Fragmentation-type ring stereospecific

Ethylenecarboxylic acids from cyclic ketotosylates Fragmentation-type stereospecific ring opening... 

The reaction of cis- and fra j-methyl-substituted l-(trimethylsiloxy)bicyclo[n.l.O]alkanes (n = 3,4,5) is stereospecific. When treated with lead(IV) acetate, followed by diazomethane, these substrates give the methyl ( )- and (Z)-alkenoates, respectively. The reaction is interpreted by assuming electrophilic attack of lead(IV) acetate on the cyclopropane carbon to cleave the ring, followed by a Grob-type fragmentation with lead(II) acetate as the leaving group. 

A synthetically useful type of a cheletropic reaction involves 2,5-dihy-drothiophene-1-1-dioxides (sulfolene dioxides). These compounds can either be made by independent synthetic routes or made by cycloaddition reactions from dienes and sulfur dioxide. At elevated temperatures the ring fragments to sulfur dioxide and a diene.For example, the two dimethyl derivatives 5 and 6 fragment to sulfur dioxide and rmAT5, ran5-2,4-hexadiene and c/5,fr<2 5-2,4-hexadiene, respectively, at temperatures of 100-150°C. The cleavages are stereospecific and involve disrotatory motion of the substituents at C-2 and C-5 similar to the pyrroline decomposition described previously. This reaction has proven to be a satisfactory... 

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