Photochemical Transformations of Some Neoclerodane and Labdane Diterpene Ketones

Abstract

Some neoclerodane (2–4) and labdane (5 and 6) diterpene ketones having a β‐ or γ‐hydroxy function, an α‐epoxy group, or an α‐olefinic double bond have been irradiated at λ = 313 nm in dry solvents (benzene or methanol). Fruticolone (2) yielded the naturally occurring 5,6‐seco‐neoclerodan‐6,1α‐olide derivative fruticolide (1), while hispanolone (5) gave the δ‐lactone 8, both transformations involving a Norrish type I photoreaction. The α,β‐unsaturated ketone derivative 4 was transformed into 7 by a stereoselective intramolecular [2+2] cycloaddition reaction involving its furanic 13(16)‐double bond. Finally, the α‐epoxy ketone 6 was rearranged to the 10(9→8)abeo‐labda‐7,9‐dione derivative 10. Apart from the interest in these reactions for synthetic purposes, the photochemical transformation of fruticolone (2) into fruticolide (1) suggests that the latter might originate from the former as an artefact during the extraction and isolation procedure, rather than from any biogenetic pathway.

Expanding Diterpene Complexity and Diversity via Photoinduced Ring Distortions

“…The key HMBC correlation peaks, such as H‐16 with C‐7, C‐15 (δC 148.9) and the C‐6 (δC 213.1) positions, allow us to confirm the proposed structure. Peaks corresponding to the nOe effect between H‐5/H‐7 and H 3 ‐20/H16, 1 H and 13 C chemical shifts of new cyclobutene strained ring, and a J H7‐H16 coupling constant (3.2 Hz) confirm the configuration proposed for 3 [24] (see Figure S3–5 for details).…”

“…Our approach aims to develop a synthetic methodology that involves photoinduced one‐step ring‐distortion reactions directly from solidagenone. Due to the presence of α,β‐unsaturated carbonyl groups and a furan moiety in the solidagenone, we claim that a [2+2]‐photocycloaddition could yield new derivative compounds with novel and complex ring architectures [23,24] . The [2+2]‐photocycloaddition enables access to highly strained ring systems and has been widely used as the key step in the synthesis of Natural and Unnatural Products [25] …”

“…Due to the presence of α,βunsaturated carbonyl groups and a furan moiety in the solidagenone, we claim that a [2 + 2]-photocycloaddition could yield new derivative compounds with novel and complex ring architectures. [23,24] The [2 + 2]-photocycloaddition enables access to highly strained ring systems and has been widely used as the key step in the synthesis of Natural and Unnatural Products. [25] Considering the previous report on the use of photochemical transformations as a tool for generating structural diversity in the NPs, [26,27] we studied the photoreactivity of solidagenone (1), evaluating different solvents, reaction times, and irradiation sources (Table 1).…”

Photoinduced Acylations Via Azolium-Promoted Intermolecular Hydrogen Atom Transfer

“…Notable early innovations include the discovery of Norrish-type photochemistry, wherein excitation to the triplet diradical state of ketones leads to rapid and uncontrolled fragmentation, abstraction, and coupling reactions. Norrish II and Norrish-Yang reactivities stem from initial intramolecular hydrogen atom transfer (HAT) and have found various synthetic applications, − including natural product total synthesis. , Norrish Type I chemistry, on the other hand, is the direct generation of an acyl and alkyl radical by ketone irradiation (Figure A). ,, Despite presenting an attractive solution to the generation of valuable radical species, this subset of photochemistry remains far less developed than its counterparts, primarily due to the uncontrolled reactivity of its initial radical products. − As such, strategies that could provide more controlled outcomes from Norrish-type processes would enable powerful new synthetic tools.…”