On the mechanism and diastereoselectivity of 2,3-dihydrobenzofuran formation from sulfinylbenzoquinones and 2-trimethylsilyloxyfuran

A mechanistic study of the reactions between 2-trimethylsilyloxyfuran 1 and (SS)-2-(arylsulfinyl)-1,4-benzoquinones 2a and 2b, giving rise to the diastereoselective formation of [3aS,8bS,SS]-3a,8b-dihydro-7-hydroxy-8-(arylsulfinyl)furo[3,2-b]benzofuran-2(3H)-ones 3a and 3b, is reported. The detection and ¹H NMR characterization of several precursors of 3a and 3b accounts for a Michael-type initial reaction which dictates the final diastereoselection of the process. A significant improvement of the stereoselectivity (up to 96% de) in the formation of the tert-butylsulfinyl substituted derivative 3c was achieved by using 2-(tert-butylsulfinyl)-1,4-benzoquinone 2c as the starting quinone.     

Formation and isomerization of cis,cis-Ir(H)2(--Ph)(CO)(PPh3)2 and cis,cis-Ir(H)(--Ph)2(CO)(PPh3)2 in oxidative addition of hydrogen and phenylacetylene to trans-Ir(CO)(--Ph)(PPh3)2

Oxidative addition of H–R (H--Ph and H₂) to trans-Ir(--Ph)(CO)(PPh₃)₂ (2) gives the initial products, cis, cis-Ir(H)(--Ph)₂(CO)(PPh₃)₂ (3a) and cis, cis-Ir(H)₂(--Ph)(CO)(PPh₃)₂ (3b), respectively. Both cis-bis(PPh₃) complexes, 3a and 3b undergo isomerization to give the trans-bis(PPh₃) complexes, trans, trans-Ir(H)(--Ph)₂(CO)(PPh₃)₂ (4a) and cis, trans-Ir(H)₂(--Ph)(CO)(PPh₃)₂ (4b). The isomerization, 3b→4b is first order with respect to 3b with k₁=6.37×10⁻⁴ s⁻¹ at 25°C under N₂ in CDCl₃. The reaction rate (k₁) seems independent of the concentration of H₂. A large negative entropy of activation (ΔS^≠=−24.9±5.7 cal deg⁻¹ mol⁻¹) and a relatively small enthalpy of activation (ΔH^≠=14.5±3.3 kcal mol⁻¹) were obtained in the temperature range 15∼35°C for the isomerization, 3b→4b under 1 atm of H₂.     

Studies on the mechanism for stereoisomerization of 1-zirconacyclopent-3-yne compounds

The stereoisomerization of 2,5-disubstituted 1-zirconacyclopent-3-yne compounds, stable five-membered cycloalkynes, has been studied with regard to the mechanism. The bimetallic complex of 1,4-bis(trimethylsilyl)butatriene was synthesized and structurally characterized, although it seems unimportant for the stereoisomerization reactions. The isomerization of trans-1,1-bis(η⁵-cyclopentadienyl)-2,5-bis(trimethylsilyl)-1-zirconacyclopent-3-yne 2a into the cis-form in benzene-d₆ solution were observed using ¹H NMR spectroscopy at 50 °C in various concentrations. The reaction was first order with respect to trans-2a. This ruled out the possibility that a bimetallic complex was responsible for the isomerization. A kinetic isotope effect was observed (k_H/k_D = 1.8), suggesting that C–H activation is involved in the rate-determining step. A mechanism via hydrogen elimination from the complex of η⁴-π,π-coordination mode is proposed.     

