Total synthesis and structure validation of (+)-bistramide C

Empirical Chiroptical Analyses of Vicinal Bromochloro Natural Products by van't Hoff's Principle of Optical Superposition: Assignment of the C-16 Configurations of Callophycols A and B

A simple empirical method is described that allows the assignment of absolute configurations of natural products containing chiral vicinal bromochloro (VBC) units, including the bromochloro substituted isoprenyl units present in the structures of antiproliferative halomon (1a) and its halogen-swapped isomer iso-halomon (1b) from the red alga, Portieria hornemannii, and callophycols A (3) and B (4) from Callophycus serratus. The relative configurations of 3 and 4, published in 2007, were incomplete: C-16 was left unassigned. It is now shown that the additivity of molar rotations, [M]D (herein, abbreviated [M])─a consequence of van't Hoff's principle of optical superposition─could be used to deconvolute rotatory contributions, designated as [MX] and [MY] of the two remotely spaced chiral substructures within 3 and 4 using simple arithmetic. Input of proxy values, [M Y1] and [MY2], for the two different VBC units in two equations for [MX] and application of a "conditional test" returns the same value for [MX] only when a proxy with the correct configuration is included. It is revealed that 3 and 4 have opposite configurations at the C-16 stereocenter: 16S and 16R, respectively. Two important implications lie in these findings: 3 and 4 appear to qualify as paired-regioisomers, coupled through a putative dyotropic rearrangement (DR), and the biosyntheses of other Callophycus secondary metabolites, now numbering over 50, are tightly controlled by stereoelectronic considerations including neighboring group interactions of the DR. It now appears, counter to earlier suggestions, that the chirality of Callophycus secondary metabolites, despite their high chemodiversity, are surprisingly highly conserved. Enantiofacial halogenation additions to the C═C double bonds of precursor alkenes appear to direct the formation of the remaining stereocenters at both the halogenated benzoate-decalin core and the distal VBC of 3 and 4. A consistent hypothesis is proposed to account for macrolactonizations in other Callophycus natural products including bromophycolides A and B. The conditional test of molar rotations was applied in a different context to understand the chiroptical properties and trends observed in the highly iodinated meroditerpenes, iodocallophycols A-E, also from Callophycus sp., resulting in the revision of the configuration of callophycol E from (10R,14R) to (10S,14S).

Catalytic, Tunable, One-Step Bismuth(III) Triflate Reaction with Alcohols: Dehydration Versus Dimerization

Bi(OTf)₃·xH₂O is a powerful catalyst for the dehydration of tertiary alcohols into alkenes in apolar solvents. The reaction proceeds smoothly and selectively, with amounts as low as 0.01 mol % catalyst, in yields up to 93%. Moreover, in polar solvents, Bi(OTf)₃·xH₂O (0.1-1 mol %) selectively catalyzes the dimerization of the alcohols instead, forming new C-C bonds, in yields up to 96%. This mild, efficient, economic, and eco-friendly method is applicable across different chemical classes and amenable to several functional groups.

Spiroacetal formation through telescoped cycloaddition and carbon-hydrogen bond functionalization: total synthesis of bistramide A

Spiroacetals can be formed through a one-pot sequence of a hetero-Diels-Alder reaction, an oxidative carbon-hydrogen bond cleavage, and an acid treatment. This convergent approach expedites access to a complex molecular subunit which is present in numerous biologically active structures. The utility of the protocol is demonstrated through its application to a brief synthesis of the actin-binding cytotoxin bistramide A.

Walking in the woods with quantum chemistry--applications of quantum chemical calculations in natural products research

This Highlight describes applications of quantum chemical calculations to problems in natural products chemistry, including the elucidation of natural product structures (distinguishing between constitutional isomers, distinguishing between diastereomers, and assigning absolute configuration) and determination of reasonable mechanisms for their formation.

Bi(OTf)3-, TfOH-, and TMSOTf-mediated, one-pot epoxide rearrangement, addition, and intramolecular silyl-modified Sakurai (ISMS) cascade toward dihydropyrans: comparison of catalysts and role of Bi(OTf)3

Catalytic quantities of bismuth(III) triflate efficiently initiate the rearrangement of epoxides to aldehydes, which subsequently react with (Z)-δ-hydroxyalkenylsilanes to afford 2,6-disubstituted 3,6-dihydro-2H-pyrans. Isolated yields of desired products using Bi(OTf)(3) were compared with yields obtained when the reactions were run with TfOH and TMSOTf in the presence and absence of several additives. These studies, as well as NMR spectroscopic analyses, indicate an initial Lewis acid/base interaction between Bi(OTf)(3) and substrates providing TfOH in situ.

