Ring cleavage transannular

Lead tetraacetate effects the ring contraction of (103) to the corresponding benzothiazines, apparently via intermediate transannular ring closure, as shown for (112) (Scheme 30), while treatment with mercuric acetate in acetic acid leads only to ring cleavage (Equation (35)) <84JOCl 16>. 

Fragmentation of Lactols to Unsaturated Medium-ring Lactones. Ring cleavage to form a medium-sized ring lactone with a transannular double bond has been observed (eq 21). ... 

Other approaches to fused -lactams have been described. An attempted transannular cyclization of the 1,4-thiazepine sulphone (155 f) to the penicillin ring system failed the products obtained were those resulting from ring cleavage and elimination." A photolytic Wolff rearrangement of... 

Upon treatment with Me3SiOTf, the 2,5-dihydrofuran-containing 14-membered marine cembranoid sarcophytoxide was converted to a 10-membered ring product as shown below <06OL2957>. Presumably, the Lewis acid promoted the cleavage of the dihydrofuran ring to provide a transient allylic cation, leading to a transannular cyclization. 

Cleavage of epoxides (6, 16-17 8, 10-12). Cleavage of epoxides catalyzed by Woelm neutral alumina is particularly useful in the case of medium-ring epoxides, which are generally rather unreactive and which are liable to undergo transannular reactions. For example, rw-cyclooctene oxide supported on neutral alumina is converted to rran.r-2-acetoxycyclooctanol in 78% yield by reaction with acetic acid at... 

Given the rich variety of electron transfer induced reactions observed for cyclopropane systems, it is not surprising to find a similarly rich reactivity for systems comprised of two fused cyclopropane rings. For bicyclobutane systems, such as the trimethyl derivative (71), nucleophilic capture leads to addition with cleavage of the transannular bond (->72). Alternatively, the initial capture and ring opening may be accompanied by dehydrogenation (—>73) [246],... 

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