Terpenes and Terpene Derivatives, XXIX. Synthesis and Olfactive Properties of (—)‐ and rac ‐Silphiperfol‐5‐en‐3‐ol and of Some Tris‐nor Derivatives

The new sesquiterpene alcohols silphiperfol‐5‐en‐3‐ol [(—)‐ 1a ] and the corresponding 7‐ epi compound (+)‐ 1b , important constituents of the essential oil of Artemisia laciniata have been synthesized in 13 steps by starting from ( R )‐(+)‐pulegone ( 6 ) in an overall yield of 8%. The key reaction is the coppercatalyzed 1,4‐Grignard addition of the iodo acetal 34 to the enone (+)‐ 4 in the presence of TMSCI and TMEDA to give the silyl ether 38 . Hydrolysis with spontaneous aldol cyclization and dehydration leads to the silphiperfolenones (+)‐ 2a , b which are separable by flash chromatography. The reduction with L‐selectride ® gives the alcohols (—)‐ 1a and (+)‐ 1b which are identical in all physical properties with the isolated natural compound. With LiAlH 4 the diastereoisomers 42a , b are formed. Olfactive evaluation of the isomers (—)‐ 1a and (+)‐ 1b has shown that the odor of the naturally occurring mixture of 1a , b with the descriptors woody, ambergris, slightly camphoraceous is correctly assigned. (—)‐ 1a represents the more woody and camphoraceous, (+)‐ 1b the more animal facette of ambergris. The odor of rac ‐ 1a , b of the diastereoisomers 42a , b and of the ketones (+)‐ 2a , b is also described as well as that of the tris‐nor‐silphiperfolene derivatives 29 – 31 .