A phosphate tether-mediated, one-pot, sequential ring-closing metathesis/cross-metathesis/chemoselective hydrogenation protocol

Synthesis of Simplified 2-Desmethyl Sanctolide A Analogs

A one-pot, sequential phosphate tether-mediated method for the synthesis of simplified 2-desmethyl sanctolide A analogs is reported. Western side-chain diversification was achieved using a pot-efficient, sequential cross metathesis (CM)/ring-closing metathesis (RCM)/H2/dephosphorylation procedure. Further diversification was achieved by Me3Al-mediated amide formation, Yamaguchi esterification, and RCM macrocyclization to access five C11/C12 Z-configured, 2-des-methyl sanctolide A analogs with improved stability.

Total Synthesis of Sanctolide A and Formal Synthesis of (2S)-Sanctolide A

Two synthetic strategies employing phosphate tether-mediated one-pot sequential protocols for the total synthesis of the polyketide nonribosomal peptide macrolide, sanctolide A, and the formal synthesis of the (2S)-epimer of sanctolide A are reported. In this work, a phosphate tether-mediated one-pot sequential ring-closing metathesis/cross metathesis/substrate-controlled "H2"/tether removal approach was developed to accomplish the total synthesis of the natural product sanctolide A.

An Iterative Phosphate Tether Mediated Approach for the Synthesis of Complex Polyol Subunits

A pot-economical approach to advanced polyol subunits is reported. The key reactions involved are iterative use of a phosphate tether-mediated one-pot sequential RCM/CM/H2 with subsequent utilization of either a regio-/diasteroselective cuprate addition or a Pd-catalyzed reductive allylic transposition. This method highlights the asymmetric synthesis of 12 complex polyol subunits in 4-6 one-pot sequential operations with a total of 12-14 reactions, of which 4-5 are catalytic, with minimal workup and purification procedures.

Unified Strategy for 1,5,9- and 1,5,7-Triols via Configuration-Encoded 1,5-Polyol Synthesis: Enantioselective Preparation of γ-Sulfonyl-α-silyloxyaldehydes and Iterative Julia-Kocienski Coupling

Diverse classes of natural products contain chiral 1,5-polyols, within which may be stereochemical triads of 1,5,9- and 1,5,7-triols. Biological activities associated with compounds containing these motifs warrant targeted synthetic strategies to access all stereoisomers of a 1,5-polyol family from cheap and easily accessible reagents while avoiding the need to determine configurations at each alcohol stereocenter. Here, we address these problems via design and implementation of an iterative configuration-encoded strategy to access 1,5-polyols with unambiguous stereocontrol; the coupling event exploits Julia-Kocienski reactions of enantiopure α-silyloxy-γ-sulfononitriles. These building blocks, bearing sulfone at one terminus and α-silyloxyaldehyde (in latent form) at the other, were prepared via asymmetric catalysis. An efficient scalable route to these building blocks was developed, leading to enantiopure samples in multigram quantities. Preliminary studies of acetals as the latent aldehyde functionality in the α-silyloxyaldehyde showed that Julia-Kocienski coupling of these building blocks was effective, but iterative application was thwarted during acetal hydrolysis, leading to use of nitrile to perform the latent aldehyde function. A variety of 1,5-polyols, including a 1,5,9,13-tetraol and a differentially protected 1,5,9-triol, were prepared, validating the approach. The accompanying paper describes the application of this configuration-encoded 1,5-polyol synthesis to 1,5,9- and 1,5,7-triols found in tetrafibricin.

Design of a Selective Ring-Closing Enyne Metathesis-Reduction for the Generation of Different Synthetic Scaffolds

A tandem process of ring-closing enyne metathesis (RCEYM)-reduction using modern ruthenium catalysts and a hydrogen donor is described. This straightforward methodology is useful for C(sp³) generation under mild reaction conditions. Variables such as solvent, catalyst, hydride source, and temperature were adjusted toward the exclusive formation of different products.

