New trans/cis tetrahydroisoquinolines. 2. trans‐ and cis‐3‐(1‐methyl‐1h‐pyrrol‐2‐yl)‐1‐(2H)‐oxo‐2‐phenethyl‐1,2,3,4‐tetrahydroisoquino‐lin‐4‐carboxylic acids and subsequent transformations

Stereochemical course of the reaction of homophthalic anhydride and N‐(1‐methyl‐1H‐pyrrol‐2‐yl‐methylidene)‐phenethylamine was studied. Mixtures of the expected trans‐ and cis‐1,2,3,4‐tetrahydroiso‐quinoline‐4‐carboxylic acids trans‐ 4 and cis‐ 4 were obtained along with by‐products 5 and 6 . The ratios of all products and the diastereomers, obtained under different reaction conditions, were established by pmr. THF as a solvent and ultrasonic treatment are applied for the first time in the reaction of this type. The reaction was made diastereoselective towards any isomer. The carboxylic group of trans‐ 4 was transformed in four steps into various cyclic amino‐methyl groups yielding numerous new tetrahydroisoquinolinones trans‐ 10a‐i incorporating a given fragment of pharmacological interest. Reduction of 10a‐i was studied.     

3‐Fluoro‐2,4‐dioxa‐7‐aza‐3‐phosphadecalin 3‐Oxides as Rigid Acetylcholine Mimetics: Conformational Analysis and Direct Evidence of the Anomeric Effect

Conformational analyses of the P(3)‐axially and P(3)‐equatorially F‐substituted (±)‐cis‐ and (±)‐trans‐2,4‐dioxa‐7‐aza‐3‐phosphadecalin 3‐oxides (3‐fluoro‐2,4‐dioxa‐7‐aza‐3‐phosphabicyclo[4.4.0]decane 3‐oxides) were performed. The results are based on independent studies in both solution and the solid state by ¹H‐ and ³¹P‐NMR experiments and computational and X‐ray crystallographic data. As expected, the axial epimers adopt neat double‐chair conformations in solution and in the crystal. Due to the anomeric effect of the electron withdrawing F‐substituent, the 2,4‐dioxa‐3‐phospha moiety in the equatorial epimers adopts a mixture of conformations in solution, mainly chair and twist‐boat; whereas a neat twist‐boat (trans‐isomer) and the unusual envelope conformation (cis‐isomer) were detected in the solid state. This is the first report of a straight visualization of these conformations and the impact of the anomeric effect in such systems.     

An Efficient Synthesis of Optically Active trans‐(3R,4R)‐3‐Acetoxy‐4‐aryl‐1‐(chrysen‐6‐yl)azetidin‐2‐ones Using (+)‐Car‐3‐ene as a Chiral Auxiliary

An efficient enantioselective synthesis of 3‐acetoxy trans‐β‐lactams 7a and 7b via [2+2] cycloaddition reactions of imines 4a and 4b , derived from a polycyclic aromatic amine and bicyclic chiral acid obtained from (+)‐car‐3‐ene, is described. The cycloaddition was found to be highly enantioselective, producing only trans‐(3R,4R)‐N‐azetidin‐2‐one in very good yields. This is the first report of the synthesis of enantiomerically pure trans‐β‐lactams 7a and 7b with a polycyclic aromatic substituent at N(1) of the azetidin ring.     

Theoretical study of the structures,conformations, and spectroscopic properties of 2‐formylthiophene‐N‐acetylhydrazone and 2‐thiophenecarboxaldehyde‐2‐thienylhydrazone

