Migration of the exocyclic double bond in terpenoid coumarins of the iresane series

In terpenoid coumarins of the iresane series with an exocyclic double bond, migration of the double bond into the ring with the retention of the configuration of the substituent in position 1 is observed in an acid medium. The reaction has been performed in CF₃COOH and has been monitored by the PMR method. Badrakemin has yielded conferol, badrakemone has yielded conferone, badrakemin acetate has yielded conferol acetate, colladonin has yielded moschatol, and farnesiferol A and gummosin have yielded the corresponding isomers with endocyclic double bonds. The rate of the reaction is affected by the nature of the substituent at C-6. The presence of a keto group increases the time of isomerization to 1.5 h as compared with the 5–10 min for compounds with an OH group at C-6. The increase in the time of the reaction leads to the formation of byproducts. The reaction does not take place in CH₃COOH.All-Union Scientific-Research Institute of Medicinal Plants, Moscow. Translated from Khimiya Prirodnykh Soedinenii, No. 5, pp. 630–637, September–October, 1979.     

Diastereoselective concise syntheses of the polyhydroxylated alkaloids DMDP and DAB

A diastereoselective concise synthesis of the iminosugars DMDP and DAB is presented starting from l-xylose and affording the two alkaloids in good yields of 35% and 22% over seven and eight steps, respectively. The Petasis borono–Mannich reaction of 3,5-di-O-benzyl-l-xylofuranose with benzylamine and (E)-styrylboronic acid served as the nitrogen-introducing key step furnishing the new C–N bond in an entirely diastereoselective manner. A chemo- and regioselective O-mesylation followed by an intramolecular S_N2-cyclisation allowed the formation of the pyrrolidine ring. Ozonolysis of the styryl double bond and subsequent reduction to form the C-5 hydroxymethyl substituent followed by hydrogenolysis of the benzyl protecting groups concluded the DMDP synthesis. Furthermore, an unexpected fragmentation process during the ozonolysis reaction also gave access to the C-5 decarbinolated DMDP derivative DAB.     

Construction of AE ring system for the C19-diterpenoid alkaloids with a 5β-hydroxyl group

An AE azabicyclic fragment, with a β-hydroxyl group at C-5 and a substituent at C-1, of the C₁₉-diterpenoid alkaloids, was synthesized. The key reactions include an intramolecular Claisen-type condensation, double Mannich reaction, and stereoselective nucleophilic addition of carbonyl group with the assistance of steric effect of 1β-substituent.     

Influence of an 8-trifluoroacetyl group on flavanol couplings

The effect of an electron attracting substituent in the Lewis acid catalyzed oligomerization of flavanols was investigated. The results showed that the presence of a COCF₃, at the 8 position of a (+)-catechin unit strongly influenced the attack of the 6 free nucleophile flavanol position by the electrophile generated from a 4-O-alkyloxy protected catechin unit. This was observed either with TiCl₄ or TMSOTf as Lewis acids in which the carbon-carbon bond formation was inhibited and the corresponding dimer was detected in small amount. On the contrary, the formation of a carbon-oxygen bond was observed and the corresponding C-4→O→C-3 ether linked procyanidin dimer was isolated in a good yield. In order to avoid the participation of the C-3 hydroxyl group in the dimerization reaction, the two flavanol units were forced into C-4→C-8 coupling by use of an internal link. The structural elucidation of the isolated compounds was achieved through MS and NMR spectroscopy.     

