Traceless Solid-Phase Synthesis of Fused Chiral Macrocycles via Conformational Constraint-Assisted Cyclic Iminium Formation

Traceless Solid-Phase Organic Synthesis

Traceless solid-phase synthesis represents an ultimate sophisticated synthetic strategy on insoluble supports. Compounds synthesized on solid supports can be released without a trace of the linker that was used to tether the intermediates during the synthesis. Thus, the target products are composed only of the components (atoms, functional groups) inherent to the target core structure. A wide variety of synthetic strategies have been developed to prepare products in a traceless manner, and this review is dedicated to all aspects of traceless solid-phase organic synthesis. Importantly, the synthesis does not need to be carried out on a linker designed for traceless synthesis; most of the synthetic approaches described herein were developed using standard, commercially available linkers (originally devised for solid-phase peptide synthesis). The type of structure prepared in a traceless fashion is not restricted. The individual synthetic approaches are divided into eight sections, each devoted to a different methodology for traceless synthesis. Each section consists of a brief outline of the synthetic strategy followed by a description of individual reported syntheses.

Configuration-Dependent Medium-Sized Ring Formation: Chiral Molecular Framework with Three-Dimensional Architecture

This report describes a configuration-dependent [6 + 8 + 5] fused ring formation via a tandem cyclic N-acyliminium nucleophilic addition reaction. Cyclization of the acyclic precursor prepared on a solid phase using l-Ser and a racemic mixture of Fmoc- trans-2-aminocyclohexanecarboxylic acid predominantly yielded the cyclic diastereomer with the (1 R,2 R)-2-aminocyclohexane moiety rather than the tricyclic diastereomer from the (1 S,2 S)-enantiomer. In contrast, the model compound prepared with d-Ser predominantly cyclized with the (1 S,2 S)-2-aminocyclohexanecarboxylic acid substrate. The outcome of the cyclization was not influenced by the type of resin, the spacer, or the N-substituent. The analogous synthesis of the [6 + 7 + 5] fused ring system yielded inseparable diastereomers in a 1:0.6 ratio.

Nα-Amino acid containing privileged structures: design, synthesis and use in solid-phase peptide synthesis

Fmoc-protected Nα-amino acid containing heterocyclic privileged structures, O-(1-methyl-5-oxo-2,5-dihydro-1H-pyrrol-3-yl)-l-serine and O-((S)-5-oxo-2,3,5,7a-tetrahydro-1H-pyrrolizin-7-yl)-l-serine, were synthesized on the solid phase from simple commercially available building blocks under mild conditions. The amino acid side-chain is composed of tetramic acid, a natural product derived privileged structure. The key transformation was the formation of cyclic enol ethers via nonclassical Wittig olefinations of the esters. Solid-phase synthesis represents a method of choice, particularly for the synthesis of peptides. This route is compatible with traditional Merrifield solid-phase peptide synthesis (SPPS), as documented on the preparation of the pentapeptide Leu-enkephalin amide H-Tyr-Gly-Gly-Phe-Leu-NH2 with Phe or Tyr replaced by a novel amino acid.