Cyclic Allene Approach to the Manzamine Alkaloid Keramaphidin B

Total Synthesis of Lissodendoric Acid A

We describe a full account of our synthetic strategy leading to the first total synthesis of the manzamine alkaloid lissodendoric acid A . These efforts demonstrate that strained cyclic allenes are valuable synthetic building blocks and can be employed efficiently in total synthesis.

Diels-Alder Cycloadditions of Oxacyclic Allenes and α-Pyrones

Reactions of α-pyrones with oxacyclic allenes in Diels-Alder trappings are described. We investigate regioselectivity trends and perform competition experiments to assess the influence of structural and electronic features on relative reaction rates. We also demonstrate the stereospecific trapping of an oxacyclic allene, which proceeds in high optical yield. This study provides insight into strained cyclic allene reactivity, as well as new synthetic tools for the rapid construction of complex, heterocyclic scaffolds.

Access to Complex Scaffolds Through [2 + 2] Cycloadditions of Strained Cyclic Allenes

We report the strain-induced [2 + 2] cycloadditions of cyclic allenes for the assembly of highly substituted cyclobutanes. By judicious choice of trapping agent, complex scaffolds bearing heteroatoms, fused rings, contiguous stereocenters, spirocycles, and quaternary centers are ultimately accessible. Moreover, we show that the resulting cycloadducts can undergo thermal isomerization. This study provides an alternative strategy to photochemical [2 + 2] cycloadditions for accessing highly functionalized cyclobutanes, while validating the use of underexplored strained intermediates for the assembly of complex architectures.

Isolable Spirocyclic Silylone: π-Delocalized Spiro[3.3]heptasila-2,6-diylidone

Strained cyclic tetrylones are important synthons due to various synthetic applications. Connecting two cyclic tetrylone rings through a single shared quaternary group 14 element atom to form a spirocyclic molecule has been unexplored both theoretically and experimentally. The formation of a spirocyclic motif has been a synthetic challenge. In contrast, the reaction of amidinato disilicon(I) 1, (Me3P)2SiCl4, and KC8 afforded π-delocalized spiro[3.3]heptasila-2,6-diylidone2 and tetrasilacyclobutadiene byproducts 3 and 4. Compound 2 is the smallest spirocyclic tetrylone derivative, which is composed of a σ-type lone pair and delocalized π bond in each all-silicon spirocyclic ring. The electronic property is supported by its coordination with a W(CO)5 moiety.

Evaluation of Retro-Aldol vs Retro-Carbonyl-Ene Mechanistic Pathways in a Complexity-Generating C-C Bond Fragmentation

We report an experimental and computational investigation of the likely mechanism of a cascade reaction. The reaction involves an intramolecular Diels-Alder reaction, followed by a C-C bond cleavage, to afford a complex bridged bicyclic product. As multiple reaction pathways could be envisioned for the latter step, the mechanism of the C-C bond cleavage step was investigated. Two reasonable reaction pathways were evaluated. Both computations and experiments indicate that the C-C bond cleavage step proceeds by a retro-carbonyl-ene pathway rather than a retro-aldol pathway. This report underscores the synergy between computational and experimental studies and establishes the mechanism of an interesting complexity-generating transformation.

Total Synthesis of Keramaphidin B and Ingenamine by Base-Catalyzed Diels-Alder Reaction Using Dynamic Regioselective Crystallization

The control of the selectivity is a central issue in the total synthesis of complex natural products. In this paper, we report the total synthesis of (±)-keramaphidin B and (±)-ingenamine. The key reaction is a DMAP-catalyzed Diels-Alder reaction in which the regioselectivity is completely controlled by dynamic crystallization. Our synthesis successfully demonstrates that dynamic crystallization can be an alternative when the selectivity is not controlled by either kinetic or thermodynamic approaches in solution.

Total Synthesis of Njaoamine C by Concurrent Macrocycle Formation

In conceptual terms, the first total synthesis of the cytotoxic marine natural product njaoamine C differs from all known approaches toward related alkaloids of the manzamine superfamily in that both macrocyclic rings enveloping the diazatricyclic core are concomitantly formed; this goal was reached by double ring closing alkyne metathesis (dRCAM). The success of this maneuver does not merely reflect a favorable preorientation of the four alkyne chains that need to be concatenated in the proper pairwise manner but is also the outcome of dynamic covalent chemistry involving error correction by the chosen "canopy" molybdenum alkylidyne catalyst. The end game downstream of dRCAM capitalizes on the striking chemoselectivity of palladium-catalyzed hydrostannation, which selects for (hetero)arylalkynes even in the presence of sterically much more accessible dialkylalkynes or alkenes; for this preference, the method complements the classical repertoire of hydrometalation and semireduction reactions.