Synthesis of PPAR agonist via asymmetric hydrogenation of a cinnamic acid derivative and stereospecific displacement of (S)-2-chloropropionic acid

The synthesis of the peroxime proliferator activated receptor (PPAR) alpha,gamma-agonist (1) was accomplished with high enantio- and diastereoselectivity by employing an asymmetric hydrogenation strategy, of an alpha-alkoxy cinnamic acid derivative, to set the C-2 chiral center. A diastereospecific S(N)2 displacement under mild basic conditions established the C-10 stereochemistry without any detectable racemization of the two epimerizable chiral centers.     

Synthesis and Kinetic Evaluation of Inhibitors of the Phosphatidylinositol-Specific Phospholipase C from Bacillus cereus

Substrate analogues of phosphatidylinositol (1) were synthesized and evaluated as potential inhibitors of the bacterial phosphatidylinositol-specific phospholipase C (PI-PLC) from Bacillus cereus. The chiral analogues of the water-soluble phospholipid substrate 5 were designed to probe the effects of varying the inositol C-2 hydroxyl group, which is generally believed to serve as the nucleophile in the first step of the hydrolysis of phosphatidylinositols by PI-PLC. In the analogues 6-9, the C-2 hydroxyl group on the inositol ring of the phosphatidylinositol derivatives was rationally altered in several ways. Inversion of the stereochemistry at C-2 of the inositol ring led to the scyllo derivative 6. The inositol C-2 hydroxy group was replaced with inversion by a fluorine to produce the scyllo-fluoro inositol 7 and with a hydrogen atom to furnish the 2-deoxy compound 8. The C-2 hydroxyl group was O-methylated to prepare the methoxy derivative 9. The natural inositol configuration at C-2 was retained in the nonhydrolyzable phosphorodithioate analogue 10. The inhibition of PI-PLC by each of these analogues was then analyzed in a continuous assay using D-myo-inositol 1-(4-nitrophenyl phosphate) (25) as a chromogenic substrate. The kinetic parameters for each of these phosphatidylinositol derivatives were determined, and each was found to be a competitive inhibitor with K(i)'s as follows: 6, 0.2 mM; 10, 0.6 mM; 8, 2.6 mM; 9, 6.6 mM; and 7, 8.8 mM. This study further establishes that the hydrolysis of phosphatidylinositol analogues by bacterial PI-PLC requires not only the presence of a C-2 hydroxyl group on the inositol ring, but the stereochemistry at this position must also correspond to the natural myo-configuration. For future inhibitor design, it is perhaps noteworthy that the best inhibitors 6 and 10 each possess a hydroxyl group at the C-2 position. Several of the inhibitors identified in this study are now being used to obtain crystallographic information for an enzyme-inhibitor complex to gain further insights regarding the mechanism of hydrolysis of phosphatidylinositides by this PI-PLC.     

Asymmetric synthesis of all the known phlegmarine alkaloids

The asymmetric synthesis of all four of the known natural phlegmarines and one synthetic derivative has been accomplished in 19-22 steps from 4-methoxy-3-(triisopropylsilyl)pyridine. Chiral N-acylpyridinium salt chemistry was used twice to set the stereocenters at the C-9 and C-2' positions of the phlegmarine skeleton. Key reactions include the use of a mixed Grignard reagent for the second N-acylpyridinium salt addition, zinc/acetic acid reduction of a complex dihydropyridone, and a von Braun cyanogen bromide N-demethylation of a late intermediate. These syntheses confirmed the absolute stereochemistry of all of the known phlegmarines.     

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.     

Synthesis of novel cyclic oligosaccharides: beta-1,6-thio-linked cycloglucopyranosides

[structure: see text] A protocol for the synthesis of novel cyclic beta-1,6-S-linked glucopyranosides is developed. The key intermediate is a linear thiooligosaccharide bearing an iodo group at C-6 of the nonreducing sugar and a thioacetyl group at the anomeric center of the reducing end sugar. The crucial macrocyclization step was achieved through base-promoted intramolecular S(N)2 glycosylation in remarkably high yields (92-95%) and with well-controlled stereochemistry.     

