A Failed Approach to the Oxetan-3-one System

In a recent approach to the preparation of dihydro-3(2H)-furanones by chemists from Thailand it was shown that consecutive, one-pot conventional treatment of 1,3-dithiane in THF with equimolar n-butyllithium, an epoxide, n-butyllithium, an aldehyde or ketone and, finally, methanesulfonyl chloride gave a spirocyclic dithioacetal in good (ca. 70%) yield. Hydrolysis with HgO/HgCl2 gave the corresponding dihydrofuranone. 

An attempt to extend this approach to the preparation of the oxetan-3-one system, i.e. use of an aldehyde in place of an epoxide, failed. Instead, a 2-acyl-3-substituted-l,4-dithiepane derivative was obtained. 

Elucidate the above reaction sequences and give a mechanism for formation of the ring expanded product. 

---