Synthesis of saccharides and related polyhydroxylated natural products. 1. Simple alditols

Total Synthesis of (-)-Batrachotoxin Enabled by a Pd/Ag-Promoted Suzuki-Miyaura Coupling Reaction

Batrachotoxin is an extremely potent cardio- and neurotoxic steroidal alkaloid found in certain species of frogs, birds, and beetles. The steroidal 6/6/6/5-membered carbocycle (ABCD-ring) is U-shaped and functionalized with two double bonds, a six-membered C3-hemiacetal across the AB-ring, a seven-membered oxazepane on the CD-ring, and a dimethylpyrrolecarboxy group at the D-ring carbon chain. These structural features present an unusual and formidable synthetic challenge. Herein we report a total synthesis of batrachotoxin based on a newly devised convergent strategy through a 22-step sequence. Enantiopure AB-ring and D-ring fragments were prepared and subjected to a crucial C(sp2 )-C(sp2 ) coupling reaction. Although both C(sp2 ) centers were sterically encumbered by proximal tetrasubstituted carbon atoms, Ag2 O strongly promoted the Pd(PPh3 )4 -catalyzed Suzuki-Miyaura coupling reaction at room temperature, thereby connecting the two fragments without damaging their preexisting functionalities. Subsequent treatment with t-BuOK induced Dieckmann condensation to cyclize the C-ring. The judiciously optimized functionalizations realized oxazepane formation, carbon chain extension, and pyrrole carboxylic acid condensation to deliver batrachotoxin.

Ring Opening of Epoxides by Pendant Silanols

We present a new ring-opening reaction of epoxides by pendant silanols, catalyzed by either Ph₃C⁺BF₄^- or BINOL-phosphoric acid. Silanol epoxides derived from trans-allylic alcohols, cis-allylic alcohols, trans-homoallylic alcohols, and cis-homoallylic alcohols were all compatible and gave products from either endo- or exo-ring opening. With silanol epoxides derived from 4-alkenyl silanols, an unusual rearrangement to tetrahydrofuran products was observed. The utility of this methodology was demonstrated in a short preparation of protected d-arabitol.

Advances in Total Synthesis of Some 2,3,5-Trisubstituted Tetrahydrofuran Natural Products

2,3,5-Trisubstituted tetrahydrofuran moiety is ubiquitous in natural products. These have served as appealing candidates for total synthesis due to their varied bio- and pharmaceutical activities. This tutorial review delineates the ingenious efforts by many researchers in the total synthesis of selected natural products based on a common 2,3,5-trisubstituted tetrahydrofuran core structure. Many of the syntheses display nuanced interplay between new methods and the ingenuity of planned strategies achieved through catalysis or cascade chemistry. In some cases, the chiron approach has come quite handy, wherein the structural features and the stereochemistry in select molecules could map well with naturally available starting materials. This compilation also aims to enhance the diversity space based on these natural products and further interest in sustainable total synthesis.

Stereoselective Synthesis of D-5-Homo-4-selenoribose as a Versatile Intermediate for 4'-Selenonucleosides

Stereoselective synthesis of D-5-homo-4-selenoribose, serving as a versatile intermediate for the synthesis of 4'-selenonucleosides 12a-c, was accomplished using Sharpless asymmetric epoxidation, regioselective cleavage of the α,β-epoxide, and stereoselective reduction of the ketone as the key steps.

Bioinspired iterative synthesis of polyketides

Diverse array of biopolymers and second metabolites (particularly polyketide natural products) has been manufactured in nature through an enzymatic iterative assembly of simple building blocks. Inspired by this strategy, molecules with inherent modularity can be efficiently synthesized by repeated succession of similar reaction sequences. This privileged strategy has been widely adopted in synthetic supramolecular chemistry. Its value also has been reorganized in natural product synthesis. A brief overview of this approach is given with a particular emphasis on the total synthesis of polyol-embedded polyketides, a class of vastly diverse structures and biologically significant natural products. This viewpoint also illustrates the limits of known individual modules in terms of diastereoselectivity and enantioselectivity. More efficient and practical iterative strategies are anticipated to emerge in the future development.

A facile stereoselective total synthesis of (R)-rugulactone

An efficient and novel synthesis of (R)-rugulactone has been achieved employing Sharpless asymmetric epoxidation of allyl alcohols followed by selective hydride reduction of epoxy alcohols and olefin cross metathesis reactions.

Spongipyran Synthetic Studies. Total Synthesis of (+)-Spongistatin 2

Evolution of a convergent synthetic strategy to access (+)-spongistatin 2 (2), a potent cytotoxic marine macrolide, is described. Highlights of the synthesis include: development of a multicomponent dithiane-mediated linchpin union tactic, devised and implemented specifically for construction of the spongistatin AB and CD spiro ring systems; application of a Ca(II) ion controlled acid promoted equilibration to set the thermodynamically less stable axial-equitorial stereogenicity in the CD spiroketal; use of sulfone addition/Julia methylenation sequences to unite the AB and CD fragments and introduce the C(44)-C(51) side chain; and fragment union and final elaboration to (+)-spongistatin 2 (2) exploiting Wittig olefination to unite the advanced ABCD and EF fragments, followed by regioselective Yamaguchi macrolactonization and global deprotection. Correction of the CD spiro ring stereogenicity was subsequently achieved via acid equilibration in the presence of Ca(II) ion to furnish (+)-spongistatin 2 (2). The synthesis proceeded with a longest linear sequence of 41 steps.

