Synthesis of 12-aza analogs of epothilones and (E)-9,10-dehydroepothilones

N-Boc-12-aza-epothilone analog (azathilone) 1 is a potent inhibitor of human cancer cell growth and represents a structurally new class of natural product-derived microtubule-stabilizing agents. Compound 1 has been prepared employing a convergent strategy that is based on the consecutive assembly of building blocks 3, 4, and 19 into diene 20 and subsequent RCM-mediated macrocycle formation. The aldol reaction between aldehyde 3 and ketone 4 delivered the required 6R,7S diastereoisomer 5 with good selectivity and provided a reliable entry into the stereoselective synthesis of carboxylic acid 12. RCM with diene 20 was highly E-selective, thus giving efficient access to (E)-9,10-dehydro-1 (2). The latter is a key analog in SAR studies with 1.     

Synthesis of the alkaloid tyroscherin by an aldol/Curtius strategy

The alkaloid tyroscherin (2), which contains a vicinal anti-amino alcohol subunit was prepared from 4-hydroxyphenylpropionic acid (5) and meso-diol 9. After desymmetrization of diol 9 and suitable protecting group manipulations, one terminus was extended via a Claisen rearrangement giving rise to enoate ent-15. The missing carbon on the other end could be incorporated using MeMgCl/CuBr·SMe₂ leading eventually to aldehyde ent-22. The acylated oxazolidinone 32 derived from acid 5 and aldehyde ent-22 were combined in an aldol reaction. A subsequent Curtius rearrangement on the carboxylic group furnished the amino function of tyroscherin (2). In a proof of concept study the same strategy was used to prepare tyroscherin analog 28.     

The cooperative effect of a hydroxyl and carboxyl group on the catalytic ability of novel β-homoproline derivatives on direct asymmetric aldol reactions

Novel β-homoproline derivatives, 2-hydroxy-2-(pyrrolidin-2-yl)acetic acids (R,S)- and (S,S)-1a-d, were synthesized. All of the prepared compounds were used as organocatalysts in the direct asymmetric aldol reaction of 4-nitrobenzaldehyde with several ketones. Among these catalysts, (R)-2-hydroxy-2-((S)-pyrrolidin-2-yl)acetic acid (R,S)-1a showed good catalytic ability in the formation of aldol product 13 (up to 69% ee, 95% yield), which was similar to the results catalyzed by l-proline (71% ee, 96% yield). Relatively low yields and low enantioselectivities were observed in aldol reactions catalyzed by (S,S)-1a, for example, 13 was obtained in 55% yield and 13% ee. The aldol reaction catalyzed by the methyl-protected carboxylic acid 1b and esters 1c,d produced much lower chemical yields and enantioselectivities during the formation of 13. The cooperative effect of the (R)-configured hydroxyl group and the carboxyl group was found to play an important role in inducing enantioselectivity in the aldol reaction. Relatively high diastereoselectivities (anti:syn = 85:15) and enantioselectivity (anti, 83% ee) were observed in the aldol reactions of 4-nitrobenzaldehyde with cyclohexanone, which was catalyzed by (R,S)-1a.     

Asymmetric aldol reactions using catalytic d(+)-proline: a new, economic and practical approach to a commonly employed C1-C6 keto-acid synthon of the epothilones

A new approach to ketoacid 4, a common C1-C6 fragment used in the total synthesis of epothilones was initiated by direct aldol reaction of acetone with a pivaldehyde-like substance 5, catalyzed with d-proline, leading to a 2,6-diketoalcohol with better than 99% ee. Further intramolecular closure of the diketone 8 followed by oxidation of the silyl protected hydroxycyclohexenone 14 led to the desired product 4. None of the steps have been optimized, yet the overall yield for the four-step process is 31%. The use of commercially available d-proline to construct the chiral center of 4 under very mild reaction conditions provided an economical and practical method for its construction.     

Construction of polyheterocyclic spirotetrahydrothiophene derivatives via sulfa-Michael/aldol cascade reaction

A series of polyheterocyclic spirotetrahydrothiophene derivatives were obtained in moderate to excellent yields via a catalyst-free sulfa-Michael/aldol cascade reaction of chalcones 1 and commercially available 1,4-dithiane-2,5-diol 2 under mild conditions. We also present the first asymmetric sulfa-Michael/aldol cascade reaction of chalcones 1 and commercially available 1,4-dithiane-2,5-diol 2 with moderate to good enantioselectivities catalyzed by readily available chiral phase-transfer catalysts (PTCs).     

Synthesis of the optically active key intermediate of FR901483

Efficient synthesis of the tricyclic key intermediate 2 for (−)-FR901483 1 was accomplished. The precursor of the intramolecular aldol reaction 4b is constructed by the Ugi 4CC reaction and subsequent intramolecular Dieckmann condensation. This approach allows a fully stereocontrolled total synthesis of (−)-FR901483, which would provide various derivatives.     

