Hydrotalcite catalysis for the synthesis of new chiral building blocks

The use of hydrotalcites for the synthesis of two chiral building blocks in a simple way is described as a new and green methodology. The synthesis of these compounds implies a regioselective Baeyer–Villiger reaction in a very selective way with ulterior opening and lactonisation. This methodology should be considered green for the use of hydrogen peroxide as the only oxidant and hydrotalcites as the catalyst, and because no residues are produced apart from water. The procedure is very adequate for using in gram scale, in order to increase the value of the obtained compounds. The conditions are excellent and can be applied for nonstable compounds, as they are very mild. The synthesised compounds are magnific starting materials for the synthesis of biologically active or natural compounds. The use of a cheap, commercial and chiral compound as carvone disposable in both enantiomeric forms adds an extra value to this methodology.     

Applications of chiral sulfoxides in enantioselective synthesis of diols and total synthesis of natural products

The sulfoxide mediated enantioselective reduction of carbonyl compounds was extended to the synthesis of enantiomerically pure syn and anti diols, including C₂ symmetric diols. Several applications related to the syntheses of natural products are described. Finally, this sulfoxide mediated enantioselective synthesis was extended to the transformation of ethyl oxalate or other oxalic acid derivatives to enantiomerically pure syn and anti 1,2‐diols. © 2002 Wiley Periodicals, Inc. Heteroatom Chem 13:443–452, 2002; Published online in Wiley Interscience (www.interscience.wiley.com). DOI 10.1002/hc.10079     

New uses of amino acids as chiral building blocks in organic synthesis

[reaction: see text] N,N-Dibenzylamino aldehydes have emerged as a highly useful class of chiral building blocks in synthetic organic chemistry. We envisioned the transformation of the N,N -dibenzylamino aldehydes to the corresponding aldimines followed by diastereoselective methylene transfer with a sulfonium ylide to obtain alpha-amino aziridines in high yields.     

Asymmetric synthesis of alkaloids using polyfunctionalized chiral building blocks

Enantiopure N‐sulfinyl‐δ‐amino‐β‐ketoesters and isoquinolones, prepared from sulfinimines (N‐sulfinyl imines) are a new class of polyfunctionalized chiral building blocks. These building blocks provide easy access to enantiomerically pure, functionalized piperidines and tetrahydroisoquinolines with a minimum of chemical manipulation and protecting‐group chemistry. © 2002 Wiley Periodicals, Inc. Heteroatom Chem 13:486–492, 2002; Published online in Wiley Interscience (www.interscience.wiley.com). DOI 10.1002/hc.10090     

Improved synthesis of the enantiomers of propafenone using chiral building blocks

Summary A short and efficient synthesis of the enantiomers of the antiarrhythmic drug propafenone (1) is described using both (R)-isopropylideneglycerol tosylate and (S)-glycidyl tosylate as chiral building blocks. The key step of the high yield synthesis is the acetalization of the carbonyl group in 1-(2-hydroxyphenyl)-3-phenyl-1-propanone (2) which allows application of mild reaction conditions in the subsequent alkylation of the phenolic hydroxy group.With our best wishes dedicated to Prof. Dr.H. Achenbach on the occasion of his 65^th birthday     

Preparation and use of enantioenriched 2-aryl-propylsulfonylbenzene derivatives as valuable building blocks for the enantioselective synthesis of bisabolane sesquiterpenes

We have demonstrated that different enantioenriched 2-arylpropylsufonylbenzene derivatives are very useful building blocks for the synthesis of aromatic bisabolane sesquiterpenes. Their preparation and the exploitation of their chemical reactivity have been comprehensively investigated. Accordingly, the naturally occurring bisabolane sesquiterpenes (−)-curcuphenol, (−)-xanthorrhizol, (+)-glandulone A, (+)-curcudiol, (+)-turmerone and (+)-curcudiol-10-one were synthesized in high enantiomeric purity. It is worth noting that the compounds (+)-curcudiol-10-one and (+)-glandulone A were prepared in enantioenriched form for the first time. Through the proposed synthetic approaches, we were able to confirm both chemical structures and the absolute configurations previously assigned to the two aforementioned sesquiterpenes.     

