Transition State of Aliphatic Claisen Rearrangement

The Claisen rearrangement is a suprafacial, concerted, nonsynchronous pericyclic process that is considered occasionally as an intramolecular 8, 2 alkylation (Eq. 3.1.18) [2]. When the Claisen rearrangement can produce enantiotopic faces at both terminals of allyl vinyl ether, the rearrangement can proceed through two pairs of chiral transition states to prepare two racemic diastereomers bearing newly created stereocenters of the products 15 and 16 (Eq. 3.1.19). Achiral allyl vinyl ether 14 can provide two enantiomeric chair-like transition states chair 1 and 2, both of which lead to the racemic, diastereomeric aldehyde 15. Similarly, the enantiomeric boat-like transition states boat 1 and 2 provide racemic diastereomer 16. The two transition states are essentially different in energy and the ratio 15/16 reflects the transition state geometry. 

Schmid has examined the rate and stereochemistry of rearrangement of four crotyl propenyl ethers 14a-14d [25, 26]. As a result, the E,E isomer 14a is found to rearrange an order of magnitude faster than the Z,Z isomer 14b. The E,E and Z,Z isomers rearrange through a chair-like transition state to give the xyn diastereomers 15a and 15b as the major product. Moreovei the Z,E and E,Z isomers give the anti diastereomers 15c and 15d as the predominant stereoisomer Since all 

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