone was partially epimerized to the corresponding trans-fused ketone by prolonged adsorption onto a silica gel plate. These results may support a recently proposed biosynthetic pathway of seiridiasteriscane A.
Asunto(s)
Alcanos/química , Compuestos Bicíclicos con Puentes/química , Ciclohexenos/química , Modelos Químicos , Análisis Espectral/métodosRESUMEN
A tandem Bischler-Napieralski/semipinacol rearrangement reaction has been developed for the purpose of assembling a bis(spirocyclic) indole framework, a privileged structural unit of aspidofractinine-type monoterpenoid indole alkaloids, and was used in combination with a subsequent Mannich reaction to expeditiously construct the central bridged bicyclo[2.2.1]heptane ring system of these molecules with contiguous quaternary centers. The development of this novel strategy culminated in the collective total synthesis of four aspidofractinine alkaloids.
RESUMEN
An enantioselective aldehyde α-alkylation/semipinacol rearrangement was achieved through organo-SOMO catalysis. The catalytically generated enamine radical cation serves as a carbon radical electrophile that can stereoselectively add to the alkene of an allylic alcohol and initiate ensuing ring-expansion of cyclopropanol or cyclobutanol. This tandem reaction enables the production of a wide range of nonracemic functionalizable α-quaternary-δ-carbonyl cycloketones in high yields and excellent enantioselectivity from simple aldehydes and allylic alcohols. As a key step, the intramolecular reaction was also successfully applied in the asymmetric total synthesis of (+)-cerapicol.
RESUMEN
An enantioselective sulfenylation/semipinacol rearrangement of 1,1-disubstituted and trisubstituted allylic alcohols was accomplished with a chiral Lewis base and a chiral Brønsted acid as cocatalysts, generating various ß-arylthio ketones bearing an all-carbon quaternary center in moderate to excellent yields and excellent enantioselectivities. These chiral arylthio ketone products are common intermediates with many applications, for example, in the design of new chiral catalysts/ligands and the total synthesis of natural products. Computational studies (DFT calculations) were carried out to explain the enantioselectivity and the role of the chiral Brønsted acid. Additionally, the synthetic utility of this method was exemplified by an enantioselective total synthesis of (-)-herbertene and a one-pot synthesis of a chiral sulfoxide and sulfone.
RESUMEN
A recent computational analysis of the stabilizing intramolecular OHâ¯O contact in 1,2-dialkyl-2,3-epoxycyclopentanol diastereomers has been extended to thiiriane, aziridine and phosphirane analogues. Density functional theory (DFT), second-order Møller-Plesset perturbation theory (MP2) and CCSD(T) coupled-cluster computations with simple methyl and ethyl substituents indicate that electronic energies of the c i s isomers are lowered by roughly 3 to 4 kcal mol-1 when the OH group of these cyclopentanol systems forms an intramolecular contact with the O, S, N or P atom on the adjacent carbon. The results also suggest that S and P can participate in these stabilizing intramolecular interactions as effectively as O and N in constrained molecular environments. The stabilizing intramolecular OHâ¯O, OHâ¯S, OHâ¯N and OHâ¯P contacts also increase the covalent OH bond length and significantly decrease the OH stretching vibrational frequency in every system with shifts typically on the order of -41 cm-1.
Asunto(s)
Aziridinas/química , Ciclopentanos/química , Compuestos Epoxi/química , Modelos Teóricos , Sulfuros/química , Teoría Funcional de la Densidad , Enlace de Hidrógeno , Espectroscopía de Resonancia Magnética , Modelos Moleculares , Conformación Molecular , Estructura MolecularRESUMEN
Sonochemical 2,3-dideoxygenation of ecdysteroids with the Δ 2(3)-bond generation and activation of the C-C bonds of the steroid core in the poststerone derivatives, that causes the skeletal rearrangement have been carried out for the first time. Thus, the ultrasonically assisted reaction of 2,3-dimesyloxy derivatives of ecdysteroids with the NaI-Zn-DMF system gives rise to their 2,3-dideoxy-Δ2(3)-analogues with yields 70-92%. In the case of 2,3-dimesyloxypoststerone as the initial ecdysteroid substrate the reaction is accompanied by the activation of the allyl moiety and semipinacol rearrangement, resulting in the C13-C14 bond migration with C/D rings contraction/expansion and providing novel short chain (8R)-13(14â¯ââ¯8)-abeo-isomer.
RESUMEN
An efficient oximation method has been developed on the basis of NO+ -initiated semipinacol rearrangement and subsequent proton elimination. The procedure enabled the rapid construction of a series of oximes and oxime ethers with spiro quaternary stereocenters from allylic silyl ethers. Additional features of this reaction include wide substrate tolerance as well as the commercial availability of the safe nitrosation reagent NOBF4 . The key N-heterotricyclic cores of three natural alkaloids, tuberostemoninolâ B, (+)-quebrachidine, and an insecticide, were also constructed efficiently by this method.
RESUMEN
An enantioselective cascade aerobic oxidation and semipinacol rearrangement reaction of 2-aryl-3-alkyl-substituted indoles via visible-light-driven cooperative organophotoredox and H-bonding catalysis is reported. The current method provides an expedient and sustainable approach to furnish a variety of valuable chiral 2-aryl-2-alkyl-substituted indolin-3-ones in 64-90 % yield with 58-94 % ee. Preliminary control experiments present important insights into the stereochemistry.
RESUMEN
ß-Functionalized ketones are a highly important and valuable class of compounds that have gained increasing attention from organic chemists due to their intensive uses as versatile synthetic intermediates and building blocks in complex molecule assembly and natural product synthesis. Accordingly, there is continuing interest in the development of new approaches for the synthesis of ß-functionalized ketones. In recent years, radical-mediated 1,2-rearrangement reactions of allylic alcohols, which proceed through cationic (semipinacol) rearrangements or radical (neophyl) rearrangements, have presented an attractive and powerful strategy to access various diversely ß-functionalized ketones. Interestingly, this strategy allows for the direct installation of a variety of valuable functional groups at the ß-position of cyclic and acyclic ketones by employing different radical precursors. However, a review focusing on the preparation of ß-functionalized ketones by radical-mediated 1,2-rearrangement reactions of allylic alcohols has not been summarized to date. This Minireview highlights recent progress made in this highly active and interesting research area, and the corresponding mechanisms will also be discussed.
RESUMEN
A copper-catalyzed enantioselective arylative semipinacol rearrangement of allylic alcohols was developed. In the presence of a catalytic amount of an in situ generated chiral copper-bisoxazoline complex, reaction of allylic alcohols with diaryliodonium salts afforded spirocycloalkanones in high yields with high diastereo- and enantioselectivities. A two-point binding model engaging the carbon-carbon double bond and the proximal hydroxyl group was proposed to be responsible for the highly efficient chirality transfer.
RESUMEN
Total syntheses of the biologically important and structurally unique tetracyclic diterpenes conidiogenone, conidiogenol, and conidiogenoneâ B of the cyclopiane class are reported. The absolute configuration of naturally occurring conidiogenoneâ B was also corrected. The key step of our strategy involved the highly efficient construction of both ringâ C and the quaternary carbon center shared by rings A and C through a one-step regioselective and diastereoselective cycloenlargement in the form of a semipinacol-type rearrangement. In particular, the desired regioselectivity was made possible by properly adjusting the migratory aptitude of the migrating carbon atom through the introduction of an electron-donating phenylthio group at this position.