Multifunctional 1,3-diols with an allyl or a propargyl group and three consecutive stereocenters, two of which are quaternary carbon atoms, form a class of important building blocks that is required for the synthesis of many biologically significant molecules, such as sieboldine A, [1] furaquinocins, [2] and ingenol. [3] The stereoselective construction of the quaternary carbon centers, in particular, has received considerable attention .[4] However, efficient construction of adjacent multiple stereocenters with two quaternary carbon atoms is still a challenging area of study.[5] During the past several years, our studies on tandem reactions of a-heterocyclopropane alcohols have resulted in the stereoselective construction of 2-quaternary 1,3-diheteroatom units by tandem rearrangement/reduction reactions of a-heterocyclopropane alcohols.[6] However, the tandem rearrangement/alkylation reactions of these alcohols, which would present an approach to the construction of more synthetically important and versatile 1,3-diheteroatom units, has not been accessed to date. Recently, we successfully developed a novel tandem reaction of a-epoxy alcohols with RB(OH) 2 (R = allyl and allenyl). The synthetically important features of this sequence include the following: 1) smooth completion of the two different chemical transformations-the stereospecific boron-promoted semipinacol rearrangement of the a-epoxy alcohol and the subsequent allylation or propargylation of the intermediate b-hydroxy ketone; 2) diastereoselective construction of the three consecutive stereogenic centers, two of which are quaternary, with one allyl or propargyl group; 3) pivotal dual roles of RB(OH) 2 : lewis acidity and alkylating ability; it is generally assumed to act as an alkylating reagent, [7] and to our knowledge, no report on the dual nature of the RB(OH) 2 has been published previously; 4) the relative stereochemistry of C1 in the 1,3-diol product 2/2', which is independent of the relative configuration of C1 in the substrate 1, can be tuned by the substituent R 4 on 1. Consequently, this tandem transformation would offer extensive application in organic synthesis. Herein, we present the results of our investigation and its synthetic application.We initially studied the tandem reaction of a-epoxy alcohols with allylboronic acid (RB(OH) 2 , R = allyl). The aepoxy alcohol substrates 1 were prepared in racemic forms by epoxidation of the corresponding allylic alcohols with mchloroperbenzoic acid (mCPBA) or tBuO 2 H/[VO(acac) 2 ] (acac = acetylacetonoate) according to literature procedures.[8] A solution of the substrate 1 (1.0 equiv) and the freshly prepared allylboronic acid [9] (1.2 equiv) in Cl(CH 2 ) 2 Cl was stirred at ambient temperature in an argon atmosphere for 6-18 h, during which time a diastereoselective tandem semipinacol rearrangement/allylation proceeded smoothly to generate the 1-allyl-1,3-diols 2 and/or 2' in good yields (Scheme 1). Solvent effects were also observed in this reaction. For example, the reaction proceeded readily...