The mutual influence of the imine substituents of terephthal-bis-imines concerning their reactivity towards Fe2(CO)9

The reaction of terephthal-bis-imines with Fe₂(CO)₉ proceeds via a C---H activation reaction in the ortho position with respect to one of the imine functions. The corresponding hydrogen atom is shifted towards the former imine carbon atom producing a methylene group instead. The dinuclear iron complexes formed by this reaction sequence and showing no coordination of the second imine group were isolated from reactions of bis-imines with both phenyl and cyclohexyl substituents at the imine nitrogen atoms. In addition, we observed three different reaction pathways of the second imine substituent of the starting material which is obviously thus influenced by the fact that the first one is coordinating an Fe₂(CO)₆ moiety. If the organic substituent at the imine nitrogen atoms is a phenyl group the formation of a trinuclear complex is achieved in which an additional Fe(CO)₃ group is coordinating the CN double bond and one of the carbon---carbon bonds of the central phenyl ring in an η⁴-fashion. The same reaction leads to the isolation of a tetranuclear iron---carbonyl compound in which both imine substituents were transformed via the pathway described above, each building up dinuclear subunits. In contrast to this the reaction of a bis-imine with cyclohexyl groups at the imine nitrogen and thus an enhanced nucleophilicity leads to the formation of a tetranuclear complex in which only one imine group reacts under C---H activation with subsequent hydrogen migration towards the former imine carbon atom. The second imine substituent also shows a C---H activation reaction in the ortho position with respect to the imine group but the corresponding hydrogen atom is transferred to one of the aromatic carbon atom of the central phenyl ring of the ligand. The C=N double bond remains unreacted and only coordinates the second Fe₂(CO)₆ moiety via the nitrogen lone pair.     

Isomerisierung von Methoxy- und Carbomethoxysubstituierten Cycloalkyl- und Alken-radikalkationen

The very complex isomerization patterns of methoxy and carbomethoxy substituted cycloalkanes (3- to 7-membered rings) have been investigated using collisional activation, metastable ion characteristics and field ionization kinetics. The extent of isomerization depends on both the ring size and the substituent. Irrespective of the electronic properties of the substituent, ring opening involves exclusively the C-1? C-2 bond whereby linear alkene radical cations are formed. In the case of OCH₃- and COOCH₃ substituents the position of the resulting double bond (terminal or α,β-unsaturated) is determined more by the ring size of the precursor molecules and less by the electronic properties of the substituents. Contrary to these findings alklyl substituted cycloalkanes (3- to 5-membered rings) rearrange exclusively to terminal alkene radical cations. The barrier for double bond isomerization seems to be substantially influenced by substituents.     

Reactions of fluorinated silanes with 2-nitrocinnamates

The interaction of 2-nitrocinnamates with silicon reagents Me₃SiR_f (R_f = CF₃, C₂F₅, and C₆F₅) promoted either by sodium acetate in DMF or by tetrabutylammonium acetate in dichloromethane has been described. The reactions proceed as conjugate addition of fluorinated carbanion at the CC bond and afford 3-aryl-2-nitrobutanoates bearing a fluorinated substituent in good yields as diastereomeric mixtures in ratio from 1:1 to 1.6:1.     

Remote substituent effects in ruthenium-catalyzed [2+2] cycloadditions: an experimental and theoretical study

The effects of a remote substituent on the regioselectivity of ruthenium-catalyzed [2+2] cycloadditions of 2-substituted norbornenes with alkynes have been investigated experimentally and theoretically using density functional theory. Most of the cycloadditions occurred smoothly at room temperature, giving the exo cycloadducts in excellent yields. Regioselectivities of 1.2:1 to 15:1 were observed with various substituents on the C-2 position of the norbornenes. Exo-C-2-substituents usually showed greater remote substituent effects on the regioselectivities of the cycloadditions than the corresponding endo-C-2-substituents. The regioselectivity of the cycloadditions with C-2 substituents containing an exocyclic double bond (sp2 hybridized carbon at C-2) are much higher than the cycloadditions with the exo and endo 2-substituted norbornenes. Theoretical studies predicted the same trends as experiment and matched the experimental product ratios well. The nature of the regioselectivity in this reaction is discussed. Different strengths of the pi(C5-C6)-->pi(C2-Y) or pi(C5-C6)-->sigma(C2-Y) orbital interactions in 2-substituted norbornenes result in different degrees of C5-C6 double bond polarization. Stronger C5-C6 polarization will increase the difference in the activation energies between the major and minor pathways and thus lead to greater regioselectivities.     