Structural, mechanistic, and computational analysis of the effects of anomeric fluorines on anomeric fluoride departure in 5-fluoroxylosyl fluorides

The effects of fluorine substitution at the C-5 center of pyranosyl fluorides on the reactivity at the C-1 anomeric center was probed by studying a series of 5-fluoroxylosyl fluoride derivatives. X-ray structures of their per-O-acetates detailed the effects on the ground-state structures. First-order rate constants for spontaneous hydrolysis, in conjunction with computational studies, revealed that changes in the stereochemistry of the 5-fluorine had minimal effects on the solvolysis rate constants and that the observed rate reductions were broadly similar to those caused by additional fluorine substitution at C-1 but significantly less than those due to substitution at C-2. Differences in the trapping behavior of 5- versus 2-fluoro-substituted glycosyl fluorides with α- and β-glycosidases arise more from differences in steric effects and hydrogen-bonding interactions than from intrinsic reactivity differences.     

First asymmetric synthesis of 3-alkoxycarbonyl-2-amino-4-aryl-4H-naphtho[1,2-b]pyrans

The first asymmetric synthesis of 3-alkoxycarbonyl-2-amino-4-aryl-4H-naphtho[1,2-b]pyrans, by Michael addition of 1-naphthol to chirally modified arylidenecyanoacetates 6 and 7 , is described. Good yields and low diastereomeric excesses have been obtained in the 1,4-conjugate additions. The absolute stereochemistry at C-4 in major isomers of pyrans 8 and 9 has been assigned as 5 by X-ray analysis of major pyran 8 .     

Total synthesis and biological evaluation of the protein phosphatase 2A inhibitor cytostatin and analogues

The total synthesis of the natural product cytostatin is described which inhibits protein phosphatase 2A. Cytostatin has anti-metastatic properties and induces apoptosis. On the basis of this synthesis the relative and absolute configuration of cytostatin could be assigned. Key structural elements of cytostatin are an alpha,beta-unsaturated lactone group and a side chain embodying a phosphate and a rather unstable (Z,Z,E)-triene subunit. In addition, the natural product carries six stereocenters. For the construction of the stereocenters reagent-controlled transformations were used in order to ensure maximum stereochemical flexibility. The Evans syn-aldol reaction was chosen to establish the stereochemistry at C-4, C-5, C-9 and C-10; C-6 was introduced by means of the Evans asymmetric alkylation. In all cases the same chiral auxiliary was employed as stereodirecting group. The stereocenter at C-11 was established by an asymmetric reduction using CBS-oxazaborolidine. Temporary protection of the phosphate group was achieved best by using the base-labile 9-fluorenylmethyl group, which could be cleanly cleaved by an excess of triethylamine; this reaction yielded analytically pure phosphates after a simple aqueous work-up. The (Z,Z,E)-triene embodied in cytostatin was synthesized by means of a Stille coupling as key transformation. The synthesis sequence established in this way readily gave access to a series of analogues with simplified structure. Initial biological testing of these analogues proved that the alpha,beta-unsaturated lactone, the C-11-hydroxy group and a fully deprotected phosphate moiety at C-9 are essential for the PP2A-inhibitory activity of cytostatin. The rather unstable triene moiety in the side chain can be replaced by other lipophilic residues with only moderate decrease of biological activity. Other phosphatases, that is, PP1, VHR, PTP1B, CD45, were not inhibited by cytostatin or any of the analogues, demonstrating the high selectivity of this compound. These findings will be useful for the design and synthesis of cytostatin-derived chemical tools for the study of biological processes influenced by PP2A.     