2‐Formylthiophene‐N‐acetylhydrazone (Hait) and 2‐thiophenecarboxaldehyde‐2‐thienylhydrazone (Htit) in the cis and trans conformations were investigated in the gas‐phase by density functional method using B3LYP as the functional set and 6‐311++G(d,p) as the basis set. The cis and trans structures were fully optimized in the C₁ and C_s symmetries. Transition states were also modeled for the cis–trans isomerization of the title compounds and the barriers to internal rotation were calculated. This work reports the structural, energetics, and spectroscopic parameters of all the optimized geometries. Some of the structural parameters are in good agreement with experimental literature data. The computed parameters for these compounds are also in good agreement with a related molecule, namely, acetohydrazide. For both Hait and Htit, the trans conformers are more stable than the cis conformers and the energy barriers are larger compared with the energy differences between the cis and trans conformers. This accounts for Hait and Htit existing mostly in the trans conformation. © 2009 Wiley Periodicals, Inc. Heteroatom Chem 20:144–150, 2009; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/hc.20526     

New Rh(III) complexes of 5‐methyl‐5‐(pyridyl)‐2,4‐imidazolidenedione: Synthesis,X‐ray structure,electrochemical study and catalytic behaviour for hydrogenation of ketones

We describe the reaction of anion [RhCl₆]³⁻ with a series of hydantoin ligands (HL1, HL2 and HL3 = 5‐methyl‐5‐(2‐, 3‐ and 4‐pyridyl)‐2,4‐imidazolidenedione, respectively). Based on spectroscopic, cyclic voltammetric, elemental and MS analyses, the complexes have the general formula K[RhCl₂(L1)₂] ( 1 ), cis ‐ and trans ‐K[RhCl₄(HL2)₂] ( 2a and 2b ) and cis ‐ and trans ‐K[RhCl₄(HL3)₂] ( 3a and 3b ). Complexes 2a , 2b , 3a and 3b were characterized successfully using infrared, ¹H NMR and ¹³C NMR spectral analyses. Dissolution of complex 1 in dimethylsulfoxide (DMSO) led to elimination of one KL1 ligand and coordination of two DMSO molecules as ligands and transformation of this complex to cis ‐ and trans ‐[RhCl₂L1(DMSO)₂] ( 1a and 1b ). Recrystallization led to separation and isolation of crystals of 1a from the initial mixture. X‐ray analysis results showed that this complex was crystallized as solvated complex cis ‐[RhCl₂L1(DMSO)₂]DMSO. The catalytic activity of these complexes was then evaluated for the hydrogenation of various ketones.     

Synthesis and Antibacterial Activity of New 2,5‐Diaryl‐3‐styryl‐4H,2,3,3a,5,6,6a‐hexahydropyrrolo[3,4‐d]isoxazole‐4,6‐diones

The synthesis of some biologically interesting pyrrolo‐isoxazolidine derivatives has been accomplished by the 1,3‐dipolar cycloaddition reaction of substituted open chain conjugated azomethine N‐oxides 1 with substituted N‐aryl maleimides 2 leading to the formation of new stereoisomeric 2,5‐diaryl‐3‐styryl‐4H,2,3,3a,5,6,6a‐hexahydropyrrolo[3,4‐d]isoxazole‐4,6‐dione derivatives 3 in excellent yields. These stereoisomers have been characterized as cis‐ 3A and trans‐ 3B on the basis of their ¹H‐NMR spectral measurements. The synthesized compounds have been screened for their antibacterial activities and have been found to be active against the bacteria Escherichia coli and Pseudomonas aeruginosa up to a significant extent.     

Structure and Photochemical Properties of r‐1, c‐2, t‐3, t‐4‐l, 3‐Bis [2‐ (5‐R‐benzoxazolyl) ] −2,4‐di (4‐R' ‐phenyl) cyclobutane

