Tetrafibricin: Synthesis of the C1−C13, C15−C25, and C27−C40 Fragments

Recent Advances in Alkenyl sp2 C-H and C-F Bond Functionalizations: Scope, Mechanism, and Applications

Alkenes and their derivatives are featured widely in a variety of natural products, pharmaceuticals, and advanced materials. Significant efforts have been made toward the development of new and practical methods to access this important class of compounds by selectively activating the alkenyl C(sp²)-H bonds in recent years. In this comprehensive review, we describe the state-of-the-art strategies for the direct functionalization of alkenyl sp² C-H and C-F bonds until June 2022. Moreover, metal-free, photoredox, and electrochemical strategies are also covered. For clarity, this review has been divided into two parts; the first part focuses on currently available alkenyl sp² C-H functionalization methods using different alkene derivatives as the starting materials, and the second part describes the alkenyl sp² C-F bond functionalization using easily accessible gem-difluoroalkenes as the starting material. This review includes the scope, limitations, mechanistic studies, stereoselective control (using directing groups as well as metal-migration strategies), and their applications to complex molecule synthesis where appropriate. Overall, this comprehensive review aims to document the considerable advancements, current status, and emerging work by critically summarizing the contributions of researchers working in this fascinating area and is expected to stimulate novel, innovative, and broadly applicable strategies for alkenyl sp² C-H and C-F bond functionalizations in the coming years.

Total Synthesis of Cryptoconcatone D via Construction of 1,3-Diol Units Using Chiral Horner-Wittig Reagents

The modular synthesis of 1,3-polyols using a chiral phosphine oxide building block is reported. This versatile building block works in a repetitive way for the stereocontrolled synthesis of a tetraol key intermediate, which serves for the first total synthesis of the potentially anti-inflammatory natural product cryptoconcatone D. A new route toward the chiral building block is also presented: Starting from 2-deoxy-d-ribose, the optimized sequence now makes the use of the building block more attractive to practicing chemists again.

Stereocontrolled Total Synthesis of Bastimolide B Using Iterative Homologation of Boronic Esters

Bastimolide B is a polyhydroxy macrolide isolated from marine cyanobacteria displaying antimalarial activity. It features a dense array of hydroxylated stereogenic centers with 1,5-relationships along a hydrocarbon chain. These 1,5-polyols represent a particularly challenging motif for synthesis, as the remote position of the stereocenters hampers stereocontrol. Herein, we present a strategy for 1,5-polyol stereocontrolled synthesis based on iterative boronic ester homologation with enantiopure magnesium carbenoids. By merging boronic ester homologation and transition-metal-catalyzed alkene hydroboration and diboration, the acyclic backbone of bastimolide B was rapidly assembled from readily available building blocks with full control over the remote stereocenters, enabling the total synthesis to be completed in 16 steps (LLS).

Stereoselective Synthesis of 1,3,5-Trienes from Alkynones and Allyl Carbonyl Compounds Through C-C σ-Bond Cleavage under Transition-Metal-Free Conditions

An atom-economical transition-metal-free protocol for the stereoselective synthesis of conjugated trienes has been developed. These transformations were realized through base promoted C-C σ-bond cleavage reactions of allyl carbonyl compounds. This...

Inspirations from tetrafibricin and related polyketides: new methods and strategies for 1,5-polyol synthesis

Covering: up to 2019 Selective synthesis with control of remote stereogenic centers has long been a challenge in organic chemistry. In recent years the interest in this topic has been energized by isolation and synthetic studies of tetrafibricin and other natural products containing 1,5-polyols, such as amphidinol 3, marinomycins, and caylobolide. Here we discuss recent developments in 1,5-polyol synthesis, including an overview of selected bioactive natural products in this class and examples of new synthetic methodologies and strategies dedicated to remote stereocontrol in these structures. To illustrate in greater depth, we review several instructive examples of how these innovations have been applied in synthetic studies on tetrafibricin.

Stereoselective synthesis of conjugated trienes via 1,4-palladium migration/Heck sequence

An efficient 1,4-palladium migration/Heck sequence was developed for the highly stereoselective synthesis of conjugated trienes.

Unified Strategy for 1,5,9- and 1,5,7-Triols via Configuration-Encoded 1,5-Polyol Synthesis: Preparation and Coupling of C15-C25 and C26-C40 Fragments of Tetrafibricin

Diverse classes of natural products contain chiral 1,5,9- and 1,5,7-triol stereotriads, including the novel fibrinogen receptor antagonist tetrafibricin. Biological activities associated with compounds containing these motifs warrant targeted synthetic strategies to 1,5-polyol families from cheap and easily accessible reagents while avoiding the need to determine configurations at each alcohol stereocenter. In the accompanying paper, we present a solution to these problems via an iterative configuration-encoded strategy that exploits Julia-Kocienski couplings of enantiopure α-silyloxy-γ-sulfononitrile building blocks. The stereocontrol is unambiguous, and the building blocks are available in multigram quantities via asymmetric catalysis. This approach efficiently accessed a C26-C40 subunit of tetrafibricin that contains a syn, syn-1,5,9-triol and all of the stereochemistry and functionality needed to advance toward tetrafibricin. A modification afforded the anti, syn-1,5,7-triol within the C15-C25 fragment of tetrafibricin by merging 1,5-polyol synthesis with diastereoselective intramolecular conjugate addition. The union of the C15-C25 and C26-C40 fragments was achieved via a BF₃·OEt₂-mediated Mukaiyama aldol construction with high 1,3- anti stereoinduction, revealing some unexpected insights on the impact of silyl protecting groups on 1,3- anti diastereocontrol by a β-siloxyaldehyde aldol acceptor. Directed 1,3- anti reduction completed the stereostructure of the C15-C40 portion of tetrafibricin, with configurations established by a combination of NMR experiments.