Synthesis and biological evaluation of Rhizobium sin-1 lipid A derivatives

A highly convergent strategy for the synthesis of several derivatives of the lipid A of Rhizobium sin-1 has been developed. The approach employed the advanced intermediate 3-O-acetyl-6-O-(3-O-acetyl-4,6-O-benzylidene-2-deoxy-2-phthalimido-beta-d-glucopyrano-syl)-2-azido-4-O-benzyl-2-deoxy-1-thio-alpha-d-glucopyranoside (5), which is protected in such a way that the anomeric center, the C-2 and C-2' amino groups, and the C-3 and C-3' hydroxyls can be selectively functionalized. The synthetic strategy was used for the preparation of 2-deoxy-6-O-[2-deoxy-3-O-[(R)-3-hydroxy-hexadecanoyl]-2-[(R)-3-octacosanoyloxy-hexadecan]amido-beta-d-glucopyranosyl]-2-[(R)-3-hydroxy-hexadecan]amido-3-O-[(R)-3-hydroxy-hexadecanoyl]-alpha-d-glucopyranose (11) and 2-deoxy-6-O-[2-deoxy-3-O-[(R)-3-hydroxy-hexadecanoyl]-2-[(R)-3-octacosanoyloxy-hexadecan]amido-beta-d-glucopyranosyl]-2-[(R)-3-hydroxy-hexadecan]amido-3-O-[(R)-3-hydroxy-hexadecanoyl]-d-glucono-1,5-lactone (13), which contain an unusual octacosanoic acid moiety and differ in the oxidation state of the anomeric center. The results of biological studies indicate that 11 and 13 lack the proinflammatory effects of Escherichia coli lipopolysaccharides (LPS). Furthermore, 13 emulated the ability of heterogeneous R. sin-1 LPS to antagonize enteric LPS, providing evidence for the critical role of the gluconolactone moiety of R. sin-1 LPS in mediating this antagonistic effect. Compound 13 is the first example of a lipid A derivative that is devoid of phosphate but possesses antagonistic properties, making it an attractive lead compound for development of a drug to use in the treatment of Gram-negative septicemia.     

Total synthesis of the epidermal growth factor inhibitor (-)-reveromycin B

The total synthesis of the epidermal growth factor inhibitor reveromycin B (2) in 25 linear steps from chiral methylene pyran 13 is described. The key steps involved an inverse electron demand hetero-Diels-Alder reaction between dienophile 13 and diene 12 to construct the 6,6-spiroketal 11 which upon oxidation with dimethyldioxirane and acid catalyzed rearrangement gave the 5,6-spiroketal aldehyde 9. Lithium acetylide addition followed by oxidation/reduction and protective group manipulation provided the reveromycin B spiroketal core 8 which was converted into the reveromycin A (1) derivative 6 in order to confirm the stereochemistry of the spiroketal segment. Introduction of the C1-C10 side chain began with sequential Wittig reactions to form the C8-C9 and C7-C6 bonds, and a tin mediated asymmetric aldol reaction installed the C4 and C5 stereocenters. The final key steps to the target molecule 2 involved a Stille coupling to introduce the C21-C22 bond, succinoylation, selective deprotection, oxidation, and Wittig condensation to form the final C2-C3 bond. Deprotection was effected by TBAF in DMF to afford reveromycin B (2) in 72% yield.     

Studies toward the synthesis of pinolidoxin, a phytotoxic nonenolide from the fungus Ascochyta pinodes. Determination of the configuration at the C-7, C-8, and C-9 chiral centers and stereoselective synthesis of the C(6)-C(18) fragment

The absolute stereochemistry at the C-7, C-8, and C-9 chiral centers of pinolidoxin (1) has been determined by chemical and spectral methods. First, the synthesis of four stereoisomeric fully benzoylated 2,3-erythro-1,2,3,4-heptanetetrols, corresponding to the C(6)-C(18) portion of the natural substance, has been accomplished starting from meso-tartaric acid. As next step, the selection of the synthetic tetrabenzoate possessing "natural" stereochemistry (10a'), suitable for absolute configuration determination, has been carried out by correlation with its "natural" homologue derived from degradation of pinolidoxin. Determination of the stereochemistry at the title chiral centers has been carried out by application of the Mosher's method both to 7a', a compound stereochemically related to 10a', and to pinolidoxin itself. The stereoselective synthesis of a protected form of the C(6)-C(18) portion of pinolidoxin, to be used in its total synthesis, has also been accomplished starting from commercially available D-erythronolactone.     

