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.