Tandem Electrocyclic Ring Opening/Radical Cyclization: Application to the Total Synthesis of Cribrostatin 6

A concise total synthesis of cribrostatin 6 (1), an antimicrobial and antineoplastic agent, was accomplished using a tandem electrocyclic ring opening/radical cyclization sequence. Specifically, intermediate 4 underwent a 4π-electrocyclic ring opening, radical cyclization, and homolytic aromatic substitution sequence followed by an oxidation to afford the natural product 1 in one pot. Owing to the rapid buildup of complexity in the key step, 1 could be synthesized from commercially available starting materials in only four linear steps. Application of this chemistry to the concise syntheses of analogs of cribrostatin 6 (1) is also reported.     

Tandem Radical Cyclization for the Construction of Difluoro‐Containing Oxindoles and Quinoline‐2,4‐diones

Cu‐catalyzed direct difluoromethylation of activated alkenes through a difluoromethyl radical addition/cyclization to afford difluorinated oxindoles and quinoline‐2,4‐diones has been developed. This method provides convenient access to a variety of oxindoles and quinoline‐2,4‐diones under mild conditions via a proposed tandem radical cyclization process, while tolerating various functional groups well. Moreover, a facile method to construct diverse difluorinated quinoline‐2,4‐diones by visible‐light photoredox catalysis under mild conditions was presented.     

Highly Functionalized Cyclopentane Derivatives by Tandem Michael Addition/Radical Cyclization/Oxygenation Reactions

Densely functionalized cyclopentane derivatives with up to four consecutive stereocenters are assembled by a tandem Michael addition/single‐electron transfer oxidation/radical cyclization/oxygenation strategy mediated by ferrocenium hexafluorophosphate, a recyclable, less toxic single‐electron transfer oxidant. Ester enolates were coupled with α‐benzylidene and α‐alkylidene β‐dicarbonyl compounds with switchable diastereoselectivity. This pivotal steering element subsequently controls the diastereoselectivity of the radical cyclization step. The substitution pattern of the radical cyclization acceptor enables a switch of the cyclization mode from a 5‐exo pattern for terminally substituted olefin units to a 6‐endo mode for internally substituted acceptors. The oxidative anionic/radical strategy also allows efficient termination by oxygenation with the free radical 2,2,6,6‐tetramethyl‐1‐piperidinoxyl, and two C?C bonds and one C?O bond are thus formed in the sequence. A stereochemical model is proposed that accounts for all of the experimental results and allows the prediction of the stereochemical outcome. Further transformations of the synthesized cyclopentanes are reported.     

Construction of Bridged Polycyclic Systems by Polyene Cyclization

A stereoselective construction of bridged tri‐ and tetracyclic systems embedded in some 3,5‐dimethylorsellinic acid (DMOA)‐derived meroterpenoids was achieved by exploiting a polyene cyclization of suitably functionalized epoxyallylsilanes. Both the olefinic substituent on the epoxide and allylic trimethylsilyl (TMS) group were found to play pivotal roles in the success of the present reaction. The fact that the cyclization of monocyclized byproducts did not proceed strongly suggests that the reaction could be a concerted transformation.     

Studies on julimycins—VI The structures of julichromes Q1·2, Q2·2, Q3·5, Q3·5, Q4·5, Q2·5 and Q5·5

Julichromes Q_1·2 and Q_2·2 are identified as the dehydration products of julimycin B-II. The structures of julichromes, which commonly have a new anthraquinonyl Q₅ unit, are confirmed by their preparation from known julichromes. The conversion reaction, Q₂ → Q₅, which involves an intra-molecular redox is probably concerned in the biosynthesis of this unit.     

Zur Struktur eines ungewöhnlichen Tetramers des Lithiumphenolats: [C6H5OLi · C4H8O]4 · C6H5OH

The Structure of an unusual Tetramere of Lithium Phenoxide: [C₆H₅OLi · C₄H₈O]₄ · C₆H₅OH Single crystals of lithium phenoxide have been obtained from THF. In the structure (P 2₁/n, Z = 4, a = 11.69 Å, b = 21.15 Å, c = 18.55 Å, β = 91.11°) four lithium atoms and four phenoxide oxygen atoms are cubically arranged. Further, each lithium atom coordinates the oxygen atom of a tetrahydrofuran molecule. The ideal cubeform structure is disturbed by one phenol molecule which is coordinated in addition to four phenoxide and four THF molecules. Hence, one edge of the cube (Li4? O4) is substituted by the coordination of the phenol oxygen atom O5 with Li4 and hydrogen bonding between O4 and the hydroxy group of phenol. Van der Waals forces are the only interaction between these complexes.     