Promiscuity of carbonic anhydrase II. Unexpected ester hydrolysis of carbohydrate-based sulfamate inhibitors

Carbonic anhydrases (CAs) are enzymes whose endogenous reaction is the reversible hydration of CO(2) to give HCO(3)(-) and a proton. CA are also known to exhibit weak and promiscuous esterase activity toward activated esters. Here, we report a series of findings obtained with a set of CA inhibitors that showed quite unexpectedly that the compounds were both inhibitors of CO(2) hydration and substrates for the esterase activity of CA. The compounds comprised a monosaccharide core with the C-6 primary hydroxyl group derivatized as a sulfamate (for CA recognition). The remaining four sugar hydroxyl groups were acylated. Using protein X-ray crystallography, the crystal structures of human CA II in complex with four of the sulfamate inhibitors were obtained. As expected, the four structures displayed the canonical CA protein-sulfamate interactions. Unexpectedly, a free hydroxyl group was observed at the anomeric center (C-1) rather than the parent C-1 acyl group. In addition, this hydroxyl group is observed axial to the carbohydrate ring while in the parent structure it is equatorial. A mechanism is proposed that accounts for this inversion of stereochemistry. For three of the inhibitors, the acyl groups at C-2 or at C-2 and C-3 were also absent with hydroxyl groups observed in their place and retention of stereochemistry. With the use of electrospray ionization-Fourier transform ion cyclotron resonance-mass spectrometry (ESI-FTICR-MS), we observed directly the sequential loss of all four acyl groups from one of the carbohydrate-based sulfamates. For this compound, the inhibitor and substrate binding mode were further analyzed using free energy calculations. These calculations suggested that the parent compound binds almost exclusively as a substrate. To conclude, we have demonstrated that acylated carbohydrate-based sulfamates are simultaneously inhibitor and substrate of human CA II. Our results suggest that, initially, the substrate binding mode dominates, but following hydrolysis, the ligand can also bind as a pure inhibitor thereby competing with the substrate binding mode.     

Synthetic Studies on the Incorporation of N-Acetylallosamine in Hyaluronic Acid-Inspired Thiodisaccharides

Two approaches for the synthesis of the thiodisaccharide β-S-GlcA(1→3)β-S-AllNAc are described here. The target disaccharide was a C-3 epimer and thio-analogue of the hyaluronic acid repetitive unit, tuned with a thiopropargyl anomeric group for further click conjugation. Thus, we analysed and tested two convenient sequences, combining the two key steps required to introduce the thioglycosidic bonds and consequently reach the target molecule: the S_N2 substitution of a good leaving group (triflate) present at C-3 of a GlcNAc derivative and the introduction of the anomeric thiopropargyl substituent. The use of a 2-azido precursor showed to be a convenient substrate for the S_N2 step. Nevertheless, further protecting group manipulation and the introduction of the thiopropargyl anomeric residue were then required. This approach showed to provide access to a variety of thiodisaccharide derivatives as interesting building blocks for the construction of neoglycoconjugates.     

Total synthesis and NMR investigations of cylindramide

Cylindramide (1) was built up from three components: a hydroxyornithine derivative 7, a tetrazolylsulfone 8, and a substituted pentalene subunit 9. Derivative 7 was prepared in a six-step reaction sequence involving the Wittig reaction and a Sharpless asymmetric dihydroxylation starting from N-Boc-3-aminopropanal (12). Tetrazolylsulfone 8 was accessible in four steps from dioxinone 22. The synthesis of the pentalene fragment 9 started from cycloocta-1,5-diene 26, that was converted into enantiopure bicyclo[3.3.0]octanedione 29. The latter was functionalized to give derivative 9. The total synthesis was accomplished by inducing C-C bond formation by Sonogashira coupling of derivatives 9 and 7 followed by olefination with tetrazolylsulfone 8 under Julia-Kocienski conditions, macrocyclization, and subsequent Lacey-Dieckmann condensation to form the tetramic acid unit. As indicated by extensive 1H and 13C NMR spectroscopic investigations (DQF-COSY, ROESY spectra), the stereochemistry of synthetic cylindramide (1) corresponds with that of the naturally occurring product. ROE data were used for molecular modeling of the lowest-energy structures for cylindramide.     

A synthetic study on antiviral and antioxidative chromene derivative

An efficient synthesis of the antiviral and antioxidative chromene (1) was achieved. A small amount of chromene 1 could be derived from plastoquinones 2 and 3, the major constituents of the brown alga, Sargassum micracanthum. By the following synthetic scheme involving its application, many kinds of analogs can be synthesized for evaluation of their biological activity and mechanistic study. The total synthesis of 1, started from geranyl acetate and protected 2-bromo-6-methylhydroquinone, was executed with Sharpless asymmetric dihydroxylation for introduction of the terminal diol system and base-catalyzed sigmatropic rearrangement for construction of the chromene skeleton as the crucial steps. The stereochemistry at C-11' was reconfirmed by this synthesis.     

