Abstract
Convenient syntheses of (±)-frontalin (I) and (±)-brevicomins (Ila) and (Ilb) were achieved from pent-4-en-1 -ol (2) and pent-4-yn-1-ol (3). In a few simple and unambiguous steps, the alkenol (2) and the alkynol (3) were transformed into the acetal bromide (7) and the alkenyl bromides (14) and (17), respectively. Acylation of the Grignard reagents of these bromides provided the corresponding methyl ketones (8), (15), and (18), the key intermediates for the synthesis of the title bicyclic acetals. The ketone (8) was converted into the olefin (9) which, on epoxidation followed by acid hydrolysis, yielded (±)- frontalin, whereas epoxidation of the alkenones (15) and (18) and subsequent cyclisation afforded exo and endo– brevicomin, stereoselectively.