P-Tether-Mediated, Iterative SN2'-Cuprate Alkylation Strategy to Skipped Polyol Stereotetrads: Utility of an Oxidative "Function Switch" with Phosphite-Borane Tethers

The development of a P-tether-mediated, iterative S_N2'-cuprate alkylation protocol for the formation of 1,3-skipped polyol stereotetrads is reported. This two-directional synthetic strategy builds molecular complexity from simple, readily prepared C₂-symmetric dienediols and unites the chemistry of both temporary phosphite-borane tethers and temporary phosphate tethers-through an oxidative "function switch" of the P-tether itself-to generate intermediates that were previously inaccessible via either method alone.

P-Stereogenic Bicyclo[4.3.1]phosphite Boranes: Synthesis and Utility of Tunable P-Tether Systems for the Desymmetrization of C2-Symmetric 1,3-anti-Diols

The development of P-stereogenic bicyclo[4.3.1]phosphite borane tether systems for the desymmetrization of C₂-symmetric dienediols is reported. This report highlights preliminary studies including tether installation and removal as well as chemoselective functionalization of the exocyclic olefin via diimide reduction or cross-metathesis. Most notably, a divergent oxidation strategy allows for the transformation of the bicyclic phosphite borane complexes to the corresponding phosphate or thiophosphate systems, highlighting the electronic attenuation of this P-tether system.

Modular Synthesis of Novel Macrocycles Bearing α,β-Unsaturated Chemotypes through a Series of One-Pot, Sequential Protocols

A series of one-pot, sequential protocols was developed for the synthesis of novel macrocycles bearing α,β-unsaturated chemotypes. The method highlights a phosphate tether-mediated approach to establish asymmetry, and consecutive one-pot, sequential processes to access the macrocycles with minimal purification procedures. This library amenable strategy provided diverse macrocycles containing α,β-unsaturated carbon-, sulfur-, or phosphorus-based warheads.

Total Synthesis of Cryptocaryol A by Enantioselective Iridium-Catalyzed Alcohol C-H Allylation

The polyketide natural product cryptocaryol A is prepared in 8 steps via iridium catalyzed enantioselective diol double C-H allylation, which directly generates an acetate-based triketide stereodiad. In 4 previously reported total syntheses, 17-28 steps were required.

A Pot-Economical Approach to the Total Synthesis of Sch-725674

A pot-economical total synthesis of antifungal Sch-725674, 1, is reported. The approach takes advantage of a number of one-pot, sequential transformations, including a phosphate tether-mediated one-pot, sequential RCM/CM/chemoselective hydrogenation protocol, a one-pot tosylation/acrylation sequence, and a one-pot, sequential Finkelstein reaction/Boord olefination/acetonide deprotection procedure to streamline the synthesis route by reducing isolation and purification procedures, thus saving time. Overall, an asymmetric route has been developed that is efficiently accomplished in seven pots from phosphate (S,S)-triene and with minimal purification.

Synthetic Studies to Lyngbouilloside: A Phosphate Tether-Mediated Synthesis of the Macrolactone Core

A concise synthetic pathway to the originally assigned structure of lyngbouilloside macrolactone (3) is reported. The core macrocycle 3 was synthesized via a phosphate tether-mediated, one-pot, sequential RCM/CM/chemoselective hydrogenation reaction, Roskamp homologation, and a high yielding Boeckman acylketene cyclization.

Phosphate tethers in natural product synthesis

Recent advances in phosphate tether-mediated natural product synthesis are reviewed. Synthetic approaches toward dolabelide C, (-)-salicylihalimide A, (-)-tetrahydrolipstatin, and (+)-strictifolione are included. In addition, current efforts in method development are briefly reviewed, including a detailed study on the effect of stereochemical complexity on the phosphate-mediated, diastereoselective ring-closing metathesis reaction and recent advances in multi-reaction, one-pot sequential processes mediated by the phosphate tether. Overall, this review seeks to highlight the utility of phosphate triesters to serve as multifunctional tethers with protecting group and latent leaving group characteristics and the ability to orchestrate multiple, orthogonal reaction pathways to allow for the facile synthesis of complex, bioactive small molecules and their analogs.

