Toward the enantioselective total synthesis of lyngbyatoxin A: on the stereocontrolled introduction of the quaternary stereogenic centre

This paper deals with an approach to the enantioselective total synthesis of Lyngbyatoxin A, with focus on the stereocontrolled introduction of the quaternary stereogenic centre. The key step in the synthesis involves an enantiospecific Lewis-acid mediated rearrangement of chiral vinyl epoxides carrying a 7-substituted indole moiety.     

Toward a total synthesis of peloruside A: enantioselective preparation of the C8-C19 region

An efficient synthetic sequence toward the C8-C19 region of peloruside A has been developed. The route is highlighted by a selective electrophilic cyclization reaction, a single-step epoxide ring-opening/methylation sequence, and a stereoselective Mukaiyama aldol reaction. [reaction: see text]     

Toward an enantioselective synthesis of (-)-zampanolide: preparation of the C9-C20 region

Progress toward the synthesis of the microtubule-stabilizing agent, (-)-zampanolide, is reported. Construction of the 2,6-cis-tetrahydropyran ring was accomplished utilizing ether transfer methodology in conjunction with an intramolecular radical cyclization reaction. Efficient installation of the C16-C20 side chain relied on a one-pot cross-metathesis/olefination sequence, Sharpless epoxidation, and selective reduction of a vinyl epoxide.     

Promising anticancer drug thapsigargin: A perspective toward the total synthesis

The guaianolide ring containing sesquiterpene thapsigargin is found in the roots and fruits of Mediterranean plant Thapsia garganica L. It is known for its activity as a potent antagonist for Ca²⁺-ATPase (sarco–endoplasmic reticulum Ca²⁺-ATPase) inhibition. Recently, a prodrug mipsagargin is being investigated to target the blood vessel of the cancer cells for the treatment of tumors. The limited natural supply (low isolation and only localized growth (Mediterranean area)) from the natural sources strongly urges for the development of chemical synthetic strategies to access these natural products. This review pertain the various strategies used so far in the thapsigargin’s synthesis, focusing on major contributions in the total synthesis till date.     

Sequential desymmetrization-fluorination: enantioselective synthesis of fluorinated cyclitols

An asymmetric synthesis of enantioenriched, highly functionalized fluorinated carbocycles has been developed based on an enantioselective Sharpless dihydroxylation of cyclohexadienylsilanes, combined with a diastereoselective electrophilic fluorodesilylation. Several parameters define the level of diastereocontrol for the fluorination step. These include the relative stereochemistry of the starting endocyclic allylsilanes and the structural features of the reactants. As expected for an S(E)2' mechanism, the fluorodesilylation occurred with clean transposition of the double bond, with the attack of the electrophilic fluorinating agent taking place preferentially anti to the silyl group. For the fluorination step, the best selectivities were observed for the monocyclic anti,syn benzyl-protected precursors and for the syn,syn starting allylsilane. Full NMR spectroscopic analysis of the fluorinated monocyclic compounds, which possess an endocyclic double bond flanked by two electronegative groups (the methoxy group and the fluorine substituent) revealed that a subtle combination of steric (1,3-axial/pseudoaxial interaction) and stereoelectronic effects (pi-sigma* interaction) favours the preferential conformers featuring the methoxy group in a pseudoaxial position.     

An enantioselective synthesis of tarchonanthuslactone

An enantioselective synthesis of tarchonanthuslactone has been achieved in eight steps from ethyl sorbate. The asymmetry of the route was introduced via a Sharpless asymmetric dihydroxylation allowing access to either enantiomer. The synthesis utilizes a palladium-catalyzed reduction and a diastereoselective base-catalyzed acetal formation as the key steps. The pyran ring of tarchonanthuslactone was established by a Still-olefination/lactonization sequence. DCC-mediated attachment of dihydrocaffeic acid completed the synthesis of tarchonanthuslactone in a 19% overall yield.     

Organosilanes in synthesis: application to an enantioselective synthesis of methyl-l-callipeltose

[reaction: see text] Methyl-l-callipeltose, the carbohydrate associated with callipeltoside A, has been synthesized in eight steps and 23% overall yield from enantioenriched allylsilane 6 and acetaldehyde. The key steps are a highly diastereoselective formal [4 + 2] annulation and a Cr(VI)-catalyzed oxidative C-C bond cleavage to produce lactone 11.     

Strategies for the enantioselective synthesis of spirooxindoles

Oxindoles and spirooxindoles are important synthetic targets that are often considered to be prevalidated with respect to their biological activity and applications for pharmaceutical lead discovery. This review features efficient strategies for the enantioselective synthesis of spirocyclic oxindoles, focusing on reports in 2010 and 2011. Although enantioselective synthesis remains an ongoing challenge, exciting recent advances in this area feature spirooxindoles with greater complexity, up to eight stereogenic centers, more practical synthetic methods, and new catalytic activation strategies. Developments in catalyst systems and reaction conditions have shown that many reactions can be optimized to control selectivity and provide access to isomeric products, which are important for biological testing. This review is organized based on two primary disconnection strategies, and then further subdivided into the type and ring size of the spirocycle that is generated. Strategies are also compared for the synthesis of non-spirocyclic 3,3'-disubstituted oxindoles.     