Aminobenzoates as building blocks for natural product assembly lines

The ortho-, meta-, and para- regioisomers of aminobenzoate are building blocks for a wide range of microbial natural products. Both the ortho-isomer (anthranilate) and PABA derive from the central shikimate pathway metabolite chorismate while the meta-isomer is not available by that route and starts from UDP-3-aminoglucose. PABA is largely funnelled into folate biosynthesis while anthranilate is the scaffold for biosynthetic elaboration into many natural heterocycles, most notably with its role in indole formation for tryptophan biosynthesis. Anthranilate is also converted to benzodiazepinones, fumiquinazolines, quinoxalines, phenoxazines, benzoxazolinates, quinolones, and phenazines, often with redox enzyme participation. The 5-hydroxy form of 3-aminobenzaote is the starter unit for ansa-bridged rifamycins, ansamitocins, and geldanamycins, whereas regioisomers 2-hydroxy, 4-hydroxy and 2,4-dihydroxy-3-aminobenzoate are key components of antimycin, grixazone, and platencin and platensimycin biosynthesis, respectively. The enzymatic mechanisms for generation of the aminobenzoate regioisomers and their subsequent utilization for diverse heterocycle and macrocycle construction are examined.     

Highly enantioselective synthesis of multifunctionalized allylic building blocks via oxazaborolidine-catalyzed borane reduction

A simple and convenient synthesis of optically active alkenyl β-hydroxy sulfides with high enantiomeric excess by CBS-oxazaborolidine-catalyzed borane reduction of the corresponding β-keto sulfides and its application to synthesis of chiral alkenic diols have been established.     

Catalytic enantioselective synthesis of secondary alkylboronate building blocks with and without metals

With or without you: Chiral secondary alkylboronates can now be accessed by highly enantioselective catalytic methods including conjugate addition under metal-free conditions with an NHC catalyst, and also iridium-catalyzed hydrogenation. These methods reinforce the potential of secondary alkylboronates as ideal and universal chiral building blocks for bond formation to sp(3) carbon atoms.     

Chemistry of epoxysulfones: straightforward synthesis of versatile chiral building blocks

[reaction: see text] Several chiral building blocks have been obtained easily in large quantities from an epoxysulfone (9) that could be obtained in both enantiomeric forms from accessible starting materials.     

Preparation of chiral building blocks for the enantioselective total synthesis of ent-kauranoids by the pig liver esterase-catalyzed asymmetric hydrolysis of a dialkyl malonate-type prochiral diester

The preparation of chiral building blocks, suitable for use in the enantioselective total synthesis of kauranoids and ent-kauranoids, is reported herein. The pig liver esterase-catalyzed asymmetric hydrolysis of dimethyl 3,3-dimethyl-2-methylenecyclohexane-1,2-dicarboxylate, a malonate-type prochiral diester, afforded the corresponding half-ester in 96% yield and with 99% enantiomeric excess. The absolute configuration of the half-ester was determined by X-ray crystallographic analysis of its derivative and its enantiodivergent transformations are also described herein.     

Glycosyl thioimidates as versatile building blocks for organic synthesis

This review discusses the synthesis and application of glycosyl thioimidates in chemical glycosylation and oligosaccharide assembly. Although glycosyl thioimidates include a broad range of compounds, the discussion herein centers on S-benzothiazolyl (SBaz), S-benzoxazolyl (SBox), S-thiazolinyl (STaz), and S-benzimidazolyl (SBiz) glycosides. These heterocyclic moieties have recently emerged as excellent anomeric leaving groups that express unique characteristics for highly diastereoselective glycosylation and help to provide a streamlined access to oligosaccharides.     