Studies on the mechanism for stereoisomerization of 1-zirconacyclopent-3-yne compounds

The stereoisomerization of 2,5-disubstituted 1-zirconacyclopent-3-yne compounds, stable five-membered cycloalkynes, has been studied with regard to the mechanism. The bimetallic complex of 1,4-bis(trimethylsilyl)butatriene was synthesized and structurally characterized, although it seems unimportant for the stereoisomerization reactions. The isomerization of trans-1,1-bis(η⁵-cyclopentadienyl)-2,5-bis(trimethylsilyl)-1-zirconacyclopent-3-yne 2a into the cis-form in benzene-d₆ solution were observed using ¹H NMR spectroscopy at 50 °C in various concentrations. The reaction was first order with respect to trans-2a. This ruled out the possibility that a bimetallic complex was responsible for the isomerization. A kinetic isotope effect was observed (k_H/k_D = 1.8), suggesting that C–H activation is involved in the rate-determining step. A mechanism via hydrogen elimination from the complex of η⁴-π,π-coordination mode is proposed.     

Natural product synthesis from (8aR)- and (8aS)-bicyclofarnesols: synthesis of (+)-wiedendiol A, (+)-norsesterterpene diene ester and (−)-subersic acid

Both enantiomers (8aR)-7 and (8aS)-7 of bicyclofarnesol were synthesized from the enzymatic resolution products (1R,4aR,8aR)-1,2,3,4,4a,5,6,7,8,8a-decahydro-5,5,8a-trimethyl-2-oxo-trans-naphthalene-1-methanol-2-ethylene acetal (8aR)-5 (98% ee) and acetate of (1S,4aS,8aS)-1,2,3,4,4a,5,6,7,8,8a-decahydro-5,5,8a-trimethyl-2-oxo-trans-naphthalene-1-methanol-2-ethylene acetal (8aS)-6 (>99% ee), respectively. The formal synthesis of (+)-wiedendiol 1 was achieved via a coupling reaction of an ate complex derived from 1,2,4-trimethoxybenzene with allyl bromide (8aS)-8 derived from (8aS)-7. The total synthesis of (+)-norsesterterpene diene ester 2 was achieved, based on the synthesis of (13E,10S)-α,β-unsaturated aldehyde 12, derived from (8aS)-7, followed by the selective construction of the (3E,5E)-diene moiety including a C(2)-stereogenic centre in (+)-2. The total synthesis of (−)-subersic acid 3 was carried out based on a Stille coupling between allyl trifluoroacetate congener 25c, derived from (8aR)-7, corresponding to the diterpene part, and aryl stannane congener 26 in the presence of Pd catalyst and CuI as an additive.     

Stereoselective reduction of chiral trans-3-acetyl-4-alkylpyrrolidin-2-ones

A number of trans-3-acetyl-4-alkylpyrrolidin-2-ones 8a–d and 11a,b were prepared and reduction of the keto group, carried out with KBH₄ in CH₃OH, led mainly to 3-hydroxyethylpyrrolidin-2-ones 13a–d and 19a,b with high stereoselection. Configuration of the newly formed stereogenic centre on the hydroxyethyl chain was assigned by ¹H NMR data supported by molecular mechanic calculations and eventually confirmed by X-ray diffraction analysis of p-iodobenzoate derivative 15.     

Silicon-carbon unsaturated compounds. 73. Photolysis of cis- and trans-1,2-dimethyl-1,2-diphenyl-1,2-disilacyclohexane in the presence of tert-butyl alcohol and acetone

Irradiation of cis-1,2-dimethyl-1,2-diphenyl-1,2-disilacyclohexane (1a) in the presence of tert-butyl alcohol in hexane with a low-pressure mercury lamp bearing a Vycor filter proceeded with high stereospecificity to give cis-2,3-benzo-1-tert-butoxy-1,4-dimethyl-4-phenyl-1,4-disilacyclooct-2-ene (2a), in 33% isolated yield, together with a 15% yield of 1-[(tert-butoxy)methylphenylsilyl]-4-(methylphenylsilyl)butane (3). The photolysis of trans-1,2-dimethyl-1,2-diphenyl-1,2-disilacyclohexane (1b) with tert-butyl alcohol under the same conditions gave stereospecifically trans-2,3-benzo-1-tert-butoxy-1,4-dimethyl-4-phenyl-1,4-disilacyclooct-2-ene (2b) in 41% isolated yield, along with a 12% yield of 3. Similar photolysis of 1a and 1b with tert-butyl alcohol-d₁ produced 2a and 2b, respectively, in addition to 1-[(tert-butoxy)(monodeuteriomethyl)(phenyl)silyl]-4-(methylphenylsilyl)butane. When 1a and 1b were photolyzed with acetone in a hexane solution, cis- and trans-2,3-benzo-1-isopropoxy-1,4-dimethyl-4-phenyl-1,4-disilacyclooct-2-ene (4a and 4b) were obtained in 25% and 23% isolated yield. In both photolyses, 1-(hydroxymethylphenylsilyl)-4-(methylphenylsilyl)butane (5) was also isolated in 4% and 5% yield, respectively. The photolysis of 1a with acetone-d₆ under the same conditions gave 4a-d₆ and 5-d₁ in 18% and 4% yields.     