Linchpin dienes: key building-blocks in the synthesis of polyenic frameworks

This review focuses on the preparation of dienic linchpin reagents and on their use in the synthesis of polyenic frameworks.

Asymmetric alcohol C-H allylation and syn-crotylation: C9-C20 of tetrafibricin

The C9-C20 segment of the fibrinogen receptor inhibitor tetrafibricin was prepared in 10 steps (longest linear sequence). Ruthenium catalyzed enantioselective syn-crotylation is used to construct C9-C13. Iridium catalyzed asymmetric alcohol C-H allylation of a commercial malic acid derived alcohol is used to construct C14-C20. Recovery and recycling of the iridium catalyst is described.

Enantio- and diastereoselective synthesis of N-acetyl dihydrotetrafibricin methyl ester

A highly diastereoselective synthesis of N-acetyl dihydrotetrafibricin methyl ester (34) is described. The synthesis features three enantioselective double allylboration reactions and an intramolecular hydrosilylation/Fleming-Tamao oxidation sequence to establish seven of the hydroxy-bearing stereocenters of 34. Especially noteworthy is the fragment-assembly double allyboration reaction of 2 and 7 using reagent 3, which provides the advanced intermediate 6 with >20:1 diastereoselectivity.

Development of a Double Allylboration Reagent Targeting 1,5-syn-(E)-Diols: Application to the Synthesis of the C(23)-C(40) Fragment of Tetrafibricin

Interest in the synthesis of the C(23)-C(40) fragment 2 of tetrafibricin prompted us to develop a new method for the synthesis of 1,5-syn-(E)-diols. Toward this end, the kinetically controlled hydroboration of allenes 6, 33, ent-39, 42 and 45 with the Soderquist borane 25R were studied. Tetrabutylammonium allenyltrifluoroborate 45 gave superior results and was utilized in a double allylboration sequence with two different aldehydes to provide the targeted 1,5-syn-(E)-diols in generally high yields (72-98%), and with high enantioselectivity (>95% e.e.), diastereoselectivity (d.r. >20:1), and (E)/(Z) selectivity (>20:1). This new method was applied to the synthesis of the C(23)-C(40) fragment 2 of tetrafibricin.

C(21)-C(40) of tetrafibricin via metal catalysis: beyond stoichiometric chiral reagents, auxiliaries, and premetalated nucleophiles

The C(21)-C(40) fragment of fibrinogen receptor inhibitor tetrafibricin was prepared in 12 steps from propane diol (longest linear sequence). In this approach, 6 C-C bonds are formed via asymmetric iridium catalyzed transfer hydrogenative carbonyl allylation and 2 C═C bonds are formed via Grubbs olefin cross-metathesis.

Synthesis of C1-C20 and C21-C40 fragments of tetrafibricin

Efficient syntheses of suitably functionalized top and bottom fragments of tetrafibricin are described. The bottom fragment is prepared by two consecutive Kocienski-Julia couplings, while the top fragment synthesis features a dithiane alkylation and a Horner-Wadsworth-Emmons reaction.

Enantio- and diastereoselective synthesis of (E)-1,5-syn-diols: application to the synthesis of the C(23)-C(40) fragment of tetrafibricin

A highly stereoselective synthesis of (E)-1,5-syn-diols 6 is described. The kinetically controlled hydroboration of allenyltrifluoroborate 8 with Soderquist borane 2 provides the (Z)-allylic trifluoroborate 9, which undergoes sequential allylboration with two different aldehydes to provide (E)-1,5-syn-diols 6 in 72-98% yields with >95% ee and >20:1 dr. Application of this method to the synthesis of the tetrafibricin C(23)-C(40) fragment 19 is described.

Fluorous Mixture Synthesis of Four Stereoisomers of the C21-C40 Fragment of Tetrafibricin

Four stereoisomers of the C21-C40 fragment are synthesized in a single exercise with the aid of fluorous tagging to encode configurations at C37 and C33. After demixing and detagging, the isomers were found to have substantially identical (1)H NMR spectra. However, there were some small but reliable differences in their (13)C NMR spectra.

Total synthesis of polyrhacitides A and B by use of an iterative strategy for the stereoselective synthesis of 1,3-polyol arrays

The first total syntheses of polyrhacitides A and B, secondary metabolites from Chinese medicinal ants, were accomplished via an iterative approach to the 1,3-polyol motif. For the construction of all stereogenic centers, the catalytic asymmetric Overman esterification was utilized.

1,n-glycols as dialdehyde equivalents in iridium-catalyzed enantioselective carbonyl allylation and iterative two-directional assembly of 1,3-polyols

Unstable? We're able! 1,n-Glycols serve as synthetic equivalents to unstable dialdehydes in two-directional carbonyl allylation from the alcohol oxidation level under iridium-catalyzed transfer hydrogenation conditions. Iterative asymmetric allylation employing 1,3-propanediol enables the rapid assembly of protected 1,3-polyol substructures with exceptional levels of stereocontrol.

SULFANYLATION OF 1,3-DITHIANE ANIONS BY 5-(ALKYLSULFANYL)-1-PHENYLTETRAZOLES

An unusual reaction between 1,3-dithiane anions and by 5-(alkylsulfanyl)-1-phenyltetrazoles has been discovered in which the dithiane anion formally displaces the 1-phenyltetrazole ring from sulfur to provide a sulfanylated dithiane.

Stereoselective synthesis of the C(1)-C(19) fragment of tetrafibricin

[reaction: see text] A stereoselective synthesis of the C(1)-C(19) fragment of tetrafibricin has been accomplished via a highly diastereoselective double allylboration reaction of 6-8 and an iodonium ion promoted urethane cyclization for the installation of the C(15) alkoxy function in 3.