The face selectivity directed by an allylic group of a diene in the diels-alder reaction is reversed from that of a dienophile

The Diels-Alder reaction of 2-alkoxy hepta-3,5-dienes with N-phenyl-maleimide gives good diastereofacelectivity; and a face selectivity rule is formulated.     

Reactions of carbon monosulphide. An experimental kinetic technique and a study of the reaction of CS with O2

A flow-tube technique has been developed for studying the kinetics of elementary reactions of gaseous monomeric CS. Carbon monosulphide is generated by means of a low-power electrodeless discharge in a dilute CS₂/Ar stream, to which reactive gases may be added through a moveable injector. Detection of CS at pressures of less than 2 × 10^?4 torr is achieved using multi-path ultraviolet absorption. The reaction of CS with O₂ has been shown to be very slow in the temperature range 300 K to 670 K.     

An unusual reaction product from a 1,3-diborolene and tetracarbonylnickel. Structure of a bis(allyl)nickel complex

The bis(allyl)nickel complex (η³-3-vinyl-2,4,5,6-tetraethyl-1-oxa-2,6-diboracyclohexenyl)(η³-2,3,4,5,6-pentaethyl-1-oxa-2,6-diboracyclohexenyl)nickel(IV) is formed by initial insertion of CO from Ni(CO)₄ into the five-membered 1,3-diborolene I, to give a six-membered ring. Subsequent exchange of the CHCH₃ group of I for the oxygen atom of the inserted CO and migration of a hydrogen atom from the

group of one ring to that of the other results in formation of the bis[1-oxa-2,6-diboracyclohexenyl]nickel, IV, having one vinyl and nine ethyl substituents. An X-ray structural analysis of IV shows the non-planarity of the C₃B₂O rings; the boron and oxygen atoms lie 0.4 and 0.7 Å, respectively, away from the best plane through the allyl carbon atoms. IV crystallizes in the space group P2₁/n with a = 9.065(2), b = 16.264(3), c = 10.187(2) Å, β = 104.28(1)°, and Z = 2.     

Molecular basis of Celmer's rules: role of the ketosynthase domain in epimerisation and demonstration that ketoreductase domains can have altered product specificity with unnatural substrates

BACKGROUND: Polyketides are structurally diverse natural products with a range of medically useful activities. Non-aromatic bacterial polyketides are synthesised on modular polyketide synthase multienzymes (PKSs) in which each cycle of chain extension requires a different 'module' of enzymatic activities. Attempts to design and construct modular PKSs that synthesise specified novel polyketides provide a particularly stringent test of our understanding of PKS structure and function. RESULTS: We show that the ketoreductase (KR) domains of modules 5 and 6 of the erythromycin PKS, housed in the multienzyme subunit DEBS3, exert an unexpectedly low level of stereochemical control in reducing the keto group of a synthetic analogue of the diketide intermediate. This led us to construct a hybrid triketide synthase based on DEBS3 with ketosynthase domain ketosynthase (KS)5 replaced by the loading module and KS1. The construct in vivo produced two major triketide stereoisomers, one expected and one surprising. The latter was of opposite configuration at three out of the four chiral centres: the branching alkyl centre was that produced by KS1 and, surprisingly, both hydroxyl centres produced by the reduction steps carried out by KR5 and KR6 respectively. CONCLUSIONS: These results demonstrate that the epimerising activity associated with module 1 of the erythromycin PKS can be conferred on module 5 merely by transfer of the KS1 domain. Moreover, the normally precise stereochemical control observed in modular PKSs is lost when KR5 and KR6 are challenged by an unfamiliar substrate, which is much smaller than their natural substrates. This observation demonstrates that the stereochemistry of ketoreduction is not necessarily invariant for a given KR domain and underlines the need for mechanistic understanding in designing genetically engineered PKSs to produce novel products.     

An allenic Pauson-Khand-type reaction: a reversal in pi-bond selectivity and the formation of seven-membered rings

[reaction: see text] Treatment of alkynyl allenes with [Rh(CO)(2)Cl](2) results in a formal [2 + 2 + 1] cycloaddition reaction. This reaction occurs with complete regioselectivity for a variety of substrates affording only 4-alkylidene cyclopentenones in good yields. Moreover, seven-membered rings have been prepared in high yields.     

