Reactions of Chiral Nitrones

The stereofacial differentiation of the diastereotopic sides of nitrones by removable auxiliaries demands the positioning of the chiral group to be within the fV-substituent. In order to exhibit a spatial differentiation, a preferred conformation of this substituent is crucial. Carbohydrates, due to their exo-anomeric effect revealed to be efficient auxiliaries for these transformations. So far the reactivity and efficiency of more than 40 carbohydrate compounds have been examined. When optically active nitrones from inexpensive carbohydrates are applied, generally the optically purities of the N-substituted isoxazolidines are determined after removal of the chiral auxiliary. 

1 Reactions of Nitrones with Chiral Nitrogen-Substituents 

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To explore synthetic methods for optically active compounds, we next turned our attention to systematic studies on diastereoselectivity in this tandem process, using acyclic secondary allyl alcohols (Z)-12 and (E)-12. Treatment of nitrone lb with alcohol (Z)-12 in the presence of a catalytic amount of TiCLt and MS4A led to smooth transesterification and intramolecular cycloaddition even at room temperature, affording 13b accompanied by a small amount of 14b (Table 6.4, entry 1). The bulkier nitrone Ic also reacted with (Z)-12 under the same conditions, giving 13c and 14c in an excellent combined yield with high diastereofacial selectivity (entry 2). It was also found that reactions of chiral nitrones Im and In with (Z)-12 proceeded smoothly to give 13m and 13n as the major cycloadducts in excellent yields with high diastereofacial selectivity, which was independent of the chirality of nitrones Im and In (entries 3 and 4). 

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