Zum verhalten von mono- und diolefinen gegenüber thallium(III)trifluoracetat

Cis-butene-2 yields with Tl(III)trifluoroacetate by cis-addition of two trifluoroacetoxygroups the bis-2,3-trifluoroacetate 1a of the mesobutane diol, whereas the trans-butene-2 affords the bis-2,3-trifluoroacetate of the racemic threo-compound as expected 2a. Cyclohexene is transformed by Tl(III)trifluoroacetate to the cyclotrimeric form 3a of cyclopentylaldehyde. Cis-cyclooctene is oxidized to trifluoroacetate 4a of cyclooctene-1-ol-3. The homoallylic alcohol 5 (cyclohexene-1-ol-4) suffers a remarkable—but not unexpected—fragmentation to Z-trifluoroacetoxy-hexene-4-al-55a. By treating the conjugated dienes cyclohexadiene and cyclopentadiene with this reagent only cis-addition of two trifluoroacetoxygroups takes place under formation of the mixture of cis-1,4-bis-trifluoroacetoxy-cyclohexene-2(8a) and cis-1,2-bis-trifluoroacetoxy-cyclohexene-3(8b) on the one and cis-1,4-bis-trifluoroacetoxy-cyclopentene-2(9a) plus cis-1,2-bis-trifluoroacetoxy-cyclopentene-3(9b) on the other side. 1,3-Butadiene and 2,3-dimethyl-butadiene are transformed under solely 1,2-addition of the trifluoroacetoxygroups to the bis-trifluoroacetoxy compounds 6a and 7a.     

Dimerisation des polyphenyl-allenes—III : Formation baso-catalysee de dimeres polycycliques du triphenyl-allene

Thermal opening of the 3trans dimer of triphenylallene 1 is conrotatory, and generates the diradical 2Z,Z. This intermediate, through ortho reclosures involving the two types of substituing phenyl groups (modes a or b), can lead to the bicyclic compounds 4a and 4b. The existence of an equilibrium between the 3trans dimer and its less stable isomers 4a and 4b is substantiated by the formation of naphthalenic derivatives 9a and 9b when the reaction is carried out in the presence of strong bases, which give rise to irreversible prototropic shifts 4 → 9. A weak base such as quinoline allows only a less complete isomerization of polyenes 4a and 4b, leading to their isomers 8a and 8b. The latter then thermally cyclize, in a classical way, and lead respectively to the tetracyclic compounds 12cis and 11b. The occurrence of such an equilibrium between 3trans and 4a,b accounts for the previously reported obtention of the dihydronaphthacene 14, in the precise instance of triphenyl-allene 1 dimerization, and more generally corroborates the recently proposed mechanism of naphthacenic compounds production from chlorinated polyphenyl-allenes.     