r‐1, c‐2, t‐3, t‐4‐1,3‐Bis[2‐(5‐R‐benzoxazolyl)]‐2,4‐di(4‐R'‐phenyl)cyclobutane (IIa: R=R' = H; IIb: R=Me, R'= H; IIc: R = Me, R' = OMe) was synthesized with high stereo‐selectivity by the photodimerization of trans‐l‐[2‐(5‐R‐benzoxazolyl)]‐2‐(4‐R'‐phenyl) ethene (Ia: R=R' = H; Ib: R = Me, R' = H; Ic: R = Me, R' = OMe) in sulfuric acid. The structures of IIa–IIc were identified by elemental analysis, IR, UV, ¹H NMR, ¹³C NMR and MS. The molecular and crystal structure of IIc has been determined by X‐ray diffraction method. The crystal of IIc (C₃₄H₃₀N₂O₄. 0.5C₂OH) is monoclinic, space group P2₁/n with cell dimensions of a = 1.5416(3), b =0.5625(1), c = 1.7875(4) nm, β = 91.56 (3)°, V= 1.550(1) nm³, Z = 2. The structure shows that the molecule of IIc is centrosymmetric, which indicates that the dimerization process is a head‐to‐tail fashion. The selectivity of the photodimerization of Ia–Ic has been enhanced by using acidic solvent and the reaction speed would be decreased when electron donating group was introduced in the 4‐position of the phenyl group. That the photodimerization is not affected by the presence of oxygen as well as its high stereo‐selectivity demonstrated that the reaction proceeded through an excited singlet state. It was also found that under irradiation of short wavelength UV, these dimers underwent photolysis completely to reproduce its trans‐monomers, and then the new formed species changed into their cis‐isomers through trans‐cis isomerization.     

1,3‐dipolar cycloaddition reactions of 2‐substituted azomethine N‐oxides with N‐benzyl maleimides leading to the synthesis of stereoisomers

The azomethine N‐oxides ( 1 ) on reacting with N‐benzylmaleimide ( 2 ) provide a mixture of stereoisomers 2,3‐diphenyl‐5‐benzyl‐4H‐2,3,3a,5,6,6a‐hexahydropyrrolo[3,4‐d]isoxazole‐4,6–dione derivatives ( 3 ) in good yields. These isomers have been assigned cis and trans configurations ( 3‐A and 3‐B ) with respect to proton C₃‐H on the azomethinic carbon on the basis of their PMR and H‐NMR COSY data. The ratio between cis and trans isomers has been found to be dependent on substituents present at ortho position of C‐phenyl aldehydic moiety. The salient feature of these 1,3‐dipolar cycloaddition reactions lies in that the benzylic protons on N‐benzyl moiety suffer gem coupling, indicating magnetic nonequivalence. J. Heterocyclic Chem.,, (2012).     

Mass spectrometric studies of cis‐ and trans‐1a,3‐disubstituted‐1,1‐dichloro‐4‐formyl‐1a, 2,3,4‐tetrahydro‐lH‐azirino[1,2‐a]1,5]benzodiazepines

The mass spectrometric behaviour of four cis‐ and trans‐1a,3‐disubstituted‐1,1‐dichloro‐4‐formyl‐1a,2,3,4‐tetrahydro‐1H‐azirino [1, 2‐a][1,5]benzodiazepines has been studied with the aid of mass‐analysed ion kinetic energy spectrometry and exact mass measurements under electron impact ionization. All compounds show a tendency to eliminate a chlorine atom from the aziridine ring, and then eliminate a neutral propene or styrene from the diazepine ring to yield azirino [1,2‐b][1,3] benzimidazole ions. These azirino [1,2‐a][1,5]‐benzodiazepimes can also eliminate HCl, or Cl plus HCl simultaneously to undergo a ring enlargement rearrangement to yield 1,6‐benzodiazocine ions, which further lose small molecular fragments, propyne or phenylacetylene, with rearrangement to give quinoxaline ions.     

