An efficient method
for the preparation of arylnaphthalene lignans
(ANLs) was developed, which is based on the Photo-Dehydro-Diels–Alder (PDDA) reaction. While intermolecular PDDA
reactions turned out to be inefficient, the intramolecular variant
using suberic acid as tether linking two aryl propiolic esters smoothly
provided naphthalenophanes. The irradiations were performed with a
previously developed annular continuous-flow reactor and UVB lamps.
In this way, the natural products Alashinol D, Taiwanin C, and an
unnamed ANL could be prepared.
Naphthalenophanes are a special type of cyclophanes. While in the past (1,5)-, (1,6)-, and (1,8)naphthalenophanes were successfully prepared by using the photo-dehydro-Diels-Alder (DDA) reaction, access to (1,7)naphthalenophanes by this method was hitherto unknown. After numerous unsuccessful attempts to prepare these compounds by thermal DDA, we found that the photoinitiated variant (PDDA) represents a very efficient method to [k](1,7)naphthalenophanes 13. The scope ranged from highly strained (k = 11, 12) to macrocyclic products (k = 22, 24). The extraordinary reactivity could be explained by folded ground-state geometries of diketones 12 used as reactants of the PDDA. Furthermore, we calculated the ring-strain energies with the help of an isodesmic reaction and evaluated structural and spectroscopic (NMR) consequences of ring strain.
We report on the triplet sensitization of the intramolecular Photo-Dehydro-Diels-Alder (PDDA) reaction of two diaryl suberates bearing methyl propiolate chromophors. Compared with the non-sensitized irradiation, considerably increased yields could be observed. Moreover, it is possible to use the more efficient UVA lamps instead of UVB lamps. Among three investigated sensitizers (xanthone, benzophenone, thioxanthone) xanthone gave the best results.
The total syntheses of three arylnaphthalene lignans (ANLs) were developed: Vitrofolal E (1), Noralashinol C (2), and Ternifoliuslignan E (3). These natural products have in common a missing substituent in 2‐position of the naphthalene moiety (2H‐ANLs). The key step of these syntheses is a regioselective intramolecular Photo‐Dehydro‐Diels‐Alder (PDDA) reaction with (1,7)naphthalenophanes as primary products. A further improvement of the photochemical step was achieved by triplet sensitization with xanthone, allowing the use of more efficient UVA lamps. It should be noted that this work is a continuation of a previous publication about the total synthesis of lignans using the PDDA reaction.[1]
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