Novel methods for the reduction of α, β-epoxy ketones, α, β-epoxy esters (glycidic esters), and their congeners to β-hydroxy carbonyl compounds (aldols) by the use of organoselenium reagents are described. The reagents, a sodium phenylseleno (triethyl) borate complex Na [PhSeB (OEt) 3] easily prepared by reduction of (PhSe) 2 with NaBH4 in EtOH and benzeneselenol (PhSeH) generated in situ from the borate complex by addition of acetic acid, have been demonstrated to serve as excellent reducing agents for these transformations. The organoselenium-mediated reduction of α, β-epoxy carbonyl compounds regiospecifically occurs at the α-carbon to produce a wide variety of cyclic (intramolecular) aldols as well as acyclic (intermolecular) ones in excellent yields. Quantitative mechanistic studies have revealed that the organoselenium-mediated reduction proceeds via an α-substitution process in contrast to the common electron transfer reducing agents.Application of the methods to natural product synthesis such as santanolides (dehydroisoerivanin, isoerivanin, ludovicin C, and 1α, 3α-dihydroxyarbusculin B), diarylheptanolides (yashabushiketol, yashabushiketodiol A and yashabushiketodiol B), plant toxins picrotoxinin and picrotin, and a corn host-specific pathotoxin PM-toxin A is described.