Computational Study on the Unconventional Hydrogen‐Bonded F–H···C Systems

The F–H···YZ₂ (Y = C, Si, BH, A1H;Z = H, PH₃) systems were examined using density functional theory calculations. The main focus of this work is to demonstrate that the chemistry of Y(PH₃)₂ exhibits a novel feature which is a central Y atom with unexpected high basicity. Further, the hydrogen bond strength can be adjusted by the substitution of H atoms of YH₂ by PH₃ groups. The FH···C(PH₃)₂ system has the strongest hydrogen bond interaction, which is larger than a conventional hydrogen bond. In addition to electrostatic interaction, donor‐acceptor interaction also plays an important role in determining the hydrogen bond strength. Therefore, a carbon atom can not only be the hydrogen bond acceptor but also can create an unusual stabilized hydrogen bond complex. Also, X₃B–YZ₂ (X = H, F; Y = C, Si, BH, A1H;Z = PH₃, NH₃) systems were examined, and it was found that the bond strength is controlled predominately by the HOMO‐LUMO gap (ΔIP). The smaller the ΔIP, the larger the bond dissociation energy of the B–Y bond. In addition, NH₃ is a better electron‐donating group than PH₃, and thus forms the strongest donor‐acceptor interaction between X₃B and Y(NH₃)₂.     

Zur Chemie in Schmelzlösungen. V. Darstellung kristalliner Halogenowolframate und Halogenomolybdate MeO · MeWO4 · nMeX2 bzw · MeO · MeMoO4 · nMeX2

Preparation and crystal growth of halogenotungstates and halogenomolybdates from molten salt solutions are described. Characteristic properties of this new class of compounds are communicated.     

The O–H···O, O–H···N and C–H···O hydrogen bonds in 1,4-dimethylpiperazine mono-betaine monohydrate

1,4-Dimethylpiperazine mono-betaine (1-carboxymethyl-1,4-dimethylpiperazinium inner salt, MBPZ) crystallizes as monohydrate. The crystals are orthorhombic, space group Pccn. Two MBPZ molecules and two water molecules form a cyclic oligomer, (MBPZ·H₂O)₂. The O–H···O and O–H···N hydrogen bonds are of 2.769(1) and 2.902(1) Å, respectively. The dimers interact with the neighboring molecules through the C–H···O hydrogen bonds of 3.234(1) Å. The piperazine ring assumes a chair conformation with the N(4)–CH₃ and N⁺(1)–CH₂COO⁻ groups in the equatorial position and the N⁺(1)–CH₃ group in the axial one. The FTIR spectrum is compared with that calculated by the B3LYP/6-31G(d,p) level of theory.     

Zur Struktur von zwei Hydraten des Natriumphenolats: C6H5ONa · H2O und C6H5ONa · 3 H2O

The Structures of two Hydrates of Sodium Phenoxide: C₆H₅ONa · H₂O and C₆H₅ONa · 3 H₂O In the monohydrate of sodium phenoxide sodium is coordinated by 4 oxygen atoms having an average distance Na? O of about 2.631 Å being arranged in form of a distorted tetrahedron. The oxygen atoms of water and phenoxid serve as bridging ligands. Hence, the structure can be considered as a network with a general formula [Na^[4]O]. Moreover, the oxygen atoms are linked via hydrogen bonding. In the trihydrate of sodium phenoxide sodium is surrounded with 5 oxygen atoms with an average distance of 2.39 Å forming a tetragonal pyramide. The oxygen of the phenoxide, however, does not participate in the coordination of the sodium ion. The coordination polyhedrons are connected by sharing edges and verteces. The resulting layer can be described by the general formula [Na^[5]O₂^[2]O^[2]O^[1]]. Via hydrogen bonding the phenoxide ions are attached to this layer.     

Crystal chemistry of MX · MgX2 · 6H2O type compounds

The following MX · MgX₂ · 6H₂O compounds (double salt hexahydrates) were synthesized by variation of the M⁺ and X^? ions: CsCl · MgCl₂ · 6 H₂O, Li(H₂O)Cl · MgCl₂ · 6H₂O, NH₄Br · MgBr₂ · 6 H₂O, RbBr · MgBr₂ · 6 H₂O, CsBr. MgBr₂ · 6 H₂O, KI · MgI₂ · 6 H₂O, NH₄I. Mgl₂ · 6 H₂O and RbI · MgI₂ · 6H₂O. By X-ray analysis of powder samples the lattice parameters and the space group were determined. On the basis of the results thus obtained, an identification with structural types was carried out. In accordance with the findings, the structure is made up of (M⁺)X₆^?octahedra which are linked into perovskite type units by sharing vertices. Their interstices are occupied by the Mg(H₂O)₆²⁺ octahedra. A “tolerance factor” t which has been calculated on the basis of the proportion of radii and which attains values between 1.045 and 1.061 is a criterion for the upper limit of the area of existence of this structure. Carnallite has a higher to value and, therefore, a different structure.