A Stereoselective Formal Synthesis of Leucascandrolide A

Scalable Total Synthesis of Leucascandrolide A Macrolactone Using a Chiral Phosphoric Acid/CuX Combined Catalytic System

A scalable total synthesis of leucascandrolide A macrolactone has been accomplished with a longest linear sequence of 17 steps from readily available feedstocks in 31.2% yield. The key steps in this synthesis are the enantioselective allylation reaction by chiral phosphoric acid (CPA)/CuBr cooperative catalysis and the diastereoselective catalytic crotylation in the presence of CPA with CuCl. These catalytic reactions can be performed on a gram scale to afford the desired products with excellent stereoselectivities.

Rhodium-Catalyzed Cyclization of Terminal and Internal Allenols: An Atom Economic and Highly Stereoselective Access Towards Tetrahydropyrans

A comprehensive study of a diastereoselective Rh-catalyzed cyclization of terminal and internal allenols is reported. The methodology allows the atom economic and highly syn-selective access to synthetically important 2,4-disubstituted and 2,4,6-trisubstituted tetrahydropyrans (THP). Furthermore, its utility and versatility are demonstrated by a great functional-group compatibility and the enantioselective total synthesis of (-)-centrolobine.

Synthesis and Analysis of Natural-Product-Like Macrocycles by Tandem Oxidation/Oxa-Conjugate Addition Reactions

As traditional small-molecule drug discovery programs focus on a relatively narrow range of chemical space, most human proteins are viewed as unreachable targets. Consequently, there is a strong interest in expanding the chemical space in drug discovery beyond traditional small molecules. Here, a strategy for the preparation of a broad natural-product-like macrocyclic library by using the tandem allylic oxidation/oxa-conjugate addition and macrocyclization reactions is reported. Cheminformatic analyses demonstrate that this tetrahydropyran-containing macrocyclic library shows a significant overlap with natural products in the chemical space. This approach can be used for designing libraries that may probe more deeply into natural-product-like space.

An Enantioselective Cross-Dehydrogenative Coupling Catalysis Approach to Substituted Tetrahydropyrans

An enantioselective cross-dehydrogenative coupling (CDC) reaction to access tetrahydropyrans has been developed. This process combines in situ Lewis acid activation of a nucleophile in concert with the oxidative formation of a transient oxocarbenium electrophile, leading to a productive and highly enantioselective CDC. These advances represent one of the first successful applications of CDC for the enantioselective couplings of unfunctionalized ethers. This system provides efficient access to valuable tetrahydropyran motifs found in many natural products and bioactive small molecules.

Asymmetric organocatalytic methods for the synthesis of tetrahydropyrans and their application in total synthesis

Recent advancement in the area of asymmetric organocatalysis led to the development of new methodologies for the construction of valuable enantiopure molecules, including various heterocycles. As one of the latter class of compounds tetrahydropyrans (THPs) constitute a core structure of a wide array of bioactive natural products. A noticeable growth has been observed in the asymmetric synthesis of THPs using small organic molecules as catalysts. This Tutorial Review describes the organocatalytic methods available to furnish THPs as well as the application of these methodologies in the total synthesis of THP-based natural products.

Advances in the synthesis of glycosidic macrolides: clavosolides A-D and cyanolide A

Covering: 2005 to 2016Clavosolides A-D and cyanolide A are glycosidic macrolides and represent a new family of marine natural products. They possess a number of unusual structural features and have attracted considerable interest from the synthetic community. This review presents a comprehensive survey of all aspects of the clavosolides A-D and cyanolide A. Specific topics include isolation, structure determination, biological activity, and synthetic approaches.

The stereodivergent formation of 2,6-cis and 2,6-trans-tetrahydropyrans: experimental and computational investigation of the mechanism of a thioester oxy-Michael cyclization

Computational and synthetic studies have elucidated the origins of stereodivergence in an oxy-Michael synthesis of 2,6-disubstituted tetrahydropyrans.

