The preparation of some chiral Lewis acids and their use as-catalysts in the addition of allylsilanes and allylstannanes to aldehydes are presented. In particular we have reviewed the application of chiral acyloxyborane (CAB), 1, chiral titanium and zirconium complexes 3-9, chiral silver complex 10, bis(oxazoline)zinc chiral complex 11 and chiral indium complex 12 for promoting the allylation reactions. Various modifications of the catalysts and of the reaction conditions adopted in order to enhance the applicability of the allylation reaction are discussed.
Arylazo sulfides 1 behave as arylating agents of a number of stabilized carbanions, such as the conjugate bases of malononitrile, ethyl cyanoacetate, diethyl malonate and ethyl acetoacetate. In line with previous results, the effectiveness of the S(RN)1 arylation has been found to depend both on the electrophilicity of the aryl radical and on the HOMO energy of the carbanion participating in the propagation cycle.
A synthetic route for the preparation of covalently-linked porphyrin-anthraquinone compounds as models for the phototrap in photosynthesis is described. 5-(2-anthraquinone)-15-(N,N-dimethylaminobenzal)-2,8,12,18-tetraethyl-3,7,13,17-tetramethylporphyrin (DPQ) and its derivatives are synthesized by condensation of 3,3'-diethyl-4,4'-dimethyl-2,2'-dipyrrolylmethane with an appropriate aldehyde in good molecular orientation and higher yield. The structures of new compounds are determined H-1 NMR, UV-visible spectroscopy, FTIR spectroscopy, mass spectrometry and elemental analytical data. Some of the photophysical properties of the compounds are reported with reference to their use as models of the reaction centre in photosynthesis.
An efficient and large-scale synthesis of indoles 3 and 4 bearing in 2-position opitically active substituents has been carried out according to the methodology introduced by Fiirstner. Furthermore, starting from indole-2-carbaldeyde, 6, the corresponding chiral indole Schiff bases 7, 8 and 9 have been synthesized. The cobalt complex, prepared from 9 and cobalt(II) acetate, has been shown to catalyze the reduction of acetophenone with NaBH4 in an e.e. of 22%.
A new preparation of (1S,2R,4S)-(-)-4-acetyl-1,2-epoxy-1-methylcyclohexane, a unique synthon for the synthesis of the sex pheromone of the southern green stinkbug, Nezara viridula (L.), is described. The steroselective formation of the epoxide ring has been accomplished by intramolecular substitution of a selenone group.
A new series of chromophores is presented where a positively charged N-alkylpyridinium acceptor and a negatively charged dicyanomethanide donor are spaced by a thiophene-based bridging unit; the unique combination of these fragments results in dramatically enhanced first molecular hyperpolarisabilities (mu beta as high as 13450 x 10(-48) esu).
A free-radical ring expansion by one carbon atom has been achieved by thermal decomposition of thiohydroxamic esters of (1-alkoxycarbonyl-2-oxocycloalkyl)acetic acids, 8. Thus, by decomposition of thiohydroxamic esters 8a-f in boiling toluene, ethyl 3-oxocycloalkenecarboxylates 9a-f are obtained as principal ring-enlarged products in 27-58% yield; accompanied by ethyl 1-(4-methyl-2-thiazolylthio)-3-oxocycloalkane-1-carboxylates 10c,d (24% yield) with the ring expanded by one carbon atom. Unrearranged ethyl 1-(4-methyl-2-thiazolylthiomethyl)-2-oxocycloalkane-1-carboxylates) 11a-f are formed as side products (8-17% yield). Ring expansion involves the following sequence of reactions: 3-exo-cyclization (to C=O group), cleavage of the cyclopropane ring, intermolecular addition to the thione group of thiohydroxamic esters and elimination reaction.
The first example of the enantioselective ring opening of 1,2-epoxides by a ketone enolate is described. Using (R,R)-(salen)Cr(III)X (X=Cl, OTf) complexes 9 and 10 as the chiral Lewis acid (LA), the addition of the lithium enolate of acetophenone, 1, to cyclohexene oxide, 2, and cyclopentene oxide, 3, gave the corresponding gamma-hydroxy ketones (gamma-HKs) 4 and 5 in low to moderate yields and in up to 62-84% ee.
The reactions of mono(cyclopentadienyl)titanium(IV)trichloride and bis(cyclopenladienyl)titanium(lv)/zirconium(IV) dichloride with ligands (LH) derived hom 3-phenyl/substituted phenyl-4-amino-5-mercapto-1,2,4-triazole and acetylferrocene have been studied and complexes of types [Cp2M(L)CI], [CpM(L)(2)Cl] [M = Ti(IV)/Zr(IV)], and [CpTi(L)Cl-2] were obtained. Tentative structural conclusions are drawn for these complexes based upon elemental analysis, electronic conductance, magnetic moment and spectral (electronic, IR, H-1- and C-13 NMR) data.
The hydrolysis of adenosine-5'-monophosphate and deoxyadenosine-5'-monophosphate has been studied with lanthanide(III) metal complexes of 2-carboxyethylgermanium sesquioxide (Ge-132) by NMR and HPLC and by measuring the liberated inorganic phosphates.
