Formation of Stereogenic Centers inside the Dihydropyrone Ring

5) gold-catalyzed intermolecular reactions of propionic acid derivatives with aUcenes [44], and 

Methods (3 and 4) are not asymmetric (Section 2.3.2) as they do not involve chiral mediators or catalysts. The chirality of the starting molecules is merely transmitted to the final products. In contrast, method (5) may be asymmetric provided that a chiral gold catalyst is used. However, the ee s achieved up to now are still unsatisfactory [44]. None of these three methods has been used as yet for the synthesis of natural dihydropyranones. 

Cycloadditions of the [4+2] type (hetero-Diels-Alder, HDA reactions) have been used in a number of cases for the preparation of dihydropyrones. However, the only really useful applications are the asymmetric versions, which lead to optically enriched adducts. These will be discussed in Section 23.2.7. 

Formation of Stereogenic Centers inside the Dihydropyrone Ring 

Naturally occurring 5,6-dihydropyran-2-ones are chiral. The stereogenic carbons responsible for this chirality may reside in the dihydropyrone ring itself (C-5/C-6) and/or in the side chains. The methods used to generate the stereogenic side-chain carbons may involve any known stereoselective reaction and are not necessarily 

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Formation of Stereogenic Centers inside the Dihydropyrone Ring I 57... 

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