The origins of the stereodivergence in the thioester oxy-Michael cyclization for the formation of 4-hydroxy-2,6-cis- or 2,6-trans-substituted tetrahydropyran rings under different conditions was investigated both computationally and experimentally. Synthetic studies showed that the 4-hydroxyl group was essential for stereodivergence. When the 4-hydroxyl group was present, TBAF-mediated conditions gave the 2,6-trans-tetrahydropyran and trifluoroacetic acid-mediated conditions gave the 2,6-cis-tetrahydropyran. This stereodivergence vanished when the hydroxyl group was removed or protected. Computational studies revealed that: (i) the trifluoroacetic acid catalysed formation of 2,6-cis-tetrahydropyrans was mediated by a trifluoroacetate-hydroxonium bridge and proceeded via a chair-like transition state; (ii) the TBAF-mediated formation of 2,6-trans-tetrahydropyrans proceeded via a boat-like transition state, where the 4-hydroxyl group formed a crucial hydrogen bond to the cyclizing alkoxide; (iii) both reactions are under kinetic control. The utility of this stereodivergent approach for the formation of 4-hydroxy-2,6-substituted tetrahydropyran rings has been demonstrated by the total syntheses of the anti-osteoporotic natural products diospongin A and B.

Approach to the Synthesis of the C1-C11 and C14-C18 portion of Leucascandrolide A

An asymmetric synthesis of the C1 to C11 and C14 to C18 fragments of the macrocyclic portion of the antibiotic Leucascandrolide A was achieved in 21 total steps from an achiral dienoate. The key 4-hydroxy-2,5-pyran portion of the natural product was established by oxy-Michael cyclization of a 5,7,9,11-tetraol intermediate, which in turn was established by an iterative asymmetric-hydration of dienoates. Alternative strategies for establishing the polyol stereochemistry were explored.

Catalytic SN2'- and Enantioselective Allylic Substitution with a Diborylmethane Reagent and Application in Synthesis

A catalytic method for the site- and enantioselective addition of commercially available di-B(pin)-methane to allylic phosphates is introduced (pin=pinacolato). Transformations may be facilitated by an NHC-Cu complex (NHC=N-heterocyclic carbene) and products obtained in 63-95 % yield, 88:12 to >98:2 S(N)2'/S(N)2 selectivity, and 85:15-99:1 enantiomeric ratio. The utility of the approach, entailing the involvement of different catalytic cross-coupling processes, is highlighted by its application to the formal synthesis of the cytotoxic natural product rhopaloic acid A.

Total Synthesis of Clavosolide A via Tandem Allylic Oxidation/Oxa-Conjugate Addition Reaction

The tandem allylic oxidation/oxa-conjugate addition reaction promoted by the gem-disubstituent effect in conjunction with the NHC-mediated oxidative esterification was explored for the facile synthesis of clavosolide A.

Synthesis of α,α'-trans-oxepanes through an organocatalytic oxa-conjugate addition reaction

Oxepanes are found in a wide range of natural products; however, they are challenging synthetic targets due to enthalpic and entropic barriers. Organocatalytic oxa-conjugate addition reactions promoted by the gem-disubstituent (Thorpe–Ingold) effect stereoselectively provided α,α′-trans-oxepanes. In addition, the potential of an organocatalytic tandem oxa-conjugate addition/α-oxidation was demonstrated in a rapid generation of molecular complexity. These organocatalytic oxa-conjugate addition reactions would provide powerful tools for the synthesis of natural products that contain highly functionalized oxepanes.

Strategies for the construction of tetrahydropyran rings in the synthesis of natural products

This review focuses on the methodology used for the construction of tetrahydropyran (THP) rings in the synthesis of natural products over the last seven years. While methods like cyclisation onto oxocarbenium ions, reduction of cyclic hemi-ketals, Michael reactions, hetero-Diels-Alder cycloadditions and cyclisations onto epoxides continue to find application, several other strategies including metal-mediated cyclisations, ring-closing metathesis, radical cyclisations and carbocation cyclisations have also found use. This review is intended to provide an overview of the area for those who are unfamiliar, and to refresh and remind those who do work in the area of the exciting developments in the field.

Synthesis of complex oxygenated heterocycles

Versatile and chemoselective preparation of substituted oxygenated heterocycles is described. Highly diastereoselective metal-catalyzed syntheses of trans-2,6- and cis-2,6-disubstituted tetrahydropyrans (THPs) are presented, along with an easy one-pot access to various ring size benzoannulated spiroketals.

Synthesis of a Library of 1,5,2-Dithiazepine 1,1-Dioxides. Part 1: A One-Pot Sulfonylation/Thia-Michael Protocol

A novel one-pot sulfonylation/intramolecular thia-Michael protocol is reported for the synthesis of 1,5,2-dithiazepine 1,1-dioxides. Sulfonylation between cysteine ethyl ester/cysteamine and 2-chloroethanesulfonyl chloride, followed by in situ intramolecular thia-Michael addition, was achieved and afforded the titled 1,5,2-dithiazepine-1,1-dioxide scaffolds. Diversification was demonstrated for future library synthesis.