Studies toward the Total Synthesis of Arylnaphthalene Lignans via a Photo-Dehydro-Diels-Alder (PDDA) Reaction

Recent advancements in the chemistry of Diels-Alder reaction for total synthesis of natural products: a comprehensive review (2020-2023)

Despite being discovered nearly a century ago, the Diels-Alder (DA) reaction remains a crucial tool in the total synthesis of natural products. It accommodates a broad range of building blocks with varying complexity and levels of derivatization, allowing the formation of six-membered rings with precise stereochemistry. This, in turn, simplifies the synthesis of core structures found in many natural products. In recent years, modifications to the traditional Diels-Alder reaction have expanded its scope. These modifications include the inverse electron demand Diels-Alder reaction, dehydro Diels-Alder reaction, hetero-Diels-Alder reaction, photoenolization Diels-Alder reaction, asymmetric Diels-Alder reaction, and domino Diels-Alder reaction have been employed to extend the scope of this process in the synthesis of natural products. This review discusses the application of the Diels-Alder reaction in the total synthesis of natural products from 2020 to 2023, along with select methodologies that are inspired by or can be used to synthesize natural products.

Total synthesis of Comfreyn A and structural analogues via two photochemical key steps

In this work the influence of o-fluorine substituents on the photo-dehydro-Diels-Alder (PDDA) reaction was investigated and the findings of this study were applied to the total synthesis of natural products. The reactant molecules consisted of two alkyl arylpropiolates, connected by a suberic acid tether and bearing fluorine substituents in each of the o-positions. While quantum chemical calculations suggested that a fluorine substituent prevents an attack of the adjacent carbon atom in the second C-C bond forming step of the PDDA reaction, this attack took place nevertheless. The subsequent fluoride elimination, assisted by protic solvents or trialkylsilanes, resulted in an "Umpolung" of the 4-position of the cycloallene intermediate enabling the introduction of nucleophiles at this position. The nucleophilic replacement of the second fluorine substituent could also be triggered photochemically. After removal of the tether, the two arene moieties stand nearly perpendicular to each other and a selective excitation of the naphthalene moiety was possible. This led to an intramolecular photoinduced electron transfer (PET) followed by a nucleophilic replacement of the fluoride according to a SR+N1Ar* mechanism. The formed phenolic hydroxyl group underwent spontaneous lactonization with the adjacent ester group. Based on these results, the first total synthesis of the lignan Comfreyn A and some structural analogues were developed.