Enantioselective syntheses of (+)-xylarenal A and ent-xylarenal A

[reaction: see text] The total synthesis of the sesquiterpenoid xylarenal A is reported. This first synthetic entry to an eremophilane terpenoid with an exocyclic vinyl aldehyde unit involves the use of the bicyclic enone (+)-3, which after a gamma-oxidation and alpha'-allylation leads to the formation of the ketone (+)-8. After its acylation, an oxidative cleavage of the allyl side chain followed by alpha-methylenation of the resulting aldehyde gives (+)-xylarenal A (1). The synthesis of (-)-xylarenal A from (-)-3 is also reported. Moreover, the first total synthesis of the trinoreremophilane (+)-1alpha-hydroxyisoondetianone (5) is described.     

Concise syntheses of coronarin A, coronarin E, austrochaparol and pacovatinin A

Total syntheses of (+)-coronarin A (1), (+)-coronarin E (2), (+)-austrochaparol (3) and (+)-pacovatinin A (4) were achieved from the synthetic (+)-albicanyl acetate (6). Dess-Martin oxidation of (+)-albicanol (5) derived from the chemoenzymatic product (6) gave an aldehyde (7), which was subjected to Julia one-pot olefination using beta-furylmethyl-heteroaromatic sulfones (8 or 9 ) gave (+)-trans coronarin E (2) and (+)-cis coronarin E (12) with high cis-selectivity. The synthesis of (+)-coronarin A (1) from (+)-trans coronarin E (2) was achiev-ed, while (+)-cis coronarin E (12) was converted to the natural products (+)-(5S,9S,10S)-15,16-epoxy-8(17),13(16),14-labdatriene (13) and (+)-austrochaparol (3). By the asymmetric synthesis of (+)-3, the absolute structure of (+)-3 was determined to be 5S, 7R, 9R, 10S configurations. Homologation of (+)-albicanol (5) followed by allylic oxidation gave (7 alpha)-hydroxy nitrile (17), which was finally converted to the natural (+)-pacovatinin A (4) in 8 steps from (+)-albicanol (5).     

Total synthesis of (+)-benzastatin E via diastereoselective Grignard addition to 2-acylindoline

[reaction: see text] A stereoselective total synthesis of (+)-benzastatin E (1) is described. The synthesis involves a diastereoselective Grignard addition to 2-acylindoline 2, which is derived from commercially available (S)-2-indolinecarboxylic acid (3). The unknown absolute configuration of (+)-1 is determined as (9S,10R).     

Total synthesis of (+)-phorboxazole A exploiting the Petasis-Ferrier rearrangement.

A highly convergent, stereocontrolled total synthesis of the potent antiproliferative agent (+)-phorboxazole A (1) has been achieved. Highlights of the synthesis include: modified Petasis-Ferrier rearrangements for assembly of both the C(11-15) and C(22-26) cis-tetrahydropyran rings; extension of the Julia olefination to the synthesis of enol ethers; the design, synthesis, and application of a novel bifunctional oxazole linchpin; and Stille coupling of a C(28) trimethyl stannane with a C(29) oxazole triflate. The longest linear sequence leading to (+)-phorboxazole A (1) was 27 steps, with an overall yield of 3%.     

Relative and Absolute Configuration of Allohedycaryol. Enantiospecific Total Synthesis of Its Enantiomer

The enantiomer of (+)-allohedycaryol, a germacrane alcohol isolated from giant fennel (Ferula communis L.), has been synthesized, thereby elucidating the relative and absolute stereochemistry of the natural product. The synthesis of (-)-allohedycaryol started from (+)-alpha-cyperone (5) which was available in relatively large quantities via alkylation of imine 7 derived from (+)-dihydrocarvone and (R)-(+)-1-phenylethylamine. In a number of steps 5 was converted into the mesylate 4with a regio- and stereoselective epoxidation as the key step. A Marshall fragmentation of 4 was used to prepare the trans,trans-cyclodeca-1,6-diene ring present in allohedycaryol. The conformation of synthetic (-)-allohedycaryol was elucidated via photochemical conversion into a bourbonane system. The synthesis of (-)-allohedycaryol also showed that natural (+)-allohedycaryol has the opposite absolute stereochemistry to that normally found in higher plants.     

Total synthesis of (+)-spongistatin 1. An effective second-generation construction of an advanced EF Wittig salt,fragment union,and final elaboration

A stereocontrolled, total synthesis of (+)-spongistatin 1 (1) has been achieved. Union of a second-generation EF Wittig salt (+)-3 with the advanced ABCD aldehyde (-)-4, followed by regioselective macrolactonization and global deprotection afforded (+)-spongistatin 1 (1). The longest linear sequence, 29 steps, proceeded in 0.5% overall yield.     

