Spirastrellolide E: Synthesis of an advanced C(1)-C(24) southern hemisphere

Synthesis of the Indolizidine Core of Virosinine A via a Microwave-Promoted Cascade Cyclization Involving Iminyl Radicals

The indolizidine core of virosinine A was synthesized by means of a microwave-promoted cascade reaction featuring 5-exo-trig iminyl radical cyclization, thiyl radical elimination, and intramolecular imine alkylation. The resulting bicyclic iminium ion underwent stereoselective reduction by Red-Al to deliver the target compound. DFT calculations suggested that both the radical cyclization and thiyl radical elimination steps are reversible at high reaction temperatures.

Synthesis and Biological Evaluation of the Southern Hemisphere of Spirastrellolide A and Analogues

The synthesis and biological evaluation of truncated spirastrellolide A analogues comprised of the southern hemisphere against protein phosphatase 2A are described. A convergent synthesis was designed featuring two gold-catalyzed cyclization reactions, specifically, a dehydrative cyclization of monoallylic diols for the synthesis of the tetrahydropyran (A-ring) and a regioselective spiroketalization for the efficient generation of the [6,6]-spiroketal (B, C-ring system). The synthesis of the southern hemisphere of spirastrellolide A was achieved involving the longest linear sequence of 19 steps. A total of eight spirastrellolide A analogues were synthesized, and preliminary PP2A enzyme assay inhibition studies were performed for the first time on analogues of the southern hemisphere. Several analogues showed inhibition, which is a positive indication and perhaps suggests that the unsaturated spiroketal fragment might be crucial to induce PP2A inhibition.

Total Synthesis of Pericoannosin A

The first total synthesis of pericoannosin A (1) containing 15 steps in the longest linear sequence with an overall yield of 5.5% is reported. The hybrid peptide-polyketide was isolated from the endophytic fungus Periconia sp. F-31 and bears a unique tricyclic core structure. The key steps are a glycolate aldol reaction and a Diels-Alder reaction utilizing an Evans auxiliary for controlling the stereochemistry. Furthermore, a late-stage equilibration was employed.

Synthesis of Vinyl Cyclopropanes via Anion Relay Cyclization

A method where an allyl alcohol is formed from a Tsuji-Trost allylation between a vinyl epoxide and an acyl containing nucleophile is described. Subsequently, a retro-Claisen condensation is utilized as a means of through-space anion relay. The anion relay results in the formation of a reactive carbanion and simultaneously activates an allylic alcohol toward intramolecular Tsuji-Trost cyclopropanation. Hence, in one pot, Tsuji-Trost allylation, retro-Claisen activation, and Tsuji-Trost cyclopropanation are combined to access synthetically useful vinyl cyclopropanes from vinyl epoxides using a mild and operationally simple procedure.

Type II Anion Relay Chemistry: Conformational Constraints To Achieve Effective [1,5]-Vinyl Brook Rearrangements

The design, synthesis, and evaluation of bifunctional linchpins, conformationally anchored on six-membered rings to achieve efficient [1,5]-Brook rearrangements involving vinyl silanes have been achieved. The restrained linchpins were subsequently exploited in type II anion relay chemistry (ARC) to permit both alkylations and Pd-mediated cross-coupling reactions (CCR) of sp² stabilized carbanions. DFT calculations were then employed to understand the mechanism and reactivity trends of [1,4]- and [1,5]-vinyl Brook rearrangements to provide insight on the role of the required copper reagent and the substrate geometry.

Synthesis of the C9-C25 Subunit of Spirastrellolide B

The synthesis of the C9-C25 subunit of the marine natural product spirastrellolide B is reported. The key synthetic features included the union of the two key fragments 5 and 6 via a Suzuki-Miyaura coupling reaction and a late-stage, one-pot sequential deprotection/cascade Achmatowicz rearrangement-spiroketalization to install the key spirocyclic intermediate present in the C9-C25 fragment of spirastrellolide B. The synthesis of the C9-C16 fragment 6 was accomplished via a phosphate tether mediated ring-closing metathesis (RCM), a subsequent hydroboration-oxidation protocol, followed by other stereoselective transformations in a facile manner. The spirocyclic intermediate was further functionalized utilizing a Lindlar/NaBH4 reduction protocol to furnish the C9-C25 subunit 3.

Synthesis of a C(1)-C(23) fragment for spirastrellolide E: development of a mechanistic rationale for spiroketalization

Synthetic analysis of spirastrellolide E envisioned to entail a cross-metathesis union of the northern and southern hemispheres followed by a Sharpless epoxidation/methylation sequence to achieve the C(22,23) stereogenicity leads to the design of a C(1)–C(23) advanced southern hemisphere exploiting a gold-catalyzed directed spiroketalization as a key step. Stereochemical analysis of this strategic transformation provides insight on the impact of the directing group carbinol stereogenicity on the reaction efficiency and, in turn, permits the conversion of the minor isomer of the spiroketal precursor to the requisite congener for successful spiroketalization.