Intermolecular cycloadditions nitronate stereoselectivity

Abstract This review is devoted to the stereoselectivity of intermolecular (intramolecular cycloadditions are not included) 1,3-dipolar cycloadditions of sugar-derived nitrones. Stereoselective cycloaddition (transformation of isoxazolidine followed by reduction of the N O bond to produce both an amino and a hydroxy function) allows the synthesis of tailor-made products of possible biological interest such as pol>4iydroxylated pyrrolidines, pyrrolizidines, indolizidines, fi-aminocarbonyl compounds, and disaccharides. Attention is focused on the preparation of isoxazolidinyl nucleosides and to the catalysis of the cycloaddition by Lewis acids. This review has concentrated on the new developments achieved from 1999 to February 2007. 

Stereoselectivity of l S-Dipolar Cydoaddition. The stereoselectivity of the intermolecular cycloaddition of an acyclic nitrone to an alkene is difficult to predict, and wotdd appear to be susceptible to minor structural changes in either component (13). The chiral 2,2-dimethyl-l,3-dioxolan-4-yl nitrone showed only modest astereoface selectivity in its addition to methyl crotonate (14). However, the more hindered tetramethyl-l,3-dioxolan-4-yl nitrone was more selective. 

In intramolecular [3+ 2]-cycloaddition reactions, silyl nitronates also lead to substantially higher stereoselectivity than intermolecular reactions (see, e.g., Scheme 3.178) (193). 

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