Unexpected formation of derivatives of cyclobutene and thiacyclobutane from the reaction between dilithiated benzylacetylene and isothiocyanates

Reaction of dilithiated benzylacetylene PhCH(Li)-CC-Li with one equivalent of an isothiocyanate RNCS followed by successive addition of t-BuOH, a solution of t-BuOK in DMSO and methyl iodide gives exclusively a cyclobutene or a thietane derivative if R=t-Bu or Ph, respectively, and mixtures of the expected thiophene, the cyclobutene and thietane derivatives if R=Me.