1,2-Acetoxy shift

The migration of oxygen from a quaternary center in a cyclohexadienone may be preferred to a carbon shift, when present as an ether or ester function rather than free hydroxy. Thus the p-quinol acetate (117) yields the orcinol monoacetate (118 79%) on treatment at room temperature with trifluoroacetic anhydride, and the p-quinol ether (119) forms the resorcinol diethyl ether (120 71%) in ethanolic sulfuric acid. In the second case, hemiketalization must intervene also some methyl shift (12%) is observed. With the quinol (121), treatment with acetic anhydride-sulfuric acid leads to the lactone (122) acetylation or lactonization probably precedes oxygen shift. A number of related examples can be found in the steroid area. - Thermal 1,3-shifts of p-quinol acetates can also be induced acetate (117) yields catechol acetate (123 50-60%, 45 °C) by way of isomerization of the first-formed acetate (124). In the o-quinol acetate series, 1,2-acetoxy shift is seen in (125) (126 92%) and in (127) (128 90%), both in... 

On successive treatment with trimethyloxonium tetrafluoroborate, triethylamine and aqueous acid, oxime acetate 83 prepared from cyclohexanone oxime and Ac20 yields a-acetoxy ketone 87 (Scheme 7). A similar result is obtained with other symmetric ketones including 4-heptanone, deoxybenzoin and dibenzyl ketone. 87 can also be formed by the reaction of cyclohexanone and O-acetyl-Af-methylhydroxylamine hydrochloride upon hydrolysis. If the reaction time is prolonged prior to hydrolysis, ketoamide 89 is isolated as the main product. The key step of the reaction is the [l,3]-acetoxy shift of 7V-acetoxyenamine 85 to the a-acetoxyimine 86. The formation of 89 provides support for the intermediacy of acetoxyimine 86 in the reaction sequence. The evidence available suggests that the [l,3]-acetoxy migration proceeds via a Claisen-type rearrangement (Scheme 7)78. 

A number of o-quinol diacetates have been prepared and they rearrange smoothly to triacetoxyben-zenes o — o migration is preferred unless the latter position is blocked or sterically crowded, when acetoxy shift is directed to another aryl site. The range of possibilities is indicated by the conversions of the o-quinol diacetates (131)-(134) to the 1,2,3- or 1,2,4-triacetoxybenzenes (135)-(138), in 56-90% yields. 

A single quinolone-fused oxepine 45 was reported in a study on the gold (I)-catalyzed formation of functionalized quinolones (130L4234).The starting 2-alkynylarylazide derivatives such as 44 undergo a 1,3-acetoxy shift, followed by cyclization of the azide and a ring-expansion via a 1,2-shift. 

The acetoxy group is introduced exclusively at the benzylic carbon. This is in accord with the intermediate being a weakly bridged species or a benzylic cation. The addition of bromide salts to the reaction mixture diminishes the amount of acetoxy compound formed by shifting the competition for the electrophile in favor of the bromide ion. Chlorination in nucleophilic solvents can also lead to solvent incorporation, as, for example, in the chlorination of 1-phenylpropene in methanol.37... 

Photochemical changes in both II and the acetoxy derivative have been monitored in fluid solution and incorporated in a polymer film. Fig. 5 shows the spectral changes accompanying photochemical transformation of the acetoxy derivative. Then changes may be interpreted in terms of scheme 3, which proposes a photochemical 1,3 acyl shift to form "in situ" an ultraviolet stabilizer chromophore which also has a carbonyl functionality. 

Two examples among many may suffice to demonstrate the diverse spectroscopic variability of chemically modified flavin. Fig. 7 1185) shows the hypsochromic shift of 25 nm (solely) of the UV-peak of a flavin pentapeptide, isolated from monoamine oxidase by performic acid oxidation the visible peak is not influenced. Fig. 7 2,69) demonstrates the dramatic change of 8-acetoxy flavin upon conversion to the anion... 

Nevertheless, acetoxycarbenes are less reactive than analogous chlorocarbenes, as shown by the data in Table 5, where we compare 1,2-H shift rate constants for phenoxymethyl-, cyclobutyl-, and isopropyl halo and acetoxy carbenes. Decreased rates of 1,2-H shifts are observed, due to the influence of the acetoxy spectator substituents, with the kinetic suppression reaching a factor > 900 for the comparison of cyclobutylacetoxycarbene with cyclobutylchlorocar-bene.81... 

The synthesis of acetoxydiazirines90 made available a number of precursors for acetoxy and acyloxycarbenes.81,91,92 The 1,2-H shifts of alkylacetoxycar-benes were discussed above, in Section VI.D. Here, we consider phenylacyloxycarbenes, 82, which rearrange only by 1,2-acyl migrations to diones, Eq. 30. 

In contrast to the cyclopropyl- and cyclobutylhalo- (and acetoxy)carbenes, the cyclopentylhalo- and acetoxycarbenes (91), and their corresponding benzo derivatives, the indanylcarbenes (94), prefer 1,2-H shifts over 1,2-C shifts Eqs. 35 and 36.115 In the absence of strain relief to drive ring expansion, as with the cyclobutylcarbenes, 17, where kc exceeds ku, the 1,2-H shift is preferred.115... 

A 2 1 (- )-90-LAH reagent was employed in the asymmetric synthesis of a cij-diol (91) by reduction of c/j-2-acetoxy-6-phenylcyclohexanone (99,100). Diol 91 is of interest as the tetrahydro derivative of a metabolite obtained from the microbial oxidation of biphenyl. Diol 91 was obtained in 46% e.e. as determined by NMR in the presence of a chiral shift reagent. It was shown to have the absolute stereochemistry (lS,2/ )-dihydroxy-3(S)-phenylcyclohexane by oxidation to ( + )-2-(S)-phenyladipic acid of known absolute stereochemistry. 

Recently, new examples of asymmetric induction in the Pummerer reaction of chiral sulfoxides have been described. Oae and Numata (301) reported that the optically active a-cyanomethyl p-tolyl sulfoxide 275 undergoes a typical Pummerer rearrangement upon heating with excess of acetic anhydride at 120°C, to give the optically active a-acetoxy sulfide 276. The optical purity at the chiral a-carbon center in 276, determined by means of H- NMR spectroscopy using a chiral shift reagent, was 29.8%. 

In practice, however, the annulation does not take place because the aldol condensation is essentially a reversible reaction and strain between the two cyclopentane rings shifts the equilibrium towards the reactants rather than towards the products. In order to overcome this difficulty, the synthetic scheme was substantially modified and 3-acetoxy-2-ethoxypropene (4) was used as the umpoled C3 fragment and the aldol condensation was substituted by a Wittig reaction, as... 

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