Synthesis of a biphenyl-based axially chiral amino acid as a highly efficient catalyst for the direct asymmetric aldol reaction

A biphenyl-based axially chiral amino acid (S)-2 has been designed and synthesized. The new amino acid (S)-2 has been found to be a more efficient catalyst than (S)-1 in the direct asymmetric aldol reaction of acetone with aldehydes. For instance, the use of only 0.1 mol % of (S)-2 was sufficient to complete the reaction between acetone and 4-nitrobenzaldehyde, giving the corresponding aldol adduct in good yield with an excellent enantioselectivity.     

Stereoselective synthesis of tubuvaline methyl ester and tubuphenylalanine, components of tubulysins, tubulin polymerization inhibitors

Synthetic studies of two components of tubulysins, tubulin polymerization inhibitors are described. The highly stereoselective synthesis of tubuvaline methyl ester (2) was accomplished by 1,3-dipolar cycloaddition of nitrone d-6 and acrylic acid derivatives 7 as a key step. The synthesis of tubuphenylalanine (3) was conducted by an aldol reaction of a boron enolate of (S)-4-isopropyl-3-propionyl-2-oxazolidinone (13) with aldehyde 14, readily prepared from phenylalanine, followed by Barton deoxygenation under radical conditions.     

Synthetic studies on apoptolidin: synthesis of the C12-C28 fragment via a highly stereoselective aldol reaction

The stereoselective and convergent synthesis of the C12-C28 segment 2 of the apoptosis inducing macrolide antibiotic, apoptolidin (1), is described. The synthesis involves a highly stereoselective tin(II)-mediated aldol reaction between the C17-C22 ethyl ketone 3 and the C23-C28 aldehyde 4 as the key step.     

A novel route for the construction of Taxol ABC-ring framework: skeletal rearrangement approach to AB-ring and intramolecular aldol approach to C-ring

We report here on the construction of the ABC-ring framework of (±)-Taxol using an intramolecular aldol reaction as a key step. AB-ring compound 8 was converted to ketoaldehyde 25 as a precursor of an aldol reaction via introduction of oxygen-functionalities and a methoxycarbonyl group, which can be converted to a methyl group, in the proper positions of the B-ring. An aldol reaction of ketoaldehyde with LDA led to the formation of the desired product 27, which corresponds to the ABC-ring framework of (±)-Taxol.     

Domino intramolecular enyne metathesis/cross metathesis approach to the xanthanolides. Enantioselective synthesis of (+)-8-epi-xanthatin

The first total synthesis of (+)-8-epi-xanthatin (1) has been achieved in 14 steps starting from the commercially available ester 24, which was converted into aldehyde 23 in six steps. An enantioselective aldol reaction of 23 gave 30, which was transformed into triflate 22 in four steps, setting the stage for a palladium-catalyzed carbonylation reaction to form acrylate 34. Compound 34 was then subjected to a deprotection/lactonization sequence to furnish enyne 21, which underwent a domino enyne ring-closing metathesis/cross metathesis process to form a seven-membered carbocycle and (E)-conjugated dienone, thereby completing the synthesis of 1. This domino ruthenium-catalyzed metathesis reaction thus serves as an efficient method to construct the core of xanthanolide and other sesquiterpene lactones.     

Inherently chiral calix[4]arene-based bifunctional organocatalysts for enantioselective aldol reactions

Two optical pure m-dimethylamino substituted inherently chiral calix[4]arene derivatives 8a and 8b bearing an l-prolinamido group have been synthesized by two routes, and structurally studied by the usual spectroscopic methods and X-ray crystallographic analysis. It was found that both of 8a and 8b could be utilized as bifunctional organocatalysts to efficiently promote the aldol reactions between aromatic aldehydes and ketones in the presence of acetic acid. Especially, with 8a as the catalyst, the reaction between 4-nitrobenzaldehyde and cyclopentanone at −20 °C gave the anti-aldol product up to 94% ee, while the anti-aldol product in up to 94:6 dr and 79% ee was obtained when 4-cyanobenzaldehyde was used as the aldol donor. Moreover, it was also demonstrated that the inherently chiral calixarene skeleton with (cS)-conformation in 8a was identified as the matched configuration of the stereogenic elements, and the inherently chiral moiety might play an important role in helping to stereocontrol the reaction.     

Novel construction of the brassinolide side chain

A stereoselective synthesis of brassinolide, which involves construction of the side chain by a highly stereoselective aldol reaction between 20S-6β-methoxy-3α,5-cyclo-5α-pregnane-20-carboxaldehyde 2 and ketone 3 or 4 catalyzed by l-proline, is described.     