Recherches sur la formation et la transformation des esters LXXII. Aryl(ou aralcoyl ou alcoyl)amino-2-tétrahydro-m-thiazines ou aryl(ou aralcoyl ou alcoyl)amino-2-dihydro-Δ2-m-thiazines et dérivés

3-Aminopropanol reacts with aryl(or aralkyl or alkyl)isothiocyanates R? N?C?S to yield the corresponding thio-ureas R? NH? CS? NH? (CH₂)₃OH which, refluxed with hydrochloric acid, are cyclized by elimination of water. The cyclization products are identical with the hydrothiazines resulting by elimination of sulfate or phosphate from the sulfuric or phosphoric monoesters of these thio-ureas. The resulting hydrothiazines are either 2-(R-imino)-tetrahydro-m-thiazines (I) or 2-(R-amino)-dihydro-Δ²-m-thiazines (II). Their structure has been established by comparison of their spectra with those of model compounds in one of which the C?N double bond is certainly endocyclic (2-methyl-dihydro-Δ²-m-thiazine), the other presenting an exocyclic C?N double bond (3-methyl-2-phenylimino-tetrahydro-m-thiazine). When R is an aryl group, the C?N double bond is exocyclic (structure I with >C?N? Ar), and one may presume that this structure is stabilized by resonance. When R is an aralkyl or an alkyl group, the C?N double bond is endocyclic (structure II). The nmr spectra were taken with three types of solvent: CDCl₃ or CCl₄; (CD₃)₂SO; CF₃COOH. In CF₃COOH solution the benzylic protons of the hydrothiazine with R = pF? C₆H₄CH₂? couple with NH (J=5,5cps) which confirms the endocyclic position of the C?N double bond in this case.     

Peculiarities of electroreduction of trans-2-allyl-6-methyl(allyl,phenyl)-1,2,3,6-tetrahydropyridines

Electrochemical reduction of trans-2-allyl-6-R-1,2,3,6-tetrahydropyridines (R = Me, All, and Ph) on the mercury cathode in anhydrous DMF (with 0.1 M Bu₄NClO₄ as the supporting electrolyte) resulted in catalytic hydrogen evolution, while in the case of anhydrous DMF the electrochemical activity of the endocyclic double bond was dictated by the nature of the R substituent at the carbon atom neighboring the double bond. The electrocatalytic hydrogenation of the piperideines under study on the Ni (Ni_disp/Ni) cathode in 40% aqueous DMF in the presence of a tenfold excess of AcOH yielded the corresponding trans-2-propyl-6-R¹-piperidines (R¹ = Me, Pr, and Ph). Using trans-2,6-diallyl-1,2,3,6-tetrahydropyridine as an example, the conditions (with annealed copper as the cathode) for selective hydrogenation of the double bonds in allyl substituents with preservation of the endocyclic double bond were found.     

Tetramethylammonium hydroxide (TMAH) thermochemolysis of 2-arylcoumaran lignin model compounds

Phenolic 2-arylcoumarans 1–6 were used to examine the behaviors of β-5 subunits in lignin during tetramethylammonium hydroxide (TMAH) thermochemolysis. Products were monitored by gas chromatography/mass spectrometry. The process predominantly provided dimeric products with the opened hydrofuran ring. Substituent changes at the γ-position of ring A and at the 5-position of ring B had a large effect on the product compositions. 2-Arylcoumarans 1 and 6 with the γ-CH₂OH substituent predominantly gave 2,3,3′,4′-tetramethoxystilbenes involving the elimination of the γ-CH₂OH substituent, while 2–5 with the γ-CH₃ substituent gave a mixture of 2,3,3′,4′-tetramethoxy-α-methylstilbenes and α-methoxy-α-(3′,4′-dimethoxyphenyl)-β-(2,3-dimethoxyphenyl)propanes. Substituent –CHCHCH₃ on ring B remained unaffected. Substituents –CHCHCH₂OH and –COOH on ring B produced the corresponding methyl ether and ester, respectively, by methylation. The –CHCHCHO substituent on ring B was converted to the –CHO substituent.     