An enantioselective total synthesis of (+)-geissoschizine

[formula: see text] A concise asymmetric synthesis of the indole alkaloid (+)-geissoschizine (1) has been completed. The synthesis features the highly diastereoselective vinylogous Mannich reaction of 3 with 4 to give 5, which is elaborated into the key tetracyclic intermediate 7 in two steps. Following the stereoselective introduction of the ethylidene moiety to give 9, reduction of the lactam and radical decarboxylation via an acyl selenide gave 12, which was converted into (+)-geissoschizine by formylation. The synthesis requires only 11 chemical operations and proceeds in an overall yield of 17%.     

Stereoselective synthesis of tetrahydrofuran lignans via BF(3) x OEt(2)-promoted reductive deoxygenation/epimerization of cyclic hemiketal: synthesis of (-)-odoratisol C, (-)-futokadsurin A, (-)-veraguensin, (+)-fragransin A(2), (+)-galbelgin, and (+)-talaumidin

A versatile route to the synthesis of 2,5-diaryl-3,4-dimethyltetrahydrofuran lignans, (-)-odoratisol C (1), (-)-futokadsurin A (2), (-)-veraguensin (3), (+)-fragransin A2 (4), (+)-galbelgin (5), and (+)-talaumidin (6), is described. Central to the synthesis of the lignans is BF(3) x OEt(2)-promoted deoxygenation/epimerization of the hemiketal 9a followed by stereoselective reduction of the oxocarbenium ion intermediates 8a,b.     

Stereoselective synthesis of (-)-1-epi-ventiloquinone L and (+)-ventiloquinone L, the monomeric unit of cardinalin 3

A stereoselective synthesis of (-)-1-epi-ventiloquinone L and (+)-ventiloquinone L, the monomeric unit of cardinalin 3 has been described. The synthesis is completed in 7 steps with 10.5% and 13% overall yields for (-)-1-epi-ventiloquinone L and (+)-ventiloquinone L respectively. The key steps involve D?tz benzannulation of carbene 5 with alkyne 6 to give a substituted naphthalene moiety and oxa-Pictet-Spengler reaction to install the 1,3-dimethylpyran moiety.     

Diastereoselective one-pot Wittig olefination-Michael addition and olefin cross metathesis strategy for total synthesis of cytotoxic natural product (+)-varitriol and its higher analogues

A stereoselective route for the total synthesis of anticancer marine natural product (+)-varitriol (1) is detailed herein. The impressive biological activity and interesting structural features of natural (+)-varitriol fuelled us to undertake the synthesis of some higher analogues (1a-j) of this molecule. The key features of the synthetic strategy include one-pot Wittig olefination followed by a highly diastereoselective oxa-Michael addition to assemble stereochemically pure tetrasubstituted THF moiety of the natural varitriol and olefin cross metathesis to couple the aromatic part with tetrasubstituted THF moiety. The total synthesis of title natural product is efficient with 21.8% overall yield for 9 linear steps from D-ribose and thus facilitates the more scaled-up practical route for the synthesis of 1 and its analogues as well. The synthetic (+)-varitriol (1) and its analogues were screened for their cytotoxicity. The present synthetic approach paves the way for preparation of numerous analogues of the title natural product for drug development.     

Asymmetric total synthesis of (+)-swainsonine

A concise asymmetric synthesis of (+)-swainsonine (ent-1) is described starting from 2, which was readily prepared from commercially available l-glutamic acid. The method features installation of the indolizidine ring via an intramolecular cyclisation of α-sulfinyl carbanion as a key step. (+)-Swainsonine was obtained in 11.8% overall yield in 10 steps.     

Asymmetric total synthesis of (-)-spirofungin A and (+)-spirofungin B

[chemical reaction: see text]. The stereocontrolled total synthesis of (-)-spirofungin A (1) and (+)-spirofungin B (2a), polyketide-type antibiotics having various antifungal activities, has been achieved employing the Weinreb amide 8, the alkyne 9, and the vinyl boronate 5 readily available from the common intermediate 10. The first synthesis proceeded with a longest linear sequence of 31 steps, affording (-)-1 and (+)-2a in 7.9% and 5.2% overall yields, respectively.     

A short enantioselective synthesis of (+)-eleutherin, (+)-allo-eleutherin and a formal synthesis of (+)-nocardione B

A short enantioselective synthesis of (+)-eleutherin, (+)-allo-eleutherin and the formal synthesis of (+)-nocardione B is described. The synthesis is completed in six steps in overall yields of 8% for eleutherin and 14% for allo-eleutherin. The synthetic strategy features an efficient combination of the Dötz annulation reaction with a chiral alkyne and an oxa-Pictet Spengler reaction as the keys steps in the stereodivergent synthesis of (+)-eleutherin and (+)-allo-eleutherin. The synthesis of (S)-(+)-2-(2′-hydroxypropyl)-5-methoxy-1,4-naphthoquinone entails the formal synthesis of (+)-nocardione B.     