Diastereoselective Diels–Alder cycloaddition of [(1R)-10-(N,N-diethylsulfamoyl)isobornyl] 2H-azirine to nucleophilic 1,4-disubstituted 1,3-dienes

Chiral [(1R)-10-(N,N-diethylsulfamoyl)isobornyl] 2H-azirine 1 [Timén, A. S.; Somfai, P. J. Org. Chem. 2003, 9958–9963; Timén, A. S.; Fisher, A.; Somfai, P. Chem. Commun. 2003, 1150–1151]. was combined to a number of 1,4-disubstituted-2-aza-1,3-dienes 2a–g [Alves, M. J.; Durães, M. M.; Gil Fortes, A. Tetrahedron 2004, 6541–6553] to give cycloadducts 8a–g as major isomers. High to good diastereofacial differentiation of the two faces of the azirine is observed when R¹,R² = Ar 2a–e; diastereoselectivity drops drastically when R¹ = Me or H 2f,g. Cycloaddition of the azirine 1 to E,E-1,4-diacetoxy-1,3-butadiene shows complete diastereoselectivity giving cycloadduct 11a.     

Synthesis of 5-substituted-3-phenacyloxime-1,2,4-oxadiazoles from acetylated or benzoylated benzoylthiacetamide with hydroxylamine hydrochloride : Spectral evidence for a tetrahedral carbinolamine intermediate

Acetylated and benzoylated benzoylthiacetamide 1a,b reacts with hydroxylamine hydrochloride in refluxing ethanol to yield 3 - acetylamino - and 3 - benzoylamino - 5 - phenyl - isoxazoles 2a,b, while 3 -phenacyloxime - 5 - substituted - 1, 4a,b are formed in the presence of sodium acetate or in pyridine.When refluxed in ethanol containing small amounts of concentrated hydrochloric acid, the above 1,2,4-oxadiazoles 4a, b rearrange to the corresponding isoxazoles 2a, b.The reaction in pyridine has been studied at 35° by using ¹H and ¹³C NMR spectroscopy. It has been possible to detect an adduct between the carbinolamine intermediate, resulting from the addition of hydroxylamine to the keto carbonyl group, and the starting thiamide.     

A convenient synthesis of chiral amino acid derived 3,4-dihydro-2H-benzo[b][1,4]thiazines and antibiotic levofloxacin

A series of 3,4-dihydro-2H-benzo[b][1,4]thiazine derivatives 8a-g were synthesized via- a copper-catalyzed intramolecular N-aryl amination reaction on substituted 2-(2-bromophenylthio)-ethanamines which were synthesized by the nucleophilic substitution reaction of 2-bromobenzenethiol with Boc-protected amino alcohol derivatives. This strategy provides a short and an efficient entry to (S)-3-methyl-1,4-benzoxazine 12, an advanced synthetic intermediate for the synthesis of levofloxacin.     

1-Methyl-2-((1E,3E)-4-phenylbuta-1,3-dienyl)pyridinium iodide: Synthesis,characterization, and X-ray analysis

1-Methyl-2-((1E,3E)-4-phenylbuta-1,3-dienyl)pyridinium iodide (1) is synthesized and characterized by UV-Vis, FT-IR, ¹H and ¹³C NMR, and single crystal X-ray diffraction. Compound 1 crystallizes in an orthorhombic Iba2 space group with the unit cell parameters a = 12.8236(11) Å, b = 20.1216(17) Å, c = 11.2071(10) Å, V = 2891.8(4) ų and Z = 8. X-ray diffraction indicates that the molecule of 1 displays E,E configurations with respect to the two middle C=C double bonds. The molecular structure of 1 is twisted and the dihedral angle between the pyridinium and phenyl rings is 15.0(2)°. In the crystal packing, cations form stacks in an anti-parallel V-shaped manner along the a axis through π-π interactions between pyridinium and phenyl rings.     

