Total synthesis and absolute configuration of malyngamide W

A concise enantioselective synthesis of malyngamide W (1) and its 2'-epimer was described. The strategy was based on three key steps: (1) ozonolysis of compound 11 which was derived from (R)-(-)-carvone 8, followed by copper-iron-catalyzed rearrangement to give the key cyclohex-2-enone intermediate 5, (2) Nozaki-Hiyama-Kishi coupling reaction between aldehyde 4 and iodide 14 to afford alcohol 3, and (3) asymmetric (R)-CBS reduction of the ketone functionality in compound 21 to establish the C-2' chiral center in the target compound 1. The absolute configuration of malyngamide W (1) was thus confirmed via the synthesis of 1 and 2'-epi-1.     

Total synthesis and correct absolute configuration of malyngamide U

The enantioselective synthesis of the previously proposed structure of malyngamide U (1) was accomplished in 18 steps from (S)-(+)-carvone. The key steps involved a hydroxymethylation of (S)-(+)-carvone and an asymmetric Henry reaction of aldehyde (+)-5, as well as condensation with the acid 3. The 1H and 13C NMR data of the synthetic compound 1 were not consistent with the data of the reported malyngamide U. The C-2' epimer of compound 1 was therefore synthesized by a similar reaction sequence. While the NMR data of C-2' epimer 23 were in full agreement with those of the reported product, the discrepancy in the specific rotation data suggested the correct structure of malyngamide U should be structure 2, in which the absolute configuration of the amine part was enantiomeric with that in compound 23. Then the correct absolute configuration of revised malyngamide U (2) was confirmed by the similar synthesis from (R)-(-)-carvone.     

Integral stereocontrolled synthesis of a spiro-norlignan, sequosempervirin A: revision of absolute configuration

A novel synthetic path to sequosempervirin A was established by employing a samarium diiodide promoted intramolecular Barbier-type reaction of the lactonic iodide, in which the key structural feature, a spiro[4.5]decane ring system, could be constructed by controlling the stereochemistry of the hydroxyl group at the 8-position. The absolute configuration of natural sequosempervirin A was revised to be 4S based on this synthesis.     

Synthesis of new 2',3'-dideoxy-6',6'-difluoro-3'-thionucleoside from gem-difluorohomoallyl alcohol

[reaction: see text] 2',3'-Dideoxy-6',6'-difluoro-3'-thionucleoside 1b, an analogue of 3Tc that has high biological activities against HIV and HBV, has been synthesized from gem-difluorohomoallyl alcohol 3 in an efficient way. The key intermediate 4-amino-3,3-difluorotetrahydrothiophen-2-ylmethyl benzoate 15 was prepared from 2,2-difluoro-1-[(4R)-2,2-dimethyl-1,3-dioxolan-4-yl]but-3-en-1-ol 3 in 11 steps. The construction of pyrimidine ring with the amino group of compound 15 gave the target compound 1.     

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.     

Total synthesis of the marine sponge metabolites (+)-rottnestol, (+)-raspailol A and (+)-raspailol B

The asymmetric syntheses of (+)-rottnestol (1) and the related marine sponge metabolites (+)-raspailols A (5) and B (6) are described. The key step in each of these sequences was a Stille coupling to form the C9-C10 sp2-sp2 bond and connect the polyene sidechains to the appropriate optically active tetrahydropyran core. For rottnestol (1), both C12 epimers were synthesised by a coupling between stannane 7 and (R)- or (S)-8 followed by acid hydrolysis which allowed for the assignment of the absolute configuration at the remote C12 stereocentre as R upon comparison of chiroptical data of the synthetic material with that reported for the natural product. In accord with this, (12R)-raspailol A (5) was synthesised from stannane 7 and sidechain 9 and this compound also compared well with the data for natural material including sign and absolute value of the specific rotation. Finally, the same C12 epimer of raspailol B (6) was secured via a union between stannane 10 and iodide 9 and this also possessed a similar rotation to that described for the natural product. Thus, all three compounds appear to possess the (12R) configuration, while that of the core tetrahydropyran ring is the same as proposed originally.     

Synthesis, Crystal Structure and Spectrometry Study of Chiral N-Ethyl-N''''-(2'''',3'''',4'''',6''''-tetra-O-acetyl-β-D-glycosyl)-thiocarbamide

1INTRODUCTIONManynomadicsugarsperformimportantbio-logicalfunctions[1].Theycancontrolvariousgeneexpressionstoadjusttheupgrowth,developmentandreactionoforgans[2].Glycosylisothiocyanateshavebeenwidelyusedasvaluableintermediatesinthesynthesisofglycosylderivatives[3].Theiso-thiocyanatesandglycosylisothiocyanateshavebeenthefocusofsyntheticattentionduringrecentyearsbecauseoftheirpotentialpharmacologicalproperties[4].Theyhavealsoattractedconsiderableinterestduetotheanti-HIVactivityshownby1-deoxy…     

Crystal Structure of (6S,9S,13R,14S)-7,8-Didehydro-3,7-dimethoxy-17-methylmorphinan-4,6-diol

