Abstract
Synthesis of (–)‐(1R,2S)‐Norephedrine HomologuesThe amino function of esters of some simple natural amino acids I is blocked in the form of a cyanoenamine by means of 2‐oxocyclopentanecarbonitrile, so that the corresponding cyanoenamino esters II are obtained. The reaction of a disubstituted lithium amide with II leeds to the cyanoenamino‐amides VI. The amide function present in VI is then transformed into an aromatic ketone by means of phenyllithium, to give the (benzoylalkyl)aminocyclopentenecarbonitriles VII. Reduction of Compounds VII with NaBH4 in EtOH −80° affects only the keto function and leads to the [(α‐hydroxybenzyl)alkyl]amino‐cyclopentenecarbonitriles VIII. The amino function is then deprotected by acid hydrolysis to give the amino‐alcohols IX with yields close to 50%; in every amino‐alcohol IX, the erythro isomer, homologous to natural (–)‐(1R,2S)‐norephedrine is the more abundant or the single product. All the polyfunctional compounds prepared conserve optical activity; it has been demonstrated that the amino‐alcohols IX are pure enantiomers and that no racemisation lakes place at any step of their synthesis.
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