Novel thiosemicarbazone derivatives containing benzimidazole moiety: Green synthesis and anti-malarial activity

A series of (E)-2-[5-chloro-1-(1H-benzo[d]imidazol-2-yl)ethylidene] N-(substituted) hydrazine carbothioamide (7a–7t) and (E)-2-[1-(1H-benzo[d]imidazol-2-yl)ethylidene] N-(substituted) hydrazine carbothioamide (8a–8t) were prepared via the synthesis of 1-(substituted-1H-benzimidazol-2-yl) ethanol (3a–3b) which was synthesized by the condensation of substituted o-phenylenediamine (2a–2b) with dl-lactic acid (1) followed by oxidation with sodium hypochlorite in mild acidic condition to form the corresponding ketones 4a–4b. Final compounds were formed by condensation of 4a–4b with different thiosemicarbazides 6a–6t. A total of 40 compounds were synthesized and characterized by FT-IR, ¹H NMR, ¹³C NMR, Mass spectral technique and elemental analysis, in addition they were evaluated for anti-malarial properties. Among the compounds tested 7o, 7p, 7q, 7r, 7s, 8e and 8h exhibited good antimalarial activity in vitro.     

An improved synthesis of ethyl cis-5-iodo-trans-2-methylcyclohexanecarboxylate, a potent attractant for the Mediterranean fruit fly

Both racemic ethyl 5-iodo-2-methylcyclohexanecarboxylate (1), known as Mediterranean fruit fly attractant ceralure B₁, and its (−)-(1R,2R,5R) enantiomer 1a were conveniently synthesized from commercially available racemic trans-6-methyl-3-cyclohexenecarboxylic acid 2 or its (1R,6R) enantiomer 2a. Key steps included an asymmetric Diels-Alder reaction using a sultam auxiliary and cyclization of the unwanted trans-5-iodo-trans-2-methylcyclohexanecarboxylic acid (8) to the intermediate lactone 7 (or 8a to 7a). The new method may circumvent chromatographic separations and seems amenable to scale-up.     

Installation of carbon chain onto 2-cyclohexene-1,4-diol monoacetate

Alkylation of cyclohexenyl monoacetate 1 with R₂Cu(CN)(MgCl)₂ or RMgBr/CuCN (cat.) in Et₂O produced trans 1,2-isomers 4, while arylation and alkenylation of 1 was accomplished with lithium borates 5 and a nickel cat. to afford trans 1,4-isomers 3 selectively. Furthermore, several transformations of the products were carried out to demonstrate synthetic advantages of the present reactions.     

Synthesis of 1,4-disilacyclohexa-2,5-dienes

Title compounds of the type 2,3,5,6-tetraphenyl-1,4-di-X-1,4-di-Y-1,4-disilacyclohexa-2,5-diene wherein X=Y=NMe₂ (4); X=NMe₂, Y=Cl (cis, trans-5); X=NMe₂, Y=Me [(trans)-6] and X=t-Bu, Y=Cl (trans-8) were synthesized from Si₂(NMe₂)₅Cl, sym-Si₂(NMe₂)₄Cl₂, sym-Si₂(NMe₂)₄Me₂, and sym-Si₂Cl₄(t-Bu)₂, respectively, in the presence of diphenylacetylene at 200 °C. Similarly the analogous title compound from the combination of 1-phenyl-1-propyne and Si₂(NMe₂)₅Cl [X=Y=NMe₂ (cis and trans-7) was synthesized. In all cases where cis/trans diastereomers could arise from two different silicon substituents (5, 6, 8) the trans isomer was the sole or dominant product. Evidence for the intermediacy of the silylene Si(NMe₂)₂ in these reactions was gained from a trapping experiment. Compound 4 upon treatment with SiCl₄, SiBr₄ or PI₃ provided the corresponding 1,1,4,4-tetrahalo derivatives 9a-c, respectively. Treatment of 4 with MeOH or PhOH gave the 1,1,4,4-tetramethoxy and tetraphenoxy analogues 9d and 9e, respectively. The tetrachloro derivative 9a upon LAH reduction led to the corresponding tetrahydro compound 10, while the reaction of 9a with H₂O gave the tetrahydroxy derivative 11. Allowing (trans)-6 to react with SiCl₄ provided a ca. 1:1 cis/trans ratio of the derivative 12 in which X=Cl, Y=Me, and possible pathways that rationalize this loss of stereochemistry are proposed. Synthesis of trans-13 in which X=t-Bu, Y=H was achieved by LAH reduction of 8. All of the title compounds except 8 experience free phenyl rotation at room temperature. At −30 °C this rotation in 8 is essentially halted. The molecular structures of 4, 8, 9c, 9e, 10 and 13 were determined by X-ray crystallography.     

