Aluminium chloride , ring-opening with

Ionic ring-expansions of 2-substituted thietanes in many cases probably proceed via the ion 106, which can open to a five-membered cyclic cation. The ring-expansion of 74 by such a mechanism has already been discussed in Section The conversion of 2-methylthietane to thiophene and di- and tetrahydro-thiophenes with triphenylmethyl cations, chloranil, and aluminum oxide also may occur via similar ions. 2-Methylthietane also yields 3-butenethiol when heated with alumina. ° The isomerization of hydroxy-benzodihydrothiophenes by aluminium chloride is believed to involve common thietane intermediates that subsequently undergo ring expansion. ... 

A somewhat similar problem is revealed by the acid-catalyzed ring opening of oxetans (104). These are formed by photochemical [2 + 2] additions, and with boron fluoride or aluminium chloride they supply 3-furylcarbinols (105) mixed, however, with the isomeric 2-furylcarbinols (106). With toluene-sulfonic acid only the expected 3-furylcarbinol results, but this is changed into the 2-isomer when treated with one of the Lewis acids. The rearrangement is believed to be effected by dissociation into furan and a carbenium ion... 

Furan and the simple alkyl-furans are relatively stable to aqueous mineral acids, though furan is instantly decomposed by concentrated sulfuric acid or by Lewis acids, such as aluminium chloride. Furan reacts only slowly with hydrogen chloride, either as the concentrated aqueous acid or in a non-hydroxylic organic solvent. Hot, dilute aqueous mineral acids cause hydrolytic ring opening. 

With these mechanisms the cis isomer 2 cannot be formed. The formation of this isomer when aluminium chloride is used as catalyst can be explained [11] by simultaneous elimination of the isopropyl group and cleavage of ring B (followed by the reformation of ring B). An alternative mechanism in which the opening of ring B precedes deisopropylation has been proposed [13]. 

Dichlorothietans may be hydrolysed on silica gel to j8-thiolactones or the chlorine atoms may be removed without ring opening by reduction with lithium aluminium hydride. An aldol condensation of an aldehyde with thietan-2-one went in low yield. j3-Propiothiolactones (thietan-2-ones) are cleaved by MeSCl and MeCOSCl to mixed disulphides of jS-mercaptocarboxylic acid chlorides. The polymerization rates of several )3-propiothiolactones substituted with an arylsulphonamido-group have been determined. ... 

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