Studies directed toward the synthesis of hamigeran B: a catalytic oxidative cyclization

An approach to the synthesis of hamigeran B is described. Key steps include a Tius-Nazarov cyclization and a palladium-catalyzed oxidative cyclization of an α-hydroxyenone.     

Studies toward the synthesis of pinnatoxins: the B,C,D-dispiroketal fragment

A stereoselective synthesis of the tricyclic B,C,D-dispiroketal fragment of pinnatoxins is described. Both products of an efficient enzymatic resolution reaction are used to set the stereochemistry of C12 and C23, which have opposite configuration. The configuration of the ketal centers is established upon an optimized thermodynamically controlled spiroketalization.     

Studies directed toward the synthesis of the massileunicellins

An approach to the massileunicellins is described that employs a cycloaldol reaction to assemble the isobenzofuran bicyclic core. A stereoselective rearrangement-epoxidation-oxidation cascade and a chelation controlled addition to a hindered acyl furan are used to install the C3, C11, C12, and C13 oxygens. The synthesis establishes eight of the nine stereocenters present in the isobenzofuran core of the massileunicellins.     

Studies directed toward the synthesis of rhizopodin: stereoselective synthesis of the C1-C15 fragment

A stereoselective synthesis of the C1-C15 fragment of a G-actin binding natural macrodiolide, rhizopodin was achieved using, as key steps, highly stereoselective acetate aldol reactions to build the C1-C7 fragment, one pot oxazole synthesis and an asymmetric Keck allylation reaction to build the C8-C15 fragment and finally, a Stille reaction to couple both the fragments.     

Studies directed toward the total synthesis of discodermolide: asymmetric synthesis of the C1-C14 fragment

[structure: see text] A convergent and stereoselective assembly of the C1-C14 subunit of marine natural product (+)-discodermolide has been completed. The approach employs chiral allylsilane bond construction methodology to establish four of the eight stereogenic centers. Key fragment coupling is achieved via an efficient stereoselective acetate aldol reaction between C1-C6 and C7-C14 subunits.     

Studies directed toward the construction of the polypropionate fragment of superstolide A

[reaction: see text] Highly stereoselective approaches directed toward the synthesis of the C18-C27 fragment of superstolide A are disclosed taking ketone 6 and aldehyde 7 as the only sources of chirality.     

Studies directed toward the synthesis of protein-bound GPI anchor

Mannobioside-linked phosphoethanolamine 10, a prototype model of the GPI anchor, was synthesized via glycosidation of the monosaccharide donor and acceptor, and subsequent phosphorylation. In order to test the reactivity of the amino group involved in 10 against the activated amino acid esters, 10 was reacted with N-protected amino acid pentafluorophenyl esters in the presence of HOBt. The reactions gave the aminoacylated products in moderate yields. When Fmoc-Ser-OPfp 12 and Fmoc-Cys(SBu^t)-OPfp 14 were reacted with 10, byproducts 19, 20 and 21 derived from N- and O-acylation were produced. In contrast, reactions of 10 and N-protected amino acid thioesters were promoted with AgNO₃, HOSu, and DIEA to afford the coupling products without the undesired O-acylation. Peptidylation of 10 with the synthesized oligopeptide thioesters 24 and 27 was also successful under the segment coupling conditions of the peptide thioester method as well as those of the native chemical ligation.     

Studies toward a synthesis of C3-epimauritine D: construction of the macrocycle

We successfully synthesized a 14-membered cyclic enamide of C3-epimauritine D from the cis-2,3-pyrrolidinediol derivative. Treatment of the pyrrolidinediol with nitrobenzonitrile in an S_NAr reaction efficiently installed the desired aryl-alkyl ether linkage on the N-heterocyclic system. Macrocyclization was successfully achieved by the use of TFFH as the peptide coupling reagent in the presence of HOAt.     

Studies directed toward the synthesis of the massileunicellins. Part 2

The fully substituted hydroisobenzofuran core of the massileunicellins containing eight contiguous stereocenters was prepared in 12 steps from (S)-(+)-carvone. Noteworthy elements of the synthesis include a one-step oxidative rearrangement/epoxidation, a novel stereoselective directed reduction of a keto diol, and a directed hydrogenation of a congested tetrasubstituted alkene.     

An efficient approach toward the synthesis of the A/B rings of ouabain

The synthesis of the highly functionalized A/B ring related to ouabain has been accomplished efficiently from commercially available α-tetralone. A key Birch reductive alkylation allows the building of an angularly substituted decalone that was adequately functionalized to produce the intermediate 2c.     

Studies directed toward total synthesis of rhodocomatulins: A regioselective synthesis of brominated hydroxyanthraquinones by anionic annulations

In this work, brominated hydroxyanthraquinones, which are the core structural motif of naturally occurring bromorhodocomatulins, were successfully synthesized using Hauser annulation reaction as the key step. When brominated Michael acceptors (brominated p-quinone monoketals) and cyanophthalides or bromocyanophthalides (Hauser donors) were reacted under the annulation conditions, brominated anthraquinones were obtained with 81–87% yields for four examples. On acidic quenching, products were obtained as solid which were separated by filtration and purified by recrystallization in acetone. No chromatography was required.     

Studies directed toward the total synthesis of the milbemycins: Synthesis of the C11 to C31 fragment of milbemycin D

A highly stereocontrolled synthesis of the C11 to C31 fragment of the potent antimicrobial agent milbemycin D has been completed. This approach utilizes a unique hydrolysis-cyclization to construct the spiroketal portion of the molecule.     

Studies directed toward the first total synthesis of acremodiol and acremonol

Studies directed toward the synthesis of acremodiol and acremonol resulted in the synthesis of two macrodiolides 1, 1a, and 2 besides 3. The attempted synthesis of 1 and 2 confirmed that the absolute stereochemistry defined in the earlier report is incorrect. Compound 1 was synthesized by RCM-mediated macrocyclization. Attempted synthesis of 2 failed to give good yields in the cyclization, and 1a and 2 were synthesized by the Yamaguchi macrolactonization method.     

Studies directed toward the synthesis of chiral tungsten and molybdenum carbonyl complexes

A major challenge that must be met for an asymmetric intermolecular Pauson-Khand reaction is to be able to limit the possible positions on the metal complex for the organic partners. Toward this end, the synthesis of monometallic systems derived from M(CO)₆ and two bidentate ligands, in which the number of possible coordination sites is reduced to two, has been investigated.     

Studies toward diazonamide A: initial synthetic forays directed toward the originally proposed structure

A brief introduction into the chemistry of diazonamide A (1) is followed by first-generation sequences to access the originally proposed structure for this unusual marine natural product. These explorations identified a route capable of delivering a model compound possessing the complete heteroaromatic core of the natural product, highlighting in the process several unanticipated synthetic challenges which led both to new methodology as well as an improved synthetic plan that was successfully applied to fully functionalized intermediates.     

Studies directed toward asymmetric synthesis of cardioactive steroids via anionic polycyclization

[reaction: see text] The use of anionic polycyclization (AP) in constructing the steroidal backbone of cardenolides was investigated. The reaction of 2-carbomethoxy-2-cyclohexenone I with the enolate of Nazarov reagent II gave, after decarboxylation and aldol condensation, steroid III with control of stereochemistry.     

Benzothiazines in synthesis: studies directed toward the synthesis of erogorgiaene

The use of benzothiazenes for the formal total synthesis of erogorgiaene and stereoselective total syntheses of two diastereomers of this natural product is described. In particular, the stereochemical course of a radical cyclization anticipated to give the correct relative stereochemistry for the synthesis of erogorgiaene is discussed utilizing both experimental and computational data.