β‐Ketonitrile‐Derived Protecting Groups of the Amino Function. Synthesis of Amino Alcohols The amino group of natural L‐amino acid esters is protected by condensation with 2‐oxocyclopentanenitrile (1) or 2‐formyl‐2‐phenylacetonitrile (10). Only the ester group of the formed cyanoenamino esters 2 and 11 reacts with nucleophilic reagents such as organometallics (RMgX, RLi), borohydrides, or metal amides, whereas the cyanoenamino group is unchanged (Schemes 1 and 2). Cyanoenamino alcohols obtained by reduction of cyanoenamino esters 2 are hydrolyzed under acidic conditions to amino alcohols with retention of the configuration of the starting amino acid. This sequence of reactions allows to prepare derivatives of L‐tyrosinol from (−)‐L‐tyrosine (see, e.g., Scheme 4). Cyanoenamino esters 11 are readily methylated at the N‐atom to give N‐methylated cyanoenamino esters (Scheme 3). This property is exploited on the way of a multistep procedure to obtain N‐methylated amino alcohols homologous to natural (−)‐(1R,2S)‐ephedrine.
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Abarbri et al. (1995) studied this question.