Rearrangements, Claisen ortho ester variant

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... 

Scheme 1. Parent Claisen rearrangement (a) and the Johnson ortho ester variant (b).

P-citronellene 605 f 612 (/Q-citronellic acid 232, 235 (/ )-citronellol 360 Claisen rearrangement 17, 87, 90, 137 f., 140, 142, 194 -, Johnson ortho ester variant see Johnson ortho ester Claisen rearrangement cobalt-mediated cyclization... 

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. 

A reaction which is related to the ortho ester Claisen rearrangement utilizes an amide acetal, such as dimethylacetamide dimethyl acetal, rather than an ortho ester in the exchange reaction with allylic alcohols.174 The stereochemistry of the reaction is analogous to that of the other variants of the Claisen rearrangement.175... 

C-IS in 55% yield (Scheme 53). In this case, the thermal Claisen rearrangement occurred stereospecifi-cally from the a-face of the bicycle, presumably via chair transition state (309) (Scheme 54). Therefore it was concluded that the ortho ester rearrangement occurred predominantly via boat transition state (310), providing the first example of a difference in transition state geometry observed in Claisen variants with the same substrate. 

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