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Cyclization side reaction

An additional cyclization side reaction can also occur, especially at elevated temperatures ... [Pg.294]

A variety of diamines to enrich the combination of monomers were also examined. Polymerization using primary dialkylamines (lld-e) and sterically demanding secondary dialkylamines (lla-c) successfully yielded polyamidines with absolute molecular weight of up to 67,200 g/mol. Surprisingly, diarylamines (llf-h) with much weaker nucleophilic ability than alkylamines also served as excellent monomers for this MCP, and the rigid monomer stmcture suppressed cyclization side-reactions. [Pg.32]

The synthesis described met some difficulties. D-Valyl-L-prolyl resin was found to undergo intramolecular aminoiysis during the coupling step with DCC. 70< o of the dipeptide was cleaved from the polymer, and the diketopiperazine of D-valyl-L-proline was excreted into solution. The reaction was catalyzed by small amounts of acetic acid and inhibited by a higher concentration (protonation of amine). This side-reaction can be suppressed by adding the DCC prior to the carboxyl component. In this way, the carboxyl component is "consumed immediately to form the DCC adduct and cannot catalyze the cyclization. [Pg.237]

Cross-linked PVP can also be obtained by cross-linking the preformed polymer chemically (with persulfates, hydrazine, or peroxides) or with actinic radiation (63). This approach requires a source of free radicals capable of hydrogen abstraction from one or another of the labile hydrogens attached alpha to the pyrrohdone carbonyl or lactam nitrogen. The subsequently formed PVP radical can combine with another such radical to form a cross-link or undergo side reactions such as scission or cyclization (64,65), thus ... [Pg.526]

Addition to 1,2-dimethyl- -piperideine or 1,2-dimethyl- -pyrroline is followed by intramolecular alkylation by the ester group as a side reaction to give 140 and 141 ( = 1, 2), respectively. Cyclization products 142 and... [Pg.284]

The acyl residue controls the formation and stability of the carbonium ion. If the carbonium ion is destabilized (by electron withdrawing groups), then cyclization to the phenanthridine nucleus will be sluggish. The slower the rate of cyclization, the greater the chance of side reactions with the cyclization reagent. Therefore, the yield of the phenanthridine will depend on the relative rates of cyclization and side reactions, which is controlled by the stability of the carbonium ion. [Pg.466]

Alkylation lo yield a leriiary amine may occur easily if the formation involves cyclization (ii). Catalysts may have a marked influence. In reductive alkylation of ammonia wilh cyclohexanones, more primary amine was formed over Ru and Rh and more secondary amine over Pd and Pt. Reduction of the ketone to an alcohol is an important side reaction over ruthenium. [Pg.83]

Reductive alkylation by alcohol solvents may occur as an unwanted side reaction 22,39), and it is to avoid this reaction that Freifelder (20) recom mends ruthenium instead of nickel in pyridine hydrogenation. Alkylation by alcohols may occur with surprising ease 67). Reduction of 18 in ethanol over 10% palladium-on carbon to an amino acid, followed bycyclization with /V,/V-dicyclohexylcarbodiimide gave a mixture of 19 and 20 wiih the major product being the /V-ethyl derivative 49,50). By carrying out the reduction in acetic acid, 20 was obtained as the sole cyclized product 40). [Pg.90]

An investigation of the thioamidation reaction on model compounds of PAN under similar conditions and a study of the functional composition of modified copolymers led to the assumption that the most probable course of the two types of side reactions causing the decrease of the number of thioamide groups are hydrolysis and cyclization reactions. [Pg.121]

Difficulties due to side reactions (cyclization) and a broad molecular weight distribution accompanying the polycondensation of active esters led to the application of methods wherein the polymers are built up stepwise. In 1968, Sakakibara et al.31) introduced the solid-phase technique using Merrifield s resin. By stepwise addition of tert-pentyloxycar-bonyl tripeptides, they have synthesized (Pro-Pro-Gly)n with n = 5, 10, 15 and 20. [Pg.160]

In the polymerization of PA-4,6, a side reaction is the cyclization of the diaminobutane endgroup2,32 ... [Pg.153]

