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

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 ).