1 INTRODUCTION (9S,13R,14S)-7,8-Didehydro-3,7-dimethoxy-4-hydroxy-17-methylmorphinan-6-one I is an impor- tant natural product that was used to ease pain, decrease blood pressure and diminish inflammation. In order to study the stereochemistry of the reduction of I with potassium boronhydride, the crystal structure of the title compound II was determined by X-ray diffraction method. 2 EXPERIMENTAL 2. 1 Instrument The melting point was measured on a Yanagimoto MP-500 apparatus…     

Total Synthesis of the Spermidine Alkaloid 13‐Hydroxyisocyclocelabenzine

A convergent total synthesis of 13‐hydroxyisocyclocelabenzine was developed. (3S)‐Methyl 3‐amino‐3‐phenylpropanoate ( 4 ) was used as the chiral building block. The 3,4‐dihydro‐4‐hydroxyisoquinolin‐1(2H)‐one derivative ( 5 ), the key fragment for the total synthesis, was prepared by a novel base‐catalyzed lactone‐lactam ring enlargement (Scheme 3). The resulting target C(13) epimers 3a / 3b from macrocyclization (Scheme 4) were separated by repeated flash chromatography. The absolute configuration of the synthetic alkaloid was determined by an X‐ray crystal‐structure analysis, which enabled us to determine the absolute configuration (9S,13R) for natural 3a with positive [α]_D.     

3’-氟-2’-O,3’-C-乙烯基键联并环尿苷的合成与表征简

设计合成了氟化并环核苷衍生物3’-氟-2’-O,3’-C-乙烯基键联的并环尿苷(1),并通过1H NMR,13C NMR和HRMS分析表征了其结构. 同时,对具有乙炔基及氟原子取代的重要中间体7的结构进行了分析确证,并提出其可能的形成机理.     

Asymmetric total synthesis of (+)-cannabisativine

The asymmetric total synthesis of natural (+)-cannabisativine 1 was completed in 19 steps and 7% overall yield. The key synthetic intermediate 29 was prepared with a high degree of stereocontrol in 12 steps starting from chiral 1-acylpyridinium salt 10. Addition of zinc enolate 11 to pyridinium salt 10 furnished dihydropyridone 12 containing two contiguous stereocenters of the correct absolute configuration. Luche reduction of ketone 16 afforded diol 17 in high yield (96%) and excellent diastereoselectivity. The Mukaiyama-Michael reaction of pyridones 27a/b with O-silyl ketene acetal 32 gave phenyl selenyl ketones 33a/b with complete stereoselectivity. Elimination of cis-beta-hydroxyselenides 34 and 35 effected the regiocontrolled preparation of tetrahydropyridine derivative 29. Several approaches to the macrocyclic ring closure of the 13-membered ring were investigated, ultimately leading to the completion of an asymmetric synthesis of the target compound with a high degree of stereocontrol.     

Crystal Structure of 5-(l-Menthyloxy)-3-Chloro-4-Pyrrolidinyl-2(5H)-Furanone

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Total synthesis of (+)-crocacin C

The first asymmetric synthesis of (+)-crocacin C (3) is described which served to confirm the absolute configuration of this compound. The key step in the sequence was the stereoselective assembly of the (E,E)-diene amide side chain by a Stille cross-coupling between the stannane 5 and vinyl iodide 6.     

Addition of difluorocarbene to 3',4'-unsaturated nucleosides: synthesis of 2'-deoxy analogues with a 2-oxabicyclo[3.1.0]hexane framework

Treatment of protected 2'-deoxy-3',4'-unsaturated nucleosides derived from adenosine and uridine with difluorocarbene [generated from bis(trifluoromethyl)mercury and sodium iodide] gave fused-ring 2,2-difluorocyclopropane compounds. Stereoselective alpha-face addition to the dihydrofuran ring resulted from hindrance by the protected beta-anomeric nucleobases. A protected uracil compound was converted smoothly into the cytosine derivative via a 4-(1,2,4-triazol-1-yl) intermediate. Removal of the protecting groups gave new difluorocyclopropane-fused nucleoside analogues. The solid-state conformation of the nearly planar furanosyl ring in the uracil compound had a shallow 2E pucker, and a more pronounced 1E conformation was present in the furanosyl ring of the cytosine derivative.     

Total synthesis of (+)-ent-cyclizidine: absolute configurational confirmation of antibiotic M146791

The first total synthesis of the enantiomer of the indolizidine alkaloid, cyclizidine, was accomplished from readily available d-serine as the starting chiron. The relevant key reactions involve the stereocontrolled construction of the indolizidine ring system with the required functionality and further elaboration to install the cyclopropyl dienyl side chain. With this total synthesis, the absolute configuration of the natural product based on a redetermination of its X-ray structure has been confirmed.     