A convenient synthesis of 1,2-difluoro-1, 2-dideoxyhexoses using xenon fluoride

A method for the addition of fluorine to the double bond in acetylated glycals using XeF₂ in the presence of BF₃ has been shown to provide a convenient route for the synthesis of acetylated 1, 2-dideoxy-1, 2-difluorosugars. The reagent attacks the double bond predominantly from the least hindered side to give a cis adduct, but the reaction also provided other isomers which have been separated by chromatography and their stereo-chemistry determined by ¹⁹F NMR spectroscopy. The proposed mechanism of the reaction involves an initial electrophilic attack by the reagent on C-2 followed by a nucleophilic attack on C-1. Deacetylation of the adducts gave 1, 2-dideoxyl-1, 2-difluorosaccharides, which are of interest in biochemical studies. A crystalline 2-deoxy-2-l-fucose was prepared by selective hydrolysis of the anomeric fluorine atom of 1, 2-dideoxy-1, 2-difluoro-l-fucose.     

Reactivity and regioselectivity in Stille couplings of 3-substituted 2,4-dichloropyridines

The influence of substituents at C-3 of 2,4-dichloropyridines on their reactivity and regioselectivity in Pd-catalyzed cross-couplings is studied. As a model reaction, the (Ph₃P)₂PdCl₂-catalyzed Stille coupling between 2-furyl(tributyl)tin and pyridines is chosen. Increased electron-withdrawing ability of a substituent at the pyridine 3-position improves the overall reactivity. Absolute selectivity for coupling at C-2 is achieved with an amino group at C-3, and the selectivity is totally reversed when the amino group is exchanged for a nitro substituent.     

Zur lokalisierung funktioneller gruppen mit hilfe der massenspektrometrie—VIII : 6-hydroxy-androstan-3,17-dione und 6,17β-dihydroxy-androstan-3-one

5β-androstan-3-ones carrying a 6α-OH group show in their mass spectra a key-ion indicating the loss of water and C-1 to C-4 as C₄H₅O? particle. 6β-OH isomers lose instead C-1 to C-4 in form of C₄H₇O?.In 6α-hydroxy-androstan-3-ones differentiation between the connection of the A/B-ring system is possible, because in 5α-isomers the loss of C-3 to C-7 occurs as a C₅H₆O₂ particle, while the 5β-isomers lose the same C atoms as a C₅H₇O? unit.Compounds with a 6β-OH group in an A/B trans connected ring system show a tendency for thermal water elimination. After rearrangement of the double bond in 4,5 position the typical fragments for 3-keto-Δ⁴-steroids are obtained.Occasionally a strong influence of a 6-OH group on fragmentation reactions in the D-ring system is observed: The presence of a 6α-OH group in an androstan-3,17-dione enhances the loss of C-16 and C-17 in the form of acetaldehydenol. Also the connection of the A/B-ring system may have a considerable influence on this type of reaction: In 6,17β-dihydroxy-androstan-3-ones only by trans connection of the A/B-ring system, C-16 and C-17 are lost with high probability after water elimination.     

Enantioselective access to tetrahydropyrano[2,3-c]pyrazoles via an organocatalytic domino Michael-hydroalkoxylation reaction

An asymmetric domino Michael-hydroalkoxylation reaction of trans-α-alkynyl-nitroolefins with N-arylpyrazolinones has been accomplished using a chiral bifunctional squaramide catalyst. Under the organocatalytic method, a broad range of tetrahydropyrano[2,3-c]pyrazoles with an exocyclic alkene at the C-6 position were prepared in high yields and excellent stereoselectivities. The presence of an exocyclic double bond and nitro group in the pyranopyrazoles provide a wide scope for further structural transformations.     

