Relevant Developments in the Use of Three-Component Reactions for the Total Synthesis of Natural Products. The last 15 Years

Multicomponent reactions (MCRs) offer a highly useful and valuable strategy that can fulfill an important role in synthesizing complex polysubstituted compounds, by simplifying otherwise long sequences and increasing their efficiency. The total synthesis of selected natural products employing three-component reactions as their common strategic MCR approach, is reviewed on a case-by-case basis with selected targets conquered during the last 15 years. The revision includes detailed descriptions of the selected successful sequences; relevant information on the isolation, and bioactivity of the different natural targets is also briefly provided.

Carbamoyl Anion Addition to Azirines

The addition of carbamoyl anions to azirines affords synthetically useful 2-aziridinyl amide building blocks. The reaction scope was explored with respect to both formamide and azirine, and the addition was found to be highly diastereoselective. A one-pot conversion of a ketoxime to an aziridinyl amide was demonstrated. The method was employed to incorporate an aziridine residue into a dipeptide segment.

Synthesis of Highly Substituted Azepanones from 2 H-Azirines by a Stepwise Annulation/Ring-Opening Sequence

Bicyclic aziridines possessing a 1-azabicyclo[4.1.0]heptan-2-one core were prepared from 2 H-azirines by a stepwise annulation sequence involving a diastereoselective allylindanation, an N-acylation, and a ring-closing metathesis to construct the six-membered ring. After hydrogenation or functionalization of the olefin, regioselective ring opening of the resulting azabicyclic compounds with carboxylic acids (or sulfur nucleophiles) afforded highly substituted azepanones possessing an ester moiety or a trifluoromethyl group and a tetrasubstituted carbon at the α and β positions of the nitrogen atom, respectively.

Ring-opening formal hetero-[5+2] cycloaddition of 1-tosyl-2,3-dihydro-1H-pyrroles with terminal alkynes: entry to 1-tosyl-2,3-dihydro 2,3-dihydro-1H-azepines

Lewis acid-catalyzed formal hetero-[5+2] cycloaddition of 2,3-dihydro-1H-pyrroles with alkynes through C(sp²)–N bond cleavage toward 2,3-dihydro-1H-azepines is described.

Aziridine electrophiles in the functionalisation of peptide chains with amine nucleophiles

We describe herein the synthesis of aziridine-containing amino acids embedded within tripeptide structures.

Site-specific integration of amino acid fragments into cyclic peptides

The concept of site-specific integration of fragments into macrocyclic entities has not yet found application in the realm of synthetic chemistry. Here we show that the reduced amidicity of aziridine amide bonds provides an entry point for the site-specific integration of amino acids and peptide fragments into the homodetic cyclic peptide architecture. This new synthetic operation improves both the convergence and divergence of cyclic peptide synthesis.

Theoretical investigations on the mechanistic pathway of the thermal rearrangement of substituted N-acyl-2,2-dimethylaziridines

The mechanism of the thermal rearrangement of substituted N-acyl-2,2-dimethylaziridines 1 has been studied using quantum chemistry methods. Geometries of reactants, transition states and products have been optimized at the B3LYP/6-311++G(2d,2p) level. Relative energies for various stationary points have been determined and reaction identified by IRC calculations. The results show that thermal rearrangements occur in three ways. Firstly, the transition state TS 1 in which a hydrogen atom of methyl groups migrates from primary carbon to oxygen of amid group to give the N-methallylamide 2. The second is via the transition state TS 2 in which the attack of oxygen to the tertiary carbon yields the oxazoline 3. The third is via the transition state TS 3 in which a hydrogen migrate from the secondary carbon to oxygen to give the vinylamide 4. In order to get insights into the factors determining the exact nature of its interactions with electrophiles, the application of reactivity parameters derived from density functional theory in a local sense, in particular the softness and Fukui function, to interpret and predict the mechanisms of the thermal decomposition of the N-acyl-2,2-dimethylaziridines 1, has been discussed.

Synthesis and ring openings of cinnamate-derived N-unfunctionalised aziridines

tert-Butyl cinnamates are aziridinated with high trans-selectivity by an N–N ylide generated in situ from N-methylmorpholine and O-diphenylphosphinyl hydroxylamine. The resulting N-unfunctionalised aziridines are shown to be versatile synthetic building blocks that undergo highly selective ring-opening reactions with a wide range of nucleophiles.

Rational Design of Sterically and Electronically Easily Tunable Chiral Bisimidazolines and Their Applications in Dual Lewis Acid/Brønsted Base Catalysis for Highly Enantioselective Nitroaldol (Henry) Reactions

A new addition to the rational design of sterically and electrically easily tunable chiral bis(imidazoline) ligands from chiral amino alcohols has been developed. Vast structural variation of chiral bis(imidazoline) ligands can be simply achieved by the choice of both the 1,2-amino alcohol and its N-1 R1 substituent. A small library of chiral bisimidazolines (1 a-h) has been constructed. The method has provided an easy and simplified route to a diverse set of air-stable and water-tolerant chiral bis(imidazoline) ligands on 10 g scales. The dual Lewis Acid/Brønsted base catalytic system generated from the (S)-1 a/Cu(OTf)2 complex and Et3N was able to catalyze Henry reactions between aldehydes and nitromethane effectively at room temperature, and also to tolerate a wide scope of aldehydes with excellent enantiomeric excesses. Not only aromatic aldehydes but also aliphatic aldehydes afforded the nitroalcohol products, with enantiomeric excesses in the 93-98% range. This dual catalytic system is among the most effective systems so far reported for the asymmetric parent Henry reactions. This work also represents the first members of the class of chiral bisimidazolines to have been demonstrated to achieve excellent enantioselectivities.

Synthesis and stereoselective functionalization of silylated heterocycles as a new class of formyl anion equivalents

The fluoride ion-induced reactivity of a series of silyl heterocycles leads to the generation of nucleophilic species capable of interacting with electrophiles, thus disclosing new classes of formyl and acyl anion synthons. Moreover, when reacting stereodefined molecules, the stereoinformation of the reacting carbon-silicon bond is transferred to the newly formed carbon-carbon bond, suggesting possible applications in stereoselective synthesis. Thus, silyl dithiolanes, oxathiolanes, dioxolanes, thiazolidines and oxazolidines can be efficiently and stereoselectively functionalized under fluoride ion conditions in the presence of electrophiles. While direct access to silyl heterocycles is generally either prevented or troublesome, a novel protocol for their synthesis has also been developed, together with a simple general access route to several functionalized and stereodefined mercaptans, building blocks for the construction of silyl heterocycles.

Novel imidazolination reaction of alkenes provides an easy access to new alpha,beta-differentiated 1,2-vicinal diamines

alpha,beta-Differentiated 1,2-vicinal diamines have been efficiently synthesized by using new electrophilic imidazolination reaction of alkenes. The hydrolysis of imidazolines was performed by treatment with 6 M HCl in THF at 70 degrees C without epimerization. Eight examples were examined to give good to excellent yields (87-96%).