Acetals migration

The postulated mechanism for the reaction involves activation of the alkyne by jt-coordination to the cationic (IPr)Au% followed by direct nucleophilic attack by the electron-rich aromatic ring to form product 111. Alternatively, two 1,2-acetate migrations give the activated aUene complex, which can be cyclised to product 110 by nucleophilic attack of the aromatic ring on the activated aUene (Scheme 2.21) [92]. 

A plausible explanation for the occurrence of acetal migration with the alditol but not with the sugar moiety of the degradation product is that the primary reaction of the acetal (for example, 39) is the fis-... 

The same authors have demonstrated that 1,3-diynes behave in predictable yet distinctive manners compared to simple enynes under electrophilic transition metal-mediated reaction conditions. This characteristic behaviour of 1,3-diynes is presumably caused by the slightly electron-withdrawing nature of the alkynyl substituent, which not only renders preferentially the formation of 5-exotype alkylidenes but also allows for the subsequent [l,3]-metallotropic shift. Several salient features of reactions with this functionality include the following (a) an acetate is more reactive than the tethered alkene as an initiator, generating [l,2]-acetate migrated alkylidene intermediate, whereas an alkene is a better terminator than an acetate/bromide to generate the cyclopropane moiety (b) allene products are not formed at all under current reaction conditions (c) 5-exo/6-endo-type alkylidene formation depends on the heteroatom substituent in the tether (d) facile metallotropic [1,3]-shift of the intermediate alkylidenes occurred whenever possible. 

Interestingly, the ribonolactone acetal (28) was converted into its enantiomer (29) by interconversion of the oxidation levels at C-l and C-5 followed by an acetal-migration step.55... 

Hughes NA (1968) Further observations on derivatives of 1,6-anhydro-P-D-talopyranose an example of acetal migration accompanying hydrolysis. Carbohydr Res 7 474-479... 

In addition to the sulfonyloxy displacement reaction which may occur on halogenation with triphenyl phosphite complexes, the possibility of acetal migration (with appropriate structure) should not be overlooked. Thus, 1,2 5,6-di-O-isopropylidene-a-D-glucofuranose with triphenyl phosphite dibromide in benzene for 48 hours afforded " (presumably) 6-bromo-6-deoxy-l,2 3,5-di-O-isopropylidene-a-D-glucofiiranose, which was hydrolyzed to 6-bromo-6-deoxy-D-glucose in 10% overall yield. In some compounds, steric factors may prevent the introduction of halogen. With vicinal glycols and triphenyl phosphite methiodide, phosphonic ester formation occurs instead of halide production, but this reaction may be eliminated by use of triphenylphosphine in carbon tetrachloride. 

As for compound IV, it might be derived from cummuleno aldehyde XX, which in turn would be produced from the first acetate migration that gave XVII (Step K of Scheme 54.6). Thus, the presence of IV provides additional support to the double acetate shift mechanism (see Scheme 54.8). 

Further exploring the approach of nucleophiles to the carbonyl center, if the nucleophile is water, then orthoacids can be formed. However, orthoacids are very unstable and generally decompose to a mixture of two products. The first product is a derivative of the starting material with the tosylate displaced by hydroxide. In the second product, the acetate migrated from C-1 to C-2. With respect to mechanistic preferences, soft nucleophiles tend to favor ring opening, whereas hard nucleophiles tend to favor orthoester-type products. 

Acid hydrolysis of 1,2 5,6-di-0-isopropylidene-3,4-dithio-D-iditol was found171 to result in acetal migration, to afford the dithioacetal 3,4-S-isopropylidene-3,4-dithio-D-iditol (90). The action of phosphorus pentasulfide on 91 gives the corresponding 3-thione, which exists as the dimeric172 1,3-dithietane 92. 

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