Highly Stereoselective Synthesis of syn-1,3-Diols through a Sequential Titanium-Mediated Aldol Reaction and LiBH4 Reduction

Conquering peaks and illuminating depths: developing stereocontrolled organic reactions to unlock nature's macrolide treasure trove

The structural complexity and biological importance of macrolide natural products has inspired the development of innovative strategies for their chemical synthesis. With their dense stereochemical content, high level of oxygenation and macrocyclic cores, we viewed the efficient total synthesis of these valuable compounds as an aspirational driver towards developing robust methods and strategies for their construction. Starting out from the initial development of our versatile asymmetric aldol methodology, this personal perspective reflects on an adventurous journey, with all its trials, tribulations and serendipitous discoveries, across the total synthesis, in our group, of a representative selection of six macrolide natural products of marine and terrestrial origin - swinholide A, spongistatin 1, spirastrellolide A, leiodermatolide, chivosazole F and actinoallolide A.

Total synthesis of the actinoallolides and a designed photoaffinity probe for target identification

The actinoallolides are a family of polyketide natural products isolated from the bacterium Actinoallomurus fulvus. They show potent biological activity against trypanosomes, the causative agents of the neglected tropical diseases human African trypanosomiasis (sleeping sickness) and Chagas disease, while exhibiting no cytotoxicity against human cell lines. Herein, we give a full account of our strategy evolution towards the synthesis of this structurally unique class of 12-membered macrolides, which culminated in the first total synthesis of (+)-actinoallolide A in 20 steps and 8% overall yield. Subsequent late-stage diversification then provided ready access to the congeneric (+)-actinoallolides B-E. Enabled by this flexible and efficient endgame sequence, we also describe the design and synthesis of a photoaffinity probe based on actinoallolide A to investigate its biological mode of action. This will allow ongoing labelling studies to identify their protein binding target(s).

Stereoselective Reductions of 3-Substituted Cyclobutanones: A Comparison between Experiment and Theory

The stereoselective reduction of carbonyls is of key importance in the total synthesis of natural products and in medicinal chemistry. Nevertheless, models for rationalizing the stereoselectivity of the hydride reductions of cyclobutanones toward cyclobutanols are largely lacking, unlike cyclohexanone reductions. In order to elucidate the factors that control the stereoselectivity of these reductions, we have investigated the effect of the reaction temperature, solvent, substituent, and type of reducing agent using a synergistic experimental-computational approach. On the experimental side, the hydride reduction of 3-substituted cyclobutanones was proven to be highly selective for the formation of cis alcohol (>90%), irrespective of the size of the hydride reagent. The pronounced selectivity can be further enhanced by lowering the reaction temperature or decreasing the solvent polarity. On the computational side, density functional theory and noncovalent interaction analysis reveal that torsional strain plays a major role in the preference for the antifacial hydride approach, consistent with the Felkin-Anh model. In the presence of the benzyloxy substituent, the high selectivity for the cis isomer is also driven by repulsive electrostatic interactions in the case of a syn-facial hydride attack. The computed cis/trans ratios are in good agreement with the experimental ones and thus show the potential of computational chemistry for predicting and rationalizing the stereoselectivity of hydride reductions of cyclobutanones.

Total synthesis of (+)-herboxidiene/GEX 1A

An efficient synthesis of herboxidiene is granted from highly stereoselective aldol reactions from two lactate-derived ketones and an oxa-Michael cyclization.

Total synthesis of (+)-herboxidiene from two chiral lactate-derived ketones

A substrate-controlled synthesis of (+)-herboxidiene from two lactate-derived chiral ketones is described. Remarkably, most of the carbon backbone was constructed through highly stereoselective titanium-mediated aldol reactions and an Ireland-Claisen rearrangement. Furthermore, an oxa-Michael cyclization and a high-yield Suzuki coupling were used to assemble the pyran ring and the diene moiety respectively.