Enantioselective total syntheses of nankakurines A and B: confirmation of structure and establishment of absolute configuration

Total syntheses of (+)-nankakurine A (2) and (+)-nankakurine B (3) were accomplished by a sequence that employs an intramolecular dipolar cycloaddition of an azomethine imine intermediate to form the azatricyclic moiety and establish the relative configuration of the spiropiperidine ring. These syntheses, together with the synthesis of the originally purported structure 1 of nankakurine A, rigorously establish the relative and absolute configuration of these structurally unusual Lycopodium alkaloids. The syntheses are sufficiently concise that gram quantities of (+)-nankakurine A (2) and (+)-nankakurine B (3) will be available for further biological studies.     

Asymmetric Syntheses of Pharmaceutically Interesting Piperidines

Alkyl-3-hydroxyl piperidine is a structural unit found in numerous bioactive natural products, drugs and drug candidates. For example, (+)-L-733 060 (1)1 and (+)-CP-99, 994 (2)2 are selective and potent neurokinin substance P receptor antagonists, which have been shown to possess potent antiemetic activity; febrifugine (3)3 and isofebrifugine (4)3 are well-known candidates of antimalarial agents isolated from Chinese medicinal plants Dichroa febrifuga. The important bioactivities, common str…     

Enantioselective total synthesis of (+)-ottelione A, (-)-ottelione B, (+)-3-epi-ottelione A and preliminary evaluation of their antitumor activity

Enantioselective total synthesis of (+)-ottelione A (1) and (-)-ottelione B (2), novel and potent antitumor agents from a freshwater plant, and (+)-3-epi-ottelione A (3), the earlier proposed stereostructure of 1, was efficiently achieved starting from the known tricyclic compound 10. The synthesis involved the following key steps: i) coupling reactions of aldehydes 8 and 9 with the aromatic portion 7 (8+7-->15 and 9+7-->27), ii) base-induced hemiacetal-opening/epimerization reactions of the cyclic hemiacetals 6 and 27 (6-->17 and 27 a-->26 a), and iii) Corey-Winter's reductive olefination of the cyclic thiocarbonates 21 and 36 (21-->22 and 36-->37). The present total synthesis fully established the absolute configuration of these natural products. The cell growth inhibition profile, COMPARE analysis, and tubulin inhibitory assay of (+)-3-epi-ottelione A (3) and its O-acetyl derivative 24 demonstrated that these unnatural substances could be prominent lead compounds for the development of anticancer agents with a novel mode of action.     

First enantiospecific total synthesis of the important biogenetic intermediates, (+)-polyneuridine and (+)-polyneuridine aldehyde, as well as 16-epi-vellosimine and macusine A

The first enantiospecific total synthesis of the alkaloids 16-epi-vellosimine (1), (+)-polyneuridine (2), (+)-polyneuridine aldehyde (3), and macusine A (4) is reported. The key oxidation was accomplished with the Corey-Kim reagent to provide the important biogenetic intermediates, 16-epi-vellosimine (1) and polyneuridine aldehyde (3), the latter of which is required for the conversion of the sarpagan skeleton into the ajmalan system in the biosynthesis of quebrachidine. [reaction: see text].     

Enantioselective Total Synthesis of (+)‐Neostenine

A chirality transfer approach using acyclic polyol intermediates for the synthesis of (+)‐neostenine ( 1 ) has been developed. The sequential Overman/Claisen rearrangement of an allylic 1,2‐diol was especially useful, installing two contiguous stereocenters with complete diastereoselectivity in a one‐pot sequence. The SmI₂‐mediated cyclization and the subsequent chemoselective reduction of a lactam moiety accomplished the first enantioselective total synthesis of (+)‐neostenine ( 1 ).     

Total synthesis of asimicin via highly stereoselective [3 + 2] annulation reactions of substituted allylsilanes

A highly stereoselective total synthesis of (+)-asimicin (1) is reported. The synthesis features two chelate-controlled [3 + 2] annulation reactions-one of which (e.g., 2 + 3) constitutes a key, convergent fragment assembly step-that establish all of the stereochemistry of the bis-tetrahydrofuran unit of the natural product.     

Catalytic Enantioselective Decarboxylative Allylations of a Mixture of Allyl Carbonates and Allyl Esters: Total Synthesis of (−)‐ and (+)‐Folicanthine

A highly enantioselective decarboxylative allylation of a mixture of enol carbonates and allyl esters has been achieved. The strategic viability of this methodology has been demonstrated through the total synthesis of cyclotryptamine alkaloids (?)‐ and (+)‐folicanthine ( 1 a ) and the formal total synthesis of (?)‐chimonanthine ( 1 b ), (+)‐calycanthine ( 1 c ), and (?)‐ditryptophenaline ( 1 d ).