Selenocysteine, called the twenty-first amino acid, is essential for ribosome-directed protein synthesis in some organisms. More than 25 selenium-containing proteins (selenoproteins) are now known. Most selenium-dependent enzymes contain selenocysteine, which is related to cysteine analog but with selenium replacing sulfur. This amino acid is encoded in a special manner by DNA. Selenomethionine is a selenide-containing amino acid that also occurs naturally, but is generated by post-transcriptional modification. Organoselenium compounds are specialized but useful collection of reagents useful in organic synthesis, although they are generally excluded from processes useful to pharmaceuticals owing to regulatory issues. Their usefulness hinges on certain attributes, including (i) the weakness of the C−Se bond and (ii) the easy oxidation of divalent selenium compounds. Vinylic selenides are organoselenium compounds that play a role in organic synthesis, especially in the development of convenient stereoselective routes to functionalized alkenes. Although various methods are mentioned for the preparation of vinylic selenides, a more useful procedure has centered on the nucleophilic or electrophilic organoselenium addition to terminal or internal alkynes. For example, the nucleophilic addition of selenophenol to alkynes affords, preferentially, the Z-vinylic selenides after longer reaction times at room temperature.The reaction is faster at a high temperature; however, the mixture of Z- and E-vinylic selenides was obtained in an almost 1:1 ratio. On the other hand, the adducts depend on the nature of the substituents at the triple bond. Conversely, vinylic selenides can be prepared by palladium-catalyzed hydroselenation of alkynes to afford the Markovnikov adduct in good yields. There are some limitations associated with the methodologies to prepare vinylic selenides illustrated above; the procedures described employ diorganoyl diselenides or selenophenol as starting materials, which are volatile and unstable and have an unpleasant odor. Also, the preparation of these compounds is complex. Selenium dioxide is useful in organic oxidation. Specifically, SeO2 will convert an allylic methylene group into the corresponding alcohol. A number of other reagents bring about this reaction. In terms of reaction mechanism, SeO2 and the allylic substrate react via pericyclic process beginning with an ene reaction that activates the C−H bond.