A flexible enantioselective synthesis of the isofurans

Recently, the isolation of a new class of human arachidonic acid tetrahydrofuran oxidation products, the isofurans (IsoF's), was reported. These new compounds are available from natural sources only in microgram quantities as mixtures. The enantioselective preparation of a versatile epoxide intermediate and its conversion to the enantiomerically pure isofurans SC-Delta(13)-9-IsoF and 15-epi-SC-Delta(13)-9-IsoF are described. This synthesis will make these metabolites available for physiological evaluation.     

Concise enantioselective synthesis of furocaulerpin

The first total synthesis of both enantiomers of furocaulerpin was accomplished in good yields with a high control of the stereogenic center by enzymatic resolution, a total control of the configuration of the central double bond and the construction of the dienyne moiety via a Stille cross-coupling.     

Toward general access to the aspidosperma-type terpenoid indole alkaloids: synthesis of the key 3,3-disubstituted piperidones through enantioselective intramolecular heck-type reaction of chloroformamides

An enantioselective intramolecular Heck-type reaction of chloroformamides has been developed for the synthesis of 3,3-disubstituted piperidones. The desired piperidone was formed in the presence of a palladium catalyst, an optically active phosphoramidite ligand, K3PO4 and Ag3PO4. The obtained piperidone was converted to epieburnamonine.     

Total synthesis of thapsigargin, a potent SERCA pump inhibitor

The enantioselective total synthesis of thapsigargin, a potent, selective inhibitor of the Ca2+ pump SERCA, is described. Starting from ketoalcohol 8, key steps involve regioselective introduction of the internal olefin at C4-C5, judicious protecting group choice to allow chelation-controlled reduction at C3, and chemoselective introduction of the angelate ester function at C3-O. A selective esterification approach completes the total synthesis in a total of 42 steps and 0.61% overall yield (88.6% average yield per step). [reaction: see text].     

Migratory hydroamination: a facile enantioselective synthesis of benzomorphans

We describe a highly efficient, general strategy for the enantioselective synthesis of benzomorphans (45-46% overall yield from commercially available material). The new synthesis demonstrates the effectiveness of an unprecedented diastereoselective cycloisomerization via migratory hydroamination and the power of palladium-catalyzed asymmetric allylic alkylation (AAA) of simple ketone enolates in the context of complex synthesis. The strategy outlined here for the enantioselective synthesis of three contiguous stereogenic centers and the novel cycloisomerization should have many applications in alkaloid synthesis.     

Carbohydrates in asymmetric synthesis: enantioselective allylation of aldehydes

An enantioselective metal-mediated addition of allylic bromides to carbonyl compounds was achieved in the presence of the inexpensive and easily accessible carbohydrates saccharose and β-cyclodextrin. The desired products were obtained in moderate to excellent yields and with up to 93% enantiomeric excess.     

A highly enantioselective benzothiepine synthesis

A highly enantioselective synthesis of benzothiepine 1a has been accomplished via an enantioenriched sulfoxide intermediate obtained by asymmetric oxidation with a chiral oxaziridine in 89:11 er. The key step is a thermodynamically controlled asymmetric cyclization reaction that produces two new stereogenic centers. The (4R,5R) isomer 1a was obtained in 98:2 er.     

Studies toward the total synthesis of popolohuanone E: enantioselective synthesis of 8-O-methylpopolohuanone E

[structure: see text]. 8-O-methylpopolohuanone E (2) was synthesized in a highly convergent manner starting from the cis-fused decalin derivative accessible from the (-)-Wieland-Miescher ketone analogue. The synthetic method features a biogenetic-type annulation of the phenolic and quinone segments to regioselectively construct the central tricyclic ring system as the key step.     

Highly enantioselective synthesis of isoxazoline N-oxides

The reaction of cinchonidine (cinchonine)-derived ammonium salts with nitroolefins in the presence of Cs2CO3 to afford optically active isoxazoline N-oxides with excellent ee and high de values has been developed.     

Formal enantioselective synthesis of (+)-compactin

[reaction: see text] The challenging structural features and important biological activity of (+)-compactin (1) explain the substantial synthetic interest that it has generated. We report a novel enantioselective approach to the advanced intermediate 2a, which constitutes a formal synthesis of (+)-1. The sequence utilizes MacMillan's organocatalytic Mukaiyama-Michael reaction, which stereoselectively adds the silyloxyfuran 6 to alpha,beta-unsaturated aldehyde 7. The chirality generated in this reaction guides the formation of the other three consecutive stereocenters found in 2a.     

Design and total synthesis of unnatural analogues of the sub-nanomolar SERCA inhibitor thapsigargin

Thapsigargin is a densely oxygenated guaianolide which displays potent sarco/endoplasmic reticulum Ca(2+) ATPase (SERCA) binding affinities. The total syntheses of designed unnatural analogues of this important natural product are described. This article constitutes the chemical synthesis behind an ongoing project. Rational modifications have been made to the lactone region of thapsigargin in order to obtain derivatives for future structure-activity relationship studies.