On the origin of siphonariid polypropionates: total synthesis of caloundrin B and its isomerization to siphonarin B

Enantioselective synthesis of the enantiomer of caloundrin B was achieved by strategic aldol coupling of an enantiopure trioxaadamantane-containing ketone with a racemic pyrone-containing aldehyde via kinetic resolution. In the presence of imidazole, ent-caloundrin B is cleanly isomerized to ent-siphonarin B confirming the proposed structure and absolute configuration for caloundrin B and establishing that it is a plausible biosynthetic product from which siphonarin B and baconipyrones A and C can originate.     

Total synthesis of (+)-massarinolin B and (+)-4-epi-massarinolin B, fungal metabolites from Massarina tunicata

The Cr(II)- and Ni(II)-mediated coupling of several tricyclic chiral aldehydes with (E)-β-iodomethacrylates (Nozaki-Hiyama-Kishi reaction) was successfully applied to the preparation of some valuable key intermediates of our synthetic strategy to the fungal metabolites (+)-massarinolin B, (+)-4-epi-massarinolin B and (+)-massarinolin C.     

Total synthesis of leustroducsin B

Leustroducsin B was synthesized via a convergent route based on division of the leustroducsin molecule into three segments A, B, and C. Two coupling reactions (Julia coupling reaction and Nozaki-Hiyama-Kishi (NHK) reaction) were employed for coupling of segments A and B: segment A1 for the Julia coupling reaction was prepared by a combination of Sharpless asymmetric epoxidation and an epoxide-cleavage reaction with an organoaluminum reagent, while segment A2 for the NHK reaction was synthesized from optically active alcohol that had previously been prepared by lipase-catalyzed kinetic resolution. Segment B, whose structure was modified with some functional groups, was synthesized from (R)-malic acid by a combination of Wittig reaction and Sharpless asymmetric dihydroxylation, and segment C, containing a cyclohexane moiety, was prepared by asymmetric Diels-Alder reaction. Segment B was first coupled with segment A1 via the Julia coupling reaction, but the yield was low due to unexpected epimerization. The NHK reaction of segment A2 proceeded to give the coupling product in good yield. This product was coupled with segment C via Wittig and Stille coupling reactions, and finally, phosphorylation was carried out by partial hydrolysis of a cyclic phosphate to give leustroducsin B.     

New strategies for the synthesis of biologically important tetrapyrroles. The "B,C + D + A" approach to linear tetrapyrroles

Linear tetrapyrroles related to phytochrome (1) were prepared in enantiospecific fashion by a new strategy beginning with ring-B,C synthons of type 19 (bis-iododipyrrins). Rings A and D were elaborated by Pd(0)-mediated coupling of 19a with the appropriate alkyne acid or amide derivatives 9 and 20, followed by intramolecular cyclization (method C: BC + D + A --> ABCD).     

Total synthesis and biological evaluation of (-)-apicularen A and its analogues

The total synthesis of (-)-apicularen A (1), a highly cytostatic 12-membered macrolide, and its analogues is described. The convergent and distinct approach not only provides 1, but also opens the opportunity to synthesize C10-C11 functional analogues of 1. The key steps of the total synthesis include assembling of iodoalkene 12 and aldehyde 13by Nozaki-Hiyama-Kishi (NHK) coupling, stereospecific construction of 2,6-trans-disubstituted dihydropyran by Pd(II)-catalyzed 1,3-chirality transfer reaction, and Yamaguchi macrolactonization. The (17E,20Z,22Z)-heptadienoylenamine moiety in the side chain is installed by an efficient Cu(I)-mediated coupling to complete the synthesis. Analogues of C11-epi-, C11-deoxy-C10-α-hydroxy-, and C10-C11 dehydrated apicularen A 3-5 were also prepared. Cytostatic activities of (-)-apicularen A and the three analogues for three different cancer cell lines are described.     

Probing host-selective phytotoxicity: synthesis of destruxin B and several natural analogues.

The syntheses of the host-selective phytotoxin destruxin B [cyclo(betaAla-Hmp-Pro-Ile-MeVal-MeAla), Hmp = (2R)-2-hydroxy-4-methylpentanoic acid], and the closely related natural analogues homodestruxin B (MeVal-->MeIle), desmethyldestruxin B (MeVal-->Val), hydroxydestruxin B (Hmp-->Dhmp, Dhmp = (2R)-2,4-dihydroxy-4-methylpentanoic acid), and hydroxyhomodestruxin B (MeVal-->MeIle, Hmp-->Dhmp) are described. In each case, the MeAla-betaAla linkage was formed by cyclization and the precursor linear hexadepsipeptides were formed by condensing two three-residue fragments. Radiolabeled samples of destruxin B, homodestruxin B, and hydroxydestruxin B were prepared by coupling [3-(14)C]-beta-alanine to the appropriate pentadepsipeptide followed by cyclization. A noteworthy feature of the synthesis involves the novel use of a Boc-hydrazide protecting group on dipeptides with a C-terminal N-methylalanine residue to inhibit the otherwise facile dioxopiperazine formation during peptide coupling.     

