Michael‐Addition von Thiocarbonsäureestern Anwendung bei der Synthese von (±)‐Jasminketolacton

Michael addition of carbothioates. Application to the synthesis of (±)‐jasmine ketolactone It is shown that the lithium enolate of S ‐ t ‐butyl thioacetate adds to 2‐cyclopentenone in the β‐position and that fluoride ions catalyze the 1, 4‐addition of the trimethylsilyl enol ether of S ‐ t ‐butyl thioacetate ( 5 ) to 2‐cyclopentenone ( 4 ) to give 6 . These novel versions of the Michael addition have been applied to a synthesis of jasmonoid compounds. Cleavage of the trimethylsilyl enol ether in 6 with tetrabutylammonium fluoride produced the corresponding ketone enolate which could be trapped in situ by alkylation with 1‐bromo‐5‐(2′‐tetrahydropyranoxy)‐2‐pentyne ( 7 ) to form 8 . Removal of the alcohol protecting group in 8 , followed by partial hydrogenation of the triple bond over Lindlar palladium and mercury ion promoted hydrolysis of the carbothioate moiety in 9 , led to 5′‐hydroxy jasmonic acid ( 10 , Scheme 3 ). 10 was converted into the S ‐(2‐pyridyl) carbothioate and cyclized in dilute benzene solution under the influence of silver ion to give (±)‐jasmine ketolactone ( 1 , Scheme 4 ), a component of the essential oil of Jasminum grandiflorum , in 72% yield. Similarly, methyljasmonate ( 2 , Scheme 2 ) was obtained from 6 by the reaction with 1‐bromo‐2‐pentyne and tetrabutylammonium fluoride followed by methanolysis and partial hydrogenation of the triple bond.