Chiral tetrahydrothiophenes and tetrahydroselenophenes are highly useful structural units.D escribed here is an ew catalytic asymmetric approach for their synthesis.W ith asuitable chiral Brønsted acid catalyst, an oxetane desymmetrization by aw ell-positioned internal sulfur or selenium nucleophile proceeded efficiently to generate all-carbon quaternary stereocenters with excellent enantioselectivities.T aming the sulfur and selenium nucleophile in the form of at hioester and selenoester,r espectively,i sc rucial to the success of this work. This approach also allows the facile synthesis of chiral tetrahydrothiopyrans.M echanistic studies,i ncluding DFT calculations,s uggested an intramolecular acyl-transfer pathway.Utilities of the chiral products are also demonstrated.Scheme 3. Synthesis of the bioactive compound IV.a)LiAlH 4 ,T HF, 0 8 8Ct oRT, 3h,84% yield;b )oxalyl chloride, DMSO, DCM, À78 8 8C; then i Pr 2 NEt, À78 8 8Cto08 8C, 48 %y ield;c)diethyl (4-(hydroxymethyl)benzyl)phosphonate, t BuOK, THF, 0 8 8Ct oRT, 4h,5 2% yield;d )Oxone, THF/H 2 O( 1:1), RT,1 2h,84% yield;e)H 2 ,Pd/C, EtOH, RT,2h, 65 %y ield. DMSO = dimethylsulfoxide, THF = tetrahydrofuran.Scheme 2. Product derivatizations.