Total synthesis of pteridic acids A and B

Pteridic acid A (1) is a spirocyclic octaketide produced by the phytoepiphytic actinomycete Streptomyces hygroscopicus TP-A0451 and possesses potent plant-growth-promoting activity comparable to that of indole-3-acetic acid. The enantioselective total synthesis of this natural product was achieved by employing the Sn(OTf)(2)-mediated Evans aldol reaction and the Fukuyama acetylenic coupling reaction as the key C--C bond-forming steps producing 1 through a 14-step sequence in 22 % overall yield from a known oxazolidinone derivative. MgBr(2)-mediated equilibration of an anomerically favored spirocyclic intermediate used for the synthesis of 1 brought about partial epimerization of the spirocenter to give the corresponding anomerically disfavored epimer, which was converted into pteridic acid B (11-epi-1), another plant-growth promoter of the same microbial origin.     

Ti(II)-mediated conversion of alpha-heterosubstituted (O,N, S) nitriles to functionalized cyclopropylamines. Effect of chelation on the cyclopropanation step

Alpha-alkoxy, amino-, and thio nitriles undergo a Ti(II)-mediated coupling with Grignard reagents to afford heterofunctionalized cyclopropylamines. A chelation effect appears to be generally responsible for the spontaneous contraction of the intermediate azatitanacycle leading to cyclopropane. By using the described reaction, 1-aminocyclopropanecarboxylic acids were prepared in four steps, in good overall yields, from the readily available benzyloxyacetonitrile.     

A convenient synthesis of a-ring-functionalized podolactones. Revision of the structure of wentilactone B

A new route to podolactones functionalized in the A ring has been achieved. Two key steps were employed in this synthesis, the construction of the bicyclic skeleton via a Mn(III)-mediated radical cyclization and the transformation of this bicyclic intermediate into the tetracyclic podolactone skeleton through a Pd (II)-mediated bislactonization of the corresponding conjugate diene. The reported synthesis of 3beta-hydroxy-13,14,15,16-tetranorlabda-7,9(11)-dien-(19,6beta),(12,17)-diolide (1) let us reassign the structure of wentilactone B, for which structure 1 was wrongly reported. [reaction: see text]     

Studies in marine polypropionate synthesis: total synthesis of (-)-baconipyrone C

[reaction--see text] An asymmetric total synthesis of the unusual siphonariid metabolite, (-)-baconipyrone C (3), is described. Key steps included a tin(II)-mediated aldol coupling for the preparation of the carboxylic acid 17 and two different boron-mediated aldol additions leading to alcohol 8. Ester formation using modified Yamaguchi conditions gave 24, leading on PMB deprotection to (-)-baconipyrone C.     

Concise total synthesis of calothrixins A and B

The concise total synthesis of calothrixins A and B has been accomplished by utilizing the one-pot formation of hexatriene as a key intermediate via the palladium-catalyzed tandem cyclization/cross-coupling reaction of triethyl(indol-2-yl)borate. In another key transformation, the indolo[3,2-j]phenanthridine core was prepared in high yield via Cu(I)-mediated 6π-electrocyclization.     

Synthesis and characterization of transition metal-vitamin B13 complexes mixed with a co-vitamin

The synthesis and structural characterization of mixed vitamin-metal complexes are reported. A complex bearing two vitamins (B3 and B13) on the same metal centre, nicotinamide(orotato)nickel(II) pentahydrate, has been prepared and characterised by single-crystal X-ray diffraction methods. A complex of Cr(III) with vitamins C and B13, bis(ascorbato)orotatochromium(III) dihydrate, has been isolated and its nature investigated by means IR, ¹³C?NMR, API-ES and thermogravimetric measurements. Theoretical calculations have been performed to propose a structure in accord with results obtained.     

Synthetic studies of halichondrin B, an antitumor polyether macrolide isolated from a marine sponge. 8. Synthesis of the lactone part (C1---C36) via horner-emmons coupling between C1---C15 and C16---C36 fragments and Yamaguchi lactonization

The lactone part (2) of halichondrin B (1) was synthesized by Yamaguchi macrolactonization of the seco-acid (3), which was synthesized via coupling of C1---C15(4) with C16---C36 (5), prepared through stereoselective construction of the E ring starting from C16---C26 (7) and C27---C36 (8).     

