Acyloxy groups

Some of the most powerful activating substituents are those m which an oxygen atom IS attached directly to the nng These substituents include the hydroxyl group as well as alkoxy and acyloxy groups All are ortho para directors... 

Hydroxyl alkoxy and acyloxy groups activate the ring to such an extent that bromina tion occurs rapidly even in the absence of a catalyst... 

The greater stability of the intermediates arising from attack at the ortho and para posi tions compared with those formed by attack at the position meta to the oxygen sub stituent explains the ortho para directing property of hydroxyl alkoxy and acyloxy groups... 

Peroxyesters decompose by an electron-transfer process catalyzed by transition metals (44,168,213) (eq. 34). This reaction has been used synthetically to bond an acyloxy group to appropriate coreactive substrates (eq. 35). 

Oxaziridines are generally formed by the action of a peracid on a combination of a carbonyl compound and an amine, either as a Schiff base (243) or a simple mixture. Yields are between 65 and 90%. Although oxygenation of Schiff bases is formally analogous to epoxidation of alkenes, the true mechanism is still under discussion. More favored than an epoxidation-type mechanism is formation of a condensation product (244), from which an acyloxy group is displaced with formation of an O—N bond. 

An acyl and an acyloxy group can be added to a double bond by treatment with an... 

When X—Y is R3Sn—H the net reaction is decarboxylation and reduction of the original acyloxy group. Halogen atom donors can also participate in such reactions. 

Hexacoordinated Sn atom with two bidentate 189 acyloxy groups. See also Table 5. 

Tetraalkylstannanes with a /i-acyloxy group undergo highly stereospecific elimination to yield Z - or E -unsaturated products, depending on whether the organotin compound has erythro- or threo-configuration, as depicted in reactions 72 and 73, respectively324. 

Evidently, this interesting reaction starts with coordination of Pd by the allylic fragment at the site of attachment of one of the acyloxy groups. Then the anion of the nitro compound is involved in C,C-coupling with olefin (18) followed by successive ionization of the nitro fragment. Finally, Pd is mediated in 0,C-coupling... 

The products have in common a bromonium ion that is formed by attack of Br2 on the face of the double bond opposite the acyloxy substituent. The two products not consistent with simple anti addition across the K bond are obtained via neighboring group participation of the acyloxy group. 

Dunitz (180) has collected X-ray crystallographic data for carbonyl compounds that possess nucleophilic atoms in proximity to C=0, and has postulated that such molecules can be used as models for the incipient transition state (reaction coordinate) for the nucleophilic addition to carbonyl compounds. Atrop-isomeric compounds have the potential, by providing a variety of such data, for understanding the incipient transition states. For example, the interaction found in the 1,4-dimethoxy-9-(2-acyloxyethyl)triptycenes (130) can be viewed as a model for SN2 type reactions where the acyloxy group is the leaving group and the methoxy is the nucleophile. In an extreme case of this sort, cyclization actually takes place. Such an example has been reported (181). 

Rate constants and Arrhenius parameters for the reaction of Et3Si radicals with various carbonyl compounds are available. Some data are collected in Table 5.2 [49]. The ease of addition of EtsSi radicals was found to decrease in the order 1,4-benzoquinone > cyclic diaryl ketones, benzaldehyde, benzil, perfluoro propionic anhydride > benzophenone alkyl aryl ketone, alkyl aldehyde > oxalate > benzoate, trifluoroacetate, anhydride > cyclic dialkyl ketone > acyclic dialkyl ketone > formate > acetate [49,50]. This order of reactivity was rationalized in terms of bond energy differences, stabilization of the radical formed, polar effects, and steric factors. Thus, a phenyl or acyl group adjacent to the carbonyl will stabilize the radical adduct whereas a perfluoroalkyl or acyloxy group next to the carbonyl moiety will enhance the contribution given by the canonical structure with a charge separation to the transition state (Equation 5.24). 

The 1,2 migration of an acyloxy group in (3-(acyloxy)alkyl radicals has been the subject of numerous synthetic and mechanistic studies [25]. These... 

Scheme 6.13 A stepwise pathway via five-membered cyclic intermediate radical 66 in the 1,2 migration of an acyloxy group...

Acyloxy-group participation in carbohydrate chemistry is reported in the prceed- 4 g paper by H. Paulsen, H.Behre and C.-P. Herald Acyloxonium-Ion-Umlage-rungen in der Kohlenhydratchemie. 

In view of the stability of the cyclic (hemiacetal) form of the aldoses, it is reasonable to assume that the open-chain aldehydo structure, formed by elimination of the nitrile group, will be more stable if cyclization is inhibited (by acyloxy groups on carbon atoms 4 or 5) than if cyclization may readily occur (hydroxyl groups on carbon atoms 4 or 5). 

When the Pd-catalyzed exchange protocol was applied to the 5-alkenyl iodides of type 51, the cyclized organozinc reagents 52 were directly obtained and functionalized by allylation with ethyl 2-bromomethylacrylate after transmetallation with CuCN 2LiCl. The five-membered ring products 53 were obtained with high trans diastereoselectivity, irrespective of the nature of the allylic substituent (R = Ph or OBz) at the allylic position (equation 21)34,39. By contrast, the zinc-induced carbocychzations were found to be moderately cis stereoselective when an acyloxy group was present at the allylic position (see Section II.B.2)29. 

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