Intramolecular cyclization of ω‐haloalkylsubstituted thiophosphorylacetonitriles: Synthesis and stereochemistry of 3‐cyano‐2‐oxo‐1,2‐thiaphosphacyclanes

Intramolecular S‐alkylation in a series of ω‐haloalkyl‐substituted thiophosphorylacetonitriles 5–7 presents an effective synthetic route to the hitherto unknown 3‐cyano‐2‐oxo‐1,2‐thiaphospholanes 14 and thiaphosphinanes 15 . The compounds were obtained as a mixture of cis‐ and trans‐isomers that were resolved to individual stereoisomers in most cases. For some of them, X‐ray diffraction analysis has been performed. It was shown that ³¹P NMR spectroscopy can be used to assign the stereochemistry of 3‐cyano‐2‐oxo‐1,2‐thiaphosphacyclanes. © 2002 John Wiley & Sons, Inc. Heteroatom Chem 13:1–21, 2002; DOI 10.1002/hc.1101     

N‐substituted bis(tetrazol‐5‐yl)diazenes: Synthesis,spectra, X‐ray molecular and crystal structures,and quantum‐chemical DFT calculations

N‐Substituted bis(tetrazol‐5‐yl)diazenes (substituents are 1‐CH₃ ( 3a ), 1‐Ph ( 3b ), 2‐CH₃ ( 3c ), and 2‐^tBu ( 3d )) were synthesized by oxidative coupling of corresponding 5‐aminotetrazoles. All compounds were characterized with ¹H and ¹³C NMR, IR‐ and UV‐spectroscopy, and thermal analysis. Crystal and molecular structures of bis(1‐phenyltetra‐ zol‐5‐yl)diazene ( 3b ) and bis(2‐tert‐butyltetrazol‐5‐yl)diazene ( 3d ) were determined by single crystal X‐ray diffraction. Molecules of these compounds are trans‐isomers in solid. According to X‐Ray data, 3b molecule is S‐trans‐S‐trans conformer, however 3d is S‐cis‐S‐cis one. Quantum‐chemical investigation of geometry and relative stability of cis‐ and trans‐isomers and stable conformations of compounds 3a–d was carried out. © 2010 Wiley Periodicals, Inc. Heteroatom Chem 21:24–35, 2010; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/hc.20574     

Synthesis and Thermal Properties of Regio‐ and Stereoregular Poly(silylene‐1,4‐phenylenevinylene)s

Polymerization of p‐(dimethylsilyl)phenylacetylene in toluene at 25 and 80°C using RhI(PPh₃)₃ as the catalyst afforded highly regio‐ and stereoregular poly(dimethylsilylene‐1,4‐phenylenevinylene)s (cis‐ 3 a and trans‐ 3 a ) containing 98% cis‐ and 99% trans‐vinylene moieties, respectively. Similarly, poly(butylmethylsilylene‐1,4‐phenylenevinylene)s ( 3 b with 91% cis‐ and 95% trans‐structures) and poly(diisopropylsilylene‐1,4‐phenylenevinylene) with 95% trans‐structure were synthesized. All polymers were soluble in common organic solvents. The trans‐type polymers showed red shifts and hyperchromic effects in the UV‐visible spectrum. The onset temperature of weight loss (T₀) of cis‐ 3 a was much higher than that of trans‐ 3 a .     

A First Approach to the cis‐trans‐cis‐Photocyclodimers of 1,2‐Naphthoquinone

On irradiation (λ=350 nm), 1,2‐naphthoquinone (=naphthalene‐1,2‐dione) monoacetals 1 are converted quantitatively to mixtures of the cis‐trans‐cis‐photocyclodimers 2 and 3 . Careful hydrolysis of each of the (parent) pentacyclic diacetals 2a and 3a affords the – rather unstable – compounds 4 and 5 , respectively.     

2,4‐Dioxa‐7‐aza‐, 2,4‐Dioxa‐8‐aza‐, and 2,4‐Dioxa‐9‐aza‐3‐phosphadecalins as Rigid Acetylcholine Mimetics: Syntheses and Characterization

