Allylic alcohol Johnson-Claisen 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. 

More recently, Kitazume and co-workers found that allylic alcohols 7 undergo the Johnson Claisen rearrangement in higher yields when the reaction is run in a sealed tube at 130 C for 12 hours.35 Under these conditions. (/ )-7 is converted into 8 with high E stereoselectivity and complete 1,3-chiral transfer. 

The C7 quaternary stereocenter of (+)-gelsemine was established utilizing a Johnson-Claisen rearrangement by S.J. Danishefsky and co-workers.The starting stereoisomeric allylic alcohols were individually subjected to the rearrangement conditions, and each gave rise to the same Y,5-unsaturated ester. 

Cook has relied on the Johnson-Claisen rearrangement for the conversion of allylic alcohol 356 to ester 358. The reaction proceeds via a boat-like transition state 357 to jdeld 2 1 diastereoselection of the exocyclic stereogenic center of 358. ... 

A salient structural feature of intermediate 18 (Scheme 2b), the retrosynthetic precursor of aldehyde 13, is its y,r5-unsaturated ester moiety. As it turns out, the Johnson ortho ester variant of the Clai-sen rearrangement is an excellent method for the synthesis of y,<5-unsaturated esters.11 In fact, the Claisen rearrangement, its many variants included, is particularly valuable in organic synthesis as a method for the stereocontrolled construction of trans di- and tri-substituted carbon-carbon double bonds.12,13 Thus, it is conceivable that intermediate 18 could be fashioned in one step from allylic alcohol 20 through a Johnson ortho ester Claisen rearrangement. In... 

The three-step procedure described for the preparation of the illustrated crotylsilanes is initiated with the hydrosilation of rac-3-butyn-2-ol. This procedure is significantly improved with respect to the positional selectivity of the hydrosilation resulting in exclusive formation of the racemic (E)-vinylsilane, and as a result the present procedure is much more amenable to scale-up than those previously described in the literature.8 The enzymatic resolution of the racemic secondary allylic alcohol (vinylsilane) has also been reported using commercially available lipase extracts. The use of a Johnson ortho ester Claisen rearrangement affords the (E)-crotylsilanes 4 in nearly enantiomerically pure form. 

Extremely useful ramifications of the Claisen rearrangement emerged with Johnson s discovery of the orthoester variant of this transformation. His approach (Scheme 2.156) involved the following sequence of steps, which were carried out in one reaction vessel (i) transesterification of the orthoester with an allylic alcohol to give 490 (ii) elimination to form the intermediate ketene acetal 491 and (iii) [3,3] sigmatropic rearrangement to yield the y, -unsaturated ester 492. The Johnson-Claisen procedure is properly considered to be one of the most efficient methods available to prepare y,(5-unsaturated esters such as 492. ° ... 

The Johnson ortho-ester variant of the Claisen rearrangement provides access to y,5-unsaturated esters.The reaction entails heating the allylic alcohol with an ort/io-ester in the presence of a carboxylic acid to form a ketene acetal, which then rearranges to the trans-unsatmated ester.An elevated reaction temperature is necessary for the in situ formation of the ketene acetal but not for the rearrangement. 

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