Orthoester rearrangement

Various nonracemic allenylstannanes have been prepared from nonracemic propargylic mesylates and (Bu3Sn)2CuLi. The stereochemistry of the displacement was shown to be anti by correlation with an allenic stannane prepared through Claisen orthoester rearrangement of a propargylic alcohol of known configuration (Scheme 33)80. 

In contrast to Mori s synthesis, Pawar and Chattapadhyay used enzymatically controlled enantiomeric separation as the final step [300]. Butanone H was converted into 3-methylpent-l-en-3-ol I. Reaction with trimethyl orthoacetate and subsequent Claisen-orthoester rearrangement yielded ethyl (E)-5-methyl-hept-4-enoate K. Transformation of K into the aldehyde L, followed by reaction with ethylmagnesium bromide furnished racemic ( )-7-methylnon-6-ene-3-ol M. Its enzyme-catalysed enantioselective transesterification using vinylacetate and lipase from Penicillium or Pseudomonas directly afforded 157, while its enantiomer was obtained from the separated alcohol by standard acetylation. 

Mukhopadhyay, B. Field, R. A., A simple one-pot method for the synthesis of partially protected mono- and disaccharide bnilding blocks nsing an orthoesterification-benzylation-orthoester rearrangement approach. Carbohydr. Res. 2003,338,2149-2152. 

Claisen orthoester rearrangement.2 This reagent can be used to prepare methyl a-substituted acrylates from allylic alcohols. 2,4,6-Trimethylbcnzoic acid is used as the acid catalyst.1... 

Johnson-Claisen Rearrangement (Johnson Orthoester Rearrangement)... 

An orthoesterification-benzylation-orthoester rearrangement approach to synthesize partially protected mono- and disaccharide building blocks was reported by Robert Field and coworkers (Scheme 2.32) [200], The reaction of L-rhamnopyranoside 120 with triethyl orthoacetate catalyzed by PTSA formed the 4-alcohol intermediate 121, which was further benzylated to furnish the fully protected intermediate 122. Without workup, 1N HC1 was added to the same vessel to rearrange the orthoester to afford the 3-alcohol 123 in a one-pot yield of 78%. Such protocol can also be applied to sugars containing a.v-diol, such as D-mannopyranoside, D-galactopyranoside, L-fucopyrano-side, and lactosyl derivatives. 

SCHEME 2.32 One-pot orthoesterification-benzylation-orthoester rearrangement methodology for the protection of the L-rhamnopyranoside 120. 

Fiirstner, A, Jeanjean, F, Razon, P, Wirtz, C, Mynott, R, Total synthesis of woodrosin I — part 2 final stages involving RCM and an orthoester rearrangement, Chem. Eur. J., 9, 320-326, 2003. 

The power of pericyclic reactions for forming new carbon-carbon bonds is nicely illustrated by the use of the Claisen orthoester rearrangement to prepare y-ketoesters with a quaternary centre next to the ketone (Scheme 9)The use of silver salts to catalyse the acetylenic oxy- Cope rearrangement for the preparation of a,0-6,e-unsaturated ketones has also been reported [equation (10)].57... 

This reaction was first reported by Johnson et al. in 1970. It is a highly stereoselective synthesis of y,5-unsaturated esters from the reaction between allylic alcohols and an orthoester in the presence of a trace amount of weak acid, such as propionic acid. Because this reaction is the modification or variant of the Claisen Rearrangement, it is often referred to as the Johnson orthoester Claisen rearrangement. Occasionally, this reaction is also known as the Claisen-Johnson orthoester rearrangement, " or Johnson orthoester protocol. This reaction involves the formation of mixed orthoester from allyl alcohol and the added orthoester, which loses an alcoholic component to form a ketene acetal then migrates to unsaturated carbonyl compounds via the Claisen Rearmagement with high syn selectivity. Posner further extended this reaction to use sulfonyl orthoester. Overall, this reaction has been applied to the synthesis of a variety of complicated natural products, such as squalenes. ... 

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