Phosphorylation of suitable piperidine precursors yielded a series of novel decalin‐type O,N,P‐heterocycles. The title compounds, P(3)‐axially and P(3)‐equatorially X‐substituted, cis‐ and trans‐configurated 2,4‐dioxa‐7‐aza‐, 2,4‐dioxa‐8‐aza‐, and 2,4‐dioxa‐9‐aza‐3‐phosphabicyclo[4.4.0]decane 3‐oxides (X=Cl, F, 4‐nitrophenoxy, and 2,4‐dinitrophenoxy), are configuratively fixed and conformationally constrained P‐analogues of acetylcholine and as such represent acetylcholine (7‐aza and 9‐aza isomers) or γ‐homo‐acetylcholine mimetics (8‐aza isomers). Being irreversible inhibitors of acetylcholinesterase (AChE), the compounds are considered to be suitable probes for the investigation of the stereochemical course of the inhibition reaction by ³¹P‐NMR spectroscopy. Moreover, the design of these mimetics will enable studies of molecular interactions with AChE, in particular, the recognition conformation of acetylcholine.     

1H, 13C, 17O NMR and quantum‐chemical study of the stereochemistry of the sulfoxide and sulfone derivatives of 3‐arylidene‐1‐thioflavan‐4‐one epoxides

The oxidation of the trans,cis‐( 2 ) and trans,trans‐epoxides ( 3 ) of differently substituted (Z)‐3‐arylidene‐1‐thioflavan‐4‐ones ( 1 ) with dimethyldioxirane (DMD) yielded the appropriate sulfoxides ( 4, 5 ) and sulfones ( 6, 7 ). The structures were elucidated by the extensive application of one‐ and two‐dimensional ¹H, ¹³C and ¹⁷O NMR spectroscopy. The conformational analysis was achieved by the application of ³J(C,H) coupling constants, NOESY responses and ab initio calculations. The preferred ground‐state conformers (twisted envelope‐A, twisted envelope‐B for 6 and twisted envelope‐A, envelope‐B for 7 ) were obtained as global minima of the theoretical ab initio MO study and also the examination of the ¹⁷O and ¹³C chemical shifts, calculated for the global minima structures of the sulfone isomers by the GIAO method. Analogous results, obtained for the sulfoxide isomers ( 4, 5 ), not only led to the preferred conformers but also gave evidence for the trans arrangement of the 2‐Ph group and the oxygen atom of the S?O group. Chemical shift differences between the isomers, sulfoxides and sulfones were corroborated by ab initio calculations of the anisotropic effects of the oxirane ring and the S?O and SO₂ groups. Copyright © 2003 John Wiley & Sons, Ltd.     

Precision Chain‐Walking Polymerization of trans‐4‐Octene Catalyzed by α‐Diimine Nickel(II) Catalysts Bearing ortho‐sec‐Phenethyl Groups

α‐Diimine nickel complexes bearing bulky ortho‐sec‐phenethyl groups (bis{[N,N′‐(4‐methyl‐2,6‐di‐sec‐phenethylphenyl)imino]‐1,2‐dimethylethane}dibromonickel ( 1 ), bis{[N,N′‐(4,6‐dimethyl‐2‐sec‐phenethylphenyl)imino]‐1,2‐dimethylethane}dibromonickel ( 2 ), bis{[N,N′‐(4‐methyl‐2‐sec‐phenethylphenyl)imino]‐1,2‐dimethylethane}dibromonickel ( 3 )) and {bis[N,N′‐(2,4,6‐trimethylphenyl)imino]‐1,2‐dimethylethane}dibromidonickel ( 4 ) are used as a precatalyst for the polymerization of trans‐4‐octene upon activation with modified methylaluminoxane. These catalysts conduct chain‐walking polymerization of trans‐4‐octene to give polymers possessing propyl and butyl branches with high molecular weight and narrow molecular weight distribution. The branching structure depends on the nickel complex as well as the polymerization temperature, and the ratio of propyl branch was increased with increasing the bulkiness of the ligand and decreasing the polymerization temperature. Consequently, the most bulky 1 among the complexes used is found to polymerize trans‐4‐octene with high 1,5‐regioselectivity at −20 °C to give poly(1‐propylpentan‐1,5‐diyl).

