Cyclization reactions diastereofacial selectivity

Padwa and Prein (105,106) applied chiral, but racemic, isomiinchnone dipoles in diastereoselective 1,3-dipolar cycloadditions. The carbonyl ylide related isomiinch-none derivative rac-70 was obtained from the rhodium-catalyzed cyclization of diazo-derivative rac-69 (Scheme 12.24) (105). The reactions of the in situ formed dipole with a series of alkenes was described and in particular the reaction with maleic acid derivatives 71a-c gave rise to reaction with high selectivities. The tetracyclic products 72a-c were all obtained in good yield with high endo/ exo and diastereofacial selectivities. In another paper by the same authors, the reactions of racemic isomilnchnones having an exo-cyclic chirality was described (106). 

The first example illustrating the use of a removable chiral auxiliary in IMDA reacticms was reported by Mukaiyama." The Diels-Alder reaction of (144) was slow and showed no diastereofacial selectivity. Conversion of (144) to the magnesium alkoxide (145), in which the dienophile is activated by chelaticm to the magnesium ion, however, increased the rate of cyclization and provided the indicated cycloadduct with 76% diastereomeric excess (rfe 88 12 mixture of diastereomers). 

Step [138], that is, an intramolecular Diels-Alder reaction (IMDA) of intermediate 249 bearing an appropriate diene and a methylene tetronate. Despite the poor diastereofacial selectivity of this step, and the low overall efficiency, Yoshii s inventive strategy remains a masterful application of stereoselection via intramolecular cyclization. 

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