Enantioselective synthesis 3,3 -sigmatropic rearrangements

Another enantioselective synthesis, shown in Scheme 13.17, is based on enantiose-lective reduction of bicyclo[2.2.2]octane-2,6-dione by baker s yeast.129 The enantiomeri-cally pure intermediate is then converted to the lactone intermediate by Baeyer-Villiger oxidation and an allylic rearrangement. The methyl group is then introduced stereoselec-tively from the exo face of the bicyclic lactone in step C-l. A final crucial step in this synthesis is a [2,3] sigmatropic rearrangement to complete sequence D. 

Enantioselective catalysis has also been used for the synthesis of optically active sulfimines [51]. By application of 5 mol % of the bisoxazoline-copper(I) catalyst 80, the sulfide 77 is oxidized catalytically to 78 which undergoes a [2,3]-sigmatropic rearrangement to give allyl amine 79 in 80% yield and with 58% ee (Eq. (20)). Other alkenes were found to give lower ee. 

The enantioselective [2,3]sigmatropic rearrangement of allylic selenimides has been achieved by application of the asymmetric synthesis of selenimides [35]. When this reaction was applied to various aryl cinnamyl selenides, the expected optically active allylic amines were obtained in good yield with moderate enan-tioselectivity via [2,3]sigmatropic rearrangement of the intermediate chiral allylic selenimides (Scheme 25). 

The enantioselective total synthesis of the cyclooctanoid natural product (+)-epoxydictymene was accomplished in the laboratory of L.A. Paquette. The entire tricyclic framework was constructed by the application of a Claisen rerrangement via a chairlike transition state. The precursor for this / 3,37-sigmatropic rearrangement was obtained by treating a lactone precursor with the solution of the Tebbe reagent in the presence of pyridine. The corresponding enol ether was formed in almost quantitative yield, and immediately after isolation it was treated with triisobutylaluminum to effect the Claisen rearrangement. 

This methodology was employed in the synthesis p,y-unsaturated-a-aminophosphonates. The reaction sequence comprised one-pot a-sulfeny-lation/olefination of aldehydes followed by [2,3]-sigmatropic rearrangement in good )delds and enantioselectivities (Scheme 8.40). ... 

A review which covers sigmatropic rearrangements in addition to Diels-Alder reactions, 1,3-dipolar cycloadditions, electrocyclic reactions, and ene reactions has appeared. The stereochemistry of [3,3]-sigmatropic reactions of chiral carbon compounds has been reviewed, as have diastereoselective Claisen rearrangements of substrates bearing chiral auxiliary and enantioselective variants of achiral substrates. Examples of [3,3]-sigmatropic rearrangements used in the synthesis of various types of 3-chromene derivatives have been reviewed. ... 

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