1,2-Acetoxy migration

A Pd-catalysed variant of the Nazarov cyclization of 1 -ethynyl-2-propenyl acetate derivatives to form cyclopentenone dierivatives involving 1,2-acetoxy migration is... 

METHOXYCARBONYL-1,1,6-TRIMETHYL-1,4,4a,5,6,7,8,8a-OCTAHYDRO-2,3-BENZOPYRONE, an intramolecular Diels-Alder reaction is responsible for the diastereoselectivity. The stereoselective 1,4-functionalization of 1,3-dienes is exemplified by a two-step process leading to cis- and trans-1-ACETOXY-4-(DICARBOMETHOXYMETHYL)-2-CYCLOHEXENE. The effectiveness of a silyl hydride in providing a means for erythro-directed reduction of a p-keto amide is applied in a route to ERYTHRO-1 -(3-HYDROXY-2-METHYL-3-PHENYL-PROPANOYLJPIPERIDINE. This is followed by an asymmetric synthesis based on a chiral bicyclic lactam leading to (R)-4-ETHYL-4-ALLYL-2-CYCLOHEXEN-1-ONE. The stereoselectivity with which acetoxy migration can operate to an adjacent radical center is reflected in the one-step reaction that gives rise to 1,3,4,6-TETRA-O-ACETYL-2-DEOXY-a-D-GLUCOPYRANOSE. 

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. 

Enamines are readily available ketone derivatives." Exposure of these compounds to certain transition metal salts has been shown to produce a-oxygenated imines which are rapidly hydrolyzed to their ketone counterparts. Thus, for example, morpholino enamines, prepared in situ, are a-acetoxylated on treatment with thallium triacetate. The process is thought to involve either direct nucleophilic extraction of an acetate unit or the intermediacy of an organothallium species which subsequendy undergoes anchimerically assisted intramolecular acetoxy migration to generate the a-acetoxyimine (Scheme IS). 

The gold complex catalysed tandem l,2-/l,2-bis-acetoxy migration in 1,4-bis-propargyl acetates to form 2,3-bis-acetoxy-1,3-dienes has been reported... 

Sometimes reduction of a ketone by NaBH4 is accompanied by hydrolysis of an ester elsewhere in the molecule. Norymberski found that a 20-keto-21-acetoxy compound with NaBH4 in methanol at 0° for 1 hour gives the 20/ ,21-diol. 50 % aqueous dimethylformamide has been used as the solvent in an attempt to prevent acetate hydrolysis, but sometimes under these conditions the 21-acetoxy group migrates to the 20-position. The rearrangement is favored by addition of the 20-acetate as seeds or by addition of... 

Displacement of an allylic halide is complicated by side reactions involving migration of the double bond. A good example is the reaction of 7a-bromo-3 -acetoxy-A -steroids (201) which gives, besides the expected... 

The synthesis of 10 features the SN2 displacement of the allylic acetate with migration of R2 from the ate complex6. Precursors 9 are prepared by the hydroboration of 3-acetoxy-l-alkynes that are available with very high enantiomeric purity via the asymmetric reduction of the corresponding l-alkyn-3-ones, and a substantial degree of asymmetric induction occurs in the conversion of 9 to 10. Best results, based on the enantioselectivity of reactions of 10 with aldehydes, are obtained when R2 is a bulky group such as isopinocampheyl (79 85 % ee)6. The yields of reactions of 10 with aldehydes are 62-76%. 

Besides aryl, vinylic and acetoxy groups also migrate. Vinylic groups migrate... 

In summary then, 1,2 free-radical migrations are much less prevalent than the analogous carbocation processes, and are important only for aryl, vinylic, acetoxy, and halogen migrating groups. The direction of migration is normally toward the... 

No products arising from a nucleophilic reaction at C-2 were observed. The reaction with acetic acid also takes place regioselectively at C-3, but the initially formed anfz-3-acetoxy product undergoes an acyl migration to give the anti-3-hydroxy-2-acetamino product (see Scheme 21). 

The reaction can also be done thermally. The stereochemistry of the thermal rearrangement of the acetoxy epoxides involves inversion at the carbon to which the acetoxy group migrates,143 and reaction probably proceeds through a cyclic TS. 

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. 

Aliphatic aldehydes react to form monoalkylidene derivatives only in presence of potassium r-butoxide. A mechanism has been postulated that also explains the observed N-deacetylation during the formation of the product. The essential step in this is the intramolecular N - O acetyl migration in the initially formed aldol. Subsequent protonation and elimination of the acetoxy group lead to the product (Scheme 26). 

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