The bismuth-mediated two-component hemiacetal/oxa-conjugate addition of delta-trialkylsilyloxy and delta-hydroxy alpha,beta-unsaturated aldehydes and ketones with alkyl aldehydes provides the syn-1,3-dioxanes in a highly efficient and stereoselective manner. The key advantages of this protocol are its operational simplicity and its ability to directly access electron-withdrawing groups without recourse to oxidation state adjustments.