Side reactions intramolecular

Concerning electrophilic side reactions, intramolecular Friedel-Crafts condensations have been reported for example, fluorenone is formed from 2-benzoylbenzenediazonium tetrafluorobo-rate.241 The strong Lewis acid boron trifluoride can also be responsible for side reactions, such as the extensive formation of tars from nitro-substituted arenediazonium tetrafluorobor-ates or the acidic hydrolysis of ester substituents, especially in the case of 2-(ethoxycar-bonyl)benzenediazonium tetrafluoroborate.105,242... 

More recent developments of cationic polymerization proceeding by the AM mechanism have opened new horizons for the synthetic application of ROP of cyclic ethers. By using this approach, main side reaction (intramolecular chain transfer to polymer leading to the formation of cyclic fraction) can be minimized and control over molecular weight, molecular weight distribution, and the structure of end-groups can be... 

Note that for 4.42, in which no intramolecular base catalysis is possible, the elimination side reaction is not observed. This result supports the mechanism suggested in Scheme 4.13. Moreover, at pH 2, where both amine groups of 4.44 are protonated, UV-vis measurements indicate that the elimination reaction is significantly retarded as compared to neutral conditions, where protonation is less extensive. Interestingy, addition of copper(II)nitrate also suppresses the elimination reaction to a significant extent. Unfortunately, elimination is still faster than the Diels-Alder reaction on the internal double bond of 4.44. 

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. 

The tertiary amine is formed in a similar manner from the imine and a secondary amine. This side reaction can be minimized by carrying out the hydrogenation in the presence of ammonia, which tends to shift the equiHbrium back towards the imine. When a compound with two or more nitrile groups is hydrogenated, the formation of both cycHc and acycHc secondary and tertiary amines is possible, depending on whether the side reaction is intramolecular or intermolecular. For example, for the hydrogenation of adiponitfile ... 

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... 

In the case of an intramolecular Wurtz reaction less side-reactions are observed this variant is especially useful for the construction of strained carbon skeletons. For example bicyclobutane 5 has been prepared from l-bromo-3-chlorocyclobutane 4 in a yield of > 90% ... 

These side reactions may occur if the /V-acyliminium ion is not trapped quickly enough by a nucleophile. So problems may arise with relatively poor nucleophiles or if there is too much steric hindrance, while in the case of intramolecular reactions, unfavorable stereoelectronic factors or intended formation of medium- or large-sized rings may play a role. The reaction conditions, such as the nature of the (acidic) catalyst and the solvent, may also be of importance. 

However, according to Mehrotra209,272-294,322 many side reactions take place Thus intramolecular reactions are due to the fact that titanium diacylates are more stable than other acylates ... 

Assuming that no intramolecular or side reactions take place and that all groups are equireactive, the polydispersity index, 7P, of hyperbranched polymers obtained by step-growth polymerization of ABX monomers is given by Eq. (2.2), where pA is die conversion in A groups.196 Note that the classical Flory relationship DPn = 1/(1 — pa) holds for ABX monomer polymerizations ... 

Cyclic structures can form as a result of side reactions. One of the most common examples is the formation of diketopiperazines during the coupling of the third amino acid onto the peptide chain (Fig. 7). Intramolecular amide bond formation gives rise to a cyclic dipeptide of a six-membered ring structure, causing losses to the sequence and regeneration of the hydroxyl sites on the resin. The nucleophilic group on the resin can lead to fiuther unwanted reactions [14]. 

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). 

Given their extraordinary reactivity, one might assume that o-QMs offer plentiful applications as electrophiles in synthetic chemistry. However, unlike their more stable /tora-quinone methide (p-QM) cousin, the potential of o-QMs remains largely untapped. The reason resides with the propensity of these species to participate in undesired addition of the closest available nucleophile, which can be solvent or the o-QM itself. Methods for o-QM generation have therefore required a combination of low concentrations and high temperatures to mitigate and reverse undesired pathways and enable the redistribution into thermodynamically preferred and desired products. Hence, the principal uses for o-QMs have been as electrophilic heterodienes either in intramolecular cycloaddition reactions with nucleophilic alkenes under thermodynamic control or in intermolecular reactions under thermodynamic control where a large excess of a reactive nucleophile thwarts unwanted side reactions by its sheer vast presence. 

Crossed Claisen reactions with two different esters, each of which has a-H atoms, are seldom useful synthetically as there are, of course, four possible products. Crossed Claisen reactions are, however, often useful when one of the two esters has no a-H atoms, e.g. HCOzEt, ArC02Et, (C02Et)2, etc., as this can act only as a carbanion acceptor. Such species are in fact good acceptors, and the side reaction of the self-condensation of the other, e.g. RCH2C02Et, ester is not normally a problem. Intramolecular Claisen reactions, where both C02Et groups are part of the same molecule [e.g. (123)], are referred to as Dieckmann cyclisations. These work best, under simple conditions, for the formation of the anions of 5-, 6- or 7-membered cyclic / -ketoesters... 

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). 

Trost and coworkers [131] synthesized oxaheterocycles by a Pd-catalyzed addition of terminal alkynes onto hydroxyalkynoates, followed by an intramolecular addition of the hydroxyl functionality on the triple bond. Simple lactonization may take place as a side reaction. 

Besides the expected intramolecular reaction, several other side reactions can occur ... 

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... 

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. 

For the formation of microgels the presence of a crosslinking monomer is not always necessary. Thus, microgels have also been detected in polymers prepared with bifunctional monomers, e.g. poly(acrylonitrile-co-vinylacetate) [39], polyethylene [40],poly(vinylchloride) [41] andpoly(vinylidenefluoride) [42].Obviously, the reason for the intramolecular crosslinking with the formation of microgels are side reactions. 

The intramolecular electron transfer leads to fast formation of semi-quinone and the lower oxidation state metal ion. The catalytic cycle is completed by fast reoxidation of the metal ion. Significant deviations from this model were observed at low dioxygen concentrations and it was suggested that another oxidation path becomes operative under such conditions. Although earlier they had been proposed to participate (10), side reactions with dehydroascorbic acid could be excluded. 

Sulfoxide-mediated intramolecular aglycone delivery has been conducted with a temporary linker formed in situ by the reaction of lanthanide triflates with the donor and acceptor-based alcohols (Scheme 4.66) [336], However, as the selectivities recorded were modest, it has to be assumed that intermolecular glycosylation was an important side reaction in this chemistry. 

Photochemical reactions of the pyrimidine polymers in solution were studied to determine the quantum yields of the intramolecular photodimerization of the pyrimidine units along the polymer chains. Photoreactions of the polymers were carried out in very dilute solutions to avoid an intermolecular(interchain) photodimerization. Quantum yields determined at 280 nm for the polymers (1-6 in Figure 1) are listed in Table I. The quantum yield of the 5-bromouracil polymer [poly(MAOU-5Br)] could not be determined because of side reactions of the base during the irradiation. 

In a recent application of this strategy, nitrocyclohexane 143 (prepared from nitrosugar 142 by intramolecular Henry reaction) was subjected to a radical denitration by HSnBu3, after protection of the hydroxyl groups to avoid side reactions. Inositol 146 was selectively obtained in good yield, once the hydroxyl protecting groups were removed (Scheme 45).101... 

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