Studied Directed toward the Synthesis of Phomenoic Acid. Part 2 . Stereocontrolled Synthesis of the C(7)‐to‐C(14) Segment

Starting from the esters (2 E ,4 S )‐ 6 and (2 E ,4 R )‐ 6 , bromo aldehydes ( S )‐ 9 and ( R )‐ 9 as well as bromo alcohols ( S )‐ 10 and ( R )‐ 10 , respectively, were prepared. Bromo alcohol ( R )‐ 8 was converted to the diol (2 E ,4 R )‐ 16 . Ozonolysis of the latter led to aldehyde ( R )‐ 17 , which was transformed, by a Wittig reaction, to (2 R ,4 E ,6 R )‐ 18 , corresponding to the C(7)‐to‐C(14) segment of phomenoic acid ( 1 ). Attempts to improve the yields by applying a Julia coupling of ( R )‐ 23 , which was prepared from (2 E ,4 R )‐ 7 , with ( R )‐ 24 were unsuccessful. Finally, the coupling of the iodo derivative (2 E ,4 S )‐ 28 with the lithiated derivative of 1,3‐dithiane 30 by the Corey‐Seebach ‘Umpolung’ led to (3 S ,4 E )‐ 32 which is a derivative of the C(7)‐to‐C(14) segment of 1 , suitable for further transformations.