Asymmetric Silaboration of Terminal Allenes Bearing α-Stereogenic Centers: Stereoselection Based on “Reagent Control”

A highly enantioface-selective silaboration of allenes having stereogenic centers at the alpha-positions of the double bonds has been achieved using a combination of a chiral silylborane (-)-2 and a chiral Pd/(R)-3 catalyst. The chiral reagent system efficiently controlled the stereochemistry of the new stereogenic centers even in the reactions of mismatched combinations. [reaction: see text]

Diastereoselective Synthesis of All Eight l-Hexoses from l-Ascorbic Acid

[reaction: see text] A novel versatile method for the synthesis of all eight diastereomerically pure L-hexoses was developed. L-Ascorbic acid was converted to two diastereomers A. These alpha-hydroxy esters were transformed into four gamma-alkoxy-alpha,beta-unsaturated esters C via the intermediates B and subsequent Wittig olefination reactions. Each one of compounds C was subjected to dihydroxylation to provide a set of two diols D. Anti/syn-differentiation in diol formation was manipulated by using (DHQD)2PHAL and (DHQ)2PHAL as chiral ligands. Further two-step reaction sequence affords all eight diastereopure L-hexoses.

Total Synthesis of Bengamide E and Analogues by Modification at C-2 and at Terminal Olefinic Positions

[Reaction: see text]. The total synthesis of the natural product Bengamide E, one of the members of a new class of antitumor natural products of marine origin, is reported based on a convergent and flexible synthetic route featuring an oxirane ring-opening reaction and an olefin cross metathesis. In a similar way, analogues structurally modified at C-2 and at the terminal vinyl positions were prepared by introduction of various nucleophiles and alkyl substituents during the epoxide opening and the olefin cross metathesis steps, respectively. These studies demonstrate the validity of our synthetic strategy, although they reveal some problems associated with the olefin cross metathesis, whose efficiency depends on the substituent at the C-2 position as well as the steric environment of the alkene.

Oxidatively truncated docosahexaenoate phospholipids: total synthesis, generation, and Peptide adduction chemistry

The recent immunological detection of extraordinarily high levels of carboxyethylpyrrole (CEP) modifications of proteins from the retinas of individuals with age-related macular degeneration provided presumptive evidence for the involvement of docosahexaenoate-derived oxidatively truncated phospholipids in retinal pathology. To facilitate the in vivo detection and characterization of the chemistry and biological activities of these postulated naturally occurring molecules, a family of oxidatively truncated phospholipids was prepared by total syntheses. Their formation in oxidation reactions of a docosahexaenoate ester of 2-lysophosphatidylcholine (DHA-PC) was also demonstrated. Free radical-induced oxidative cleavage of DHA-PC promoted by myeloperoxidase or copper ions generates similar mixtures of these phospholipids. The most abundant products were 1-palmitoyl-2-succinoyl-sn-glycero-3-phosphatidylcholine (4.7%) and 2-(6-carboxy-4-oxohex-5-enoyl)-1-palmitoyl-sn-glycero-3-phosphatidylcholine (1.7%). Both of these oxidatively truncated phospholipids are homologues of biologically active arachidonate-derived phospholipids. A minor product from DHA-PC, 2-(4-hydroxy-7-oxohept-5-enoyl)-1-palmitoyl-sn-glycero-3-phosphatidylcholine (0.4% yield), reacted with the epsilon-amino group of a peptide lysyl residue to produce a CEP derivative in 0.7% yield. These observations support the previous conclusion, based on immunological evidence, that CEPs are generated by the reaction of an oxidatively truncated phospholipid with proteins in the retina and further indicate that CEP protein modifications probably represent only a tiny fraction of the products generated upon oxidative damage of DHA-PC in photoreceptor disk membranes.

Novel bioactive phospholipids: practical total syntheses of products from the oxidation of arachidonic and linoleic esters of 2-lysophosphatidylcholine(1)

Total syntheses of nine novel phospholipids were accomplished to facilitate the identification and biological testing of compounds that are generated upon oxidative cleavage of arachidonate and linoleate esters of 2-lysophosphatidylcholine, the two most abundant polyunsaturated phospholipids in low-density lipoprotein. An efficient general synthesis exploiting 2-lithiofuran as a 4-oxo-2-butenoyl carbanion equivalent provided phospholipids containing gamma-keto-alpha,beta-unsaturated carbonyl functional arrays. By exploiting facile cis-trans isomerizations, two commercially available cis alkenes, (2Z)-2-butene-1,4-diol and 2,5-dihydrofuran, could be employed as starting materials for preparing the Horner-Wadsworth-Emmons reagent 4-(diethoxyphosphoryl)-2E-butenal, a valuable building block for the synthesis of 2,4-dienals. The reagent was exploited in a total synthesis of 13-oxotridec-9E,11E-dienoic acid, confirming the identity of this product that is generated upon autoxidation of linoleic acid and by decomposition of 13-hydroperoxy-9,11-octadecadienoate (13-HPODE), especially in the presence of redox active transition metal ions, cytochrome p-450, or hydroperoxide lyase.