Antiperiplanar rotamer

Cl -C2 bond in bwhereas in c, it is antiperiplanar to the Cl -H bond. The latter arrangement is energetically unfavored as the relatively bulky R substituent is placed directly under the plane of the pyranose ring in the a-D-anomers, while in the (3-D-anomers it will be placed over it. Therefore, in both anomers, the b rotamer is the preferred one. 

When a methylamino group was situated equatorially. Booth et al. (85) found that the preferred species was 40 (ap, — sc) having the nitrogen lone pair antiperiplanar to the endocyclic C(2)—O bond (— sc arrangement of the O—C—N—C system). For the axial methylamino group the preferred rotamer 40 (sc, sc) also had the gauche arrangement of the O—C—N—C... 

These positive results encouraged the authors to explore the activity of the chiral dicarhojg lic acids in reactions with different nucleophiles. Interesting results were obtained with dimethyl diazomethylphosphonate (up to 96% ee) and also in the trans-selective asymmetric aziridination with diazoacetamides 66. In the latter case, the traizs-selectivity was justified by an attack on the Re face of the imine differently from the attack on the Si face observed in the addition of t-butyl diazoacetate. This opposite facial enantioselectivily was due to the preference for the rotamer with the carboxamide group and aryl group of iV-Boc-imine in an antiperiplanar orientation and in this case a hydrogen bond between the amidic NH and Boc groups acted as a secondary factor. 

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