Mechanism of cyclisation with tin(IV) chloride

[![enter image description here](https://i.stack.imgur.com/lUAb3.png)](https://i.stack.imgur.com/lUAb3.png)


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](https://doi.org/10.1002/chem.200400407)) 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)?




The stereochemistry of this type of cyclization is well-understood. An extensive review may be found [here](https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2575671/). As you have realized the role of the Lewis acid stannic chloride is to generate the carbocation **1**. The vacant p-orbital is aligned in the sigma sense with the π-bond of the double bond to permit cyclization, in the Markovnikov sense, to carbocation **2**. There are two possible modes of cyclization of this carbocation to form either the less strained *cis*-ring juncture or the more strained *trans*-ring juncture. The transition state leading to the *cis*-lactone involves a steric interaction with an axial methyl group. The transition state (**2** $\rightarrow$ **3**) leading to the *trans*-lactone is less hindered and kinetically controlled. The utilization of the *tert*-butyl ester is to allow the rapid collapse of cation **3** to the *trans*-lactone **4**, thereby insuring that the ring closure is not reversible. The *tert*-butyl cation **5** is scavenged by the solvent, nitromethane.  
  



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