     

Synthesis of Multifunctionalized 1,2,3,4‐Tetrahydropyridines, 2,3‐Dihydropyridin‐4(1H)‐ones,and Pyridines from Tandem Reactions Initiated by [5+1] Cycloaddition of N‐Formylmethyl‐Substituted Enamides to Isocyanides: Mechanistic Insight and Synthetic Application

Tandem reactions for the efficient synthesis of multifunctionalized 1,2,3,4‐tetrahydropyridines, 2,3‐dihydropyridin‐4(1H)‐ones, and pyridine derivatives have been developed and reaction mechanisms have been investigated. Synthetic cascades are initiated by the Zn(OTf)₂‐mediated [5+1] cycloaddition of N‐formylmethyl‐substituted tertiary enamides to isocyanides, thus leading to the versatile heterocyclic enamino imine intermediates. Interception of the intermediates by diastereoselective reduction of imine functionality with Me₄NBH(OAc)₃ afforded 1,6‐disubstituted trans‐3‐hydroxy‐4‐arylamino‐ or ‐alkylamino‐1,2,3,4‐tetrahydropyridines, whereas acylation of the imino group followed by acidic hydrolysis produced 1,6‐disubstituted 3‐acyloxy‐2,3‐dihydropyridin‐4(1H)‐ones. Aerobic oxidation led to the aromatization followed by intermolecular acyl‐group transfer from the pyridinium nitrogen to the 3‐hydroxy moiety, thereby yielding substituted 3‐acyloxy‐4‐aminopyridines. Synthetic potentials of the resulting products have been demonstrated by expedient and highly stereoselective synthesis of cis,cis‐4,5‐dihydroxy‐2‐phenylpiperidine and trans,trans‐4‐amino‐5‐hydroxy‐2‐phenylpiperidine compounds, which are important in medicinal chemistry, through simple and practical reduction reactions.     

Analogues of α‐Campholenal (= (1R)‐2,2,3‐Trimethylcyclopent‐3‐ene‐1‐acetaldehyde) as Building Blocks for (+)‐β‐Necrodol (= (1S,3S)‐2,2,3‐Trimethyl‐4‐methylenecyclopentanemethanol) and Sandalwood‐like Alcohols

To complete our panorama in structure–activity relationships (SARs) of sandalwood‐like alcohols derived from analogues of α‐campholenal (= (1R)‐2,2,3‐trimethylcyclopent‐3‐ene‐1‐acetaldehyde), we isomerized the epoxy‐isopropyl‐apopinene (?)‐ 2d to the corresponding unreported α‐campholenal analogue (+)‐ 4d (Scheme 1). Derived from the known 3‐demethyl‐α‐campholenal (+)‐ 4a , we prepared the saturated analogue (+)‐ 5a by hydrogenation, while the heterocyclic aldehyde (+)‐ 5b was obtained via a Bayer‐Villiger reaction from the known methyl ketone (+)‐ 6 . Oxidative hydroboration of the known α‐campholenal acetal (?)‐ 8b allowed, after subsequent oxidation of alcohol (+)‐ 9b to ketone (+)‐ 10 , and appropriate alkyl Grignard reaction, access to the 3,4‐disubstituted analogues (+)‐ 4f,g following dehydration and deprotection. (Scheme 2). Epoxidation of either (+)‐ 4b or its methyl ketone (+)‐ 4h , afforded stereoselectively the trans‐epoxy derivatives 11a,b , while the minor cis‐stereoisomer (+)‐ 12a was isolated by chromatography (trans/cis of the epoxy moiety relative to the C₂ or C₃ side chain). Alternatively, the corresponding trans‐epoxy alcohol or acetate 13a,b was obtained either by reduction/esterification from trans‐epoxy aldehyde (+)‐ 11a or by stereoselective epoxidation of the α‐campholenol (+)‐ 15a or of its acetate (?)‐ 15b , respectively. Their cis‐analogues were prepared starting from (+)‐ 12a . Either (+)‐ 4h or (?)‐ 11b , was submitted to a Bayer‐Villiger oxidation to afford acetate (?)‐ 16a . Since isomerizations of (?)‐ 16 lead preferentially to β‐campholene isomers, we followed a known procedure for the isomerization of (?)‐epoxyverbenone (?)‐ 2e to the norcampholenal analogue (+)‐ 19a . Reduction and subsequent protection afforded the silyl ether (?)‐ 19c , which was stereoselectively hydroborated under oxidative condition to afford the secondary alcohol (+)‐ 20c . Further oxidation and epimerization furnished the trans‐ketone (?)‐ 17a , a known intermediate of either (+)‐β‐necrodol (= (+)‐(1S,3S)‐2,2,3‐trimethyl‐4‐methylenecyclopentanemethanol; 17c ) or (+)‐(Z)‐lancifolol (= (1S,3R,4Z)‐2,2,3‐trimethyl‐4‐(4‐methylpent‐3‐enylidene)cyclopentanemethanol). Finally, hydrogenation of (+)‐ 4b gave the saturated cis‐aldehyde (+)‐ 21 , readily reduced to its corresponding alcohol (+)‐ 22a . Similarly, hydrogenation of β‐campholenol (= 2,3,3‐trimethylcyclopent‐1‐ene‐1‐ethanol) gave access via the cis‐alcohol rac‐ 23a , to the cis‐aldehyde rac‐ 24 .     