Synthesis of the tricyclic indole alkaloids,dilemmaones A and B

Dilemmaones A-C are naturally occurring tricyclic indole alkaloids possessing a unique hydroxymethylene or methoxymethylene substituent at the C2 position of the indole core and a C6–C7 fused cyclopentanone. Dilemmaone B has been prepared in 5 steps from 5-methylindan-1-one, and dilemmaone A has been prepared in 3 steps from a common precursor, 6-bromo-5-methyl-7-nitroindan-1-one. In both syntheses, key steps include a Kosugi-Migita-Stille cross coupling and a reductive cyclization using hydrogen gas and a transition metal catalyst.     

First total synthesis of (-)-achilleol B: reassignment of its relative stereochemistry

The first total synthesis of (-)-achilleol B was achieved using a convergent approach with a longest linear sequence of 14 steps. Three key steps were employed, including an enantioselective Robinson annelation for the construction of the bicyclic moiety. The monocyclic synthon was prepared through a Ti(III)-mediated cylization of a chiral monoepoxide obtained via asymmetric dihydroxylation of geranylacetone. The asymmetric preparation of these subunits also permitted us to achieve the enantioselective synthesis of elegansidiol, achilleol A, and farnesiferol B.     

Copper(II)-mediated autoxidation of tert-butylresorcinols

Although copper(II)-mediated oxidation of phenols results in oxidative coupling rather than in oxygenation, it was recently reported that naturally occurring 5-alkylresorcinols undergo oxygenation in the presence of copper(II). To explore the generality of this reaction, the copper(II)-mediated autoxidation of 4-tert-butylresorcinol and 4,6-di-tert-butylresorcinol was investigated and was found to result in direct oxygenation at open activated positions and, at the tert-butyl-substituted positions, in oxygenation with competing loss of (as isobutylene) and 1,2-rearrangement of the tert-butyl group. 5-tert-Butyl-2-hydroxy-1,4-benzoquinone is the major product from both starting materials, and the final product mixture reflects, in part, coupling of metastable initially formed electrophilic and nucleophilic side products. Mechanisms that are consistent with the observed products and control reactions are proposed. The key step appears to be equilibration of a copper(II)-resorcinolate with a charge-transfer radical form that reacts regioselectively with O(2) as prescribed by resonance.     

Total Synthesis of Lajollamycin B

The first total synthesis of lajollamycin B, a structurally novel nitro-tetraene spiro-β-lactone/γ-lactone antibiotic, is described. The convergent synthesis involves the construction of the C8′–C11′ nitrodienylstannane and its coupling with the segment prepared from the C1′–C7′ ω-iodoheptadienoic acid and the right-hand heterocyclic fragment, which has been utilized for our previous syntheses of oxazolomycin A. The revision of the geometry of the terminal Δ^{10′, 11′}-double bond from E to Z is also described for the structure of natural lajollamycin B.     

Enantiocontrolled total syntheses of breviones A, B, and C

Enantiocontrolled total syntheses of the breviones A, B, and C have been accomplished using a highly diastereoselective oxidative coupling of an α-pyrone with a tricyclic diene prepared from an optically pure Wieland-Miescher ketone derivative through the 7-endo-trig mode of acyl radical cyclization.     

Total syntheses of epothilones B and D

Total syntheses of the microtubule stabilizing antitumor drugs epothilone B and D are described, starting from optically pure (S)-malic acid and methyl (R)-3-hydroxy-2-methylpropionate. The synthesis is highly convergent by coupling the three fragments C1-C6 (fragment D), C7-C10 (fragment C), and C11-C21 (fragment B). Key steps are two stereoselective Wittig type olefinations to generate the 12,13- and 16,17-double bonds, an enantioselective Mukaiyama aldol addition to synthesize fragment D, and a sulfone anion allyl iodide alkylation to connect fragments B and C. Finally fragment D was attached to the B + C fragment via aldol addition.     

Stereoselective total synthesis of reveromycin B and C19-epi-reveromycin B

Our studies toward the total synthesis of the reveromycin family of natural products are described herein. Our synthetic approach is efficient, stereocontrolled, and convergent and has resulted in the first synthesis of reveromycin B (4) and C19-epi-reveromycin B (55). Key steps of this successful strategy include: a modified Negishi coupling (construction of C7-C8 bond) and a Kishi-Nozaki reaction (construction of C19-C20 bond), which were employed in the attachment of the target side chains. The key building blocks for the total synthesis were thus defined as vinyl iodide 6, alkyne 7, and alkyne 8. Our synthesis illustrates the utility of the modified Negishi coupling for the construction of complex dienes, confirms the proposed stereochemistry of reveromycins and paves the way for the preparation of designed analogues for biological study.