Two-step procedure

Polyimides are synthesized by polycondensation of tetracarbox-ylic dianhydrides with diamines, and the synthetic procedure is roughly classified into three methods a two-step procedure, a one-step procedure and a three-step procedure. 

Most polyimides are difficult to process because of their insolubility and high melting temperatures, so polyimides are synthesized by a two-step procedure via a soluble polyamic acid, precursor of the polyimide (Eq. 1) [4-6]. In the first step, ringopening polyaddition of a diamine to a tetracarboxylic dianhydride is carried out in an amide type solvents, for example, N-methyl-2-pyrrolidinone (NMP) and N,N -dimethylacetamide (DMAc), at room temperature, leading to formation of a polyamic acid solution. After processing from the polyamic acid solution, the thermal conversion of the polyamic acid to the polyimide is performed by heating a at about 300°C or by chemical treatment with a mixture of carboxylic dianhydride and tertiary amine in the second step 17]. 

As solutions of polyamic acids are difficult to be store for long periods because of their low stability [8], procedures using derivatives of polyamic acids, such as polyamic acid alkyl esters [9,10], polyamic acid trimethylsilyl esters 111, 12] and polyamic acid amides [13], have been investigated. Although the solutions of these derivatives are more stable than those of polyamic acids, synthesis of polyimides via these polyamic acid derivatives is not attractive for large-scale mass production and industrial applications because more steps are involved, and the cost is high. 

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Infrared pulses of 200 fs duration with 150 of bandwidth centred at 2000 were used in this study. They were generated in a two-step procedure [46]. First, a p-BaB204 (BBO) OPO was used to convert the 800 mn photons from the Ti sapphire amplifier system into signal and idler beams at 1379 and 1905 mn, respectively. These two pulses were sent tlirough a difference frequency crystal (AgGaS2) to yield pulses... 

Rudisill and Stille developed a two-step procedure in which 2-bromo-or 2-trifluoromethanesulfonyloxyacetanilides were coupled with tri-n-butyl-stannylacetylenes in the presence of Pd(PPh3)4.[l], Cyclization was then effected with PdCl2(CH3CN)2. The conditions are compatible with a variety of carbocyclic substituents so the procedure can provide 2-substituted indoles with carbocyclic substituents. The reported yield ranges from 40% to 97% for the coupling and from 40% to 82% for cyclization. 

Commercially, sulfonic acid ion-exchange resins are used in fixed-bed reactors to make these tertiary alkyl ethers (14). Since the reaction is very selective to tertiary olefins and also reversible, a two-step procedure is also used to recover commercially pure tertiary olefins from mixed olefin process streams. The corresponding tertiary alkyl ether is produced in the olefin mixture and then easily separated from the unreacted olefins by simple fractionation. The reaction is then reversed in a second step to make a commercially pure tertiary olefin, usually isobutylene or isoamylene. 

Ring expansion of haloalkyloxiranes provides a simple two-step procedure for the preparation of azetidin-3-ols (Section 5.09.2.3.2(f)) which can be extended to include 3-substituted ethers and O-esters (79CRV331 p. 341). The availability of 3-hydroxyazetidines provides access to a variety of 3-substituted azetidines, including halogeno, amino and alkylthio derivatives, by further substitution reactions (Section 5.09.2.2.4). Photolysis of phenylacylamines has also found application in the formation of azetidin-3-ols (33). Not surprisingly, few 2-0-substituted azetidines are known. The 2-methoxyazetidine (57) has been produced by an internal displacement, where the internal amide ion is generated by nucleophilic addition to an imine. 

Seven procedures descnbe preparation of important synthesis intermediates A two-step procedure gives 2-(HYDROXYMETHYL)ALLYLTRIMETH-YLSILANE, a versatile bifunctional reagent As the acetate, it can be converted to a tnmethylenemethane-palladium complex (in situ) which undergoes [3 -(- 2] annulation reactions with electron-deficient alkenes A preparation of halide-free METHYLLITHIUM is included because the presence of lithium halide in the reagent sometimes complicates the analysis and use of methyllithium Commercial samples invariably contain a full molar equivalent of bromide or iodide AZLLENE IS a fundamental compound in organic chemistry, the preparation... 

