Synthesis of the C21–C28 segment of pectenotoxin-4

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.

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.

Stereoselective, Ag-Catalyzed Cyclizations To Access Polysubstituted Pyran Ring Systems: Synthesis of C1-C12 Subunit of Madeirolide A

The exploration into the scope of a silver-catalyzed cyclization (AgCC) of propargyl benzoates for accessing pyran ring systems has been reported. The impact of the degree of substitution, nature of the substitution on the carbon backbone/benzoate moiety, and stereochemistry has been evaluated. The application of this methodology to the synthesis of the C1-C12 southern fragment of madeirolide A is disclosed.

The conversion of allenes to strained three-membered heterocycles

The addition of heteroatoms to an allenic double bond yields strained heterocycles that serve as scaffolds for further useful transformations.

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.

Diastereoselective chelation-controlled additions to β-silyloxy aldehydes

A general diastereoselective method for the addition of dialkylzincs and (E)-di- and (E)-trisubstituted vinylzinc reagents to β-silyloxy aldehydes is presented. This method employs alkyl zinc triflate and nonaflate Lewis acids and affords chelation-controlled products (6:1 to > 20:1 dr).

Spirodiepoxide-based cascades: direct access to diverse motifs

Allene epoxide formation/opening reaction sequences enabled direct access to diverse products. Described here are a single flask procedure for allene preparation and allene oxidation/derivatization reactions that give, among others, diendiol, diyndiol, α'-hydroxy-γ-enone, dihydrofuranone, butenolide, and δ-lactone products.

Highly diastereoselective chelation-controlled additions to α-silyloxy ketones

The polar Felkin-Anh, Cornforth-Evans, and Cram-chelation models predict that the addition of organometallic reagents to silyl-protected α-hydroxy ketones proceeds via a nonchelation pathway to give anti-diol addition products. This prediction has held true for the vast majority of additions reported in the literature, and few methods for chelation-controlled additions of organometallic reagents to silyl-protected α-hydroxy ketones have been introduced. Herein, we present a general and highly diastereoselective method for the addition of dialkylzincs and (E)-di-, (E)-tri-, and (Z)-disubstituted vinylzinc reagents to α-silyloxy ketones using alkyl zinc halide Lewis acids, RZnX, to give chelation-controlled products (dr ≥18:1). The compatibility of organozinc reagents with other functional groups makes this method potentially very useful in complex molecule synthesis.

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.

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.

Overriding Felkin control: a general method for highly diastereoselective chelation-controlled additions to alpha-silyloxy aldehydes

According to the Felkin-Anh and Cram-chelation models, nucleophilic additions to alpha-silyloxy aldehydes proceed through a nonchelation pathway due to the steric and electronic properties of the silyl group, giving rise to Felkin addition products. Herein we describe a general method to promote chelation-control in additions to alpha-silyloxy aldehydes. Dialkylzincs, functionalized dialkylzincs, and (E)-disubstituted, (E)-trisubstituted, and (Z)-disubstituted vinylzinc reagents add to silyl-protected alpha-hydroxy aldehydes with high selectivity for chelation-controlled products (dr of 10:1 to >20:1) in the presence of alkylzinc halides or triflates, RZnX. With the high functional group tolerance of organozinc reagents, the mild Lewis acidity of RZnX, and the excellent diastereoselectivities favoring the chelation-controlled products, this method will be useful in the synthesis of natural products. A mechanism involving chelation is supported by (1) NMR studies of a model substrate, (2) a dramatic increase in reaction rate in the presence of an alkylzinc halide, and (3) higher diastereoselectivity with larger alkyl substituents on the alpha-carbon of the aldehyde. This method provides access to chelation-controlled addition products with high diastereoselectivity previously unavailable using achiral organometallic reagents.