Development of an access route to the c31-c52 central core of amphidinol 3

Synthesis of C-linked α-Gal and α-GalNAc-1'-hydroxyalkanes by way of C2 functionality transfer

Inspired by reports of water sculpted Tn antigen (α-GalNAc-O-Ser/Thr) epitopes and our interest in producing metabolically more stable C-linked analogs of Tn, we explored the utility of C2 functionality on α-Gal-C-alkenes to deliver hydroxyl to the pendant alkenyl chain. Toward this end, a cyclic carbonate approach gave rise to a single C-linked α-Gal-1'(S)-hydroxyethane in 3 steps, and use of a 2-(hydroxyimino)galactoside cyclization transferred an oxygen to a pendant cis-substituted C-linked alkene affording the R-configuration at the newly formed stereocenter (7:1 dr). Reduction and acetylation of the resultant isoxazoline demonstrated this approach as a viable route to C-linked α-GalNAc-1'-hydroxyalkanes.

Total Synthesis of Amphidinol 3: A General Strategy for Synthesizing Amphidinol Analogues and Structure-Activity Relationship Study

Amphidinol 3 (AM3) is a potent antifungal produced by the dinoflagellate Amphidinium klebsii. It was difficult to determine the absolute configuration of AM3 by using the scarce natural product due to the presence of numerous stereogenic centers on the acyclic carbon chain. Since the absolute configuration was partially determined on the basis of insufficient evidence, the originally proposed structure has been revised three times. Although recent progress on structure determination by computational analysis is remarkable, total synthesis is still the most reliable way to confirm structures. The first total synthesis of AM3 was achieved via expeditious assembly of three components in five steps, confirming the revised structure of AM3 after more than 20 years since its first discovery. The established synthetic route would be a general strategy for synthesizing amphidinol congeners. An artificial and simplified analogue of AM3, which elicited antifungal activity comparable to that of AM3, was designed and synthesized. This is the first example of a biologically active artificial analogue possessing a shorter polyol moiety, providing insight on the antifungal mode-of-action.

A highly convergent synthesis of the C1-C31 polyol domain of amphidinol 3 featuring a TST-RCM reaction: confirmation of the revised relative stereochemistry

The concise enantioselective synthesis of the revised C1-C31 fragment of the polyketide amphidinol 3 was accomplished in 16 steps and 12.8% overall yield. Salient features of the strategy include chemoselective Weinreb amide coupling and concomitant CBS reduction for the preparation of the C1-C15 tris-syn-1,5-diol motif and a temporary silicon-tethered ring-closing metathesis (TST-RCM) reaction in combination with a diastereoselective hydroboration for the construction of the C16-C31 polypropionate fragment. The union of the fragments was accomplished by a regioselective ring-opening of the terminal epoxide with a phenyl sulfone stabilized carbanion, which upon reduction and deprotection permits a comparison of the relative configuration with the natural product.

Isolation, structural determination and synthetic approaches toward amphidinol 3

This review highlights the isolation and the structural determination of amphidinol 3 (AM3), as well as the synthetic efforts to its preparation. The mechanism of action of AM3 will not be developed herein.

Karlotoxin synthetic studies: concise synthesis of a C(42-63) B-ring tetrahydropyran fragment

Starting from natural D-mannose, a C(42-63) B-ring tetrahydropyran fragment in karlotoxin 2 has been prepared via a common THP intermediate in a concise manner. E-selective Julia-Kocienski olefination efficiently assembled a C(51-63) chlorodiene subunit and a C(42-50) tetrahydropyran segment.

Synthesis of the bis-tetrahydropyran core of amphidinol 3

A convergent synthesis of the C31-C52 bis-tetrahydropyran core of the natural product amphidinol 3 is reported. A common intermediate was synthesized from d-tartaric acid utilizing an asymmetric glycolate alkylation/ring-closing metathesis sequence to construct the THP rings. Differential elaboration of the common intermediate allowed the synthesis of two distinct coupling partners which were joined through a modified Horner-Wadsworth-Emmons olefination to provide the bis-tetrahydropyran core.

Stereoselective synthesis of the C31-C40/C43-C52 unit of amphidinol 3

A concise synthesis of a tetrahydropyran ring system corresponding to the C31-C40 and C43-C52 units of amphidinol 3 is described. Successive chemoselective reactions, i.e., cross-metathesis to differentiate the iodoolefin from the terminal olefin and Sharpless asymmetric dihydroxylation on the resulting E-olefin, resulted in expeditious synthesis of an intermediate that was then cross-coupled to afford an E,E-diene system. Four contiguous stereogenic centers were installed via construction of the tetrahydropyran ring by means of Katsuki-Sharpless asymmetric epoxidation, 6-endo-tet cyclization, and Sharpless asymmetric dihydroxylation.

Combinatorial synthesis of the 1,5-polyol system based on cross metathesis: structure revision of amphidinol 3

Combinatorial synthesis of a 1,5-polyol system corresponding to the C1-C14 unit of amphidinol 3 (AM3) and its diastereomers was achieved via chemoselective cross metathesis as the key step. Comparison of (13)C NMR data of the synthetic specimens with that of AM3 led to a controversy regarding the originally proposed structure. From GC-MS analysis of the degradation product, the absolute configuration at C2 of AM3 has been revised to be R.

Synthesis of the C(26)-C(42) and C(43)-C(67) pyran-containing fragments of amphidinol 3 via a common pyran intermediate

The pyran-containing fragments 1 and 2 of AM3 have been synthesized from the common pyran intermediate 3. Careful orchestration of the alcohol protecting groups was necessary to facilitate deprotection of alcohol functionality in the presence of the chemically sensitive polyene chain.

Synthesis of the C1-C52 fragment of amphidinol 3, featuring a beta-alkoxy alkyllithium addition reaction

An advanced intermediate for the synthesis of amphidinol 3 has been prepared. A cross-metathesis reaction was used to couple the C1-C12 and C13-C26 segments. An unusual beta-alkoxy alkyllithium reagent was generated from this segment and added to a Weinreb amide to assemble the C1-C52 section of amphidinol 3. These compounds represent some of the most advanced intermediates reported to date for the synthesis of amphidinol 3.