Bidirectional and Convergent Routes to oligo(Tetrahydrofurans)

Bifunctionalized Allenes. Part XVI. Synthesis of 3-Phosphoryl-2,5-dihydrofurans by Coinage Metal-Catalyzed Cyclo-isomerization of Phosphorylated α-Hydroxyallenes

Phosphorylated α-hydroxyallenes 1 and 2 were smoothly converted into the corresponding 2,5-dihydrofurans 3 and 4 in an 5-endo-trig cycloisomerization reaction by using 5 mol % of coinage metal salts as catalyst. Experimental conditions such as the type of the solvent, the reaction temperature, the mol % and the type of the catalyst were optimized. This mild and efficient cyclization method can be applied to dimethyl 1-hydroxyalkyl-alka-1,2-dienephosphonates 1 and 2-diphenylphosphinoyl-2,3-dien-1-ols 2a–c and 3-diphenylphosphinoyl-3,4-dien-2-ols 2d,e, furnishing 3-phosphorylated 2,5-dihydrofurans 3 and 4 in very good yields.

Bifunctionalized allenes. Part XV. Synthesis of 2,5-dihydro-1,2-oxaphospholes by electrophilic cyclization reaction of phosphorylated α-hydroxyallenes

This paper discusses a reaction of phosphorylated α-hydroxyallenes with protected or unprotected hydroxy groups involving 5-endo-trig cyclizations. Various electrophilic reagents such as sulfuryl chloride, bromine, benzenesulfenyl and benzeneselenenyl chlorides have been applied. The paper describes the reaction of 1-hydroxyalkyl-1,2-dienephosphonates with electrophiles that produces 2-methoxy-2-oxo-2,5-dihydro-1,2-oxaphospholes due to the participation of the phosphonate neighbouring group in the cyclization. On the other hand, (1E)-alk-1-en-1-yl phosphine oxides were prepared as mixtures with 2,5-dihydro-1,2-oxaphosphol-2-ium halides in a ratio of about 1:2 by chemo-, regio, and stereoselective electrophilic addition to the C(2)-C(3)-double bond in the allene moiety and subsequent concurrent attack of the external (halide anion) and internal (phosphine oxide group) nucleophiles. The paper proposes a possible mechanism that involves cyclization and additional reactions of the phosphorylated α-hydroxyallenes.

Ion-channels: goals for function-oriented synthesis

Ion channels provide a conductance pathway for the passive transport of ions across membranes. These functional molecules perform key tasks in biological systems such as neuronal signaling, muscular control, and sensing. Recently, function-oriented synthesis researchers began to focus on ion channels with the goal of modifying the function of existing ion channels (ion selectivity, gating) or creating new channels with novel functions. Both approaches, ion channel engineering and de novo design, have involved synthetic chemists, biochemists, structural biologists, and neurochemists. Researchers characterize the function of ion channels by measuring their conductance in samples of biological membranes (patch clamp) or artificial membranes (planar lipid bilayers). At the single molecule level, these measurements require special attention to the purity of the sample, a challenge that synthetic chemists should be aware of. Ideally, researchers study the function of channels while also acquiring structural data (X-ray, NMR) to understand and predict how synthetic modifications alter channel function. Long-term oriented researchers would like to apply synthetic ion channels to single molecule sensing and to implantat these synthetic systems in living organisms as tools or for the treatment of channelopathies. In this Account, we discuss our own work on synthetic ion channels and explain the shift of our research focus from a de novo design of oligo-THFs and oligo-THF-amino acids to ion channel engineering. We introduce details about two biological lead structures for ion channel engineering: the gramicidin β(6,3) helix as an example of a channel with a narrow ion conductance pathway and the outer membrane porins (OmpF, OmpG) with their open β-barrel structure. The increase and the reversal of ion selectivity of these systems and the hydrophobic match/mismatch of the channel with the phospholipid bilayer are of particular interest. For engineering ion channels, we need to supplement the single-point attachment of a synthetic modulator with the synthesis of a more challenging two-point attachment. The successful function-oriented synthesis of ion channels will require interdisciplinary efforts that include new electrophysiology techniques, efficient synthesis (peptide/protein/organic), and good structural analysis.

Total synthesis, stereochemical assignment, and biological activity of chamuvarinin and structural analogues

A highly stereocontrolled synthesis of (+)-chamuvarinin has been completed in 1.5% overall yield over 20 steps. The key fragment coupling reactions were the addition of alkyne 8 to aldehyde 7 (under Felkin-Anh control), followed by the two step activation/cyclization to close the C20-C23 2,5-cis-substituted tetrahydrofuran ring and a Julia-Kocienski olefination at C8-C9 to introduce the terminal butenolide. The inherent flexibility of our coupling strategy led to a streamlined synthesis with 17 steps in the longest sequence (2.2% overall yield), in which the key bond couplings are reversed. In addition, a series of structural analogues of chamuvarinin have been prepared and screened for activity against HeLa cancer cell lines and both the bloodstream and insect forms of Trypanosoma brucei, the parasitic agent responsible for African sleeping sickness.

anti-Oligoannelated THF moieties: synthesis via push-pull-substituted cyclopropanes

The first synthesis of anti-fused oligoannelated THF moieties is reported. The key transformation of the synthetic sequence, consisting of cyclopropanation, reduction and oxidation, is the expansion of a push-pull-substituted three-membered ring into a five-membered enol ether system. A repetition of the sequence allows the creation of oligoacetals up to a nonacyclic system.

(-)-Complanine, an inflammatory substance of marine fireworm: a synthetic study

The synthesis of (-)-complanine, an inflammatory substance of Eurythoe complanata, was accomplished by a "chiral synthon" approach. The absolute configuration of this molecule was determined to be R.