Reactions of trimethinecyanines of the 1,3-dioxenium series with nucleophiles. The structures of the parent carbocation and of a product of its reaction with methanol

The chemistry and the regioselectivity of alcoholysis, aminolysis, and hydrolysis of symmetrical trimethinecyanine carbenium ions in 4-[3-(2,2-diorganyl-6-phenyl-4H-1,3-dioxen-4-ylidene) prop-1-enyl]-2,2-diorganyl-6-phenyl-1,3-dioxenium perchlorates (4a,b) were investigated. The structures of trimethinecyanine 4,5:4′,5′-bisannelated at the methine chain (3) and of a product of the reaction of nonannelated trimethinecyanine with the methoxide anion were established by X-ray diffraction analysis,¹H NMR spectroscopy, and quantum-chemical calculations (PM3). The nucleophile attacks the mesomeric π-conjugated system of the trimethinecyanine cation selectivily at the C(6) atom of one of the dioxenium fragrments, the second dioxenium ring being deactivated. Alcoholysis affords products of addition of the methoxy group to trimethinecyamines. In the course of aminolysis and hydrolysis, the dioxenium ring that is subjected to the attack eliminates acetone to form 1,3-dioxenylidenepropenylic derivatives of 1,3-enaminoketones and 1,3-ketoenols, respectively. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 3, pp. 521–529, March. 1999.     

On the origin of cytotoxicity of the natural product varacin. A novel example of a pentathiepin reaction that provides evidence for a triatomic sulfur intermediate

A density functional theoretical study is presented, which implicates a novel S(3)-cleavage in the decomposition of a pentathiepin. This study predicts an interconversion between a pentathiepin and an open-chain polysulfur ion intermediate from which a key determinant in the chemistry then follows. Expulsion of diatomic sulfur, S(2), is unlikely from the unimolecular collapse of the open-chain polysulfur ion. Instead, S(3) can dissociate due to an unusually long and weak sulfur-sulfur (S4-S5) bond. A mechanistic picture now emerges which predicts that the novel S-S cleavage reaction and the unanticipated S(3) fragmentation are a result of delocalization of the negative charge within the remaining carbon-sulfur fragment. The computed results presented here reveal a new aspect to the chemistry of pentathiepins, that of S(3) unit transfer, which is proposed to have significance in the mechanism of cytotoxicity of the natural product varacin, 1.     

An analysis of the difficulties associated with determining that a reaction in chemical equilibrium is incomplete

Foundations of Chemistry - There are inherent difficulties in a subject like chemistry particularly the notion of a chemical reaction. In this paper the difficulties are discussed from a teaching...     

An intramolecular darzens reaction of the ester group with the active halogen group. Preparation of 2,7-dioxabicyclo[4.1.0] heptanes from 1-bromo-4-acyloxybutane derivatives

When 1-bromo-4-acyloxybutanes having electron-withdrawing groups at 1-position, which were obtained by Cu(I)-catalyzed photochemical addition of 1-bromo-2-acyloxyethanes to electron-deficient olefins, were subjected to treatment with potassium t-butoxide, 2,7-dioxabicyclo[4.1.0] heptanes were obtained.     

Neryl- and geranyltriethylammonium halides in the allylation of sodium diethyl malonate. The effect of the leaving group of the allylating reagent on the selectivity of the reaction

In the absence of catalysts,N-neryl-andN-geranyltriethylammonium halides can allylate sodium diethyl malonate to give terpene malonate derivatives. Using the above-mentioned ammonium salts, geranyl and neryl acetates, geranyl ethyl carbonate, and geranyl diethyl phosphate as examples, it has been shown that with Pd⁰ and Pd^II catalysts, the selectivity of the formation of neryl-, geranyl-, and linalylmalonates can be governed by varying the leaving group of the allylating agent.Translated fromIzyestiya Akademii Nauk. Seriya Khimicheskaya, No. 2, pp. 270–272, February, 1994.     

Ziegler-Natta 型催化剂NdCl3·3I-PtOH-AlEt3 两组分间的反应及其反应产物对双稀烃聚合的催化活性

The reactions between the two components of the rare-earth coordination catalyst, NdCl3.3i-PrOH and AlEt3, were studied. It was found that the activated solid product was a mixture containing alkylated neodymium compounds, but no alkyl chloride was formed during the reaction. The alkyls bounded to neodymium or to the bridge between neodymium and aluminium atoms are thermally less stable than those bonded to the aluminium atom. Two possible structures of alkylated neodymium compounds were postulated. The catalytic activity in the isoprene polymerization increases with the increase of degree of alkylation and decreases in the presence of alkyl aluminium chloride.     

Change in the selectivity of the action of catalysts during the hydrogenation of the diene group. Communication 2. Modification of a nickel catalyst with cadmium

Russian Chemical Bulletin -     

Reactions of an indolinonic nitroxide with superoxide radical anion in the presence of alkylhalides. Unexpected formation of a reduced transposed product

This study concerns the reactions of 2‐methyl‐2‐phenyl‐3‐phenylimino‐2,3‐dihydroindol‐l‐oxyl and 2,2,6,6‐tetramethylpiperidine‐1‐oxyl with alkylperoxyls, generated from potassium superoxide and a series of alkylhalides, in order to evaluate possible differences in reactivity with primary, secondary and tertiary alkylperoxyls. To better understand the reactivity of the studied indolinonic aminoxyl in alkaline medium, the investigation was extended to its reactions with potassium hydroxide and potassium tert‐butoxide in different solvents.     

