Nucleophilic agent

The following acid-catalyzed cyclizations leading to steroid hormone precursors exemplify some important facts an acetylenic bond is less nucleophilic than an olelinic bond acetylenic bonds tend to form cyclopentane rather than cyclohexane derivatives, if there is a choice in proton-catalyzed olefin cyclizations the thermodynamically most stable Irons connection of cyclohexane rings is obtained selectively electroneutral nucleophilic agents such as ethylene carbonate can be used to terminate the cationic cyclization process forming stable enol derivatives which can be hydrolyzed to carbonyl compounds without this nucleophile and with trifluoroacetic acid the corresponding enol ester may be obtained (M.B. Gravestock, 1978, A,B P.E. Peterson, 1969). 

The C-S bond in purine derivatives undergoes cleavage under mild conditions by nucleophilic agents such as benzylmercaptan or glutathione in dimethylformamide with a phosphate buffer of pH 6.5 (277). The salt (110) of dithiazolylsulfide heated at 190 C yields the A-4-thiazo-line-2-thione (112) and 2-chlorothiazole (111) (Scheme 56) (278-280). 

Cyanuric fluoride is readily hydrolyzed to 2,4,6-thhydroxy-l,3,5-triaziae [108-80-5] (cyanuric acid). Cyanuric fluoride reacts faster with nucleophilic agents such as ammonia and amines than cyanuric chloride. 

Many other routes to produce thioglycohc acid have been iavestigated (9). To try to minimise by-products, nucleophilic agents other than alkah sulfhydrates have been claimed, eg, thiosulfates, sodium disulfides, thiourea, xanthogenic acid derivatives, and sodium trithiocarbonates (10). These alternative methods, which require reduction of the disulfides or hydrolysis of carboxymethylthio derivatives, seem less competitive than those usiag alkah sulfhydrates. 

Metal carboxyiates have been considered as nucleophilic agents capable of removing aHyUc chlorine and thereby affording stabilization (143). Typical PVC stabilizers, eg, tin, lead, or cadmium esters, actually promote the degradation of VDC polymers. The metal cations in these compounds are much too acidic to be used with VDC polymers. An effective carboxylate stabilizer must contain a metal cation sufftcientiy acidic to interact with aHyUc chlorine and to facihtate its displacement by the carboxylate anion, but at the same time not acidic enough to strip chlorine from the polymer main chain (144). 

The situation is similar for the derivatives obtained from the bases by the action of various nucleophilic agents. These derivatives can also be considered as ring-chain prototropic systems. There is also... 

Azauridine was also synthesized using the knowledge of the course of alkylation of 6-azauracil 2-methylmercapto derivatives (e.g., Section II,B,4,b). The 1-ribofuranosyl derivative obtained by reaction of the mercury salt of the 2-methylmercapto derivative with tri-O-benzoyl-jS-D-ribofuranosyl chloride on removal of the methyl-mercapto and then benzoyl groups yielded crystalline 6-azauridine, The main difference between uracil and 6-azauracil nucleosides consists in the preparation of cyclic nucleosides. It is known that uridine can be readily converted to cyclic nucleosides by the reaction of 2 (50-O-mesyl derivatives with nucleophilic agents, Analogous... 

The ease with which the dipolarophile interacts with vinylacetylenes depends mainly on a spatial factor. The study of the reactions of alkylthiobuten-3-ynones-l and their selenic and telluric analogs with DPNT shows that, in this case, nitrilimine also acts as a nucleophilic agent with a nucleophilic center on the carbon atom of the 1,3-dipole and always adds to the terminal carbon of the enyne system to form l,3-diphenyl-5-/ -2-pyrazolenines. The oxidation of the latter with chloranil leads to alkynylpyrazoles (65ZOR51). 

A. The Opening of the Isoxazole Ring by the Action of Nucleophilic Agents.398... 

As already noted, the lability of the isoxazole nucleus toward the action of nucleophilic agents distinguishes this heterocyclic system from those of other azoles and of pyridine. The conditions which lead to ring opening, and the products of ring cleavage are quite varied and depend on the position and the nature of substituents, although it is invariably the N—0 bond which is broken. 

