Inter‐ and Intramolecular [2+2]‐Photocycloaddition of Tetronates − Stereoselectivity, Mechanism, Scope and Synthetic Applications

Abstract

[2+2]-photocycloaddition reactions of various tetronates (3−8, 12, 16, 20, 37) have been examined. The tetronates were prepared from the corresponding tetronic acid by O-alkylation with an alkyl bromide or with an alcohol (Mitsunobu protocol). If the photocycloaddition reaction (λ = 254 nm, solvent: diethyl ether or tert-butyl alcohol) was carried out in an intramolecular fashion the tri-and tetracyclic products 21−23, 27−36 were obtained in good yields (52−75%) and with excellent simple diastereoselectivity. The reaction tolerates a high degree of substitution at both the tetronate and at the alkene double bond. High facial diastereoselectivities were observed if stereogenic centers were present in the alkene tether (e.g. 7a Ǟ 33) or if the alkenyl substituent was attached to the stereogenic C5 carbon atom of the tetronate irradiation precursor (e.g. 20 Ǟ 32). The reaction was also conducted in an intermolecular fashion. The methoxyethoxy-

Recent Advances in the Synthesis of Cyclobutanes by Olefin [2 + 2] Photocycloaddition Reactions

“…Although originally conceived to react in an intramolecular [2 + 2] photocycloaddition reaction, intermolecular reactions were found to be feasible and occurred with cyclic olefins expectedly to the cis-anti-cis products (e.g., 342 → rac - 343 , MEM = methoxyethoxymethyl; Scheme ). − Surprisingly, direct excitation at short wavelength (λ = 254 nm) turned out to be superior to a sensitized protocol. If appropriately protected, tetronic amides turned out also to be suitable substrates …”

Visible-Light-Mediated Enantioselective Photoreactions of 3-Alkylquinolones with 4-O-Tethered Alkenes and Allenes

“…Indeed, substituents at this position impair coordination to the carbonyl group and may have therefore been avoided . Still, there is a continuing interest in the formation of cyclobutanes with a high degree of substitution, and Figure depicts three representative products, rac - 1 , rac - 2 , and rac - 3 , from the recent literature that were prepared by intramolecular [2 + 2] photocycloaddition reactions. One of the few cyclobutanes obtained in enantiomercally enriched form from an α-substituted enone is dihydroquinolone 4 .…”

Formation of a Four‐Membered Ring: From a Carbonyl‐Conjugated Alkene

“…In a chiral adaptation of this method, the uracil-derived enone 140 was employed to prepare the diastereomeric cyclobutanes 141 in very good yield (Scheme 6.49). Although intermolecular reactions are possible, better yields were obtained if the reactions were conducted intramolecularly, irrespective of whether the alkenyl chain was attached to the a-, b-, or g-position of the tetronate [139]. Enantioselective access to the corresponding transproducts was feasible by epimerization in a-position to the carboxyl group [135].…”