Preparation of 1,4,5,8-tetra-tert-butyl-1,3,5,7-cyclooctatetraene by twofold SO2 extrusion

Oxidation of cis-1-transoid-1,2-cis-2-3,4,7,8-tetra-tert-butyl-9,10-dithiatricyclo[4.2.1.1^2,5]deca-3,7-diene 9,10-dioxide (2) by dimethyldioxirane gave the corresponding tetraoxide 4 quantitatively, which in turn provided 1,4,5,8-tetra-tert-butyl-1,3,5,7-cyclooctatetraene (6a) quantitatively by twofold thermal SO₂ extrusion. The molecular structure of 6a is described on the basis of DFT calculations (B3LYP/6-31G(d) level) and X-ray crystallographic analysis.     

PhP-PPh group bound to 1,8-positions of naphthalene: Preparation of cis isomer and synthesis of binuclear complex

A thermodynamically less stable cis isomer of 1,2-diphosphacycle was prepared from the corresponding trans isomer. Diphosphine, in which a PhP-PPh bond bridges the 1,8-positions of naphthalene, 1,2-diphenyl-1,2-dihydronaphtho[1,8-cd][1,2]diphosphole (1), was first prepared according to a previously reported method, and the trans isomer of 1 was irradiated in tetrahydrofuran with UV-vis light to reach equilibrium with cis-1 in a trans:cis ratio of 1:2. When a similar photochemical conversion was carried out using a saturated hexane solution of trans-1, cis-1 was precipitated in a good yield of 94%. The configuration of cis-1 was confirmed by X-ray analysis. Both cis- and trans-1 diphosphine ligands were used for the preparation of binuclear gold complexes. The crystal structure of (μ-cis-1)-[AuCl]₂ demonstrated that the two lone pairs of cis-1 are suitably directed for arrangement of the two gold centers in close proximity to each other. The two independent (μ-cis-1)-[AuCl]₂ molecules in the crystal were found to form a dimer through the multiple intermolecular interaction among the gold centers.     

Synthesis and molecular structures of dinuclear complexes with 1,2-dihydro-1,2-diphenyl-naphtho[1,8-c,d]1,2-diphosphole as a bridging ligand

A bisphosphine in which a PhP-PPh bond bridges 1,8-positions of naphthalene, 1,2-dihydro-1,2-diphenyl-naphtho[1,8-cd]-1,2-diphosphole (1), was used as a bridging ligand for the preparation of dinuclear group 6 metal complexes. Free trans-1, a more stable isomer having two phenyl groups on phosphorus centers mutually trans with respect to a naphthalene plane, was allowed to react with two equivalents of M(CO)₅(thf) (M = W, Mo, Cr) at room temperature to give dinuclear complexes (OC)₅M(μ-trans-1)M(CO)₅ (M = W (2a), Mo (2b), Cr (2c)). The preparation of the corresponding dinuclear complexes bridged by the cis isomer of 1 was also carried out starting from the free trans-1 in the following way. Mono-nuclear complexes M(trans-1)(CO)₅ (M = W (3a), Mo (3b), Cr (3c)) which had been prepared by a reaction of trans-1 with one equivalent of the corresponding M(CO)₅(thf) (M = W, Mo, Cr) complex, were heated in toluene, wherein a part of the trans-3a-c was converted to their respective cis isomer M(cis-1)(CO)₅. Each cis trans mixture of the mono-nuclear complexes 3a-c was treated with the corresponding M(CO)₅(thf) to give a cis trans mixture of the respective dinuclear complexes 2a-c. The cis isomer of the ditungsten complex 2a was isolated, and its molecular structure was confirmed by X-ray analysis, showing a shorter W?W distance of 5.1661(3) Å than that of 5.8317(2) Å in trans-2a.     

An efficient and versatile route to the synthesis of 9,10-dihydro-3-formylphenanthrenes

An innovative and efficient route to the synthesis of 9,10-dihydro-3-formylphenanthrenes 7 has been delineated through the ring transformation of 2-oxo-4-sec-amino-5,6-dihydro-2H-benzo[h]chromene-3-carbonitriles 4 with methyl glyoxaldimethylacetal 5 to masked 3-dimethoxymethyl-1-sec-amino-9,10-dihydrophenanthrene-3-carbonitriles 6 followed by deacetalation with Amberlyst 15 in excellent yields.     

