Synthese von Alkylphenolen und ‐pyrocatecholen aus Plectranthus albidus (Labiatae)

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Abstract
Synthesis of Alkylphenols and ‐catechols from Plectranthus albidus (Labiatae)In the preceding paper, we described the isolation and structure elucidation of a series of even‐numbered phenol‐ or pyrocatechol‐derived 1‐arylalkane‐5‐ones. To establish the assigned structures unambiguously and to have larger quantities available for physiological testing, the following compounds were prepared: in the alkylphenol series, 1‐(4′‐hydroxyphenyl)tetradecan‐5‐one (2a), 1‐(4′‐hydroxyphenyl)hexadecan‐5‐one (2b), and 1‐(4′‐hydroxyphenyl)octadecan‐5‐one (2c); in the alkylcatechol series, 1‐(3′,4′‐dihydroxyphenyl)decan‐5‐one (3a; not isolated as a natural compound), 1‐(3′,4′‐dihydroxyphenyl)dodecan‐5‐one (3b), 1‐(3′,4′‐dihydroxyphenyl)tetradecan‐5‐one (3c), 1‐(3′,4′‐dihydroxyphenyl)hexadecan‐5‐one (3d), 1‐(3′,4′‐dihydroxyphenyl)octadecan‐5‐one (3e), and 1‐(3′,4′‐dihydroxyphenyl)icosan‐5‐one (3f); in the alkenylphenol series, (Z)‐1‐(4′‐hydroxyphenyl)octadec‐13‐en‐5‐one (4a) and (E)‐1‐(4′‐hydroxyphenyl)octadec‐13‐en‐5‐one (4b); in the alkenylcatechol series, (E,E)‐1‐(3′,4′‐dihydroxyphenyl)deca‐1,3‐dien‐5‐one (1) and (Z)‐1‐(3′,4′‐dihydroxyphenyl)octadec‐13‐en‐5‐one (5). All compounds proved to be identical with the previously assigned structures. Compound 1 was synthesized by regioselective aldol condensation of heptan‐2‐one with (E)‐1‐(3′,4′‐dimethoxyphenyl)prop‐2‐enal (6d; Scheme 1), the phenols 2a–c and the catechols 3a–f by addition of the corresponding alkyl Grignard reagent to 5‐(4′‐methoxyphenyl)‐ or 5‐(3′,4′‐dimethoxyphenyl)pentanal (17c and 18c, resp.; Scheme 4), and the olefins 4a, 4b and 5 from 17c or 18c via the 9‐O‐silyl‐protected 13‐(4′‐methoxyphenyl)‐ or 13‐(3′,4′‐dimethoxyphenyl)tridecanals (26 and 27, resp.) and Wittig olefination as the key steps (Scheme 5).