Three-Step Synthesis of (±)-Preussin from Decanal

Organocatalytic Enantioselective Sulfa-Michael Additions to α,β-Unsaturated Diazoketones

Enantioselective sulfa-Michael additions to α,β unsaturated diazocarbonyl compounds have been developed. Quinine-derived squaramide was found to be the best catalyst to promote C-S bond formation in a highly stereoselective fashion for alkyl and aryl thiols. The easy-to-follow protocol allowed the preparation of 22 examples in enantiomeric ratios up to 97:3 and reaction yields up to 94%. The mechanism and origins of enantioselectivity were determined through density functional theory (DFT) calculations.

Alkoxyallene-based syntheses of preussin and its analogs and their cytotoxicity

Short syntheses of oxa-preussin, racemic preussin and (-)-preussin are reported. Starting from a racemic 3-nonyl-substituted methoxyallene derivative, its lithiation and addition to phenylethanal provided the corresponding allenyl alcohol that was converted into two diastereomeric dihydrofuran derivatives by silver nitrate-catalyzed 5-endo-trig cyclization. The acid hydrolysis of the enol ether moiety gave heterocyclic ketones and subsequent highly stereoselective reductions with l-selectride furnished 2-benzyl-5-nonylfuran-3-ol derivatives in good overall yield. The major all-cis-diastereomer has the skeleton and relative configuration of preussin and is hence called oxa-preussin. An analogous sequence with the same allene, but an N-sulfonyl imine as the electrophile, finally led to racemic preussin. The stereoselectivities of the individual steps are discussed in detail. With an enantiopure 2-benzyl-5-nonylpyrrolidin-3-one intermediate the preparation of (-)-preussin with an enantiomeric ratio of >95 : 5 could be accomplished in a few steps. The sign of the optical rotation of this product finally proved the absolute configurations of its precursors and demonstrated that our chiral auxiliary-based route led to the antipode of the natural product. The cytotoxicity of several of the prepared heterocycles against MCF-7 tumor cells was investigated and five compounds, including racemic and enantiopure (-)-preussin, were identified as highly cytotoxic with IC50 values in the range of 3-6 μM.

An efficient approach to trans-4-hydroxy-5-substituted 2-pyrrolidinones through a stereoselective tandem Barbier process: divergent syntheses of (3R,4S)-statines, (+)-preussin and (−)-hapalosin

Natural products of (+)-preussin and (−)-hapalosin have been asymmetrically synthesized through the stereoselective tandem Barbier process.

α,β-Unsaturated diazoketones as useful platforms in the synthesis of nitrogen heterocycles

Among the different types of diazocarbonyl substrates found in the literature to date, α,β-unsaturated diazoketones have proven to be very promising as multifunctional intermediates. Possessing a diazo group, a ketone function and a double bond all together in a single molecule, these compounds constitute versatile building blocks for synthesis. For example, double bond functionalization, followed by intramolecular insertion reactions, can be a short alternative to prepare several rings or heterocyclic compounds. Although there are many efficient methods to prepare diazoketones, very few can be extended to the synthesis of the a,β-unsaturated diazoketones; this is likely responsible for their limited application in synthesis. Unfortunately, the classical methods to prepare saturated- or aryl-diazoketones (acylation of diazomethane with acyl chlorides or mixed anhydrides) are not suitable for preparing a,β-unsaturated diazoketones, since pyrazolines (dipolar cycloaddition products from the reaction between diazomethane and the double bond) are formed. Although Danheiser's two-step detrifluoroacetylative procedure (starting from a,β-unsaturated methyl ketones) is considered the best general method, it cannot be applied to the synthesis of all types of a,β-unsaturated diazoketones. For example, the synthesis of more complex unsaturated diazoketones, as well as those with epimerizable stereocenters in the γ position, was never described before. Another point is related to the geometry of the double bond, since practically all examples described thus far refer to unsaturated diazoketones with E geometry. In recent years, our research group developed two new Horner-Wadsworth-Emmons reagents (containing a diazocarbonyl function) that could be easily applied in the one-step preparation of α,β-unsaturated diazoketones from aldehydes. Not only were we able to selectively synthesize E- and Z-unsaturated diazoketones, but also to employ these useful platforms in the short synthesis of several nitrogen heterocycles such as indolizidines, quinolizidines, piperidines, and pyrrolidines. Our purpose in this Account is to introduce this class of diazoketone and provide a brief historical overview, culminating in how we developed a general methodology to prepare them. In continuation, we wish to call of the reader's attention to these important building blocks, showing how we could apply them to the synthesis of several nitrogen heterocycles, including the very short preparation of some popular alkaloids. The reader will also notice that the combination of these three important functions in the same molecule makes these compounds special as well as provides powerful platforms to access many important molecules in a direct fashion.