Alkenes may be converted m a two-step procedure into vicinalfluora ethers [57] (equation 31). 

The second condition is that the well-dispersed slurry forms a homogeneous bed by formation of the bed under well-controlled conditions. This is achieved by a two-step procedure where the bed is formed using constant velocity of the mobile phase and then stabilizing the bed at a constant pressure (Hagel, 1989). The rationale for the first step at constant velocity is that this will create uniform drag forces from the flowing liquid on the gel particles and thus... 

Since hydrogenolysis resulted in only a 20% yield of the free aldehyde, a two-step procedure was developed in which the diphenylmethyl group was first cleaved with HF/anisole and then the unsubstituted semicarbazone was cleaved with formalin in 40-60% overall yield. 

From a practical point of view, literature data indicate that it is not necessary in most cases to isolate the title salts prior to their reaction. Tliis can be readily understood since their preparation is nearly quantitative and sulfur dioxide is the sole product evolved during their formation. In fact, it is highly advisable to recourse to a two-step procedure during the reactions between A -(l-chloroalkyl)pyridinium chlorides and amino acids (Section IV,C,6) (97BSB383). 

Furoxan dialdehyde 105 has been prepared from the bisaldoxime 14 by des-oximation with nitrous acid. The bisaldoxime 14 was prepared by a two-step procedure from nitromethane [25LA7] (Scheme 55). The benzoylated oxime 106... 

In synthesis of the alkaloid tjipanazole E (19b), the required symmetric dichloro-indolo[2,3-fl]carbazole 20a was obtained in a two-step procedure starting fi-om 4-chlorophenylhydrazine hydrochloride and 1,2-cyclohexanedione employing a Fischer indolization. Subsequent attachment of an acetyl-protected glucopyranosyl moiety to one of the nitrogens, followed by cleavage of the protective groups with ammonia in methanol, produced the desired natural product (91X7739). 

A result equivalent to an allylic substitution reaction with a chiral leaving group can also be achieved by a two-step procedure involving a conjugate addition reaction and a subsequent elimination reaction, as demonstrated by Tamura et al., wbo studied the reaction shown in Scheme 8.15 [27]. 

The two-step procedure includes formation of a N-substituted phthalimide 3, and its subsequent cleavage to the primary amine 5. Phthalimide (which can be obtained from reaction of phthalic acid with ammonia) shows NH-acidity, since the negative charge of the phthalimide anion (the conjugated base) is stabilized... 

This two step procedure appears to be by far the most convenient one for preparing exo-ch-bicyclo[3 3 0]octane-2-car-boxylic acid from the readily available starting materials The first step of the procedure is also illustrative of the method of obtaining 2-substituted bicyclo[3 3 Ojoctanes4,6 from cis cn-1,5-cyclooctadiene. 

Diphenyl-4/7-thiopyran (9) was prepared by application of the known two-step procedure involving the 2,6-dichloro intermediate from the corresponding bis-dimethylacetal (82CJC574). 

Two principle approaches from thiopyrylium salts to thiopyrans accompanied by C-substitution have been found, e.g., the one-step additions of C-nucleophiles or the two-step procedures involving primary conversions of the salts to nucleophilic intermediates followed by attacks with appropriate electrophiles. 

Cyclohexen-l-one is easily prepared by a two-step procedure from cyclohexane-1,3-dione.3 It is available from Fluka AG CH-9470 Buchs. 

PA-4,2 is an even-even PA with a very high melting temperature (390°C). It is prepared from an oxalic ester and 1,4-tetramethylenediamine in a two-step procedure a prepolymerization in solution and a polymerization of the prepolymers in the solid state. 

