New Musk Odorants: (3 E )‐4‐(2′‐Alkyl‐5′,5′‐dimethylcyclopent‐1′‐enyl)but‐3‐en‐2‐ones and (3 E )‐1‐Acetyl‐3‐alkylidene‐4,4‐dimethylcyclohexenes

Four new representatives, 18 , 23 , 29 , and 32 , of the young family of dienone musks were synthesized by two general routes. The first five‐step synthetic route leading to (3 E )‐4‐(2′‐alkyl‐5′,5′‐dimethylcyclopent‐1′‐enyl)but‐3‐en‐2‐onescommenced with the enolate alkylation of 2,2‐dimethylcyclopentanone ( 13 ), either directly with an alkyl halide or through the Reetz tert ‐alkylation. Subsequent cerium(III)‐mediated butynol Grignard reaction, dehydration with sulfuric acid, ( E )‐selective partial reduction of the triple bond with lithium aluminum hydride, and concluding pyridinium chlorochromate oxidation complemented this route. The second four‐step route leading to (3 E )‐1‐acetyl‐3‐alkylidene‐4,4‐dimethylcycloalk‐1‐enes commenced with the Woods–Grignard modification on the vinylogous ester 3‐ethoxycyclohex‐2‐enone ( 24 ) employing ethylmagnesium bromide. The product 25 was then gem ‐6,6‐dimethylated by its lithium enolate to provide 26 , which was subjected to a Schlosser–Wittig reaction to introduce the 3‐alkylidene substituent. Dirhodium(II) caprolactamate‐catalyzed allylic oxidation with tert ‐butyl hydroperoxide completed the second general synthetic scheme. The four target compounds, 18 , 23 , 29 , and 32 , were designed as diseco derivatives of a carotol lead, and they all constitute musk odorants with floral‐fruity side notes. A tert ‐butyl group at the C‐6 position of the ( E )‐hexa‐3,5‐dien‐2‐one skeleton was found to intensify the musk odor, and ( E )‐4‐(2′‐ tert ‐butyl‐5′,5′‐dimethylcyclopent‐1′‐enyl)but‐3‐en‐2‐one ( 23 ) was found the most intense and interesting odorant of the series, with a very uncommon undertone of beetroot and dried fruits.(© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2008)