Ring-opening Sn2'' reactions of 7-oxanorbornenes by organolithium reagents. Regio- and stereospecific synthesis of substituted cyclohexenediols

The nucleophilic Sn2' bridge opening of 7-oxabicyclo[2.2.1] hept-5-en-2-ols with organolithium reagents occurs in a regio- and stereospecific fashion to produce 6-substituted-cyclohex-4-en-1,3-diols, regardless of the stereochemistry at C-2. A free alcohol functionality is necessary to attain complete regiocontrol of the process. The methodology is utilized to prepare an optically pure cyclohexene derivative, (+)-(1S,3S,6R)-6-n-butyl-3-methyl-cyclohex-4-en-1,3-diol (5b), as a model system.     

de novo asymmetric synthesis of daumone via a palladium-catalyzed glycosylation

The enantioselective syntheses of daumone and two analogues have been achieved in seven to eight steps. This route relies upon a diasteroselective palladium-catalyzed glycosylation reaction for the formation of the anomeric bond. The asymmetry of the sugar and aglycone portion of daumone were introduced by Noyori reduction of an acylfuran and a propargyl ketone. A highly diastereoselective epoxidation and reductive ring opening established the desired C-2 and C-4 stereochemistry of daumone. [reaction: see text]     

Determination of the stereochemistry of anhydroerythromycin A, the principal degradation product of the antibiotic erythromycin A

Anhydroerythromycin A arises from the acid-catalysed degradation of erythromycin A both in vitro and in vivo. It has negligible antibacterial activity, but inhibits drug oxidation in the liver, and is responsible for unwanted drug-drug interactions. Its structure has 18 chiral centres common with erythromycin A, but C-9 (the spiro carbon) is also chiral in anhydroerythromycin and its stereochemistry has not previously been reported; both 9R- and 9S-anhydroerythromycin A are plausible structures. An understanding of the chirality at C-9 was expected to throw light on the mechanism of acid-catalysed degradation of erythromycin A, a subject that has been debated in the literature over several decades.We now report a determination of the three-dimensional structure of anhydroerythromycin A, including the stereochemistry at C-9, by NMR and molecular modelling. In parallel, the relative stereochemistry of anhydroerythromycin A 2'-acetate was determined by X-ray crystallography. Both compounds were shown to have 9R stereochemistry, and anhydroerythromycin A exhibited considerable conformational flexibility in solution.     

Formal total synthesis of (−)-hamigeran B from a chemo-enzymatically prepared building block with quaternary chiral center

A formal total synthesis of (?)-hamigeran B was achieved in 17 steps from commercially available ethyl 2-oxocyclopentanecarboxylate. Carbonyl reductase-catalyzed asymmetric reduction and the subsequent chemical transformations furnished an enantiomerically pure synthetic intermediate, (R)-5-formyl-2-isopropyl-5-methylcyclopent-1-en-1-yl trifluoromethylsulfonate. Suzuki-Miyaura coupling with Gao's arylboronate [2-(2-formyl-3-methoxy-5-methylphenyl)-4,4,5,5-tetramethyl-1,3,2-dioxaborolane], under PdCl₂(dppf)?CH₂Cl₂ catalysis, and the subsequent cyclization by way of intramolecular reductive SmI₂-mediated 1,2-diol formation provided a tricyclic skeleton with a tetrasubstituted double bond between C-1 and C-9b. Upon hydrogenation of this double bond, the proper stereochemistry of the remaining chiral centers was established. Exclusive addition of the hydrogen atom from the β-face occurred, owing to the shielding of the α-face with a bulky TBS protective group on the C-4 alcohol. The hydrogenation products were transformed into Clive's synthetic precursor for (?)-hamigeran B.     

1H and 13C NMR spectra of C-6 and C-9 substituted 3-azabicyclco[3.3.1]nonanes

The 1H and 13C NMR data for 3-azabicyclo[3.3.1]nonanes with OH and OMe substituents at C-6 and C-9 were measured using 1D (DEPT) and 2D (COSY, HSQC, HMBC, NOESY) experiments. Comparison of this NMR data illustrates the effects of stereochemistry and substitution at these positions.