Total syntheses of amphidinolide T1, T3, T4, and T5

A concise, flexible, and high yielding entry into the family of amphidinolide T macrolides, a series of cytotoxic natural products of marine origin, has been developed. All individual members, except amphidinolide T3 (3), derive from compound 39 as a common synthetic intermediate which is formed from three building blocks of similar size and complexity. The fragment coupling steps involve a highly diastereoselective SnCl(4) mediated reaction of the furanosyl sulfone derivative 11 with the silyl enol ether 18 and a palladium-catalyzed Negishi type coupling reaction between the polyfunctional organozinc reagent derived from iodide 32a and the enantiopure acid chloride 24b. The 19-membered macrocyclic ring is then formed by a high yielding ring closing metathesis (RCM) reaction of diene 33 catalyzed by the "second generation" ruthenium carbene complex 34. The efficiency of the RCM transformation stems, to a large extent, from the conformational bias introduced by the syn-syn-configured stereotriad at C12-C14 of the substrate which constitutes a key design element of the synthesis plan. The use of Nysted's reagent 38 in combination with TiCl(4) was required for the olefination of the sterically hindered ketone group in 36, whereas more conventional alkene formations were unsuccessful for this elaboration. Finally, it is shown that the inversion of a single and seemingly remote stereocenter (C12) in one of the building blocks not only affects the efficiency and stereochemical outcome of the RCM step but also exerts a significant influence on the course of the acyl-Negishi reaction, allowing a radical manifold to compete with productive cross coupling.     

(+)-1R,3R,5S-1,8,8-三甲基-3-硫代-3-(4-甲基苯氧基)-2,4,3-二氮磷杂二环[3,2,1]辛烷的合成和分子绝对构型

合成的标题化合物用X射线衍射方法确定其分子的绝对构型,晶体空间群为P_(2_1 2_1 2_1),晶胞参数:a=0.8365(3)、b=0.8625(3)、c=2.3513(2)nm,Z=4.结构分析表明,分子骨架中存在3个环结构,即含磷、氮原子的船式七元环、椅式六元环以及由碳原子组成的信封式五元环。P—S键为e键,P—O键为a键。     

Enantioselective synthesis of (--)-gilbertine via a cationic cascade cyclization

Described is the first enantioselective synthesis of (-)-gilbertine (2), a member of the uleine-type family, and the determination of the absolute configuration of this natural product is reported. The key step employs a cationic cascade reaction for a tetrahydropyrane and piperidine ring formation and the construction of the pentacyclic framework in one step. The synthetic strategy utilizes the Shibasaki reaction to build up the first stereogenic center. A formylation reaction of a 3-substituted cyclohexanone derivative was achieved, giving only the desired regioisomer. The Japp-Klingemann Fischer indole protocol was used successfully as a convergent synthetic approach for the construction of the desired tetrahydrocarbazole (20). Furthermore, an unexpected behavior of this 2,3-disubstituted cyclohexanone derivative during an epimerization process was investigated, resulting in different chemical behavior of the enantiomers and the racemate. The diastereomeric resolution was achieved via the cationic cascade reaction, demonstrating the versatility of this approach. Significantly, the synthetic 17-step sequence was easy to execute, giving (-)-gilbertine in 5.5% overall yield.     

Isolation, structure determination, and synthesis of allo-RA-V and neo-RA-V, RA-series bicyclic peptides from Rubia cordifolia L

Two bicyclic hexapeptides, allo-RA-V (4) and neo-RA-V (5), and one cyclic hexapeptide, O-seco-RA-V (6), were isolated from the roots of Rubia cordifolia?L. Their gross structures were elucidated on the basis of spectroscopic analysis and X-ray crystallography of compound 5. The absolute stereochemistry of compounds 4 and 5 were established by their total syntheses, and the absolute stereochemistry of compound 6 by chemical correlation with deoxybouvardin (3). Comparison of the 3D structures of highly active RA-VII (1) with less-active compounds 4 and 5 suggests that the orientation of the Tyr-5 and/or Tyr-6 phenyl rings plays a significant role in their biological activity. The isolation of peptides 4-6, along with compound 3, and the comparison of their structures seem to indicate that peptide 6 may be the common precursor to bicyclic peptides 3-5 in the plant.     

Stereocontrolled syntheses of carbocyclic C-nucleosides and related compounds.

Carbocyclic 9-deazapurine nucleosides (1-4), a spiranic pyrimidone carbocyclic compound (5), and an unusual carbocyclic isonucleoside (6) were prepared as enantiomerically pure compounds via the key intermediates 10 and 21 from 1,4-gamma-ribonolactone. The key intermediate 10 was prepared by stereoselective reduction with Bu3SnH and then converted to carbocyclic C-ribonucleosides 1, 3, and 4. 2',3'-Didehydro-2',3'-dideoxycarbocyclic 9-deazainosine (2) was prepared from a 2',3'-dimesylate 17 by treatment with Li2Te followed by an acidic deprotection. The key bicyclic intermediate 21 was prepared from a diol 20 by an intramolecular cyclization using CHI3-Ph3P-imidazole and converted to the spiranic compound 5 and an olefinic nucleoside 6 by the construction of the heterocyclic moiety followed by deprotection.