A series of theoretical studies of the SCV(C)P have been reported [38,40,70-74], which give valuable information on the kinetics, the molecular weights, the MWD, and the DB of the polymers obtained. Table 2 summarizes the calculated MWD and DB of hyperbranched polymers obtained by SCVP and SCVCP under various conditions. All calculations were conducted, assuming an ideal case, no cyclization (i.e., intramolecular reaction of the vinyl group with an active center), no excluded volume effects (i.e., rate constants are independent of the location of the active center or vinyl group in the macromolecule), and no side reactions (e.g., transfer or termination). [Pg.9]

Cyclization to six-membered rings (Eq. 15) provided modest diastereoselectivity and required the use of bulkier PhMeSiH2 to prevent olefin hy-drosilylation. Propargyl and homopropargyl amines 94 afforded a variety of heterocycles (Scheme 21), if the catalyst was added slowly over the reaction course to diminish side reactions resulting from metal coordination to the basic amine [56]. The reaction procedure was extended to the diastereoselect-ive bicyclization of dienyne substrate 95, giving 96 as product in a cascade fashion (Eq. 16) [57]. [Pg.237]

The origin of the reaction which limits the chain growth is not known. Probability of cyclization, one of the side reactions, should decrease strongly with the polymerization degree in agreement with the... [Pg.80]

The possible importance of side reactions such as Friedel Craft alkylation, inter or intramolecular Diels Alder cyclization or re-addition of HC1 in this context have also been emphasised (43). [Pg.236]

PMMA can exist in two simple stereoregular forms, isotactic and syndiotactic, but commercially available samples—prepared via free-radical initiators—tend to have tacticities lying in the range 60-70% syndiotactic triad content, the exact content depending upon the reaction temperature.426 Several terminating side reactions have been identified, the most important of which is intramolecular cyclization leading to methoxide formation, as shown in Scheme 5.427... [Pg.23]

The most studied catalyst family of this type are lithium alkyls. With relatively non-bulky substituents, for example nBuLi, the polymerization of MMA is complicated by side reactions.4 0 These may be suppressed if bulkier initiators such as 1,1-diphenylhexyllithium are used,431 especially at low temperature (typically —78 °C), allowing the synthesis of block copolymers.432,433 The addition of bulky lithium alkoxides to alkyllithium initiators also retards the rate of intramolecular cyclization, thus allowing the polymerization temperature to be raised.427 LiCl has been used to similar effect, allowing monodisperse PMMA (Mw/Mn = 1.2) to be prepared at —20 °C.434 Sterically hindered lithium aluminum alkyls have been used at ambient (or higher) temperature to polymerize MMA in a controlled way.435 This process has been termed screened anionic polymerization since the bulky alkyl substituents screen the propagating terminus from side reactions. [Pg.24]

Type C syntheses are typified by the oxidative cyclization of amidinothiono groups, and this has become the basis of a versatile synthesis of 1,2,4-thiadiazoles. This type of reaction is known for its speed and absence of side reactions. The synthesis of unsymmetrical 3,5-disubstituted-l,2,4-thiadiazoles of unambiguous structure in high yields is possible by this method. [Pg.503]


See other pages where Cyclization side reaction is mentioned: [Pg.245]    [Pg.53]    [Pg.376]    [Pg.580]    [Pg.31]    [Pg.194]    [Pg.245]    [Pg.53]    [Pg.376]    [Pg.580]    [Pg.31]    [Pg.194]    [Pg.150]    [Pg.217]    [Pg.293]    [Pg.81]    [Pg.245]    [Pg.256]    [Pg.316]    [Pg.537]    [Pg.181]    [Pg.61]    [Pg.123]    [Pg.556]    [Pg.595]    [Pg.302]    [Pg.1037]    [Pg.5]    [Pg.168]    [Pg.200]    [Pg.361]    [Pg.364]    [Pg.21]    [Pg.198]    [Pg.284]    [Pg.895]    [Pg.306]    [Pg.628]   
See also in sourсe #XX -- [ Pg.140 ]




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Cyclization reactions

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