XANTHATE TRANSFER CYCLIZATION OF GLYCOLIC ACID-DERIVED RADICALS - SYNTHESIS OF 5-MEMBERED TO 8-MEMBERED RING ETHERS

A novel method for the preparation of functionalized five- to eight-me mbered ring ethers is described. This method involves the xanthate tra nsfer radical cyclization of ken-1-oxy)-2-[(ethoxythio-carbonyl)sulfan yl]acetic acid methyl esters 6 to give good yields of xanthate-substit uted five- to eight-membered ethers 7 and/or 8, depending on the lengt h of the carbon chain. These cyclic ethers are usually obtained as mix tures of diastereomers. The cyclizations are performed at 150-160 degr ees C in tert-butylbenzene with di-tert-butyl peroxide as the initiato r. This group-transfer radical cyclization was successfully applied in a total synthesis of lauthisan (44). The use of benzoyl xanthate (56) as a catalyst allows a visible light-induced xanthate transfer cycliz ation to a tetrahydrofuran in high yield.