2-Phenylsulphonyl cyclic ethers undergo facile displacement of the sulphonyl group by alcohols, in the presence of magnesium bromide etherate and sodium bicarbonate in tetrahydrofuran, to give good yields of the corresponding acetals.
The preparation of tetrahydro-pyran and -furan ethers typically involves relatively acidic conditions 1. Since many functional groups may not be compatible with such acidity, we have examined the magnesium bromide induced displacement of the phenylsulphonyl moiety by alcohols under mildly basic conditions2, 3 (scheme).
These mild conditions tolerate a wide variety of functional groups including furans, ketones, esters, silyl ethers, acetals, alkenes and alkynes.
R~O~SO2P h
R' OH, MgBr2.Et20 , NaHCOa, THF ~. R~O~OR '
Scheme
The starting 2-phenylsuphonyl cyclic ethers 1-5, were prepared in excellent yield, as previously reported, from lactols, lactol ethers, dihydro-pyrans, or -furans. 4 Sulphone 6 was synthesised by oxidation of the corresponding sulphide with meta-chloroperbenzoic acid (mCPBA). In a typical experiment, these sulphones were converted into the corresponding acetals by stirring in tetrahydrofuran (THF) with the required alcohol, magnesium bromide etherate and sodium bicarbonate, at room temperature overnight. (Tables ).
~SO2Ph ~SO2Ph
(1) (2)
AcO~3e~,tSO2P h AcO~,,,,"k~O.."~SO2P h (4) (5) 1329 ph~'~%SO2Ph (3) OMe TBDMSO%~,,~ ~"",. 0/",, S 0 2 p h (e)