The iron-catalyzed six-membered ring carbocyclizations of 5-oxa-2,8,10-undecatriene derivatives proceed with high regio-and stereoselectivity to yield trans-3,4-disubstituted tetrahydropyrans. Substrates containing an alkyl substituent at the 4-or the 7-position of the starting triene ether cyclize with high secondary stereoinduction to produce trans,transtrisubstituted derivatives. Diastereomeric (X)-and (.X)-substrates cyclize via complementary [4+4]-and [4+2]-ene reaction pathways. Transition-metal mediated carbocyclization reactions are providing the synthetic organic chemist with an increasingly versatile array of methodologies for the construction of common ring systems. 2 We have recently reported the bpy*Fe(O) catalyzed carbocyclizations of (2E,7!)-, and (22,7E)-2,7,9-decatrien-l-01 ethers giving rise to c-and c-1,2disubstituted cyclopentanes, respectively.3 These formal iron catalyzed [4+4]-ene cyclizations proceed via initial carbon-carbon bond formation followed by the regioselective iron mediated transfer of an allylic hydrogen to the diene moiety. 4 Since many transition metal mediated carbocyclizations of acyclic substrates are limited in scope to the construction of five-membered rings, we were interested in examining the applicability of our methodology to the construction of other ring sizes and to the incorporation of additional functionality within the ring system. Toward this end we have investigated the cyclizations of triene ethers leading to the stereoselective formation of substituted tetrahydropyrans. Treatment of the (21,8E)-triene ether la with the bpy*Fe(O) catalyst under standard iron-catalyzed ene conditions (10 mol% bpy*Fe(O) benzene, 25"C, 7h), effects a formal [4+4Iene cyclization to yield a mixture of cis-and trans-enol ethers 2. Upon complete cyclization, the reaction mixture is filtered through Florisil (1:l EtOAc-hexanes) to remove the iron catalyst and the crude product is treated with excess ethylene glycol (cat. TsOH, THF, 25X, 3-8h). The trans-3,4-disubstituted pyran 3a is isolated by flash chromatography in 84% overall yield.5 Careful capillary gas chromatographic analysis of the crude acetal mixture shows that the cyclization proceeds with greater than 5O:l trans simple diastereoselectivity. Less than 1% of any cis-substituted pyrans and less than 1.5% of an isomeric trans-substituted pyran are formed. While additional alkyl substituents on the