Regiospecific acylation

Treatment of 5//-dibenz[6,/]azepine (5, R1 = H) with an excess of butyllithium furnishes the 4,5-dilithio derivative 6, which with an equivalent of an A. A-dimethylcarboxamide undergoes regiospecific acylation at C4.206 In a similar manner, 5-acyl-5f/-dibenz[6,/]azepines, e.g. 5 (R1 = Ac), with butyllithium and carbon dioxide form dibenz[6,/]azepine-4-carboxylic acids, e.g. 7 (R1 = Ac R2 = OH).207... 

The sydnone ring has also been used as an ortho-director of lithiation. Thus, on reaction with Bu Li-TMEDA, 3-phenylsydnone has been found to form a dilithio species which can be regiospecifically acylated at the ort/io-position by a weak electrophile. 

A further example of the use of phenyl sulphones in prenylation reactions is provided in the recent synthesis of deoxytrisporone (37) (Scheme 5)7 An interesting feature of this synthesis is the regiospecific acylation of the diol (35) with isobutyric anhydride. 

The formation of the cyclopropyl ketones can be explained by assuming considerable interaction between the C-Si tr-bond and an electron-deficient orbital at the y-carbon which arises from the regiospecific acylation to the olefinic bond. Cyclodesilylation of the intermediate electrophilic species leads to cyclopropyl ketone, whereas a 1,2-hydride transfer followed by -elimination of the trimethylsilyl group leads to the homoallylic ketone. 

Silyl enol ethers react with acid chlorides and Lewis acids in what is effectively a Friedel-Crafts reaction.32 Both silyl enol ethers 22 and 25 derived from the ketone 23 give regiospecific acylation with MeCOCl and TiCI4. The yields of the two 1,3-diketones 89 and 90, formed as a mixture of diastereoisomers, are excellent. 

Patil, M. L., Jnaneshwara, G. K., Sabde, D. R, Dongare, M. K., Sudalai, A., and Deshpande, V. H. 1997. Regiospecific acylations of aromatics and selective reductions of azobenzenes over hydrated zirconia. Tetrahedron Lett., 38 2137-2140. 

Since /V-acylation is a reversible process, it has allowed the regiospecific alkylation of, for example, imidazoles to give the sterically less favored derivative. This principle is illustrated in Scheme 7 (80AHC(27)24l). 

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). 

Unsubstituted cycloamyloses have been used to catalyze a number of reactions in addition to acyl group transfer. Brass and Bender (8) showed that cycloamyloses promoted phenol release from diphenyl and bis(p-nitro-phenyl) carbonates and from diphenyl and bis(m-nitrophenyl)methyl phos-phonates. Breslow and Campbell (10,11) showed that the reaction of anisole with HOCL in aqueous solution is catalyzed by cyclohexaamylose and cycloheptaamylose. Anisole is bound by the cyclodextrins and is chlorinated exclusively in the para position while bound. Cycloheptaamylose has been used to promote regiospecific alkylation followed by the highly selective oxidation shown in reaction (3) (95). In addition cycloheptaamylose effec-... 

In an effort to circumvent a homodimerization event acyl silanes have been used to promote a cross-benzoin reaction. Initial reports by Johnson and co-workers employed potassium cyanide to catalyze the regiospecific cross silyl benzoin reaction to afford a single regioisomer in good yield (Eq. 2) [45 7]. 

The 1,2,4-diazaarsoles are colorless oils or crystals. The unsubstituted compound is deprotonated by butyllithium and subsequently alkylated or acylated at N-1 <86TL2957>. The 1-acyl derivatives (9) (R = Me, Ph) readily undergo a regiospecific cycloaddition of nitrones, nitrile oxides and diazoacetic esters (Scheme 1). The cycloaddition of diphenyl nitrile imine is more general in respect to the 1-substituent (R = H, Me, Ph, COMe). The cycloreversion of the adduct (10) at higher temperatures provides an in situ access to the 1,3-diphenyl diazaarsole (11) which immediately enters another cycloaddition (Scheme 2) <86TL2957>. 

Phospholipase A2 Hydrolysis of Phospholipids Use of 31P NMR to Study the Hydrolysis, Acyl Migration, Regiospecific Synthesis, and Solubilization of Phospholipids... 

This naphthalene synthesis has several attractive features. The intermediate or-ganometallics need not be isolated it is regiospecific and aliphatic, aromatic, or heterocyclic acyl groups can be introduced efficiently. 

Hoskins and Crout6 have carried out the selective esterification of retronecine (12) at C-9 in moderate yield with simple acids using NN -dicyclohexylcarbodi-imide. With a/3-unsaturated acids and bulky a-trisubstituted acids, the use of /VJV -carbonyldi-imidazole, with prior formation of the acyl-imidazole, gave reasonable yields of the C-9 monoesters of (12). The regiospecificity of this... 

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