Cleavage acylative

Reviews cover the topics alkynethiolates in synthesis " oxygen-exchange reactions of sulphoxides sulphonyldiazomethanes C—S bond cleavage acyl isothiocyanates radical reactions of sulphur compounds addition of sulphenyl halides to olefins " sulphenamidcs mercaptoethylation of amines sulphur as a chiral centre " stereochemistry of S and S " compounds " reductive cleavage of sulphides, synthetic uses of alkene- and alkyne-thiolates, and thio-Claisen rearrangements nucleophilic displacements at sulphur in disulphides aromatic... 

Acetic acid and other carboxylic acids are protonated in superacids to form stable carboxonium ions at low temperatures. Cleavage to related acyl cations is observed (by NMR) upon raising the temperature of the solutions. In excess superacids a diprotonation equilibrium, indicated by theoretical calculations, can play a role in the ionization process. 

Keto acids are obtained by acylation of cyclopentanone enamines (see p. I3f.) with acid chlorides and subsequent base-catalyzed mro-aldol cleavage (S. Hdnig, 1960). 

Acyl cations form by coordination of an acyl chloride with aluminum chloride followed by cleavage of the carbon-chlonne bond... 

Mass Spectrometry Aldehydes and ketones typically give a prominent molecular ion peak m their mass spectra Aldehydes also exhibit an M— 1 peak A major fragmentation pathway for both aldehydes and ketones leads to formation of acyl cations (acylium ions) by cleavage of an alkyl group from the carbonyl The most intense peak m the mass spectrum of diethyl ketone for example is m z 57 corresponding to loss of ethyl radi cal from the molecular ion... 

Amides are the least reactive caiboxyhc acid deiivative and the only nucleophilic acyl substitution reaction they undeigo is hydrolysis Amides are fanly stable m water but the amide bond is cleaved on heating m the presence of strong acids 01 bases Nomi nally this cleavage produces an amine and a caiboxyhc acid... 

The saponification of 0 labeled ethyl propanoate was desenbed in Section 20 11 as one of the significant expenments that demonstrated acyl-oxygen cleavage in ester hydrolysis The 0 labeled ethyl propanoate used in this expenment was prepared from 0 labeled ethyl alcohol which in turn was obtained from acetaldehyde and 0 enriched water Wnte a senes of equations... 

The oxidation may be carried out with an inert solvent thermally (35), with a sensitizer such as bromine (36), with uv radiation (37), or over a suitable catalyst (38). Principal by-products of all these oxidation processes are the acyl fluoride products derived from oxidative cleavage of the perfluoroaLkene (eq. 

Chemical Modification. The chemistry and synthetic strategies used in the commercial synthesis of cephalosporins have been reviewed (87) and can be broadly divided into ( /) Selection of starting material penicillin precursors must be rearranged to the cephalosporin nucleus (2) cleavage of the acyl side chain of the precursor (2) synthesis of the C-7 and C-3 side-chain precursors (4) acylation of the C-7 amino function to introduce the desked acylamino side chain (5) kitroduction of the C-3 substituent and 6) protection and/or activation of functional groups that may be requked. 

Dimethylisoxazol-5-amine is easily acylated to its formyl derivative (697) which, on catalytic hydrogenation, undergoes ring cleavage and recyclization to yield 5,6-dimethyl-pyrimidin-4(3H)-one (698) other acyl derivatives give analogous 2-substituted pyrimidines... 

AP298). A similar result may be achieved by ring cleavage followed by acylation and final reclosure 54JCS665). 

Friedel-Crafts acylation usually fails (72AHC(14)43), but 3-substituted l-methyl-2,1-benzisothiazole 2,2-dioxides can be acetylated at the 5-position (73JHC249). l-Methyl-2,1-benzisothiazol-3-one can be chlorsulfonated at the 5-position (78JHC529). Vilsmeier-Haack formylation causes cleavage of the isothiazole ring (80JCR(S)197). 

Oxiranones (a-lactones) (Section 5.05.3.2.1), e.g. (6), in accord with their putative facile opening to zwitterions (Scheme 18), usually react readily with nucleophiles by alkyl-oxygen cleavage (Scheme 41) (71JA557), rather than the acyl-oxygen cleavage common for esters. 

Rearrangement involving cleavage of the C—O bond is also observed with the phenyl isocyanate adduct (59). In place of an O-acylated nitrone its product (60) of acyl migration is isolated (67JPR(36)86). 

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