Systematic Synthesis of Antitumor Annonaceous Acetogenins

Systematic synthesis of mono- and bis-THF ring cores, synthetic intermediates of antitumor annonaceous acetogenins, has been achieved by asymmetric alkynylation and subsequent stereodivergent THF ring formation as key steps. The asymmetric alkynylation of alpha-oxyaldehyde and alpha-tetrahydrofuranic aldehyde with (S)-3-butyne-1,2-diol derivatives gave good yield with very high diastereoselectivity. These adducts were converted into mono- and bis-THF cores via two kinds of one-pot THF ring formation, respectively. In addition, the total synthesis of murisolin, which shows cytotoxic activity against human tumor cell lines with potency from 10(5) to 10(6) times that of adriamycin, was also achieved.

A practical synthesis of the F-ring of halichondrin B via ozonolytic desymmetrization of a C(2)-symmetric dihydroxycyclohexene

C(2)-symmetric dihydroxycyclohexene 1 was desymmetrized via a one-pot Criegee ozonolysis/acylation protocol to afford acetal-lactone 2. Installation of the allyl side chain on the convex face of the bicyclic system and subsequent reduction provided the desired tetrahydrofuran 4 with the correct relative and absolute stereochemistries. Simple functional group manipulations led to the desired F-ring module 3 of halichondrin B.

Highly stereoselective and stereodivergent synthesis of four types of THF cores in acetogenins using a C4-chiral building block

[reaction: see text] Four stereoisomers of the THF cores, synthetic intermediates of acetogenins, have been synthesized with high diastereoselectivity by asymmetric alkynylation and subsequent stereodivergent THF ring formation. The asymmetric alkynylation of alpha-oxyaldehyde with (S)-3-butyne-1,2-diol derivatives (C4-unit) gave good yields of syn and anti adducts with >97:3 dr and 94:6 dr, respectively. These adducts were converted into the four types of THF compounds via one-pot THF formation or via intramolecular Williamson synthesis.

Gold(III) chloride catalyzed cyclization of alpha-hydroxyallenes to 2,5-dihydrofurans

[reaction: see text] Functionalized alpha-hydroxyallenes 1 were smoothly converted into the corresponding 2,5-dihydrofurans 2 by using 5-10 mol % of gold(III) chloride as catalyst. This mild and efficient cyclization method can be applied to alkyl- and alkenyl-substituted allenes at room temperature, furnishing tri- and tetrasubstituted dihydrofurans in good to excellent chemical yields and with complete axis to center chirality transfer.

Chemo-, Regio-, and Stereoselective Cyclizations of 1,3-Bis(trimethylsilyloxy)-1,3-butadienes withα-Chloroacetic Acid Chlorides andα-Chloroacetic Acetals

Treatment of 1,3-bis(trimethylsilyloxy)-1,3-butadienes with alpha-chlorocarboxylic acid chlorides resulted in chemo- and regioselective formation of 6-chloro3,5-dioxo esters, which were regioselectively converted into functionalised 3(2H)furanones. Chemo- and regioselective condensation of 1,3-bis(trimethylsilyloxy)-1,3-butadienes with alpha-chloroacetic dimethyl acetal afforded 6-chloro-5-methoxy-3-oxo esters, which could be regio- and stereoselectively transformed into 2-alkylidene-4-methoxytetrahydrofurans.

Regio- and stereoselective synthesis of Nor-nonactinic acid derivatives--kinetic reaction control in the Lewis acid mediated domino reaction of 1,3-dicarbonyl dianions with 1-bromo-2,3-epoxypropanes

Reaction of 1,3-dicarbonyl dianions with epibromohydrin derivatives results in formation of functionalized 2-alkylidene-5-hydroxymethyltetrahydrofurans. These reactions proceed by chemoselective attack of the dianion onto the carbon attached to the bromine atom and subsequent nucleophilic attack of the resultant monoanion onto the epoxide. The cyclization products, which were formed with very good regio- and stereoselectivities, are of pharmacological relevance and represent versatile building blocks for the synthesis of natural products.

Total synthesis of asimicin and bullatacin

The efficient total synthesis of asimicin, 1, and bullatacin, 2, has demonstrated the advantages of three different strategies for the synthesis of the tricyclic intermediates 6 and 7, which represent the key fragment of the bis-THF Annonaceous acetogenins. The naked carbon skeleton strategy is based on the production of all asymmetric centers by selective placement of the oxygen functions onto an unsaturated, nonfunctionalized carbon skeleton. Diversity in this approach arises from the relative timing of highly stereoselective reactions, such as the Sharpless asymmetric dihydroxylation (AD) reaction, the Kennedy oxidative cyclization (OC) with rhenium(VII) oxide, the Mitsunobu-type alcohol epimerization reaction, and the Williamson etherification reaction. The convergent strategy, which is based on the combinatorial coupling of two series of diastereomeric fragments, to produce intermediates such as 11 and 12, enjoys the advantages of both efficiency and versatility. The third approach, which is based on partially functionalized intermediates, such as 13, combines the advantages of both the linear and the convergent strategies-synthetic efficiency and diversity.

Annonaceous acetogenins: recent progress

The Annonaceous acetogenins are promising new antitumor and pesticidal agents that are found only in the plant family Annonaceae. Chemically, they are derivatives of long-chain fatty acids. Biologically, they exhibit their potent bioactivities through depletion of ATP levels via inhibiting complex I of mitochondria and inhibiting the NADH oxidase of plasma membranes of tumor cells. Thus, they thwart ATP-driven resistance mechanisms. This review presents the progress made in the chemistry, biology, and development of these compounds since December 1995.