Transformations of Functional Groups

Scheme 3.2. Transformations of Functional Groups by Nucleophilic Substitution... 

Numerous transformations of functional groups attached to 1,2,3-oxadiazoles have been reported in the last 10 years. The reactivity of substituents attached to ring carbon atoms was thoroughly discussed in CHEC(1984) and CHEC-11(1996). 

The amido-, thioamido-, sulfonamido-, and semicarbazido-benzofuroxans possessing different lateral chains (het-eroaliphatic, heterocyclic, and aromatic) were prepared by traditional methods by transformation of functional group of benzene fragment. The typical chemistry of benzofuroxanes did not take place in these cases <2002AP15>. 

B. Involving Direct Functionalization or Transformation of Functional Groups... 

Substituents can be introduced into the 4-position of sydnones by conventional electrophilic substitution but there are also several examples of electrophilic substitution following metallation at C-4 <95H(41)1525). Standard transformations of functional groups at C-4 of sydnones have also been extensively investigated. In particular, these reactions have been used to synthesize sydnones bearing a variety of heterocyclic substituents at the 4-position <92Mi 403-01). Sydnones can act as... 

The third set of procedures focuses on the vital role that organometallic reagents play in the transformation of functional groups. The first of the seven... 

Although structural modifications of the basic phenoxatellurine and phenoxatellurazine derivatives have only been studied intermittently, interest grows in a more careful study of transformation of functional groups... 

The wide potentiality of semiconductor-mediated photocatalysis was also applied for the transformation of functional groups such as selective cyclization of amino adds in aqueous suspensions [59]. 

The second route to postsynthetic modification of SAMs is the chemical transformation of functional groups present on their outermost surface this approach mostly relies on chemistries already established for the functionalization of solid supports (Fig. 4.3). Two important points to bear in mind are (1) it is essentially impossible to extensively characterize the structure of the reaction products or purify them without destroying the SAM and (2) many solution-phase reactions may be very difficult when carried out on a surface because of the steric hindrance due to the very closely packed end groups. 

As illustrated, the major steps in the conversion of 140 to 139 correspond to non-isohypsic transformations of functional groups the reduction of an aldehyde to a primary alcohol, the oxidation of a secondary alcohol to a ketone, and the oxidation of a primary alcohol to a carboxylic acid. The introduction and removal of the isopropylidene protecting groups and the use of the bacterium Aeetobacter suboxydans (a non-typical oxidizing agent) ensures selectivity in the reactions of the polyfunctional intermediate compounds. 

In much the same way, if the structure contains heteroatoms that are not a part of the heteroaromatic system, it makes sense to start the analysis by rupturing a carbon-heteroatom bond as the reverse reaction represents, essentially, a trivial transformation of functional groups. The presence of small ring fragments such as cyclopropane or epoxide rings in the structure of the target molecule almost automatically dictates the retrosynthetic scission of these moieties in the initial steps of retrosynthetical analysis, as both these groups can be easily introduced with the help of very reliable methods. 

Enzymatic resolution of racemic functionalized isoxazolines is a valuable technique for the preparation of enantio-merically enriched and pure 4,5-dihydroisoxazoles. These enzymatic resolutions exploit a variety of transformations of functional groups resident on the side-chains of the isoxazoline ring. The multipolymer enzymatic resolution of soluble polymer-supported alcohols 307 and 308 was achieved using an immobilized lipase from Candida antarctica (Novozym 435). The (R)-alcohol 309 was obtained in enantiomerically pure form (>99% ee) after its cleavage from the poly(ethylene glycol) (PEG) scaffold (Scheme 70) <2000JOG8527>. 

Dihydrooxazoles continue to occupy an important place in organic synthesis and medicinal chemistry as they have found use as versatile synthetic intermediates, protecting groups/pro-drugs for carboxylic acids, and chiral auxiliaries in asymmetric synthesis. There are several protocols in the literature for the transformations of functional groups such as acids, esters, nitriles, hydroxyl amides, aldehydes, and alkenes to 2-oxazolines. Newer additions to these methods feature greater ease of synthesis and milder conditions. 

Another fascinating, unique property of allylic systems is their propensity for allylic rearrangement with transfer of functionality from one end of the allylic system to the other and concomitant double bond shift with or without transformation of functional groups. In the latter case (X = Y equation 1) a reaction is synthetically useful only in unsymmetrical cases for a 1,3-transposition of the functional group. 

A partial transformation of functional groups should be called selective only, if the yield is (significantly) higher than the statistically expected yield. 

Further diazirines were prepared by transformations of functional groups, leaving the diazirine group intact. Esterifications, and cleavage of esters and acetals, could be carried out as well as transformation of carboxyl groups to acid chlorides and Hofmann elimination of a quaternary ammonium group. There was a report on the oxidation of hydroxyl groups... 

Miscellaneous Methods of Preparing Phosphines. - Transformation of functional groups present in alkyl- and aryl-phosphines has been widely employed in the synthesis of new systems. Further examples of azine formation... 

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