Regiospecific effect

Thus, it seemed worthwhile to compare photoinitiated reaction probabilities for linear and broadside structures. Such a steric or regiospecific effect, i.e., based on the geometric differences associated with the weak inter-molecular forces, might be present in many different environments such as liquids, surfaces, and larger complexes. 

Regiospecific effects arc illustrated in the results summarized in Tables 1 and 2. Methoxymercuration (discussed later) of methyl oleate gives methyl 9- and 10-methoxystearates in high yield (98%) and in equal proportions. When this reaction is applied to ricinoleate (12-hydroxy-9c-18 1, a p -hydroxy alkene) in the form of its aeetoxy or methoxy derivative, the two ethers are formed in very different proportions with the 9-methoxy derivative in exeess. With the corresponding derivatives of... 

Directive effects on lithiation have also been studied. The regiospecific /3-metallation of A-methylpyrrole derivatives and 2-substituted furans has been effected by employing the directive effect of the oxazolino group (82JCs(Pl)1343). 2-Substituted furans and thiophenes are metallated in the 5-position. The formation of 2-lithio-3-bromofuran on treatment of... 

Alkynes are often less accessible and less stable than the corresponding alkenes. (2) The generally slower reaction rate with alkynes usually leads to lower yields. (3) Alkenes provide better regiospecificity than alkynes because alkenes have more variation in substitution and some substituents have a pronounced directional effect. 

Figure 1.2 Effect of polar factors on regiospecificity of radical addition.

It is also possible that complexation of monomer or propagating species could influence the regiospecificity of addition. However, since the effect is likely to be an enhancement of the usual tendency for head-to-tail addition, perhaps it is not surprising that such effects have not been reported. 

The reaction of an a-halo carbonyl compound with zinc, tin, or indium together with an aldehyde in water gave a direct cross-aldol reaction product (Eq. 8.90).226,227 A direct Reformatsky-type reaction occurred when an aromatic aldehyde reacted with an a-bromo ester in water mediated by zinc in low yields. Recently, it was found that such a reaction mediated by indium was successful and was promoted by son-ication (Eq. 8.91).228 The combination of BiCl3-Al,229 CdCl2-Sm,230 and Zn-Et3B-Eb0231 is also an effective mediator. Bismuth metal, upon activation by zinc fluoride, effected the crossed aldol reaction between a-bromo carbonyl compounds and aldehydes in aqueous media. The reaction was found to be regiospecific and syn-diastereoselective (Eq. 8.92).232... 

The orthometallation-carbonylation sequence described earlier (Scheme 155) has been used to effect regiospecific methoxycarbonylation of 4-thiachromanones (Scheme 161 ).232... 

A report on a regiospecific 6-exo radical cyclization for the generation of lactams 173 highlights a remarkable halogen-substitution effect on the direct reactions of unsaturated N-H amides on vinylic halogen substituents <06OL2647>. 

Steric effects and FMO control have been combined in an elegant way to achieve regiospecific synthesis of pyrazole inhibitors of dihydroorotate dehydrogenase <2006SL901>. When the size of the propargylic acid ester 86 is increased from ethyl to diphenylmethyl, pyrazole 87 is formed from compound 85 regiospecifically (Scheme 3 Table 4) <2006H(68)1007>. 

Aromatic fluorination can be carried out by a regiospecific destannylation process shown in reaction 69. This is an effective method for producing fluorinated m-tyrosine and other radiopharmaceuticals, as shown in reaction 70. The process can be applied for radiolabelling with 18F, denoted as F in these reactions, and the products used as radioactive tracers for clinical and fundamental investigations318-321. 

The regiospecificity of the exclusive O-acylation [8] and O-phosphorylation [9] of P-dicarbonyl compounds (Chapter 3) also illustrates the effect of phase-transfer catalysts on the stereochemical course of reactions. Similarly, directed reduction of P-hydroxy ketones using tetramethylammonium trisacetoxyborohydride leads to the preferential formation of the anti dihydroxy system in high yield with a stereoselectivity >95% [10] (Section 11.4). 

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