Entropy-controlled selectivity in the vinylation of a cyclic chiral nitrone. An efficient route to enantiopure polyhydroxylated pyrrolidines

A short synthesis of 1,4-dideoxy-1,4-imino-L-arabinitol (LAB1) (4) and of 1,4-dideoxy-1,4-imino-D-galactitol (5), two azasugars active as enzymatic inhibitors, is reported. The key reaction is the addition of vinylmagnesium chloride to (3S,4S)-3,4-bis(benzyloxy)-3,4-dihydro-2H-pyrrole 1-oxide (3), a nitrone easily available from L-tartaric acid. Unexpectedly, the reaction affords the corresponding (2S,3S,4S)-1-hydroxy-2-ethenyl-3,4-bis(benzyloxy)pyrrolidine (9) in very good yield and in 93/7 diastereomeric ratio (dr) independently of the reaction temperature, thus representing a unique case of entropy-controlled reaction in a 100 K interval (from +20 degrees C to -80 degrees C). The trans intermediate 9 is converted in two steps (reduction, N-protection) into the common intermediate (2S,3S,4S)-1-(benzyloxycarbonyl)-3,4-bis(benzyloxy)-2-ethenylpyrrolidine (11). Double bond oxidation followed by reductive debenzylation opens a route to the target pyrrolidine azasugars 4 and 5.     

An anion-dependent switch in selectivity results from a change of C-H activation mechanism in the reaction of an imidazolium salt with IrH5(PPh3)2

Changing the counteranion along the series Br, BF4, PF6, SbF6 in their ion-paired 2-pyridylmethyl imidazolium salts causes the kinetic reaction products with IrH5(PPh3)2 to switch from chelating N-heterocyclic carbenes (NHCs) having normal C2 (N path) to abnormal C5 binding (AN path). Computational work (DFT) suggests that the AN path involves C-H oxidative addition to Ir(III) to give Ir(V) with little anion dependence. The N path, in contrast, goes by heterolytic C-H activation with proton transfer to the adjacent hydride. The proton that is transferred is accompanied by the counteranion in an anion-coupled proton transfer, leading to an anion dependence of the N path, and therefore of the N/AN selectivity. The N path goes via Ir(III), not Ir(V), because the normal NHC is a much less strong donor ligand than the abnormal NHC. PGSE NMR experiments support the formation of ion-pair in both the reactants and the products. 19F,1H-HOESY NMR experiments indicate an ion-pair structure for the products that is consistent with the computational prediction (ONIOM(B3PW91/UFF)).     

Two contrasting ethynyl hydroboration pathways in the formation of a novel tris-hydroboration product from reaction of dimesitylborane with 2,5-diethynylpyridine

Reaction of 2,5-diethynylpyridine with dimesitylborane, [(Mes)(2)BH](2)(Mes = mesityl = 2,4,6-Me(3)C(6)H(2)), gave the unexpected tris-hydroboration product 1-[(Mes)(2)B]-2-[Z-1-[(Mes)(2)B]ethylidene]-5-[E-[(Mes)(2)B]vinyl]-1,2-dihydropyridine, which has been structurally characterised by single-crystal X-ray diffraction.     

Unexpected dramatic substituent effect for tuning the selectivity in the double ring-closing metathesis reaction of N-containing tetraenes. An efficient synthesis of bicyclic izidine alkaloid skeletons

[reaction: see text] A double ring-closing metathesis reaction for the efficient construction of the fused bicyclic izidine alkaloid skeleton was developed. In this reaction, high selectivity was realized by tuning of electronic and steric effects of substituents in the N-containing tetraenes. It was observed that the reactivity of electron-rich carbon-carbon double bonds is higher than that of electron-deficient ones. A brief mechanistic study is also discussed.     

Radical Products in the Chemical Oxidationof Amines. An ESR Study of Secondary CationRadicals from Aniline and Derivatives of 1,2-Phenylenediamine

Summary.  The oxidation of aniline and some derivatives of 1,2-phenylenediamine was performed by the system Pb(CH₃COO)₄—CF₃COOH—CH₂Cl₂. An ESR investigation of the oxidized aniline confirmed the presence of secondary cation radical. Its structure was determined by comparison of its ESR spectrum with those of cation radicals prepared from derivatives of 1,4-phenylenediamine. The oxidation of 1,2-phenylenediamine and its derivatives leads to the formation of either primary or secondary cation radicals, the latter having the character of substituted dihydrophenazinium cation radicals. Received August 17, 2000. Accepted (revised) October 16, 2000