Use of azetidine scaffolds in stereoselective transformations (microreview)

Synthesis of optically active 2-substituted azetidine-2-carbonitriles from chiral 1-arylethylamine via α-alkylation of N-borane complexes

The base-promoted α-alkylation of N-((S)-1-arylethyl)azetidine-2-carbonitriles 3via formation of their N-borane complexes 4 was investigated. For example, treatment of diastereomerically pure borane N-((S)-1'-(4''-methoxyphenyl)ethyl)azetidine-2-carbonitrile complex (1S,2S,1'S)-4b with 1.2 equivalents of LDA at -78 °C followed by 1.3 equivalents of benzyl bromide at -78 °C and warming to room temperature produced α-benzylated (2S,1'S)-5ba in 72% yield and (2R,1'S)-5ba in 2% yield. A mechanism for this diastereoselective α-alkylation was proposed. Our method enables the production of optically active 2-substituted azetidine-2-carbonitriles, such as α-benzylated (S)-10a and (R)-10a, starting from commercially available (S)-(1-(4-methoxyphenyl)ethyl)amine.

Skeletal remodelling of α-substituted cyclic eneformamides to α-ketonyl cyclic amines

A modular dehomologation strategy, which skeletally remodels cyclic α-substituted eneformamides to one-carbon shorter α-ketonyl saturated cyclic amines is described.

Base-promoted diastereoselective α-alkylation of borane N-((S)-1'-phenylethyl)azetidine-2-carboxylic acid ester complexes

The base-promoted α-alkylation of N-((S)-1-phenylethyl)azetidine-2-carboxylic acid esters 1 was investigated. The use of diastereomerically pure borane complexes 3 as substrates, which are easily prepared from 1, dramatically improved the yields and diastereoselectivities of α-alkylated products 2. For example, the treatment of tert-butyl ester (1S,2S,1'S)-3a with 2.4 equivalents of lithium bis(trimethysilyl)amide (LiHMDS) at 0 °C followed by 2.6 equivalents of benzyl bromide afforded α-benzylated (2S,1'S)-2aa in 90% yield as almost a single diastereomer. Our method enables the production of optically active α-substituted azetidine-2-carboxylic acid esters starting from commercially available (S)-1-phenylethylamine, which is one of the least expensive chiral compounds.