The introduction of electron-accepting substituents into the isoxa-zole nucleus sharply increases its lability toward nucleophilic agents. Thus, whereas isoxazole is cleaved by ethanolic alcoholates, its 4-nitro derivative (131) requires merely heating with aniline for ring... 

The cleavage of 3-acylisoxazoles proceeds differently, the nucleophilic agent attacking the carbon atom of the carbonyl group, for example (138- 139). ... 

Quatemization of the isoxazole nitrogen atom makes the ring particularly susceptible toward nucleophilic attack there is a certain analogy here with pyridine. The cleavage of the ring proceeds extremely readily in quaternary salts of isoxazole, even occurring by the action of such weak nucleophilic agents as the anions of carboxylic acids. 

The ketocarbene 4 that is generated by loss of Na from the a-diazo ketone, and that has an electron-sextet, rearranges to the more stable ketene 2 by a nucleophilic 1,2-shift of substituent R. The ketene thus formed corresponds to the isocyanate product of the related Curtius reaction. The ketene can further react with nucleophilic agents, that add to the C=0-double bond. For example by reaction with water a carboxylic acid 3 is formed, while from reaction with an alcohol R -OH an ester 5 is obtained directly. The reaction with ammonia or an amine R -NHa leads to formation of a carboxylic amide 6 or 7 ... 

A form of vitamin B12 can produce methyl carbanion, a reactive species that is responsible for methylation of Hg (see Figure 8.2, Craig 1986, and IAEA Technical Report 137). Methyl carbanion acts as a nucleophilic agent toward Hg ions. 

Epoxldation of (12) with a nucleophilic agent occurred only from the outside of the molecule and the necessary oxidation to (7) could be carried out with RuC and NalO. The remaining steps required new reactions based on selenium chemistry and gave methylenomycin in 12% overall yield from maleic anhydride. 

The hydrolysis of silica alkoxides is a very slow reaction, a sharp acceleration is observed when the pH is shifted to acidic or alkaline side. This is caused by catalysis by protons or hydroxy groups acting as strong electrophilic or nucleophilic agents, respectively [8,18,58],... 

Functional groups were selectively introduced at the C-2 position of isophorone by epoxide ring-opening by several nucleophiles with active methylene groups. Different behavior was observed depending on the reaction conditions and the nature of the nucleophilic agents [57]. The best experimental systems involved PTC or KF-alumina under solvent-free conditions and MW irradiation (Eq. 37 and Tab. 5.15). 

C) In the presence of a nucleophilic exomolecular agent such as H20, cysteine, amino acids, or amino groups, the nucleophilic agent is added to the o-quinone structure that evolves to a modified diphenol in position 5, which produces several polymeric compounds. 

The introduction of powerful electron-withdrawing substituents (NO2, CN, a polynitrogen heterocycle) at the a-C atom of AN stabilize anions of AN, thus facilitating their generation. At the same time, stabilization of anions of AN leads to a decrease in their reactivity and such anions act as milder nucleophilic agents. It will be seen from the following that this leads to an increase in the contribution of C-alkylation. 

Main Chemical Transformations of BENA BENAs can be characterized as compounds having chameleonic activity. In other words, these compounds depending on the nature of partner can act either as (3-C nucleophiles (the attack by an electrophilic agent) or as electrophiles (the attack by a nucleophilic agent) (Scheme 3.231). 

Nucleophilic agents, in VDC polymer stabilization, 25 720 Nucleophilic aromatic substitution,... 

The subsequent exhaustive electrolysis at the potentials of the second anodic process to generate the [Zn(TPP)]2+ dication gives rise to important results. In fact, the electrogenerated dication is stable in the absence of nucleophilic agents, but, in the presence of nucleophilic agents (such as methanol), it transforms into zinc(II) 5 -methoxyl-5,l0,15,20-tetraphenyl isoporphyrin, [Zn(TPiPOCH3)]+, 98... 

The behaviour of other aromatic nitrocompounds (e.g. m-nitroanisole, 3,5-dimethoxynitrobenzene and 4-nitrobiphenyl) follows the same pattern the same short-lived absorption is produced upon exciting an aromatic compound in the presence of a variety of nucleophilic agents, whereas the lifetime of the species formed depends on the nature of the reagent. 

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