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Ammonia Synthons For The Multicomponent Assembly Of Complex Y Lactams

1615 words - 7 pages

Five-membered ring lactams, known as γ-lactams, are important structural motifs in biologically active natural products, but γ-lactams that are substituted at the 1-position (nitrogen) are less common in drug leads. Although the substituted γ-lactams and the unsubstituted γ-lactams share the main γ-lactam core, a major difference between the two is the substitution at the nitrogen; the unsubstituted γ-lactams are also known as N-H lactam. This difference is substitution is seen in Figure 1 through a natural product Heliotropamide,[1] a γ-lactam substituted at the 1-position and Salinosporamide,[2] a γ-lactam unsubstituted at the 1-position. A survey of γ-lactams in preclinical and clinical development showed a nearly two-to-one ratio of N-H lactams over their substituted counter parts. [3] This is perhaps due to synthetic limitations.
HO CO2H O
NH
O
HN HO Cl
H3CO HN HO2C
O OH HO CH OCH3 OO3
OH
Salinosporamide
Heliotropamide
Figure 1: Examples of N-substituted γ-lactam and an “N-H” γ-lactams Jared Shaw and co-workers at the University of California- Davis, USA, have
reported a multicomponent methodology for the synthesis of unsubstituted γ-lactams and their subsequent N-functionalization. By employing ammonium acetate as their starting material in a previously reported four-component reaction (4CR), the research team demonstrated a range of N-H lactam structures that could be produced. These substrates could then be functionalized at the nitrogen through acylation or arylation to provide the structural diversity unavailable through the original 4CR. [3] The mechanistically distinct four-component reaction previously reported in another study by Shaw enables the assembly of γ-lactams in a single synthetic step in high yield and diastereoselectivity. [4] This methodology allows for the outline of different substituents at several positions around the

  2
ring by the choice of building blocks used. This one-pot, four-component reaction, shown in Figure 2, forms up to three stereogenic centers in a single step.
R1 O
NH2 R2 H
R
3
O
O
O
R4
toluene (reflux) -H2O
R1 O N R4
R2 S R3
O OH
SH
Figure 2: Lactam formation four-component reaction [4]
Shaw and co-workers utilize the above discussed 4CR reaction as a starting point for the synthesis of structurally diverse γ-lactams. The synthesis of N-H lactams has proven to be a general problem for multicomponent reactions (MCRs) that employ amines. [3] Multicomponent reactions (MCRs) are convergent reactions that involve the combination of three or more starting materials in a single operation and are among the most powerful methods for efficient generation of molecular complexity. [5] The general picture of Shaw’s study is shown in Figure 3, which allows for the synthesis of lactams that are currently not available through a direct MCR. Upon the formation of the N-H lactam, the substrate is then functionalized at the nitrogen, which provides the structural diversity that is unattainable through the original...

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