The synthesis of sulfosuccinate esters—either monoesters or diesters—or the corresponding amides is usually done in a two-step procedure. In both cases, the MA is reacted in the first step with organic alcohol, organic amine, or (in the case of monoesters) with other suitable organic reactants that bear hydroxyl (or even protic) groups in the second step sulfation of the maleic ester takes place. 

Thermoplastic xylan derivatives have been prepared by in-hne modification with propylene oxide of the xylan present in the alkaline extract of barley husks [424,425]. Following peracetylation of the hydroxypropylated xylan in formamide solution yielded the water-insoluble acetoxypropyl xylan. The thermal properties of the derivative quahfy this material as a potential biodegradable and thermoplastic additive to melt-processed plastics. Xylan from oat spelts was oxidized to 2,3-dicarboxyhc derivatives in a two-step procedure using HI04/NaC102 as oxidants [426]. 

Palladium-catalyzed aminations of aryl halides is now a well-documented process [86-88], Heo et al. showed that amino-substituted 2-pyridones 54 and 55 can be prepared in a two-step procedure via a microwave-assisted Buchwald-Hartwig amination reaction of 5- or 6-bromo-2-benzyloxypyri-dines 50 and 51 followed by a hydrogenolysis of the benzyl ether 52 and 53, as outlined in Fig. 9 [89]. The actual microwave-assisted Buchwald-Hartwig coupling was not performed directly at the 2-pyridone scaffold, but instead at the intermediate pyridine. Initially, the reaction was performed at 150 °C for 10 min with Pd2(dba)3 as the palladium source, which provided both the desired amino-pyridines (65% yield) as well as the debrominated pyridine. After improving the conditions, the best temperature and time to use proved... 

A two-step procedure was required for the preparation of a diverse set of pyrrole-3-carboxylic acid derivatives. The diketone 15 was prepared using a functional homologation of a 6-ketoester 14 with different aldehydes followed by oxidation with PCC. The Paal-Knorr reaction was carried out in AcOH in a sealed tube under microwave irradiation (180 °C, 5-10 min) to give differently substituted pyrroles with a COOMe group in position 3 (Scheme 5). This group was further transformed to expand the diversity of the products prepared with this method [32]. 

The amination of aromatic hydroxy-N-heterocycles is a standard reaction in medicinal and agricultural chemistry and has heen reviewed [36]. The hitherto commonly used two-step procedure for amination of hydroxy-N-heterocycles, starts with conversion into the chloro-N-heterocycles, e.g. by treatment of the hydroxy-N-heterocycle with POCI3, PCI5, or SOCI2, followed by reaction with the amine moiety. This methodology has several drawbacks however, for example ... 

The extensive calculations of Serrano-Andres et al [31] have shown a spurious valence-Rydberg mixing in the CASSCF wave functions when valence (7t,7r )and Rydberg orbitals are optimized all together in a state average calculation it was shown that these orbitals loose their diffuse character and instead tend to provide an extra correlation to valence orbitals. To avoid such interaction, the orbitals used for the Cl treatment of the electronic spectrum were obtained by a two step procedure ... 

Since sulphones 204 are easily available compounds one would expect that they could be used as starting materials for the preparation of sulphoxides via the selective removal of one oxygen atom from the sulphonyl group (equation 112). Up to now, there is only one example reported of a direct reduction of a sulphone to a sulphoxide. The bicyclic dideuterio sulphone 205 after 24 h treatment with three-fold excess of diisobutyl aluminium hydride in boiling dichloromethane gave the corresponding sulphoxide 206 in 36% yield (equation 113). A two-step procedure for the selective reduction of sulphones to sulphoxides, which involves an initial reaction of sulphone 204 with aryldiazonium tetrafluoroborate 207 to form aryloxysulphoxonium salt 208 and its subsequent reduction (equation 114), was alluded to by Shimagaki and coworkers and... 

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