Synthesis and chemistry of tricyclic cyclopropene-tricyclo[3.2.2.0]nona-2(4),6-diene

The 1,2-bridged tricyclic cyclopropene, tricyclo[3.2.2.0^2,4]nona-2(4),6-diene (1), has been synthesized by the elimination of 2-bromo-4-chlorotricyclo[3.2.2.0^2,4]-non-6-ene (5). Cyclopropene 1 will undergo different isomerizations in ether solution and in neat conditions. Compound 1 rearranged to an anti-Bredt compound 4 via diradical mechanism in ether and tricyclic compound 6 via vinyl carbene mechanism in neat conditions. Compound 1 can be trapped with DPIBF at different temperatures yielding different results: the exo-endo adduct 2 (exo-addition from the view of the cyclopropene and endo-addition from the view of bicyclo[2.2.2]octene) is a sole product at 0°C by slowly addition of methyllithium, and the exo-endo adduct 2, endo-endo adduct 9, anti-Bredt adduct 3, and styrene 8 are isolated at ether refluxing temperature. Styrene 8 is proposed to be formed from endo-endo adduct 9 by diradical mechanism. The chemistry of exo-endo adduct 2 and endo-endo adduct 9 is as well studied. The exo-endo adduct 2 undergoes hydration in trifluoroacetic acid to generate 1,3-cis-diol 11 followed by eliminations of water and formaldehyde to give naphthalene 12. The endo-endo adduct 9 reacts with water in tetrahydrofuran-containing silica gel to yield 1,4-cis-diol 10. Both 9 and 10 react with trifluoroacetic acid to form trans-3-hydroxy trifluoroacetate 13. Compound 13 will undergo hydrolysis and isomerization to generate 1,3-cis-diol 11 in trifluoroacetic acid.     

The first asymmetric synthesis of all four isomers of cis- and trans-3,4-dihydroxy-3,4-dihydromollugin

Asymmetric synthesis of all the four stereoisomers of cis-3,4-dihydroxy-3,4-dihydromollugins 4 and 6 and trans-3,4-dihydroxy-3,4-dihydromollugins 5 and 7 was achieved. The O-methoxymethyl mollugin derivatives were dihydroxylated to (−)- and (+)-cis-3,4-dihydroxy-3,4-dihydromollugin derivatives using both AD-mix-α and AD-mix-β. Deprotection of the MOM-ethers of cis-dihydroxy compounds resulted in the targeted stereoisomers (−)-(3R,4R)-cis-3,4-dihydroxy-3,4-dihydromollugin 4, (−)-(3R,4S)-trans-3,4-dihydroxy-3,4-dihydromollugin 5, (+)-(3S,4S)-cis-3,4-dihydroxy-3,4-dihydromollugin 6 and (+)-(3S,4R)-trans-3,4-dihydroxy-3,4-dihydromollugin 7. These routes were paved with difficulties, for example, incompatibility of the substrates with AD-mixes, the unexpected formation of trans-dihydroxy compounds and failures in deprotection protocols.     

Chemo enzymatic synthesis of Rengyol and Isorengyol

Cyanohydrins 2 of O-protected 4-hydroxycyclohexanones 1 are excellent starting compounds for the synthesis of Isorengyol (I) and Rengyol (II). The cyano group of the O-benzyl derivative 2d is first converted into the corresponding aldehyde 4, which via Wittig olefination led to the vinyl compound 6. Hydroboration of the trans-derivative (trans-6) leads, after debenzylation, to Isorengyol, whereas hydroboration and debenzylation of the cis-isomer (cis-6) gives Rengyol. With hydroxynitrile lyases (HNLs) as catalysts the stereoselective preparation of cis- as well as trans-cyanohydrin 2d is possible, which enables the selective preparation of Isorengyol or Rengyol, respectively. The trans-configuration of Isorengyol and the cis-configuration of Rengyol were secured by X-ray crystal structure analysis.     

Convenient construction of a variety of glycosidic linkages using a universal glucosyl donor

This letter deals with the concept of constructing four types (cis-α, trans-α, cis-β, and trans-β) of glycosidic linkages using a universal glucosyl donor. The selectively protected universal glucosyl donor 8 was synthesized in 36% yield from d-glucose (eight steps). The donor 8 undergoes glycosidation with a primary carbohydrate alcohol 7 to give disaccharide 9 having a 1,2-cis-α-glycosidic linkage in 90% yield. The construction of the corresponding 1,2-trans-α-glycosidic linkage was performed in 68% yield (three steps) from 9. A similar glycosidation of the 2-O-(N-phenylcarbamoyl)-glucosyl donor 6 derived from 8 with 7 gave disaccharide 11 having a 1,2-trans-β-glycosidic linkage in 75% yield. The construction of the corresponding 1,2-cis-β-linkage was performed in 53% yield (three steps) from 11.     

