Prevention allyl rearrangement

Wharton reaction (1, 439 8, 245). This reaction can also be conducted under free-radical conditions of reduction of thiocarbonylimidazolide derivatives of a,/l-epoxy alcohols with tri-n-butyltin hydride.17 In some cases, inverse addition is necessary to prevent further rearrangement of the allylic alcohol initially formed. Yields are around 50-60%. The modification is not useful in the carbohydrate field, where very complex product mixtures are formed. 

The fact that cation-stabilizing R3 groups facilitate the reaction (R3 = NR2 > OR > CH3 > CP ) supports the postulation of a cationic intermediate. Substitution at the allylic C-2 position prevents the rearrangement either by steric repulsion or by the more facile Pd H elimination from the tertiary palladium intermediate. 

Wagner-Meerwein and allylic rearrangements are very common whenever a carbonium ion is formed. For example, in the simple Friedel-Crafts alkylation, the alkyl group always tends to rearrange to give the most stable carbonium ion, which then adds to the aromatic ring. In order to prevent this rearrangement,... 

The reactions with oxygen and MesSiCl, on the other hand, occurred without allylic rearrangement it is noteworthy that the presence of ether prevented any reaction of 67 with the chlorosilane (27). 

Careless attempts to carry out a Claisen rearrangement on this allyl ether often give the compound shown instead of the expected product. What is the expected product How is the unwanted product formed Addition of a small amount of a weak base, such as PhNMe2. helps to prevent the unwanted reaction. How ... 

As discussed above the kinetic stability of allylic sulfoximines prevents them from going downhill to the thermodynamically favored sulfinamides, which in turn is a precondition for their successful application as solutions for asymmetric d -synthons [ 13-15,20-32]. On the other hand, if it would be possible to make them rearrange to allylic amines with 1,3-chirality transfer, this would be an interesting entry to this class of compounds (Scheme 6). ... 

Eilbracht et al. have developed rhodium- or ruthenium-catalyzed one-pot synthesis of cyclopentanones from allyl vinyl ether via tandem Claisen rearrangement and hydroacylation [109-111]. This protocol requires elevated temperature (140-220°C) and also requires alkyl or aryl substituents at the terminal position of the allylic double bond to prevent undesirable double bond migration in the intermediary formed, unsaturated aldehyde. 

---