Total synthesis of (+)-rolliniastatin 1

Scatena G, Fernandes JB, Cass QB, Batista Jr. JM. Scope of the 2(5H)-furanone helicity rule: a combined ECD, VCD, and DFT investigation

Combination of ECD, VCD and DFT calculations revealed a possible exception to the widely used helicity rule of the 2(5H)-furanone chromophore.

[3 + 2]-Annulation Reactions of Chiral Allylsilanes and Chiral Aldehydes. Studies on the Synthesis of Bis-tetrahydrofuran Substructures of Annonaceous Acetogenins

[Structure: See text] Double asymmetric [3 + 2]-annulation reactions of chiral beta-silyloxyallylsilanes with chiral 2-tetrahydrofuranyl carboxaldehydes have been studied, leading to the stereocontrolled synthesis of six diastereomeric bis-tetrahydrofuran structures corresponding to the core subunits of members of the Annonaceous acetogenin family of natural products. Transition-state models are proposed to account for the stereoselectivity of the double-stereodifferentiating [3 + 2]-annulation reactions.

A Bidirectional Approach to the Synthesis of a Complete Library of Adjacent-Bis-THF Annonaceous Acetogenins

Thirty-six stereoisomers of bifunctional adjacent bis-THF (tetrahydrofuran) lactones have been synthesized, which can afford a complete library of the adjacent bis-THF Annonaceous acetogenins. The bis-THF lactones were synthesized, starting from the enantioselectively pure 8,9:12,13-(E,E and Z,E)-16-benzyloxy-5-hydroxy-hexadeca-1,4-olide, in a highly distereoselective manner using oxidative reactions, including rhenium(VII) oxides-mediated oxidative cyclization, Shi's asymmetric epoxidation, and Sharpless asymmetric dihydroxylation reactions. Using the nonsymmetrical bis-THF lactones, syntheses of two nonnatural acetogenins were achieved.

The sclerophytin A adventure

The structural designation A originally made by Sharma and Alam to sclerophytin A was considered to be ambiguous and so notably strained relative to B that the latter was targeted for de novo synthesis (Scheme 1). Our two successful routes began with (5S)-(d-menthyloxy)-2(5H)-furanone and involved the application of cycloaddition, Claisen ring expansion, transannular oxymercuration, and 1,2-carbonyl transposition tactics to arrive at B. It was immediately apparent from polarity considerations and spectroscopic data that the antileukemic marine metabolite in question was in need of more deep-seated structural revision. Following close re-examination of an acquired authentic sample by advanced NMR techniques, the strong inference was made that sclerophytin A actually lacked a second oxygen bridge and was in reality the triol C. This conclusion was unequivocally confirmed by diverting an advanced intermediate generated earlier into a short sequence beginning with regiocontrolled dihydroxylation and terminating with configurational inversion at the secondary carbinol center. The status of other members of this series is also presented.

Total asymmetric synthesis of the putative structure of the cytotoxic diterpenoid (-)-sclerophytin a and of the authentic natural sclerophytins A and B

An enantioselective synthetic route to the thermodynamically most stable diastereomer of the structure assigned to sclerophytin A (5) has been realized. The required tricyclic ketone 33 was prepared by sequential Tebbe-Claisen rearrangement of lactones 29 and 30, which originated from the Diels-Alder cycloaddition of Danishefsky's diene to (5S)-5-(d-menthyloxy)-2(5H)-furanone (14). An allyl and a cyano group were introduced into the resulting adduct by means of stereocontrolled allylindation under aqueous Barbier-like conditions and by way of cyanotrimethylsilane, respectively. Following stereocontrolled nucleophilic addition of a methyl group to 33, ring A was elaborated by formation of the silyl enol ether, ytterbium triflate-catalyzed condensation with formaldehyde, O-silylation, and Cu(I)-promoted 1,4-addition of isopropylmagnesium chloride. The superfluous ketone carbonyl was subsequently removed and the second ether bridge introduced by means of oxymercuration chemistry. Only then was the exocyclic methylene group unmasked via elimination. An alternative approach to the alpha-carbinol diastereomer proceeds by initial alpha-oxygenation of 37 and ensuing 1,2-carbonyl transposition. Neither this series of steps nor the Wittig olefination to follow induced epimerization at C10. Through deployment of oxymercuration chemistry, it was again possible to elaborate the dual oxygen-bridge network of the target ring system. Oxidation of the organomercurial products with O(2) in the presence of sodium borohydride furnished 72, which was readily separated from its isomer 73 after oxidation to 61. Hydride attack on this ketone proceeded with high selectivity from the beta-direction to deliver (-)-60. Comparison of the high-field (1)H and (13)C NMR properties and polarity of synthetic 5 with natural material required that structural revision be made. Following a complete spectral reassessment of the structural assignments to many sclerophytin diterpenes, a general approach to sclerophytin A, three diastereomers thereof, and of sclerophytin B was devised. The presence of two oxygen bridges as originally formulated was thereby ruled out, and absolute configurations were properly determined. Key elements of the strategy include dihydroxylation of a medium-ring double bond, oxidation of the secondary hydroxyl in the two resulting diols, unmasking of an exocyclic methylene group at C-11, and stereocontrolled 1,2-reduction of the alpha-hydroxy ketone functionality made available earlier.

Synthesis of the alleged structure of sclerophytin A. The setting of two oxygen bridges within the fused cyclodecanol B ring is not Nature's Way

[reaction: see text] The structure attributed to sclerophytin A, a cytotoxic soft coral metabolite, was synthesized in an enantioselective manner from (5S)-5-(d-menthyloxy)-2(5H)-furanone. The spectral properties and polarity of the synthetic product require that the structural assignment to the natural material be revised.

Stereocontrolled Synthesis of Key Advanced Intermediates toward Simplified Acetogenin Analogues

The stereo- and enantiocontrolled synthesis of substituted beta-hydroxy ethers based on glycol and catechol bearing an alkyne group and a series of substituents is reported. These substrates were designed to mimic the bis-THF array of annonaceous acetogenins and to provide an access to simplified and modified analogues. The key steps of the synthesis involve the condensation of the nonracemic mesylate of solketal with ethylene glycol and catechol, followed by an alkylation with a glycidyl derivative. Under appropriate conditions, the reaction is completely stereoselective and allows the synthesis of all the diastereomers. After the epoxide was opened with triethylsilylacetylene, the second epoxide was unmasked and reacted with a series of alkyl, aryl, amine, and alcohol reagents. A series of 28 analogues was prepared having a glycol or a catechol core, a stereodefined configuration of the flanking hydroxyl groups, and an acetylenic appendage suitable for a coupling to a lactone-bearing fragment.