Addition conjuguée dans le thf-hmpt d'arylacétonitriles lithies aux cyclohexèn-2 ones substituées: intervention des facteurs stériques et électroniques

The reaction of lithiated arylacetonitriles 2a-c with 2-cyclohexenones 3-9 in THF-HMPA (80:20 v/v) at ?70° leads in all cases but one to δ-ketoalkylnitriles, resulting from 1,4-addition. The only case where some 1,2-addition takes place is the reaction of 2a with 4,4-dimethyl 2-cyclohexenone 7. The formation of allylic alcoholates, products of 1,2-addition, is shown to be reversible in this solvent mixture. However, in several cases, kinetic control of 1,4-addition is demonstrated. The rate of formation of 1,4-addition products depends upon the arylacetonitrile substituents: 2a >2b >2c (i.e. the higher lying the nucleophile HOMO the faster the reaction). It is also sensitive to the cyclohexenone substituents; although methyl on C-2 and gem-dimethyl on C-5 or t-butyl on C-4 have small influence on the reaction rate, methyl on C-3 or gem dimethyl on C-4 induce a noticeable rate decrease. The addition, of 2b on 4-t-butyl 2-cyclohexenone 8 leads, at the early stage, to a predominating isomer 15b, to which the trans diaxial configuration of the substituents has been attributed by 250 MHz ¹H NMR. A single transition state model, reagent-like, where the nucleophile approaches the C-C bond by the less hindered face of the 2-cyclohexenone in a 1,2-diplanar conformation, the CN substituent being directed towards the C=0 group, accounts for all the substituent effects and the stereoselectivity observed.     

Octahedral bis(acetylide) and bis(diacetylide) complexes of ruthenium(II) with linear C2MC2 and C4MC4 chains: The first X-ray structures of the all trans bis(acetylides) Ru(CO)2(CCPh)2(PEt3)2 and Ru(CO)2(CCH)2(PEt3)2

Bis(acetylides) and bis(diacetylides) of ruthenium(II), trans-Ru(CO)₂(PEt₃)₂(CCR)₂ (1) (1a, R  Ph; 1b, R  ^tBu; 1c, R  SiMe₃; 1d, R  H) and trans-Ru(CO)₂(PEt₃)₂(CCC CR)₂ (2) (2a, R  SiMe₃; 2b, R  H) have been synthesized and characterised. The first single crystal X-ray analyses of these all trans-acetylides have revealed linear C₂RuC₂ chains in 1a and 1d.     

Reactions of dispiro[2.0.2.2]oct-7-ene. Electrophilic and free radical additions

Dispiro[2.0.2.2]oct-7-ene 1 was synthesized by debrominatioa of cis- and trans-7,8-dibromodispiro[2.0.2.2]octane 3a with LAH and by dechlorination of cis- and trans-7,8-dichlorodispiro[2.0.2.2]octane 3b with magnesium. Stepwise electrophilic additions to 1 of HBr, HI, Br₂ and Cl₂ were studied. The major products (and yields) from these reactions were: 7-bromodispiro[2.0.2.2]octane 2a (43%), 4-iodo-4,5-ethanospiro[2,3]hexane 4b (ca. 50%); trans-3a (40%); and cis-3b (20%). Free-radical addition of hydrogen bromide to 1 gave an 80% yield of 7-bromodispiro[2.0.2.2]octane 2a. At ?10°, hydroboration-oxidation of 1 was found to give mainly 7-hydroxydispiro[2.0.2.2]octane 2a in ca. 90% yield; at 25°, near equal amounts of 2c and 4-(2-hydroxyethyl) spiro[2.3]hex-4-ene 14 were obtained.