Asymmetric synthesis of a chiral building block for cyclopentanoids: a novel enantioselective synthesis of preclavulone A

A new asymmetric approach to the hydroxylactone (+)-(3aR,4R,6aS)-4-(hydroxymethyl)-3a,4-dihydro-3H-cyclopenta[b]furan-2(6aH)-one (1), a key synthetic building block for cis-1,2-disubstituted five-membered ring derivatives (i.e., isoprostanes, jasmonates, and clavulones), has been described. A remarkable control of the absolute and relative configuration of the three stereocenters was achieved. Thus, the use of the Trost's asymmetric allylic alkylation strategy secured highly enantioenriched (R)-3-(nitromethyl)cyclopent-1-ene (13), which was smoothly converted to (R)-cyclopent-2-enecarboxylic acid (15) in excellent yield and high enantiomeric purity (>98% ee). 6-exo-trig atom-transfer radical cyclizations of ((R)-cyclopent-2-enyl)methyl 2-iodoacetate (12) produced exclusively the desired cis-fused delta-lactone (4aR,7aR)-hexahydro-5-iodocyclopenta[c]pyran-3(1H)-one (11), which was subsequently elaborated to hydroxylactone 1 through a stereocontrolled Pd(II)-mediated lactonization reaction. En route to preclavulone A, a putative elusive intermediate in the biosynthesis of clavulones, the synthetic utility of compound 1 was demonstrated. The key feature in this synthesis was the installation of the lower side chain via the Knochel organozinc sp3-sp C-C coupling protocol.     

Synthesis of chiral building blocks for cephalostatin analogues

The present paper describes the synthesis of 5‐azido‐6‐ketones (14) and 6‐hydroxy‐5‐ketone (20) from Hajos Wiechert ketone as chiral building blocks for cephalostatin analogues. The synthesis of symmetric cephalostatin analogue from 6‐hydroxy‐5‐ketone has also been reported. The characterization of the each synthesized compounds was carried out by IR, ¹H‐NMR, ¹³C‐NMR and High resolution Mass Spectrometry.     

Oxazolines as chiral building blocks for imidazolium salts and N-heterocyclic carbene ligands

Enantiomerically pure imidazolium triflates can be readily prepared from bioxazolines and oxazolineimines; deprotonation of imidazolium triflate 2 gives a chiral N-heterocyclic carbene that can act as a ligand in a catalytically active palladium complex.     

Regioselective synthesis of gentisyl alcohol-type marine natural products

Gentisyl alcohol-type natural products, possessing various important biological properties, have been synthesized from 4-methoxyphenol by using a selective phenol monohydroxymethylation/monochlorination, a CAN oxidation and a sodium dithionite reduction as the key steps. The natural product synthesis is efficient, atom- and step-economical, and requires no protecting groups.     

Synthesis of 5-aminothiazoles as building blocks for library synthesis

A convenient route to 4-phenyl-5-aminothiazoles is described, which offers control over substitution at the 2-position. 2-N-Acylglycinamides were dithionated and a subsequent TFAA-mediated cyclisation step was followed by removal of the 5-N-trifluoroacetyl group providing the free amines. Though applicable generally the method was found to be most effective when introducing aromatic substituents at the 2-position, whereupon moderate overall yields of the 5-amino compounds were obtained.     

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.     

Chiral oxazolopiperidone lactams: versatile intermediates for the enantioselective synthesis of piperidine-containing natural products

Phenylglycinol-derived oxazolopiperidone lactams are exceptionally versatile building blocks for the enantioselective construction of structurally diverse piperidine-containing natural products and bioactive compounds. These lactams are readily available in both enantiomeric series by cyclocondensation of the chiral amino alcohol with a delta-oxo acid derivative and allow the substituents to be introduced at the different ring positions in a regio- and stereocontrolled manner, providing access to enantiopure polysubstituted piperidines bearing virtually any type of substitution pattern, and also quinolizidines, indolizidines, perhydroquinolines, hydroisoquinolines, as well as complex indole alkaloids. Of particular interest are cyclocondensation reactions with racemic or prochiral delta-oxo (di)acid derivatives in processes involving dynamic kinetic resolution and/or differentiation of enantiotopic or diastereotopic ester groups, as they directly lead to lactams that already incorporate the carbon substituents on the heterocyclic ring. The use of (S)-3,4-dimethoxyphenylalaninol or (S)-tryptophanol in the above cyclocondensation reactions expands the potential and the scope of the methodology, providing a straightforward route to enantiopure benzo[a]- and indolo[2,3-a]quinolizidines.