Thietanes ring-contraction

The thietane ring contraction with bicyclic ylide intermediacy is the basis of the synthesis of thiiradialene 413 by vacuum pyrolysis of tosylhydra-zone lithium salt 414 (81TL4815). 

Anions of thietane-1-oxides 589 undergo ring contraction to give cyclopropyl sulphoxides 590685 (equation 377). 

An interesting synthetic approach to thietanes is the selective desulfurization of cyclic disulfides.The treatment of dithiolanes with a diethyl-aminophosphine results in a ring contraction to thietanes, (Eq. 19). This has been demonstrated with a-lipoic acid, a coenzyme with a dithiolane structure involved in the biological oxidation of pyruvic acid. The reaction is proposed to be initiated by the electrophilic attack of the phosphorus on the ring sulfur atom, resulting in the formation of an acyclic internal phosphonium salt, which by subsequent elimination of a phosphine sulfide, closes to the four-membered ring. °... 

Thietanes can also be prepared by ring contraction of higher-membered heterocyclic rings. Thermolysis of the dithiolane dioxide 129 in benzene leads to trans-2-phenyl-3-benzoylthietane (130) and benzalacetophenone (Eq. 20). 

Heating of thietanes can often lead to a ring contraction. Thermal treatment (150°C, in vacuo) of the tosylhydrazide salt 213 gives the allene epi-sulfide 214, a unique synthesis for this molecule. The mechanism is suggested to proceed either via the intermediate bicyclobutane ylide 215 or the mesomeric structure 216. Dodson et al. have noticed the rearrangement... 

Paquette has used the chloroketal derivatives of thietane 1,1-dioxide as an especially well-suited model for the theoretical study of the thermally induced intramolecular six-electron ring contraction rearrangement, which is pointed out in Eq. (54) for the ketals of 2-bromocyclobutanone that give cyclopropylcarboxylates. Desulfurization of thietanes to cyclopropane did not prove too successful. ... 

Intramolecular interaction of thietane sulfur atoms with carbocations (or developing carbocations) can lead to either ring expansion, as in the case of (39) (66CCC1831), or to ring contraction, as with (40) (73JOC649,69JOC3998). 

Reactions initiated by electrophilic or oxidative attack at the ring sulfur atom (Scheme 4) are considered in the present review only if intermediates 19, such as 5-oxides or 5-dioxides, are not isolated but are transformed to the sulfur-free ring contraction product without prior isolation. Thus, sulfur dioxide extrusion from such isolated 5,5-dioxides as thietane 5,5-dioxide 21 or the Ramberg-BScklund reaction of a-halosulfonyl compounds (e.g., of 23) (770R1) (Scheme 5) are not included (for reviews on SO2 extrusions from heterocycles, see 90MI1 92MI1). 

Anionic rearrangement of sulfonium ylides and anions of sulfides are well exemplified. It has been reported that base-catalyzed ring contraction of 3-alkyl- and 2,3-dialkyIthietane 1-oxides, prepared by peroxyacid oxidation of the corresponding substituted thietanes, gave substituted cyclopropanes with stereoselectivity controlled by the configuration of the sulfur... 

Chloromethyl Oxirans react with barium hydroxide that is saturated with HjS to give 3-hydroxy-thietans. Aldehyde-dimedone adducts give thietans (80) when treated with SCl2. Ring-contraction of a dithiolan with tris(diethylamino)-phosphine gives a 2-propyl-thietan. Treatment of (81) with triphenylphosphine and diethyl azodicarboxylate gives (82). 

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