Unsaturated acetals, radical-chain cyclizations

Scheme 15. Radical-chain cyclization of unsaturated acetals using polarity reversal catalysis...

Thiol-catalyzed Radical-chain Cyclization of Unsaturated Acetals and Thioacetals. When the unsaturated dioxolane 1 and a radical initiator, 2,2-di(t-butylperoxy)butane (DBPB), were heated at 125 °C in octane in the presence of tri(f-butoxy) silanethiol (TBST), the spirocyclic ketal 2 was formed cleanly and isolated in 92% yield (eq 1). When the reaction was performed in the absence of TBST, compound 2 was not detected. TBST is... 

The authors used (5)-carvotanacetone (dihydrocarvone) as starting material (Scheme 34). To prepare the linearly conjugated sUylenol ether, they used the Kharash protocol and attained y-alkylation by Mukaiyama aldol reaction with trimethylorthoformate (195). The ketoacetal 295 was a-hydroxylated according to Rubottom by silylenol ether formation followed by epoxidation and silyl migration. Acid treatment transformed 296 to the epimeric cyclic acetals 297 and 298. endo-Aceta 297 was equilibrated thereby increasing the amount of exo-acetal 298. The necessary unsaturated side chain for the prospected radical cyclization was introduced by 1,4-addition of a (trimethylsilyl)butynylcopper compound. 

More complex products are obtained from cyclizations in which the oxidizable functionality and the alkene are present in the same molecule. y9-Keto esters have been used extensively for Mn(III)-based oxidative cyclizations and react with Mn(OAc)3 at room temperature or slightly above [4, 10, 11, 15], They may be cyclic or acyclic and may be a-unsubstituted or may contain an a-alkyl or chloro substituent. Cycloalkanones are formed if the unsaturated chain is attached to the ketone. y-Lactones are formed from allylic acetoacetates [10, 11]. Less acidic /3-keto amides have recently been used for the formation of lactams or cycloalkanones [37]. Malonic esters have also been widely used and form radicals at 60-80 °C. Cycloalkanes are formed if an unsaturated chain is attached to the a-position. y-Lactones are formed from allylic malonates [10, 11]. yff-Diketones have been used with some success for cyclizations to both alkenes and aromatic rings [10, 11]. Other acidic carbonyl compounds such as fi-keto acids, /3-keto sulfoxides, j8-keto sulfones, and P-nitro ketones have seen limited use [10, 11]. We have recently found that oxidative cyclizations of unsaturated ketones can be carried out in high yield in acetic acid at 80 °C if the ketone selectively enolizes to one side and the product cannot enolize... 

Free radical cycUzations. Cobaloxime in combination with a sacrificial Zn foil anode has been used to effect the reductive generation of carbon-centered radicals from bromoacetals in electrochemical proces.ses. When an unsaturated side chain is present in such acetals, cyclization may occur. 

---