Aryl groups other reactions

3-Arylanthranils (541) on thermolysis give acridones (542) (8iAHC(28)23i). 3-Phenylanthranils (543) also form acridones (544) on treatment with nitrous acid (67AHC(8)277). Related rearrangements are found with 3-heteroarylanthranils (e.g. 545— 546) (8iAHC(28)23l). 

Azolecarboxylic acids can be quite strongly acidic. Thus, l,2,5-thiadiazole-3,4-dicarboxylic acid has first and second pK3 values of 1.6 and 4.1, respectively (68AHC(9)107). The acidic strengths of the oxazolecarboxylic acids are in the order 2 5 4, in agreement with the electron distribution within the oxazole ring (74AHC(17)99). Azolecarboxylic acids are amino acids and can exist partly in the zwitterionic, or betaine, form (e.g. 547). 

Azoleacetic acids with a carboxymethyl group are also decarboxylated readily, e.g. all three thiazole isomers, by a mechanism similar to that for the decarboxylation of (3-keto acids cf. Section 3.4.3.1.2. The mechanism has been investigated in the oxazole case, (549)— (550)— -(551) (72JCS(P2)I077). 

Reactivity of Five-membered Rings with Two or More Heteroatoms 

However, in some cases carboxylic acid-derived groups can participate in ring fission-reclosure reactions. Thus, photolysis of 1,5-disubstituted tetrazole (552) gives nitrogen and appears to involve the nitrene intermediate (553), which reacts further to give (554) (77AHC(2i)323). 

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Dimethylthiophosphinic bromide has been prepared by addition of elemental bromine to tetramethyldiphosphine disulfide suspended in carbon tetrachloride.1- The reaction is rather general in that alkyl or aryl groups other than methyl can be attached to the phosphorus atom to give products of the type... 

There are many examples of such reactivity and some of these have been reviewed by Roth and coworkers, a research group that is extremely active in this area. An example that is typical of the processes encountered involves the cyclization of the diene geraniol (1). In this case the sensitizer is 9,10-dicyanoanthracene (DCA) and the reactions are carried out in methylene chloride. The authors state that a contact radical-ion parr is involved, i.e. the radical cation of the diene is in close proximity to the radical anion of the DCA. Reaction within this yields the cyclopentane derivatives 2 and 3 in the yields shown. The ring formation is the result of a five centre CC cyclization within the radical cation of 1. When a more powerful oxidant such as p-dicyanobenzene is used as the sensitizer in acetonitrile as solvent, separated radical-ion pairs are involved. This leads to intramolecular trapping and the formation of the bicyclic ethers 4 and 5 . The bicyclic ether incorporates an aryl group by reaction of the radical cation of the diene with the radical anion of the sensitizer (DCB). This type of reactivity is referred to later. Other naturally occurring compounds such as (/fj-f-bj-a-terpineol (6) and (R)-(- -)-limonene (7)... 

The reactions of 3-unsubstituted iso.xazolium salts (123) with hydroxide, alkoxide, cyanide and azide ions have also been studied, and they can in general be rationalized in terms of the ketoketenimine intermediate (124). The results of these reactions are summarized below. The application of such reactions to 3-unsubstituted isoxazolium salts bearing substituents other than alkyl and aryl groups has received little attention, but 5-aminoisoxazolium salts have been studied (74CB13). 

The diazonio group of arenediazonium salts can be replaced by alkenes and alkynes or, seen from the other reaction partner, alkenes and alkynes can be arylated with arenediazonium salts. The reactions are catalyzed by copper salts and, as found more recently, also by salts of palladium and other metals. 

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