The first methyl antimony linked dimeric tetrathiafulvalene and tetraselenafulvalenes

In quest of novel organic donors, dimeric tetrathiafulvalene (TTF) and tetraselenafulvalenes (TSFs) linked by a single or double methyl antimony bridge, MeSb(TTF)₂ (1), (MeSb)₂(TSF)₂ (cis-2 and trans-2), and MeSb(TSF)₂ (3), have been synthesized. Singly bridged 1 and 3 show three pairs of redox waves, whereas doubly bridged cis-2 and trans-2 show two pairs of redox waves similarly to TSF. The X-ray structural analyses of neutral crystals, 1, cis-2 and trans-2, have succeeded. In their structures, antimony and chalcogen atoms form close intermolecular contacts useful in constructing supramolecular networks.     

Promoting effects of the hydroxymethyl group on the fluorescent signaling recognition of anions by thioureas

A series of novel fluorescent naphthylthioureas 1-4 with hydroxymethyl groups was designed and synthesized. Upon complexation with anions, 1-4 showed strong fluorescence enhancements in the order: 1>2>3≈4, which is consistent with the number of hydroxymethyl groups contained in their structures. Hydroxymethyl groups have an important influence on the compounds’ trans-trans or trans-cis conformations, and their action to promote the fluorescence signaling recognition of the thioureas for anions might be caused by their preorganizing the intramolecular protons of the receptor in favor of sites of the trans-trans conformation ready for hydrogen bond formation with the anions. Thioureas 1 to 4 had favorable selectivities for certain anions, which relied on the net charge and Brφnsted basicity of the anions.     

Electrostatic forces that determine O,O-trans versus O,S-cis conformers in the aminated porphyrin complexes of Cd(N-NHCO-2-C4H3O-tpp)(OAc) and Cd(N-NHCO-2-C4H3S-tpp)(OAc)

Two new diamagnetic, mononuclear and aminated porphyrin complexes of O,O-trans-Cd (3-trans) and O,S-cis-Cd (4-cis) have been synthesized and characterized by ¹H, ¹³C NMR spectroscopy. The crystal structures of (acetato)(N-2-furancarboxamido-meso-tetraphenylporphyrinato)cadmium(II) [Cd(N-NHCO-2-C₄H₃O-tpp)(OAc); 3-trans] and (acetato)(N-2-thiophenecarboxamido-meso-tetraphenylporphyrinato)cadmium(II) [Cd(N-NHCO-2-C₄H₃S-tpp)(OAc); 4-cis] were determined. The coordination sphere around Cd²⁺ is a distorted square-based pyramid in which the apical site is occupied by a bidentate chelating OAc⁻ group for 3-trans and 4-cis. The plane of three pyrrole nitrogen atoms [i.e., N(1), N(2), N(4) for 3-trans and N(1), N(2), N(3) for 4-cis] strongly bonded to Cd²⁺ is adopted as a reference plane 3N. The N(3) and N(4) pyrrole rings bearing the 2-furancarboxamido (Fr) and 2-thiophenecarboxamido groups in 3-trans and 4-cis, respectively, deviate mostly from the 3N plane, thus orienting separately with a dihedral angle of 33.4° and of 31.0°. In 3-trans, Cd²⁺ and N(5) are located on different sides at 1.06 and −1.49 Å from its 3N plane, while in 4-cis, Cd²⁺ and N(5) are also located on different sides at 1.04 and −1.53 Å from its 3N plane. An attractive electrostatic interaction between the Cd²⁺ and O(4)⁻ atoms in furan stabilizes the O,O-trans conformer of 3. A repulsive electrostatic interaction between Cd²⁺ and S(1)⁺ destabilizes the O,S-trans conformer of 4. Both of these repulsive and the mutually attractive interactions between S(1)⁺ and O(3)⁻ atoms favor the O,S-cis rotamer of 4 both in the vapor phase and in low polarity solvents. NOE difference spectroscopy, HMQC and HMBC were employed for the unambiguous assignment of the ¹H and ¹³C NMR resonances of 3-trans and 4-cis in CDCl₃ at 20 and −50 °C.     