(5-methyl-1,3,4-thiadiazol-2-ylsulfanyl) acetohydrazide, 1, has been prepared and allowed to react with substituted benzaldehydes or acetophenones to yield the corresponding hydrazones 2a-j. These hydrazones, on condensation with 2-hydrazino-4-methylquinoline under microwave irradiation and by conventional methods, yielded the tetrazines 3a-j. In comparison with the conventional method the reaction rate is enhanced about 250 times by using microwaves and yields are improved. 1 on reaction with chloroacetic acid, dichloroacetic acid, and chloroacetyl chloride yielded the pyridazinones 4-6, while on reaction with phenyl isothiocyanate and phenyl isocyanate yielded the thiosemicarbazide 7a and semicarbazide 7b which have been elaborated further to the thiazole 8a, the oxazole 8b, the thiazolidinone 9 and the triazole 10.
In this paper we highlight the general strategies established over the time for the functionalization of poly(organophosphazenes) in order to modify both the chemical structures and the chemicophysical properties of these substances. These modifications have been obtained by reacting poly(dichlorophosphazene) with difunctional nucleophiles thus introducing selected chemical functions into the polyphosphazene substrates or by carrying out suitable functionalization processes on pre-synthesized phosphazene macromolecules. Particular attention has been paid to the chemical modification of polyphosphazenes containing 2,2,2-trifluoroethoxy, phenoxy, 4-ethylphenoxy, 4-sec-butylphonoxy and 4-methoxyphenoxy substituents as these groups were attached to phosphazene homopolymers and copolymers that have been considered in the past for industrial development.
In this paper we report some of our recent results in the isolation and structure characterisation of bioactive secondary metabolites from marine invertebrates.
1,3-dipolar cycloaddition of azomethine ylides has been used to prepare a series of fullero-pyrrolidines in which C-60 is covalently linked to a ferrocene or a ruthenium(II) trisbipyridine electron donor. Photoinduced optical absorption studies clearly show that in both cases intramolecular electron transfer from the donor to the fullerene acceptor moiety occurs with concomitant formation of a long-lived charge separate pair.
The molecular mechanics (MM) and dynamics (MD) procedures have been used to study the structure and conformational behaviour of alpha- and beta-amanitin. Results show that both systems are rigid molecules for which conformational flexibility is essentially limited to the side-chain motion. Different intra-and inter-molecular hydrogen bonds were found that render to alpha-amanitin more hydrophobic Chan beta-amanitin. An i-->i+3 hydrogen bond interaction was localised only in beta-amanitin. A satisfactory agreement is observed between our structures and those coming from available experimental measurements.
The reactions between nucleophiles and aromatic (or heteroaromatic) substrates represent a means of outmost importance for modifying or functionalising organic compounds. In this perspective, a number of viable pathways for aromatic nucleophilic substitutions is summarised. Particular attention is placed on the peculiar reactivity of the thiophene ring and, within this field, examples ale also presented relevant to the exploitation of inter-mediates derived from reactions of nitrothiophenes with nucleophiles.
Although the discovery of stereoisomerism is due to van't Hoff and Le Eel (1875), the hypothesis of tetrahedral carbon, on which the stereoisomerism is actually based, is older. The latter is due to Kekule (1862), who then built the tetrahedral carbon molecular models, and applied them to solve the problem of the structure of benzene. The molecular models of Kekule were used by the most talented pupils of his school (Dewar, Koemer, van't Hoff), and were applied to diverse structural problems. Among: these contributions, of great relevance is the work of Emanuele Paterno, who grew up in the laboratory of S. Cannizzaro in Palermo, and who came also in contact with the school of Kekule. Paterno, using the molecular models of Kekule, in 1869 applied for the first time the theory of the tetrahedral carbon to aliphatic organic compounds, and discussed the conformational isomerism of these compounds. Only a few years later, van't Hoff and Le Eel succeeded in applying the hypotesis of Kekule to the problem of optical antipodes, and discovered stereoisomerism.
A theoretical study of the first oxidation step of [3.2.1] bridged bicyclic disulphides to thiosulphinates was performed with hydrogen peroxide and performic acid in order to understand the exo/endo selectivity. Ab initio molecular orbital calculations were carried out with the 3-21G* basis set. Key equilibrium geometries were optimized at the RHF and MP2 level. Transition states single-point energy calculations were performed at MP2 level with RHF geometries. The entrance channel for the oxygen transfer showed the peroxide bond oriented-along the direction of the localized p-pure lone pair. This highly-oriented reaction path was probably responsible for the stereoselective attack on the exo face. Study of the substituent effect on stereoselective oxidation was attempted. A comparison between calculated and experimental structure of the reaction product was reported. The relative stability of diastereomers was calculated in the gas phase and in solution.
The binding with human serum albumin (HSA) of Cibacron Blue F3GA, Levafix Brilliant Blue EB, the structure of which has been carefully investigated, Reactive Orange 046, and Reactive Red 022 dyes, immobilised on sepharose Ct 6B, has been quantitatively investigated by frontal affinity chromatography. This technique allowed the calculation of the association constants of The ligand-HSA complexes, on the basis of the dependence of the HSA elution volume on its initial concentration. The values obtained were high and comparable for Cibacron and Levafix dyes (5.4 and 5.7x10(6) M-1, respectively) and significantly smaller for Reactive Orange 046 and Reactive Red 022 (0.23 and 0.4x10(6) M-1). A possible interpretation of these data, based on the different structures of the bonded dyes, is proposed.
