Total synthesis and determination of the absolute configuration of guadinomines B and C2

This article describes the determination of the absolute configurations of the guadinomines, which are novel cyclic guanidyl natural products that are inhibitors of the type III secretion system (TTSS) of bacteria. Any compound that interrupts the TTSS of bacteria is potentially an ideal anti-infectious drug. The reliable asymmetric synthesis of guadinomines has revealed their absolute configurations, which could not have been defined without this synthetic approach. Our report not only describes the asymmetric total synthesis of the title compounds, but also demonstrates the novel concise synthesis of tri-substituted piperazinone cores as optically pure forms. The novel feature of our method is an intramolecular S(N)2 cyclization that uses PPh(3) and I(2) to construct the unique 5-membered cyclic guanidine substructure.     

Density functional study on the mechanism of bicyclic guanidine-catalyzed Strecker reaction

As a direct and viable synthesis of amino acids, the small organic molecule catalyzed asymmetric Strecker reactions have been explored successfully in recent years. For these catalysts, the active sites may be a guanidine group or similarly a urea group. In an effort to elucidate the reaction mechanism, we have investigated the bicyclic guanidine-catalyzed Strecker reaction of HCN and methanimine using density functional theory with the B3LYP method. Assisted by guanidine, two competitive pathways to aminoacetonitrile were rationalized. The aminoisoacetonitrile may not form due to the instability of the product.     

Total synthesis of (-)-galanthamine and (-)-lycoramine via catalytic asymmetric hydrogenation and intramolecular reductive Heck cyclization

A synthetic strategy featuring efficient ruthenium-catalyzed asymmetric hydrogenation of racemic α-aryloxy cyclic ketone via dynamic kinetic resolution and palladium-catalyzed intramolecular reductive Heck cyclization has been developed for the asymmetric total synthesis of (-)-galanthamine (20.1%, 12 steps) and (-)-lycoramine (40.2%, 10 steps).     

Total synthesis of an antitumor antibiotic,Fostriecin (CI-920)

The total synthesis of an antitumor antibiotic, fostriecin (CI-920), via a highly convergent route is described. A characteristic feature of the present total synthesis is that the synthesis was achieved via a coupling procedure of three segments A, B, and C. The unsaturated lactone moiety of fostriecin, corresponding to segment A, was constructed from a known Horner-Emmons reagent, and the stereochemistry of the C-5 position was introduced by asymmetric reduction with (R)-BINAl-H. Segment B having a series of stereogenic centers was synthesized from (R)-malic acid and the stereogenic centers at the C-8 and C-9 positions were prepared by a combination of Wittig reaction and Sharpless asymmetric dihydroxylation reaction. The conjugated Z,Z,E-triene moiety of fostriecin, corresponding to segment C, was eventually constructed by Wittig reaction and Stille coupling reaction. The phosphate moiety, which is known to be essentially important for the antitumor activity, was introduced via two routes: (i) direct phosphorylation of the monohydroxyl derivative in which other hydroxyl groups are protected with silyl groups; (ii) cyclic phosphorylation and selective cleavage of the cyclic phosphate derivative. Although the former route is basically the same as those reported by other groups, the latter route is novel and more effective than the former one. The present total synthesis would serve as a versatile synthetic route to not only fostriecin, but also its various analogues including stereoisomers.     

Asymmetric aziridination of cyclic enones using chiral diamine catalysts and its application to the total synthesis of (-)-agelastatin A

The asymmetric aziridination of cyclic enones with N-tosyloxycarbamates, using N-neopentyl 1,2-diphenylethylenediamine as a catalyst, and its application to the formal total synthesis of (-)-agelastatin A, using a one-pot silylation-selenylation procedure and the regioselective aziridine-opening by an azide anion as key steps, are described.     

Asymmetric synthesis of (2S,3R)-capreomycidine and the total synthesis of capreomycin IB

A 27 step total synthesis of the tuberculostatic macrocyclic peptide antibiotic capreomycin IB has been accomplished. The synthesis features the use of an enolate-aldimine condensation between a chiral glycine aluminum enolate and the benzyl imine of 3-tert-butyldimethylsiloxy-propanal as a means of preparing the cyclic guanidine amino acid (2S,3R)-capreomycidine. Additionally, a Hofmann rearrangement was exacted on a late-stage pentapeptide in order to transform an asparagine residue into a diaminopropanoic acid residue.     

Stereoselective total synthesis of arenastatin A, a spongean cytotoxic depsipeptide

A highly stereoselective total synthesis of arenastatin A, an extremely potent cytotoxic cyclic depsipeptide from marine sponge, was developed. The desired 7,8-β-epoxide in arenastatin A was constructed by asymmetric sulfur ylide-mediated epoxidation in good yield and highly stereoselective manner.     

