Stereoselectivity nitrile ylide 1,3-dipolar cycloadditions

The amino acid derived chiral oxazolidinone 188 is a very commonly used auxiliary in Diels-Alder and aldol reactions. However, its use in diastereoselective 1,3-dipolar cycloadditions is less widespread. It has, however, been used with nitrile oxides, nitrones, and azomethine ylides. In reactions of 188 (R = Bn, R =Me, R = Me) with nitrile oxides, up to 92% de have been obtained when the reaction was performed in the presence of 1 equiv of MgBr2 (303). In the absence of a metal salt, much lower selectivities were obtained. The same observation was made for reactions of 188 (R = Bn, R = H, R = Me) with cyclic nitrones in an early study by Murahashi et al. (277). In the presence of Znl2, endo/exo selectivity of 89 11 and up to 92% de was observed, whereas in the absence of additives, low selectivities resulted. In more recent studies, it has been shown for 188 (R =/-Pr, R = H, R =Me) that, in the presence of catalytic amounts of Mgl2-phenanthroline (10%) (16) or Yb(OTf)3(20%) (304), the reaction with acyclic nitrones proceeded with high yields and stereoselectivity. Once again, the presence of the metal salt was crucial for the reaction no reaction was observed in their absence. Various derivatives of 188 were used in reactions with an unsubstituted azomethine ylide (305). This reaction proceeded in the absence of metal salts with up to 60% de. The presence of metal salts led to decomposition of the azomethine ylide. 

Treatment of (Z)-A-(4-nitrobenzyl)benzimidoyl chloride with base generates the nitrile ylide 1018, which undergoes a 1,3-dipolar cycloaddition with 5,6-dihydropyran-2-one to afford the exo-adduct tetrahydropyran-2-one 1019 as the exclusive product (Scheme 264) <1998T11613>. A 1,3-dipolar cycloaddition of a chiral cyclic nitrone with 5,6-dihydropyran-2-one proceeds with high stereoselectivity to the co-tetrahydropyran-2-one <2000TA2015, 2001TA3163>. 

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