Photocyclization reactions. Part 8. Synthesis of 2‐quinolone,quinoline and coumarin derivatives using trans‐cis isomerization by photoreaction

2‐Quinolone 2 , quinoline 3 , coumarin (2H‐1‐benzopyran‐ 2 ‐one) 5 , and 2H‐1‐benzopyran hemiacetal 6 were synthesized by photocyclization reaction of traans‐o‐aminocinnamoyl derivatives trans‐ 1 and trans‐o‐hydroxycinnamoyl derivatives trans‐ 4 . The reaction proceeds through trans‐cis isomerization followed by intramolecular cyclization.     

Influence of the stereochemical configuration on the radical polymerization of methacrylic monomers: cis‐ and trans‐(2‐cyclohexyl‐1,3‐dioxan‐5‐yl) methacrylate

The synthesis of two new isomeric monomers, cis‐(2‐cyclohexyl‐1,3‐dioxan‐5‐yl) methacrylate (CCDM) and trans‐(2‐cyclohexyl‐1,3‐dioxan‐5‐yl) methacrylate (TCDM), starting from the reaction of glycerol and cyclohexanecarbaldehyde, is reported. The process involved the preparation of different alcohol acetals and esterification with methacryloyl chloride of the corresponding cis and trans 5‐hydroxy compounds of 2‐cyclohexyl‐1,3‐dioxane. The radical polymerization reactions of both monomers, under the same conditions of temperature, solvent, monomer, and initiator concentrations, were studied to investigate the influence of the monomer configuration on the values of the propagation and termination rate constants (k_p and k_t ).The values of the ratio k_p /k_t ^1/2 were determined by UV spectroscopy by the measurement of the changes of absorbance with time at several wavelengths in the range 275–285 nm, where an appropriate change in absorbance was observed. Reliable values of the kinetics constants were determined by UV spectroscopy, showing a very good reproducibility of the kinetic experiments. The values of k_p /k_t ^1/2, in the temperature interval 45–65 °C, lay in the range 0.40–0.50 L^1/2/mol^1/2s^1/2 and 0.20–0.30 L^1/2/mol^1/2s^1/2 for CCDM and TCDM, respectively. Measurements of both the radical concentrations and the absolute rate constants k_p and k_t were also carried out with electron paramagnetic resonance techniques. The values of k_p at 60 °C were nearly identical for both the trans and cis monomers, but the termination rate constant of the trans monomer was about three times that of the cis monomer at the same temperature. © 2000 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 38: 3883–3891, 2000