Stereoselective rearrangement

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. 

Keywords 17a-hydroxy-20-ketosteroid, acyloin rearrangement, substitutive rearrangement, stereoselective, gas-solid reaction... 

A short review of the first 100 years of ketene chemistry covers haloketenes, Wolff (g) rearrangements, stereoselective nucleophilic attack, dimerization, cycloadditions, ketene-Claisen and -Cope reactions, bisketenes, and free radical processes.12 ... 

Finally, a,P-epoxysilanes rearrange stereoselectively under boron trifluoride catalysis to give silyl enol ethers (Equations Si5.24 and Si5.25). 

By contrast, diaslereomeric mixtures of 2-substituted 1-acetoxy-l-vinylcyclohexanes (or 1-ben-zoyloxy compounds) were found to rearrange stereoselectively to the 2-substituted ( )-2-ace-toxy (or benzoyloxy)ethylidenecyclohexanes under palladium(II) catalysis23. The stereochemical course of the reaction could be explained by the assumption that palladium(II) preferred to coordinate to the equatorial vinyl group anti with respect to the substituent at C-2. Both wnn -conformers, either from cis- or tram-substituted 1-acetoxy-l-vinylcyclohexene, furnished the same racemic cyclohexyl derivative with an -exocyclic double bond. 

Thus, l-alkynyl-2-methylcyclopentanols react with chlorodiphcnylphosphine in the presence of base to give transient phosphinites, which rearrange stereoselectively to allenyl phosphine oxides I24... 

Marshall, J. A. Stereochemical control in the ester enolate Claisen rearrangement. Stereoselectivity in silyl ketene acetal formation. Chemtracts Org. Chem. 1991,4,154-157. 

Dubreuil, D., Cleophax, J., De Almeida, M. V., Verre-Sebrie, C., Liaigre, J., Vass, G., Gero, S. D. Stereoselective synthesis of 6-deoxy and 3,6-dideoxy-D-myoinositol precursors of deoxy myoinositol phosphate analogs from D-galactose. Tetrahedron 1997, 53,16747-16766. Paquette, L. A., Kinney, M. J., Dullweber, U. Practical Synthesis of Spirocyclic Bis-C,C-glycosides. Mechanistic Models in Explanation of Rearrangement Stereoselectivity and the Bifurcation of Reaction Pathways. J. Org. Chem. 1997, 62,1713-1722. 

Claisen rearrangement The Ireland-Claisen rearrangement Stereoselective Reduction of p-Hydroxy Ketones... 

The stereochemical course of the photoinduced walk rearrangement sensitized by benzophenone was examined for the optically active ester (-)-47 and nitrile (+)-48 (76). Similarly, as in the corresponding thermal rearrangement (Figure 5), the inversion process is preferred in both systems (stereoselectivity for 47 >92%, for 48 >76%). In nitrile 48 an additional racemization made of the starting material due to a one-center epimerization at C-7 competes with the rearrangement. Stereoselective diradical processes of the triplet states were proposed to explain these results. 

For a discussion of stereoelectronic effects in this case see also ref 621. In a similar way, -branched dithioesters can be rearranged stereoselectively to a-allyl-/l-hydroxy dithioesters. The relative configurations have not been assigned. The order of selectivity follows the degree of steric hindrance of the substituents R1 bound to the stercogenic center (Table 26)623. 

P-Pinene, a byproduct of the wood and paper industry, is the starting material for an enantioselective synthesis of (i )-citronellal Thermal cycloreversion leads to myrcene. The allylamine obtained by lithiation of myrcene with butyllithium and diethylamine involving an intermediate lithium chelate rearranges stereoselectively in the presence of a chiral catalyst containing the BlNAP-ligand (2,2 -bis-(di-phenylphosphino)-l,r-binapthyl = BINAP) (telomerization) to the enamine, which then readily undergoes acid-catalyzed hydrolysis to (7 )-(+)-citronellal with high enantiomeric excess. 

Vinyl acetals - readily prepared from allyl acetals by double bond isomerization - are rearranged stereoselectively in the presence of a Lewis acid to aldol ethers. 

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