Total synthesis of (+)-laurencin: an asymmetric alkylation-ring-closing metathesis approach to medium ring ethers

[formula: see text] The enantioselective total synthesis of (+)-laurencin 1 is achieved in 18 steps from (S)-(+)-4-benzyl-3-benzyloxyacetyl-2-oxazolidinone. The key steps in this synthesis are an asymmetric glycolate alkylation leading to acyl oxazolidinone 2 and a subsequent ring-closing olefin metathesis to construct the oxocene core of 1. The approach to medium ring ethers utilized in this synthesis provides a general and efficient route to the cyclic core of other marine natural products.     

Total synthesis of (+)-batzelladine A and (-)-batzelladine D, and identification of their target protein

Asymmetric total synthesis of batzelladine A (1) and batzelladine D (2) has been achieved. Our synthesis of batzelladines features 1) stereoselective construction of the cyclic guanidine system by means of successive 1,3-dipolar cycloaddition reaction and subsequent cyclization, 2) direct esterification of the bicyclic carboxylic acid 35 with the guanidine alcohol 8 or 59 to construct the whole carbon skeleton of batzelladines, and 3) one-step formation of the alpha,beta-unsaturated aldehyde 53 from the primary alcohol 47 with tetra-n-propylammoniumperruthenate (TPAP), providing an efficient route to the left-hand bicyclic guanidine alcohol of batzelladine A (1). With the synthetic compounds 1 and 2 in hand, their target protein was examined by using immobilized CD4 and gp120 affinity gels. The results indicated that batzelladines A (1) and D (2) bind specifically to CD4.     

Enantioselective total syntheses of (-)-taiwaniaquinone H and (-)-taiwaniaquinol B by iridium-catalyzed borylation and palladium-catalyzed asymmetric α-arylation

We report a concise, enantioselective total synthesis of (-)-taiwaniaquinone H and the first enantioselective total synthesis of (-)-taiwaniaquinol B by a route that includes asymmetric palladium-catalyzed α-arylation of a ketone with an aryl bromide that was generated by sterically controlled halogenation via iridium-catalyzed C-H borylation. This asymmetric α-arylation creates the benzylic quaternary stereogenic center present in the taiwaniaquinoids. The synthesis was completed efficiently by developing a Lewis acid-promoted cascade to construct the [6,5,6] tricyclic core of an intermediate common to the synthesis of a number of taiwaniaquinoids. Through the preparation of these compounds, we demonstrate the utility of constructing benzylic quaternary stereogenic centers, even those lacking a carbonyl group in the α-position, by asymmetric α-arylation.     

Enantioselective synthesis of cyclic amides and amines through mo-catalyzed asymmetric ring-closing metathesis

First, an efficient method for the synthesis of optically enriched N-fused bicyclic structures is reported. Through Mo-catalyzed desymmetrization of readily available achiral polyene substrates, 5,6-, 5,7-, and 5,8-bicyclic amides can be synthesized in up to >98% ee. The effects of catalyst structure, olefin substitution, positioning of Lewis basic functional groups and ring size are examined and discussed in detail. In the second phase of investigations, a catalytic asymmetric method for highly enantioselective (up to 97% ee) synthesis of small- and medium-ring unsaturated cyclic amines is reported; optically enriched products bear a secondary amine or a readily removable Cbz or acetamide unit. Regio- and diastereoselective functionalizations of olefins within the optically enriched amine products have been carried out. Both catalytic asymmetric methods include transformations that lead to the formation of trisubstituted as well as disubstituted cyclic alkenes. The protocols outlined herein afford various cyclic amines of high optical purity; such products are not easily accessed by alternative protocols and can be used in enantioselective total syntheses of biologically active molecules.     

Asymmetric total synthesis of the antimalarial drug (+)-mefloquine hydrochloride via chiral N-amino cyclic carbamate hydrazones

Mefloquine hydrochloride is an important antimalarial drug. It is currently manufactured and administered in racemic form; however there are indications regarding the biological activity of the two enantiomers that suggest the superiority of the (+)-form. The asymmetric total synthesis of the (+)-enantiomer of mefloquine hydrochloride is described. The key asymmetric transformation utilized is a novel asymmetric Darzens reaction of a chiral α-chloro-N-amino cyclic carbamate hydrazone derived from an N-amino cyclic carbamate (ACC) chiral auxiliary.     

