A s y m m e t r i c [ 3 , 3 ] -S i g m a t r o p i c R e a r r a n g e m e n t s Udo Nubbemeyer Abstract: The synthesis of new complex structures is still a challenge in preparative organic chemistry. Focusing on the generation of defined stereogenic centers, the [3,3]-sigmatropic rearrangements are known as reliable reactions. Always, a highly ordered transition state must be passed through, which allows the shift of chiral information from the reactant into the nascent product. Generally, the complete [1,3]-and, frequently, the [1,4]-chirality transfer enables one to predict the configuration of the new centers.This review focuses on Claisen and Cope rearrangements, which adopt the chiral information via a so termed asymmetric induction. This means, that the directing chiral subunit is placed outside of the six centers of the rearrangement system being reorganized during the course of the [3,3]-sigmatropic reaction.
The zwitterionic Claisen rearrangement of optically-active N-allyl pyrrolidines and various acid chlorides proceeds with high simple diastereoselection (internal asymmetric induction) and high 1,2-asymmetric induction, generating a new C-C bond adjacent to a chiral C-O function. The resulting gamma,delta-unsaturated amides were cyclized to the corresponding optically active gamma-butyrolactones, which are useful intermediates in natural product synthesis. On one hand, a diastereoselective iodocyclization of several lactones led to tetrahydrofurans with a substitution pattern representing a key intermediate of an oxa-prostaglandin synthesis. On the other, a one-pot procedure of a Swern oxidation and consecutive Grignard reaction of one gamma-lactone allowed a diastereoselective chain elongation. The final oxidation/cyclization sequence completed a highly efficient synthesis of the (+)-dihydrocanadensolide or its C-3 epimer, respectively.
Abstract:The zwitterionic aza-Claisen rearrangement of optically active 3-pyrrolidine acryl esters and various acid chlorides to generate optically active azoninones proceeds with high simple diastereoselectivity (internal asymmetric induction) and a complete 1,3-chirality transfer. The reaction path observed depends on the substitution pattern of the allylic system: while the more electron-rich alkylated allyl amine formed predominantly von Braun type products, the a,P-unsaturated esters could be rearranged with high yields. The azoninones thus obtained were treated with electrophiles, inducing regio-and diastereoselective transannular ring contractions. The resulting indolizidinones should be useful key intermediates in alkaloid synthesis.
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