Selective attack

The hydroxyl groups of glucose (and, of course, other saccharides) must be regio- and stereo-selectively attacked, if this most abundant natural carbon compound is to be used as starting material. We shall first show with a few selected examples, how this can be achieved (A.H. Haines, 1976 J. Lehmann, 1976 L. Hough, 1979). 

A similar intramolecular oxidation, but for the methyl groups C-18 and C-19 was introduced by D.H.R. Barton (1979). Axial hydroxyl groups are converted to esters of nitrous or hypochlorous acid and irradiated. Oxyl radicals are liberated and selectively attack the neighboring axial methyl groups. Reactions of the methylene radicals formed with nitrosyl or chlorine radicals yield oximes or chlorides. 

Substituted allylic alcohols are carbonylated using the o.vidizing system of PdCl2 and CuCU in the presence of HCl and oxygen at room temperature and 1 atm of CO to give the 7-lactone 16 in moderate ylelds[20]. Carbonylation of secondary and tertiary allylic alcohols catalyzed by Pd2(dba)j and dppb affords the 7-lactone 17 by selective attack of CO at the terminal carbon under fairly severe conditionsf21]. 

Clearly, the proportion of substitution occurring adjacent to the pyridinic nitrogen atom is increased by protonation. Also noteworthy are the high proportion of ortho substitution product and the selective attack at C-3 in the iV-phenyl derivative. 

Slides Microstructures of oxide layers and oxide-resistant coatings on metals and alloys selective attack of eutectic alloys [5]. 

Parting—the selective attack of one or more constituents of a solid solution alloy. 

Selective ether cleavage comes about during the substitution step, which obeys an Sn2 mechanism. Therefore, selective cleavage requires selective attack by Y on one of the electrophilic carbons in the protonated ether. Determine if selective attack is likely by examining the shape of the lowest-unoccupied molecular orbital (LUMO) in protonated ethyl propyl ether. Is this orbital larger near one carbon than the other If so, what product combination will result What other atom(s) contribute to the LUMO What would happen if 1 attacked this atom(s) ... 

Thus, the acidity oi a lactam is evidently not a reliable quantity for predicting the course of the methylation. The acidity gives information only as to the reaction velocity. In this connection the reaction course of isomethylreductone (6) is illuminating, " With diazomethane in ether containing 1 mole of water, the enolraethyl ether (7) is formed. However, if water is present only in traces, then the alcoholic hydroxyl group is selectively attacked to give 8. 

It is not worth while, therefore, to give a digest of experimentally determined corrosion rates , but Table 2.21 indicates some sources of such data and their nature. (Some references to data on compatibility of fused salts with non-metallic materials have been included for the sake of completeness.) It should be remembered, that in the case of alloys, failure usually arises from selective attack which causes porosity of the container, even though the wall may appear on casual inspection to be quite sound... 

Where cracks follow intergranular paths due to electrochemical effects related to grain boundary heterogeneity it is likely that selective attack should be observed in such locations by exposure to potent environments even in the absence of applied stress. This has been observed with, low-... 

General corrosion damage was the cause of failure of an A1 alloy welded pipe assembly in an aircraft bowser which was attacked by a deicing-fluid — water mixture at small weld defects . Selective attack has been reported in welded cupro-nickel subjected to estuarine and seawater environments . It was the consequence of the combination of alloy element segregation in the weld metal and the action of sulphate reducing bacteria (SRB). Sulphide-coated Cu-enriched areas were cathodic relative to the adjacent Ni-rich areas where, in the latter, the sulphides were being continuously removed by the turbulence. Sulphite ions seemed to act as a mild inhibitor. 

Test name Ref. Usual solution composition Test procedure Quantitative measure Potential range (V vs. S.H.E.) Species selectively attacked... 

Oxidation reactions were used in the synthesis of porphyrin d, the metal-free ligand system of naturally occurring heme d,. In a total synthesis of porphyrin d,12d oxo functions were introduced into isobacteriochlorin 3 by selenium dioxide oxidation to yield 4. The selenium dioxide selectively attacks the 3- and 8-positions of the partially reduced pyrrole rings of the chromophore. In another synthesis23a c of porphyrin d, an isobacteriochlorin 5, derived by... 

There has been a plethora of recent hterature regarding the synthetic manipulations of the 2(lH)-pyrazinone skeleton. Even though the addition-elimination reactions at the C-3 position to decorate the pyrazinone scaffold are well documented [24], the versatihty of such approaches can be found somewhat limited. Selective attack of nucleophiles on the chloroimine group of the pyrazinone system can generate 3-alkoxy- and 3-amino-pyrazinones (Scheme 9) [27,28]. The 3-CN group was introduced via a Rosemund-von Braun reaction with copper(I)cyanide under harsh conditions (heating in NMP at 150 °C) [27] (Scheme 9). 

Bacteria are vulnerable to the selective attack of chemotherapeutic agents because of the many metabolic and molecular differences between them and animal cells. The biology of vims replication, with its considerable dependence on host-cell energy-producing, protein-synthesizing and biosynthetic enzyme systems, severely limits the opportunities for selective attack. Another problem is that many vims diseases only become apparent after extensive viral multiplication and tissue damage has been done. 

In the proposed mechanism (Scheme 5-33), both the phosphite and the aldehyde are coordinated to the catalyst before nucleophilic attack forms the P-C bond. Slow addition of the aldehyde was found to improve ees it was suggested that this minimizes unselective attack of the activated phosphite on free aldehyde instead of the desired selective attack on complexed aldehyde [26]. In related chemistry with para-anisaldehyde, Shibuya found that ees also depended on the rare earth (La, Eu, Sm) in the heterobimetallic catalyst [24]. 

Track-Etched Track-etched membranes (Fig. 20-66) are now made by exposing a thin polymer film to a collimated beam of radiation strong enough to break chemical bonds in the polymer chains. The film is then etched in a bath which selectively attacks the damaged polymer. The technique produces a film with photogenic pores. 

In the approach using bis(tetrazolyl)morpholinophosphine for synthesizing bis (deoxyribonucleoside) phosphates it is not necessary to protect the 3 -hydroxyl group of die second deoxyribonucleoside (selective attack on the 5 -OH).[983,[1003 In this reaction bis(tetrazolyl)morpholinophosphine is superior to other bis(azolyl)moipholinophosphines or azolylchloromorpholinophosphines.11 13... 

In the reactions mentioned in the preceding sections, several "stereoselective processes have been involved. Various examples have verified that the extension of singly-occupied MO determines the favorable spatial direction of interaction with other species. If there are two such nonequivalent directions in the molecule, the reaction will become stereoselective. Two or more hydrogen atoms attached to the same carbon atom are in some cases nonequivalent. Such a nonequivalence becomes a cause of stereoselectivity and has been explained theoretically. Also several cases have been mentioned in which some nucleophiles selectively attack the molecule from a certain spatial direction. 

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