A General Entry to Antifeedant Sesterterpenoids: Total Synthesis of (+)‐Norleucosceptroid A, (−)‐Norleucosceptroid B,and (−)‐Leucosceptroid K

The first asymmetric total synthesis of the antifeedant terpenoids (+)‐norleucosceptroid A, (−)‐norleucosceptroid B, and (−)‐leucosceptroid K has been accomplished. This highly concise synthetic route was guided by our efforts to develop a platform for the collective synthesis of a whole family of antifeedant natural products. The synthesis features a Hauser–Kraus‐type annulation followed by an unprecedented, highly efficient intramolecular dilactol aldol‐type condensation reaction to produce the 5,6,5 skeleton. The developed synthetic route proceeds for norleucosceptroid A and B in 16 steps (longest linear sequence) from known compounds.     

A General Entry to Antifeedant Sesterterpenoids: Total Synthesis of (+)‐Norleucosceptroid A, (−)‐Norleucosceptroid B,and (−)‐Leucosceptroid K

The first asymmetric total synthesis of the antifeedant terpenoids (+)‐norleucosceptroid A, (?)‐norleucosceptroid B, and (?)‐leucosceptroid K has been accomplished. This highly concise synthetic route was guided by our efforts to develop a platform for the collective synthesis of a whole family of antifeedant natural products. The synthesis features a Hauser–Kraus‐type annulation followed by an unprecedented, highly efficient intramolecular dilactol aldol‐type condensation reaction to produce the 5,6,5 skeleton. The developed synthetic route proceeds for norleucosceptroid A and B in 16 steps (longest linear sequence) from known compounds.     

Enantioselective synthesis of (−)-α-conhydrine via cyclic sulfate methodology

An asymmetric synthesis of (−)-α-conhydrine is described using the Sharpless asymmetric dihydroxylation and the regiospecific nucleophilic opening of a cyclic sulfate as the key steps.     

Acyclic and cyclic aminophosphonic acids: asymmetric syntheses mediated by chiral sulfinyl auxiliary

Aminophosphonic acids have become increasingly important in different fields of chemistry, medicine and agriculture. This account outlines the results obtained in the author’s laboratory on the asymmetric synthesis of acyclic and cyclic aminophosphonic acids mediated by chiral sulfinyl auxiliary. A key reaction in the synthesis of enantiopure α- and β-aminoalkanephosphonic acids involving a highly diastereoselective addition of phosphite anions or α-phosphonate carbanions to enantiopure sulfinimines is discussed. The asymmetric cyclopropanation of enantiopure α-phosphorylvinyl sulfoxides with sulfur ylides is presented as a platform for developing a new approach to optically active β-aminocyclopropanephosphonic acids. It is exemplified by the total synthesis of enantiopure β-amino-γ-phenylcyclopropanephosphonic acid - a constrained analogue of the GABA_B antagonist phaclofen.     

Development of a strategy for the asymmetric synthesis of polycyclic polyprenylated acylphloroglucinols via N-amino cyclic carbamate hydrazones: application to the total synthesis of (+)-clusianone

A broadly applicable asymmetric synthetic strategy utilizing N-amino cyclic carbamate alkylation that provides access to the various stereochemical permutations of a common structural motif found in many polycyclic polyprenylated acylphloroglucinols is described. The utility of this methodology is demonstrated through the first asymmetric total synthesis of the antiviral agent (+)-clusianone.     

Tandem enantioselective conjugate addition--cyclopropanation. Application to natural products synthesis

A tandem asymmetric conjugate addition-cyclopropanation was developed, in which a cyclic or linear enone was converted to a TMS-protected 3-substituted-cyclopropanol in an efficient one-pot reaction. These compounds were then selectively cleaved to yield alpha-methyl-beta-alkyl ketones, alpha-methylene-enones, or chain extended gamma-alkyl-enones. This methodology was applied to the formal total synthesis of (-)-(S,S)-clavukerin A and (+)-(R,S)-isoclavukerin.     

Enantioselective synthesis of marine indole alkaloid hamacanthin B

An enantioselective total synthesis of hamacanthin B (1) is described. This synthesis is based on the asymmetric synthesis of (S)-2-azido-(indol-3-yl)ethylamine 7, which is coupled with the 3-indolyl-alpha-oxoacetyl chloride 8 and subsequently used in a successful intramolecular Staudinger-aza Wittig cyclization to form the central dihydropyrazinone ring. The stereochemistry of naturally isolated hamacanthin B is revealed as the (S)-configuration.     

Progress toward the total synthesis of callipeltin A (I): asymmetric synthesis of (3S,4R)-3,4-dimethylglutamine

[reaction:see text] During the total synthesis of the novel cyclic depsipeptide callipeltin A (1), the unit (3S,4R)-3,4-dimethylglutamine, was successfully synthesized by asymmetric Michael addition and subsequent electrophilic azidation. The key feature of this approach is the generation of three adjacent stereogenic centers using the same camphorsultam chiral auxiliary.