Total Synthesis of Pericoannosin A

Recent applications of asymmetric organocatalytic annulation reactions in natural product synthesis

The past two decades have witnessed remarkable growth of asymmetric organocatalysis, which is now a firmly established synthetic tool, serving as a powerful platform for the production of chiral molecules. Ring structures are ubiquitous in organic compounds, and, in the context of natural product synthesis, strategic construction of ring motifs is often crucial, fundamentally impacting the eventual fate of the whole synthetic plan. In this review, we provide a comprehensive and updated summary of asymmetric organocatalytic annulation reactions; in particular, the application of these annulation strategies in natural product synthesis will be highlighted.

Applications of the Horner–Wadsworth–Emmons Olefination in Modern Natural Product Synthesis

The Horner–Wadsworth–Emmons (HWE) reaction is one of the most reliable olefination reaction and can be broadly applied in organic chemistry and natural product synthesis with excellent selectivity. Within the last few years HWE reaction conditions have been optimized and new reagents developed to overcome challenges in the total syntheses of natural products. This review highlights the application of HWE olefinations in total syntheses of structurally different natural products covering 2015 to 2020. Applied HWE reagents and reactions conditions are highlighted to support future synthetic approaches and serve as guideline to find the best HWE conditions for the most complicated natural products.

1 Introduction and Historical Background

2 Applications of HWE

2.1 Cyclization by HWE Reactions

2.2.1 Formation of Medium- to Larger-Sized Rings

2.2.2 Formation of Small- to Medium-Sized Rings

2.3 Synthesis of α,β-Unsaturated Carbonyl Groups

2.4 Synthesis of Substituted C=C Bonds

2.5 Late-Stage Modifications by HWE Reactions

2.6 HWE Reactions on Solid Supports

2.7 Synthesis of Poly-Conjugated C=C Bonds

2.8 HWE-Mediated Coupling of Larger Building Blocks

2.9 Miscellaneous

3 Summary and Outlook

The Total Synthesis of Chondrochloren A

The first total synthesis of chondrochloren A is accomplished using a 1,2‐metallate rearrangement addition as an alternative for the Nozaki‐Hiyama‐Kishi reaction. This transformation also avoids the inherent challenges of this polyketide segment and provides a new, unprecedented strategy to assemble polyketidal frameworks. The formation of the Z‐enamide is accomplished using a Z‐selective cross coupling of the corresponding amide to a Z‐vinyl bromide.

Desymmetrization of C2 -Symmetric Bis(Boronic Esters) by Zweifel Olefinations

anti‐Configured 1,3‐dimethyl deoxypropionate motifs are important sub structures in natural products. Herein, we describe a bidirectional approach for the rapid construction of natural products featuring such motifs by using C₂‐symmetrical 1,3‐bis(boronic esters). As for its application in convergent syntheses it was important to establish a selective mono‐Zweifel olefination we describe the scope and limitations by using different 1,3‐bis(boronic esters) and nucleophiles. This protocol takes advantage of the combination of the Hoppe–Matteson–Zweifel chemistry, which was elegantly put into practice by Aggarwal et al. In order to show its applicability the total syntheses of two natural products, serricornin and (+)‐invictolide, were performed.

Towards the total synthesis of chondrochloren A: synthesis of the (Z)-enamide fragment

The stereoselective synthesis of the (Z)-enamide fragment of chondrochloren (1) is described. A Buchwald-type coupling between amide 3 and (Z)-bromide 4 was used to generate the required fragment. The employed amide 3 comprising three chiral centers was obtained through a seven-step sequence starting from ᴅ-ribonic acid-1,4-lactone. The (Z)-vinyl bromide 4 is accessible in four steps from 4-hydroxybenzaldehyde. The pivotal cross coupling between both fragments was achieved after extensive experimentation with copper(I) iodide, K₂CO₃ and N,N′-dimethylethane-1,2-diamine.

Formal Total Synthesis of Pericoannosin A

A concise formal total synthesis of pericoannosin A, by the synthesis of an advanced intermediate of pericoannosin A, was achieved in eight steps from commercially available isoprene in a 21.7% overall yield. Key transformations for this expedited route include an enantioselective organocatalytic Diels-Alder reaction to construct the C ring, a diastereoselective reduction (under Felkin-Ahn control), and a hydroboration/oxidation sequence for chain homologation. This work represents the second synthetic effort toward pericoannosin A, the only reported natural product based on a hexahydro-1H-isochromen-5-isobutylpyrrolidin-2-one core.

Polyoxygenated Tertiary Alcohols: A Kiyooka Approach

A Kiyooka aldol approach for the stereoselective synthesis of tertiary alcohols is presented. This approach allows for the incorporation of different substituents at all three remaining positions at the chiral center bearing the tertiary alcohol. To demonstrate the validity of this approach different chiral alcohols were depicted and the relationship of double bond geometry of the ketene acetal and the diastereoselectivity was established.