A modular phosphate tether-mediated divergent strategy to complex polyols

An efficient and divergent synthesis of polyol subunits utilizing a phosphate tether-mediated, one-pot, sequential RCM/CM/reduction process is reported. A modular, 3-component coupling strategy has been developed, in which, simple “order of addition” of a pair of olefinic-alcohol components to a pseudo-C₂-symmetric phosphoryl chloride, coupled with the RCM/CM/reduction protocol, yields five polyol fragments. Each of the product polyols bears a central 1,3-anti-diol subunit with differential olefinic geometries at the periphery.

An efficient, modular approach for the synthesis of (+)-strictifolione and a related natural product

An efficient, library amenable, "pot economical" total synthesis of (+)-strictifolione and the related natural product, (6R)-6[(E,4R,6R)-4,6-dihydroxy-10-phenyl-1-decenyl]-5,6-dihydro-2H-2-pyrone, are reported. This modular approach takes advantage of two consecutive phosphate tether-mediated, one-pot, sequential protocols, followed by a final cross metathesis to deliver both antifungal natural products in a three-pot process from the respective enantiomeric (R,R)- and (S,S)-trienes with minimal purification. A salient feature of this route is that additional protecting groups are not required as a result of the orthogonal protecting- and leaving-group properties innate to phosphate triesters.

Phosphate-tether-mediated ring-closing metathesis for the preparation of complex 1,3-anti-diol-containing subunits

An array of examples of diastereoselective, phosphate-tether-mediated ring-closing metathesis reactions, which highlight the importance of product ring size and substrate stereochemical compatibility, as well as complexity, is reported. Studies focus primarily on the formation of bicyclo[n.3.1]phosphates, involving the coupling of C₂-symmetric dienediol subunits with a variety of simple, as well as complex, alcohol partners.

Tandem catalysis in domino olefin cross-metathesis/intramolecular oxa-conjugate cyclization: concise synthesis of 2,6-cis-substituted tetrahydropyran derivatives

Herein, we describe the concise synthesis of 2,6-cis-substituted tetrahydropyran derivatives based on a domino olefin cross-metathesis/intramolecular oxa-conjugate cyclization (CM/IOCC) reaction. We have found that the domino CM/IOCC of δ-hydroxy olefins with α,β-unsaturated carbonyl compounds (e.g., trans-crotonaldehyde or N-acryloyl-2,5-dimethylpyrrole) could be efficiently achieved in the presence of the second-generation Hoveyda-Grubbs catalyst under elevated temperature conditions, directly affording 2,6-cis-substituted tetrahydropyrans in excellent yields with synthetically useful diastereoselectivity ("auto-tandem catalysis"). In addition, we have found that the domino CM/IOCC of δ-hydroxy olefins with α,β-unsaturated carbonyl compounds could be achieved simply by performing CM in the presence of a Brønsted acid in CH(2)Cl(2) at 25-35 °C, which delivered 2,6-cis-substituted tetrahydropyrans in good yields with excellent diastereoselectivity ("orthogonal-tandem catalysis"). To understand the mechanism of auto-tandem catalysis in the domino CM/IOCC reaction, we have investigated the role of ruthenium hydride complexes in the IOCC of a ζ-hydroxy α,β-unsaturated ketone as a model case.

Evolution of a protecting-group-free total synthesis: studies en route to the neuroactive marine macrolide (-)-palmyrolide A

A full account of our synthetic work toward the first total synthesis of the neuroactive marine macrolide (-)-palmyrolide A is described. Our first-generation approach aimed to unlock the unknown C(5)-C(7) stereochemical relationship via the synthesis of four diastereomers of palmyrolide A aldehyde, a known degradation product. When these efforts provided inconclusive results, recourse to synthesizing all possible stereocombinations of the 15-membered macrolide was undertaken. These studies were critical in confirming the absolute stereochemistry, yielding the first total synthesis of (+)-ent-palmyrolide A. Subsequent to this work, the first protecting-group-free total synthesis of natural (-)-palmyrolide A is also reported.