Stereochemical studies—XIII: Cyclic aminoalcohols and related compounds—VI synthesis of the four 2-amino-4-t-butylcyclopentanol isomers

From diethyl 3-t-butyladipate (5), via cis- and trans-4-t-butylcyclopentene-1,2-oxide (31, 32) as key compounds, the syntheses of cis-2-amino-cis-4-t-butylcyclopentanol (1), cis-2-amino-trans-4-t-butylcyclopentanol (2), trans-2-amino-cis-4-t-butylcyclopentanol (3) and trans-2-amino-trans-4-t-butylcyclopentanol (4) have been achieved. 1, 3 and 4 were also synthesized from the corresponding 2-hydroxy-4-t-butylcarboxylic acids by Curtius degradation of the hydrazides (11, 18, 19). The steric course of process leading to the above compounds is discussed.     

New diastereomeric bis-sesquiterpenes from Hainan marine sponges Axinyssa variabilis and Lipastrotethya ana

Three unprecedented diastereoisomeric dimers, cis-dimer A (1), cis-dimer B (2) and trans-dimer C (3), exhibiting a bis-bisabolene skeleton, and a new sesquiterpene, dehydrotheonelline (4), their potential precursor, have been isolated from the South China Sea sponges Axinyssa variabilis and Lipastrotethya ana, along with known related sesquiterpenes. The structure of the novel molecules has been determined by extensive NMR spectral analysis.     

Aryl-2,3-oxaphosphabicyclo[2.2.2]octene derivatives—the precursors of oxoarylphosphine oxides (aryl metaphosphonates)

The Baeyer-Villiger oxidation of 7-phosphanorbornene 7-oxides with sterically demanding substituents on the phosphorus atom (4a-d) by m-chloroperbenzoic acid afforded the title products (5a-d) as a mixture of two regioisomers (A and B). Isomer A, the result of thermodynamic control, was stable, while isomer B, the product of kinetic control, underwent decomposition and/or epoxidation. Single crystal X-ray analysis of P-(2,4,6-triisopropylphenyl) oxaphosphabicyclooctene (5Ac) was not only useful in the evaluation of its structure, but, for the first time in the literature, a low-coordinated arylmetaphosphonate (15c) formed by fragmentation on X-ray irradiation could also be detected. The precursors (5Aa-c) were utilized in the thermoinduced and UV light-mediated fragmentation-related phosphorylations of alcohols. Beside the well-known elimination-addition mechanism via the metaphosphonate intermediate (15), a novel addition-elimination route involving a species with a pentavalent pentacoordinated phosphorus atom (16) was also substantiated.     

Enantioselective synthesis of 2-[(3-ethyl-4-piperidyl)methyl]indoles from a phenylglycinol-derived lactam: formal synthesis of Strychnos alkaloids

A diastereodivergent synthesis of enantiopure cis- and trans-2-[(3-ethyl-4-piperidyl)methyl]indole (cis-1b and trans-1b) from a common phenylglycinol-derived oxazolopiperidone lactam 3 is reported. The key step is a stereocontrolled conjugate addition, either under kinetic or thermodynamic control, of the dilithium salt of 2-(2-indolyl)-1,3-dithiane to unsaturated lactam 3.     

Synthesis, structure, and catalytic activity of titanium complexes with new chiral 11,12-diamino-9,10-dihydro-9,10-ethanoanthracene-based ligands

A new series of organo-titanium complexes have been prepared from the reaction between Ti(NMe₂)₄ and C₂-symmetric ligands, (R,R)-11,12-bis(pyrrol-2-ylmethyleneamino)-9,10-dihydro-9,10-ethanoanthracene (1H₂), and (R,R)-bis(diphenylthiophosphoramino)-9,10-dihydro-9,10-ethanoanthracene (2H₂), (R,R)-11,12-bis(mesitylenesulphonylamino)-9,10-dihydro-9,10-ethanoanthracene (3H₂) and (R,R)-bis(diphenylthiophosphoramino)-1,2-cyclohexane (4H₂). Treatment of Ti(NMe₂)₄ with 1 equiv of 1H₂ gives, after recrystallization from a benzene solution, the binuclear double helicate titanium amide (1)₂[Ti(NMe₂)₂]₂⋅(5) in 71% yield. While under similar reaction conditions, reaction of Ti(NMe₂)₄ with 1 equiv of 2H₂, 3H₂ or 4H₂ gives, after recrystallization from a toluene or benzene solution, the mononuclear single helicate titanium amides (2)Ti(NMe₂)₂ (6), (3)Ti(NMe₂)₂ (7) and (4)Ti(NMe₂)₂ (8), respectively, in good yields. All new compounds have been characterized by various spectroscopic techniques, and elemental analyses. The solid-state structures of complexes 5-8 have further been confirmed by X-ray diffraction analyses. The titanium amides are active catalysts for the polymerization of rac-lactide, leading to the isotactic-rich polylactides.     

