Acetals rearrangements

Silver trifluoroacetate is a suitable catalyst for various cationic rearrangements involving multiple carbon-carbon bonds [49 5(1] In the presence of silver trifluoroacetate, 2 propynyl acetates rearrange to the butadienyl acetates to give dienes that are useful in Diels-Alder reactions [49] (equation 22)... 

Optimized rearrangement conditions in presence of excess silylketene acetal. Rearrangement in the presence of dihydropyrane, re-addition of PhSeOH. 

The silyl ketene acetal rearrangement can also be carried out by reaction of the ester with a silyl triflate and tertiary amine, without formation of the ester enolate. Optimum results are obtained with bulky silyl triflates and amines, e.g., f-butyldimethylsilyl triflate and (V-methyl-Af, /V-dicyclohcxylaminc. Under these conditions the reaction is stereoselective for the Z-silyl ketene acetal and the stereochemistry of the allylic double bond determines the syn or anti configuration of the product.243... 

Entry 6 is analogous to a silyl ketene acetal rearrangement. The reactant in this case is an imide. Entry 7 is an example of PdCl2-catalyzed imidate rearrangement. Entry 8 is an example of an azonia-Cope rearrangement, with the monocylic intermediate then undergoing an intramolecular Mannich condensation. (See Section 2.2.1 for a discussion of the Mannich reaction). Entry 9 shows a thioimidate rearrangement. 

Ireland-Claisen (silyl ketene acetal) rearrangement... 

A tricyclic acetate rearranged in benzene at 110 CC in a /Moluenesulfonic acid catalyzed reaction yielding cycloundecanone 2.231... 

Allylic esters of fluoroacetic acid were used in the Ireland silyl ketene acetal rearrangement procedures by the Welch group at Albany [164]. For example, Eq. (53) shows a highly diastereoselective rearrangement which formed an early stage in syntheses of 2,3-dideoxy-2-fluoro-3-C-methyl pentose nucleosides [165, 166]. If a stereoselective synthesis of a functionalised monofluorocompound is... 

Allylic acetals rearrange under acid catalysis to produce tetrahydrofurans. 

In the presence of tetrakis(triphenylphosphine)palladium a-cyano allylic acetates rearrange to y-acetoxy-a,/3-unsaturated nitriles. The products can he converted into furanc derivatives (equation II).2... 

J. O. Buchanan and A. R Edgar, Acetoxoniuin ions from acetoxyoxiranes and ccthoesten Their conversion into ethylidene acetals, rearrangement, and solvolysis, Carbohydr. Res. 49 289 (1975). 

The Johnson Claisen rearrangement,31 also called the ortho ester Claisen rearrangement, involves the reaction of an allylic alcohol with an ortho ester, often triethyl orthoacetate, in the presence of a carboxylic acid such as propionic acid, usually at temperatures between 120 and 140 JC (Scheme 3).1 The intermediate kctcnc acetal rearranges stereoselectively. 

The acetonation of 2,4-benzylidene-D-sorbitol has been studied by Vargha135 and by Laan and Dekker 88 when copper sulfate is employed as the catalyst a mono- and a di-isopropylidene derivative (m. p. 179° and 131°, respectively) are obtained. An interesting point about the latter compound is that, unless acetal rearrangement occurs during the acetonation stage, it must contain at least one isopropylidene group which is not united to adjacent positions in the hexitol molecule. 

Still and Schneider employed Ireland-Claisen rearrangement in the total synthesis of ( )-frullanolide (ll)6 (Scheme 1.3f). The key step of the synthesis is efficient Ireland-Claisen rearrangement of the P-pyrrolidinopropionate ester 12. The triethylsilylketene acetal rearranged in toluene at reflux and the pyrrolidine moiety was eliminated after stirring with a mixture of dimethyl sulfate and potassium carbonate in methanol to afford the a-substituted acrylic ester (13). Saponification followed by iodolactonization gave the iodolactone 14, which upon treatment with DBU led to ( )-frullanolide. 

Epoxycyclohexane, conformational analysis, 11 Epoxy-derivatives of enol acetates, rearrangement, 367-388, 445... 

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