Synthesis of a platform to access bistramides and their analogues

The platform C14-C40, which can be used to prepare bistramide C and 39-oxobistramide K, was synthesized in 19 steps with an overall yield of 6.2%. Furthermore, the chemoselective reduction of the ketone at C-39 was performed giving an easy access to bistramides A, B, D, K, and L. Finally, the versatility of the synthesis of the C14-C40 fragment can allow the preparation of a large variety of stereoisomers to produce bistramide analogues.

Synthesis of a 35-member stereoisomer library of bistramide A: evaluation of effects on actin state, cell cycle and tumor cell growth

Synthesis and preliminary biological evaluation of a 35-member library of bistramide A stereoisomers are reported. All eight stereoisomers of the C1-C13 tetrahydropyran fragment of the molecule were prepared utilizing crotylsilane reagents 9 and 10 in our [4+2]-annulation methodology. In addition, the four isomers of the C14-C18 gamma-amino acid unit were accessed via a Lewis acid mediated crotylation reaction with use of both enantiomers of organosilane 11. The spiroketal subunit of bistramide A was modified at the C39-alcohol to give another point of stereochemical diversification. The fragments were coupled by using a standard peptide coupling protocol to provide 35 stereoisomers of the natural product. These stereochemical analogues were screened for their effects on cellular actin and cytotoxicity against cancer cell lines (UO-31 renal and SF-295 CNS). The results of these assays identified one analogue, 1.21, with enhanced potency relative to the natural product, bistramide A.

Making mixtures to solve structures: structural elucidation via combinatorial synthesis

A domino Horner-Wadsworth-Emmons olefination strategy has been used to prepare homologous series of (polyen)ones, and through combinatorial elaboration, corresponding families of highly branched hydrocarbons. Gas chromatography-mass spectrometry of the mixtures has enabled the rapid and unambiguous identification of several highly branched alkanes of geochemical importance. This is the first example of the use of combinatorial synthesis for the elucidation of structural connectivity.

Asymmetric synthesis of naturally occurring spiroketals

Spiroketals are widely found as substructures of many naturally occurring compounds from diverse sources including plants, animals as well as microbes. Naturally occurring spiroketals are biologically active and most of them are chiral molecules. This article aims at reviewing the asymmetric synthesis of biologically active spiroketals for last 10 years (1998-2007).

Marine natural products

This review covers the literature published in 2005 for marine natural products, with 704 citations (493 for the period January to December 2005) referring to compounds isolated from marine microorganisms and phytoplankton, green algae, brown algae, red algae, sponges, coelenterates, bryozoans, molluscs, tunicates and echinoderms. The emphasis is on new compounds (812 for 2005), together with their relevant biological activities, source organisms and country of origin. Biosynthetic studies, first syntheses, and syntheses that lead to the revision of structures or stereochemistries, have been included.

Marine natural products: synthetic aspects

An overview of marine natural products synthesis during 2005 is provided. In a similar vein to earlier installments in this series, the emphasis is on total syntheses of molecules of contemporary interest, new total syntheses, and syntheses that have resulted in structure confirmation or stereochemical assignments.

Spiroketals via oxidative rearrangement of enol ethers

Oxidative rearrangement of cyclic enol ethers leads to alpha-alkoxyesters. In the presence of a neighboring spiroether, this approach provides a stereoselective access to spiroketals. A modified proposal for the biosynthesis of acutumine is presented.

Total Synthesis of Bistramide A

An asymmetric synthesis of the marine metabolite bistramide A is reported. The synthesis relies on the utility of three different organosilane reagents to construct all principle fragments and 8 of the 11 stereogenic centers of the natural product. [structure: see text].

Enantioselective total synthesis of bistramide A

The enantioselective synthesis of bistramide A has been achieved with a longest linear sequence of 18 steps. The synthetic strategy involves the use of a distereoselective glycolate alkylation, an aldol addition of a chlorotitanium enolate of N-acylthiazolidinthione, and a Sharpless asymmetric epoxidation to synthesize the three key fragments.

Synthesis of derivatives of potent antitumor bistramides D and A leading to the first crystal structure of natural bistramide D

We report a crystalline derivative of bistramide D synthesized from natural bistramide A, and its structure was determined by X-ray analysis.

Systematic Assignment of the Configuration of Flexible Natural Products by Spectroscopic and Computational Methods: The Bistramide C Analysis

[reaction: see text] The combination of NMR NOE, chemical shift, and J-coupling measurements with molar rotation and circular dichroism (CD) determinations, including RI-DFT BP86/aug-cc-pVDZ calculations, reduced a candidate pool of 1024 possible stereoisomers of (+)-bistramide C to a single absolute configuration assignment for the 10 stereogenic carbons of the marine natural product.