Diastereoselective synthesis of the pectenotoxin 2 non-anomeric AB spiroacetal

Rhodium-catalysed vinyl 1,4-conjugate addition coupled with Sharpless asymmetric dihydroxylation in the synthesis of the CDE ring fragment of pectenotoxin-4

Our synthesis of the CDE ring fragment of pectenotoxin-4 utilised two key steps to make the complex bicyclic ketal unit: (i) a rhodium-catalysed vinyl group 1,4-addition as the major C-C bond forming step; (ii) a stereoselective Sharpless Asymmetric Dihydroxylation (SAD) of the resulting 1,1-disubstituted homoallylic alcohol. Subsequent acid-catalysed cyclisation afforded the desired [5,6]-bicyclic ketal of the target molecule. This methodology was shown to be compatible with the desired E ring fragment 35 in order to construct the CDE fragment 37 of pectenotoxin-4.

Cobalt versus Osmium: Control of Both trans and cis Selectivity in Construction of the EFG Rings of Pectenotoxin 4

Catalytic oxidative cyclisation reactions have been employed for the synthesis of the E and F rings of the complex natural product target pectenotoxin 4. The choice of metal catalyst (cobalt- or osmium-based) allowed for the formation of THF rings with either trans or cis stereoselectivity. Fragment union using a modified Julia reaction then enabled the synthesis of an advanced synthetic intermediate containing the EF and G rings of the target.

Function-Oriented Studies Targeting Pectenotoxin 2: Synthesis of the GH-Ring System and a Structurally Simplified Macrolactone

A chemical foundation for function-oriented studies of pectenotoxin 2 (PTX2) is described. A synthesis of the bicyclic GH-system, and the design and synthesis of a PTX2-analogue, is presented. While maintaining critical features for actin binding, and lacking the Achilles’ heel for the natural product’s anticancer activity (the AB-spiroketal), this first-generation analogue did not possess the anticancer properties of PTX2, an observation that indicates the molecular significance of features present in the natural product’s CDEF-tetracycle.

A bidirectional synthesis of spiroacetals via Rh(ii)-catalysed C-H insertion

Acyclic methylene acetals bearing two diazoester subunits have been converted to [5,5]-spiroacetals via bidirectional C-H insertion under Rh(ii) catalysis. Using a chiral Rh(ii) catalyst, the major diastereomer can be produced in high enantiomeric excess (89%).

Generation, structure and reactivity of tertiary organolithium reagents

Tertiary organolithium reagents have great potential in natural product synthesis. Recent progress in organolithium generation has made them accessible. Their utility and stereoselectivity in synthesis is discussed.

Interplay of cascade oxidative cyclization and hydride shifts in the synthesis of the ABC spiroketal ring system of pectenotoxin-4

Concepts: The formation of stereochemically defined bis-THF units through a double cyclization and a hydride-shift-initiated route to spiroketals is described (see scheme; Xc=chiral auxiliary). The resulting sequence has been used in a synthesis of the C1-16 fragment of the naturally occurring antitumor agent pectenotoxin-4.

Synthesis of the C1-C26 hexacyclic subunit of pectenotoxin 2

Synthesis of the C1-C26 hexacyclic subunit of pectenotoxin-2 (PTX-2) is described that features a stereoselective annulation to generate the C-ring by triple asymmetric Nozaki-Hiyama-Kishi coupling followed by oxidative cyclization. Preparation of the C1-C14 AB spriroketal-containing subunit employs a recently developed metallacycle-mediated reductive cross-coupling between a TMS-alkyne and a terminal alkene.

A convergent route to the CDEF-tetracycle of pectenotoxin-2

A convergent synthesis of the CDEF-tetracyclic region of pectenotoxin-2 (PTX-2) is described. The synthetic pathway derives from a head-to-tail union of 2 equiv of linalool to establish a stereodefined DEF-tricyclic aldehyde. Subsequent coupling with a "northern" fragment enolate, followed by a tandem Sharpless epoxidation/cyclization, delivers the C10-C26 polycyclic region of the natural product.

Highly diastereoselective domino synthesis of 6-spirosubstituted pyrido[2,3-d]pyrimidine derivatives in water

A highly diastereoselective domino reaction of 2,6-diaminopyrimidine-4-one with structurally diverse aryl aldehydes and various barbituric acids in water under microwave irradiation is described. The products are 6-spiro-substituted pyrido[2,3-d]pyrimidines with high diastereoselectivities (up to 99: 1) in which the major diastereomer bears a cis relationship between substituents at the 5- and 7-positions. Furthermore, the mechanism for diastereoselectivity was confirmed by DFT (B3LYP) calculations.

Spirodiepoxide strategy to the C ring of pectenotoxin 4: synthesis of the C1-C19 sector

The synthesis of a C1-C19 precursor to pectenotoxin 4 is presented. The strategy employed the functionalized allene shown. Key features include: olefin metathesis of two simple fragments to prepare the left portion of the allene-precursor, diastereoselective propargylation of an epoxy aldehyde to form the right portion, use of the DMDO-stable m-fluorobenzyl ether, and an allene spirodiepoxidation/C-ring formation cascade.

Recent synthetic approaches toward non-anomeric spiroketals in natural products

Many natural products of biological interest contain [6,5]- and [6,6]-spiroketal moieties that can adopt various configurations, benefiting or not from anomeric conformation stabilizing effects. The spiroketal fragments are often important for the biological activity of the compounds containing them. Most stable spiroketal stereoisomers, including those benefiting from conformational anomeric effects (gauche conformers can be more stable than anti conformers because of a contra-steric stabilizing effect), are obtained easily under acidic conditions that permit acetal heterolysis (formation of tertiary oxycarbenium ion intermediates). The synthesis of less stable stereoisomers requires stereoselective acetal forming reactions that do not permit their equilibration with their most stable stereoisomers or, in the case of suitably substituted derivatives, concomitant reactions generating tricyclic products that quench the less stable spiroketal conformers. Ingenuous approaches have been recently developed for the synthesis of naturally occurring [6,6]- and [5,6]-nonanomeric spiroketals and analogues. The identification of several parameters that can influence the stereochemical outcome of spirocyclization processes has led to seminal improvements in the selective preparation of the non-anomeric isomers that are discussed herein. This review also gives an up-dated view of conformational anomeric effect which represents a small fraction of the enthalpic anomeric effect that makes gem-dioxy substituted compounds much more stable that their 1,n-dioxy substituted isomers (n > 1). Although models assuming sp3-hybridized oxygen atoms have been very popular (rabbit ears for the two non-bonding electron pairs of oxygen atom), sp2-hybridized oxygen atoms are used to describe the conformational anomeric effect.

Direct kinetic formation of nonanomeric [6.5]-spiroketals in aqueous media

The direct kinetic formation of spiroketals from mixed ketal-alcohol precursors under acid catalysis was studied using four differently substituted systems. In all cases, the exclusive formation of the anomeric isomer was observed under equilibrating conditions. However, the formation of the nonanomeric spiroketal isomer was observed if the reaction was performed under kinetic conditions using an appropriately tuned acid. Water had a dramatic accelerating effect on the spiroketalization reactions that were performed in THF, and the highest yields of the nonanomeric products were obtained in aqueous THF. The nonanomeric/anomeric product ratio was also strongly affected by the substituents and the stereochemistry of the starting alcohol.