Efficient schemes for selective N-protection of 1,4,7-triazacyclononane and I ,5,9-triazacyclododecane and for synthesis of related bis-coronands are based upon the synthetic intermediacy of tricyclic orthoamides.
The inversion of the rotational barrier of a methyl group can be accomplished by a suitable arrangement of water molecules, as shown by X‐ray crystal structure investigations of the orthoamide 1. In the trihydrate, the methyl group adopts a nearly elipsed conformation, whereas in the anhydrous compound the normal staggered conformation is found. The H…O bond strengths of the C−H·O hydrogen bonds required for this inversion (ca. 1.8 kcal nol−1) were confirmed by quantum‐chemical calculations only when correlation effects were taken into consideration.
Die Rotationsbarriere einer Methylgruppe auf den Kopf stellen können geeignet angeordnete Wassermoleküle, wie Kristallstrukturuntersuchungen des Orthoamids 1 ergaben. Im Trihydrat 1′ nimmt die Methylgruppe nahezu ekliptische Konformation ein, während für die wasserfreie Verbindung die übliche gestaffelte Konformation gefunden wurde. Die für diese Umkehrung erforderliche H … O‐Bindungsstärke der C‐H … O‐Brücken von ca. 1.8 kcal mol−1 ergeben quantenmechanische Berechnungen erst bei Berücksichtigung von Korrelationseffekten.
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