I have taken this photo from synarchive: https://synarchive.com/syn/325
I tried reading the research paper this synthesis was found in (Couladouros, E. A.; Vidali, V. P. Chem. Eur. J. 2004, 10 (15), 3822–3835) as well as some of the references of that paper but I could not figure this mechanism out.
Could it just be the Lewis acid attaches to alcohol, causing the alcohol to leave; alkene attacks carbocation, ester attacks the carbocation formed (losing tBu in the process)?