Selective and efficient transformation of N-(4-substituted benzoyl)-α-dehydroarylalanine alkyl esters into 4,5-dihydrooxazole derivatives via photoinduced electron transfer

The irradiation of N-(4-substituted benzoyl)-α-dehydroarylalanine alkyl esters (1) in methanol containing triethylamine (TEA) was found to quantitatively give cis- and trans-4,5-dihydrooxazole derivatives (2), which were described as being formed via electron transfer from TEA to the excited-state (E)-1 followed by kinetically-controlled cyclization of the (E)-1-derived anion radical. A product composition analysis showed that the cis-2/trans-2 composition ratio is greatly varied depending on the stereoelectronic properties of the substituents, the polarity of protic solvents and the concentration of TEA.     

Synthesis and thermal behavior of cis- and trans-1-tert-butyl-4,5-dimethyl-2-phenyl-2-(trimethylsiloxy)-1-(trimethylsilyl)-1-silacyclohex-4-ene

The cothermolysis of benzoyl(tert-butyl)bis(trimethylsilyl)silane with 2,3-dimethylbutadiene in a sealed tube at 140 °C for 24 h afforded cis- and trans-1-tert-butyl-4,5-dimethyl-2-phenyl-2-(trimethylsiloxy)-1-(trimethylsilyl)-1-silacyclohex-4-ene (2 and 3) in a ratio of approximately 1:1 in 66% combined yield. When cis-silacyclohex-4-ene 2 was heated in a sealed tube at 250 °C for 24 h, dyotropic ring contraction took place to give 1-[(tert-butyl)(trimethylsiloxy)(trimethylsilyl)silyl]-3,4-dimethyl-1-phenylcyclopent-3-ene (4), but not trans-2-tert-butyl-4,5-dimethyl-2-phenyl-1-(trimethylsiloxy)-1-(trimethylsilyl)-1-silacyclohex-4-ene (6). The thermolysis of trans-silacyclohex-4-ene 3 under the same conditions, however, afforded two products, 1-silyl-1-phenylcyclopent-3-ene 4 and trans-1-tert-butyl-4,5-dimethyl-2-phenyl-1-(trimethylsiloxy)-2-(trimethylsilyl)-1-silacyclohex-4-ene (5). The theoretical calculations were carried out to characterize the transition states and other local minima, and to evaluate the activation energies for the dyotropic rearrangement of 2 to 4 and 6, and 3 to 4 and 5. The energy barriers between 2 and 4, between 3 and 4, and between 3 and 5 were evaluated to be 188, 191, 192 kJ mol⁻¹, respectively. The energy barrier between 2 and 6, however, was calculated to be 201 kJ mol⁻¹ or higher. These results are consistent with the experimental finding that the thermal isomerization of 2 affords only 4, but 3 produces both 4 and 5.     

The case of a Cd2Cu2 complex containing an apparently C3-symmetric ligand: Erroneous ligand-structure assignment by X-ray diffraction data analysis

A simplified synthesis of a previously reported Cd₂Cu₂ complex 4 was developed in order to have a more readily available source of 1,3,5-tris(trifluoromethyl)-cyclohexane-cis,cis-1,3,5-triol 1, a unique tridentate ligand, only tentatively identified elsewhere. Attempts to liberate the ligand from 4 resulted in the sole formation of 2,4,6-tris(trifluoromethyl)-tetrahydropyran-cis,cis-2,4,6-triol (2); no traces of 1 were detected. A newly conducted X-ray analysis of complex 4 led to diffraction data that could be explained either by the previously reported structure for 4 or by the statistically disordered structure 5, containing ligand 2 rather than 1. Whereas refinement of both models led to equivalent merit factors, mass spectral data and elemental analysis of the crystals revealed unequivocally the sole presence of the tetrahydropyran derivative within the complex and proved thus the erroneous structure assignment 4 previously published for compound 5.     

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.