Total syntheses of zaragozic acids A and C by a carbonyl ylide cycloaddition strategy

A carbonyl ylide cycloaddition approach to the squalene synthase inhibitors zaragozic acids A and C is described. The carbonyl ylide precursor 8 was synthesized starting from di-tert-butyl D-tartrate (47) via an eleven-step sequence involving the regioselective reduction of the mono-MPM (MPM=4-methoxybenzyl) ether 48 with LiBH4 and the diastereoselective addition of sodium tert-butyl diazoacetate to alpha-keto ester 10. The reaction of alpha-diazo ester 8 with 3-butyn-2-one (40) in the presence of a catalytic amount of [Rh2(OAc)4] gave the desired cycloadduct 59 as a single diastereomer. The dihydroxylation of enone 59 followed by sequential transformations permitted the construction of the fully functionalized 2,8-dioxabicyclo[3.2.1]octane core 5. Alkene 79 derived from 5 serves as a common precursor to zaragozic acids A (1) and C (2), since the elongation of the C1 alkyl side chain can be attained by olefin cross-metathesis, especially under the influence of Blechert's catalyst (85).     

Total synthesis of the squalene synthase inhibitor zaragozic acid C

Zaragozic acids and squalestatins were documented by Merck, Glaxo, and Tokyo Noko University/Mitsubishi Kasei Corporation as part of a program aimed at identifying novel inhibitors of squalene synthase, as well as farnesyl transferase. These natural products have attracted considerable attention from numerous synthetic chemists because of their therapeutic potential and novel architecture. This review highlights our total syntheses of zaragozic acid C by two convergent strategies. The key steps in our first-generation synthesis involve 1) simultaneous creation of the C4 and C5 quaternary stereocenters through the Sn(OTf)2-promoted aldol coupling reaction between the alpha-keto ester and silyl ketene thioacetal derived from L- and D-tartaric acids, respectively; and 2) construction of the bicyclic core structure via acid-catalyzed internal ketalization under kinetically controlled conditions. The second-generation strategy relies on a tandem carbonyl ylide formation/1,3-dipolar cycloaddition approach and features elongation of the C1 alkyl side chain through an olefin cross-metathesis as well as high convergency and flexibility.     

Total synthesis of zaragozic acid C by an aldol-based strategy

A total synthesis of zaragozic acid C by a convergent strategy is described in which the key features include (1) the simultaneous creation of the C4 and C5 quaternary stereogenic centers by a Sn(OTf)₂-promoted aldol coupling reaction between an α-keto ester and a silyl ketene thioacetal derived from l- and d-tartaric acids, respectively, (2) the direct introduction of lithium acetylide as the C1 side chain equivalent onto the fully functionalized aldehyde, and (3) construction of the bicyclic core structure by acid-catalyzed internal ketalization under kinetically controlled conditions.     

Chemo- and diastereoselective synthesis of spiro-dioxolanes from intermolecular carbonyl ylide cycloaddition with aryl aldehyde

Reaction of cyclic diazoamides and aromatic aldehydes having electron-donating or -withdrawing groups in the presence of rhodium(II) acetate catalyst at room temperature afforded the spiro-indolodioxolanes. A novel three-component reaction involving intermolecular carbonyl ylides has been demonstrated in a chemo- and diastereoselective manner.     

Total synthesis of gambierol: the generation of the A-C and F-H subunits by using a C-glycoside centered strategy

Gambierol, a representative of the marine ladder toxin family, consists of eight ether rings, 18 stereocenters, and two challenging pyranyl rings having methyl groups that are in a 1,3-diaxial orientation to one another. Herein we describe the generation of gambierol's A-C and F-H ring systems and demonstrate the versatility of the glycosyl anhydride, enol ether-olefin RCM strategy to fused polycyclic ethers. This work has both enabled us to generate sufficient quantities of the gambierol precursors and has enabled us to better understand the chemical transformations that were key to these efforts. Fundamental work included efforts to C-glycosides and C-ketosides, Claisen rearrangements, and enol ether-olefin RCM reactions.     

Surprising 1,7-cyclization of vinyl carbonyl ylides generated from reaction of indanetrione with vinyl diazo compounds

Reactions of tricarbonyl compounds with vinyl diazo compounds 2 were carried out. Reaction of 1,2,3-indanetrione with 2a,b,c gave the spiroindan-1,3-dione-2,2′-benzodihydrooxepin 7a,b,c, but not normal products oxirane and dihydrofuran derivatives expected from intermediate vinyl carbonyl ylides 4. Formation of 7 requires isomerization of vinyl carbonyl ylides 4 bearing a (Z)-cyanostyryl group to unstable (E)-form 5 and subsequent cyclization to oxepin 6 followed by a 1,5-hydrogen shift. However, reaction of 2 with six-membered cyclic tricarbonyl compounds 1,2,3-trioxo-2,3-dihydrophenalene 11 and dimethylalloxane 13 gave the dioxole 12 and the dihydrofuran 14, respectively, typical products expected from vinyl carbonyl ylides.     

Synthesis of Amathaspiramides by Aminocyanation of Enoates

Concise routes for the total and formal syntheses of the amathaspiramides were developed through a formal [3+2] cycloaddition between lithium(trimethylsilyl)diazomethane and α,β‐unsaturated esters. The effectiveness of this new cycloaddition for the construction of Δ²‐pyrazolines containing a α‐tert‐alkylamino carbon center and subsequent facile protonolytic N? N bond cleavage allows the synthesis of a key intermediate of the amathaspiramides and other α,α‐disubstituted amino acid derivatives.     

Stereoselective Synthesis of Tetrahydroindolizines through the Catalytic Formation of Pyridinium Ylides from Diazo Compounds

Commercially available iron(III) and copper(I) complexes catalyzed multicomponent cycloaddition reactions between diazo compounds, pyridines, and electrophilic alkenes to give alkaloid‐inspired tetrahydroindolizidines in high yield with high diastereoselectivity. Hitherto, the catalytic formation of versatile pyridinium ylides from metal carbenes has been poorly developed; the broad utility demonstrated herein sets the stage for the invention of further multicomponent reactions in future.     

Microwave-assisted cycloaddition of diisopropyl diazomethylphosphonate to electron-deficient alkenes: synthesis of multifunctionalized phosphonopyrazolynes and phosphonopyrazoles

An efficient method for the synthesis of functionalized phosphonopyrazolines has been developed. The procedure involved the microwave-assisted cycloaddition of diisopropyl diazomethylphosphonate to dipolarophiles, such as α,β-unsaturated-nitriles and -esters, performed in neat conditions. By oxidative aromatization the phosphonopyrazolines, thus obtained, were converted into the corresponding phosphonopyazoles. Alternatively, phosphonopyazoles could be obtained by a one-pot, two step procedure directly from dipolarophiles, thus avoiding the isolation of the intermediate pyrazolines. Full spectroscopic characterization of the compounds has been also reported.