Preparation and reactions of 3-phosphinyl-1-aza-1,3-butadienes. Synthesis of phosphorylated pyridine and pyrazole derivatives

3-Phosphinyl 1-aza-1,3-butadienes 2 are obtained by aldol condensation between hydrazonoalkyl phosphine oxides and N,N-dimethylformamide dimethyl acetal. Transamination reaction of these azadienes with amines yields functionalized 1-aza-1,3-butadienes 3. Cycloaddition processes of these azadienes 2a with electron-poor dienophiles to give phosphorylated pyridine derivatives 9 and 15 are also reported, while intramolecular cyclization reaction of heterodiene 2b affords phosphorylated pyrazole 17.     

Concise route to defined stereoisomers of the hydroxy acid of the chondramides

The use of Kobayashi vinylogous aldol reaction in the reaction with acetaldehyde led to anti-aldol product 11. After reductive removal of the chiral auxiliary, the primary alcohol was converted to the allyliodide 14. This compound could be engaged in an Evans alkylation reaction, leading eventually to hydroxy acid 19. Inclusion of a Mitsunobu inversion reaction on the sequence starting with ent-11 led to hydroxy ester 30, featuring a 6,7-syn-configuration. These hydroxy acids should help to elucidate the correct stereostructure of the chondramide depsipeptides.     

Selective sulfonylation of 4-C-hydroxymethyl-β-l-threo-pento-1,4-furanose: synthesis of bicyclic diazasugars

Hydroxymethylation of α-d-xylo-pentodialdose 6 using excess formaldehyde and sodium hydroxide in THF-water (one pot aldol and crossed Cannizzaro reactions) followed by hydrogenolysis of C3-O-benzyl group afforded triol 8. The regio-selective α- and β-sulfonylation of hydroxymethyl groups in 8 afforded 9a (α-sulfonylation) and 14 (β-sulfonylation) in good yield. The cleavage of the 1,2-acetonide functionality, individually in 9a and 14, followed by reaction with 1,3-diaminopropane gave in situ formation of sugar aminals, that undergo concomitant nucleophilic displacement of the sulfonyloxy group by amino functionality to give hitherto unknown bicyclic diazasugars 4 and 5, respectively, with a hydroxymethyl substituent at C-7.     

Novel reduction of perfluoroalkyl ketones with lithium alkoxides

The reaction of tert-butyl N-(2-bromophenyl)carbamate (1) with ethyl perfluorooctanoate in the presence of tert-butyllithium did not give the desired N-(2-perfluorooctanoylphenyl)carbamate (3) but gave 1-hydroxy-1H-perfluorooctyl compound (4), which was supposed to be formed by the reduction of 3. A similar reaction of 2,2,2-trifluoroacetophenone with tert-butyllithium did not gave any reduction product. Detailed investigation showed that lithium ethoxide worked as the reducing agent of this abnormal reduction. By the reaction of lithium isopropoxide, an aldol product from 2,2,2-trifluoroacetophenone with acetone was isolated, while perfluoroheptyl or perfluoropropyl phenyl ketones were reduced by this alkoxide in a high yield without formation of the aldol adduct.     

Domino cyclocondensation of arylaldehydes with 2-acetylpyridine

A novel synthetic route for the preparation of cyclohexane skeleton 3 was developed via the one-pot domino Claisen-Schmidt condensation reaction/Michael addition/double aldol reaction of 1 equiv of functionalized arylaldehyde 1 with 3 equiv of 2-acetylpyridine (2a).     

Syntheses of 1′,2′-cyclopentyl nucleosides as potential antiviral agents

To discover novel nucleosides as potential antiviral agents, 1′,2′-cyclopentyl nucleosides were designed as hybrids of sofosbuvir and GS-6620. An asymmetric aldol condensation reaction was used as the key transformation to prepare the versatile 1′,2′-cyclopentyl ribose 6, which is useful to explore diverse bases at 1′ and its utility was demonstrated via the syntheses of nucleosides 9 and 11. The 2′-β-methyl-1′,2′-cyclopentyl ribonucleoside scaffold was exemplified via a C-nucleoside which was prepared using a RCM reaction as the key step leading to novel nucleoside 35.     

Synthesis,characterization, and crystal structure of 2′,2″-dihydroxy-3,3‴-bichalcone and its related chalcone–flavanone and biflavanone analogs

The synthesis of 3,3′-diformyl-1,1′-biphenyl 1 and its reaction in basic media with 2′-hydroxyacetophenone and paeonol, respectively, are described. The products, a bichalcone 2, it’s partially cyclized analog, a flavanone–chalcone 3 and the fully cyclized biflavanone 4, are reported as products for the first reaction, while only the chalcone 5 for the second. The crystalline and molecular structure, solved by X-ray diffraction analysis, for compounds 2 and 5 are also presented in this work.