The adsorption of carbon monoxide on a platinum electrode

The adsorption of CO from 0.5 M H₂SO₄ solution on platinum has been studied using CO labelled with C-14. The adsorption of CO on Pt occurs in the potential range of hydrogen adsorption as well as in the double layer region. In the whole potential range the rate of adsorption follows first order kinetics. From the surface concentrations and charges for oxidation of adsorbed species it follows that the product of chemisorption consists at least of two kinds of species. One of them is the COOH radical probably formed by the reaction of CO with water.     

Carbon-13 NMR studies on some 5-substituted quinoxalines

Eleven 5-substituted quinoxalines (NO₂, NH₂, COOH, OCH₃, CH₃, OH, F, Cl, Br, I, CN, the latter five not reported previously) have been synthesized by standard methods. Their ¹³C NMR spectra have been measured in DMSO-d₆ and assigned on the basis of substituent parameters, by line widths and by intensities. The chemical shifts compare favorably with those calculated using benzene substituent parameters, and are very close to those of corresponding carbons in 1-substituted phenazines. The correlation with the chemical shifts of the corresponding positions in 1-substituted naphthalenes is also close except for those of carbons 4a and 8a in the quinoxalines which, due to their proximity to nitrogen, are downfield (in some cases 12 ppm) of the signals of the corresponding carbons in naphthalene. 5-Fluoroquinoxaline was also measured in CDCl₃, CD₃COCD₃, CD₃CN, CD₃OD, C₆D₆ and CD₃COOD. In all solvents an abnormally low ²J(CF) (～ 12 Hz) was found for C-4a and no C? F spin-spin splitting could be detected for the three-bond coupling of C-8a. Similar abnormalities were found in 2-fluoroaniline and 2-fluoroacetanilide. There are linear relationships between the Q parameter of the substituent and the chemical shift of carbons 4a, 5 and 6. A linear relationship also exists between the chemical shift of C-8 (‘para’ position) and the Hammett σ_p parameter of the substituent.     

Substituent and coupling effects in the IR spectra of arylidenecyclanones

By studying the IR spectra of arylidenecyclanones possessing exocyclic CC double bonds, a correlation has been found between the wave number and intensity values of the ν CO and ν CC vibrations and the substituent constants. Transmissive factors of the CC double bond have been calculated and these indicated that the most pronounced conjugation occurs in the case of the indanones and the smallest for 1-thiochromanones. It has also been established that the substituent effect transmitted by the CC double bond is mainly of a resonant character. Coupling of the ν CO and ν CC vibrations has been studied by the method of Taylor and Smith. It has been concluded that in the case of arylidene-indanones possessing strong coupling, of the two intense bands between 1600 and 1800 cm⁻¹ the one found at higher wave number is a result of an out-of-phase (ν_as) coupling of the ν CO and ν CC vibrations while the other originates from an in-phase (ν_s) coupling. Coupling was less in the case of the other compounds investigated. Our studies prove that for the evaluation of the substituent effects the coupling phenomena should be taken into consideration.     

Cage expansion of alkyne clusters. An example of ligand assistance by alkyne substituents

Starting from dicobalt hexacarbonyl complexes of ene-ynes or hetero-ene-ynes, FeCo₂C₂-clusters can be prepared in high yield by metal fragment condensation with (biscyclooctene)(tricarbonyl)iron. The reaction takes place at temperatures below 0°C and allows the introduction of terminal alkynes into the cluster framework. It has been shown that the ease of these reactions is due to the free double bond in α-position of the coordinated triple bond, thus providing direct evidence for a ligand-assisted cage expansion reaction, unprecedented in this field of chemistry. The crystal structure of a FeCo₂C₂-cluster bearing a chiral Fe(CO)₃-diene substituent has been determined.