organolithium reagents (R'Li) to the ate complex R'RRSe−Li+. Selenoxides (R−Se(O)−R) are the selenium equivalents of sulfoxides. They can be further oxidized to selenones R−Se(O)2R, the selenium analogues of sulfones. Selenenic acid (RSe−OH) are intermediates in the oxidation of selenols. They occur in some selenoenzymes, such as glutathione peroxidase. Seleninic acids (RSe(O)OH) are analogues of sulfinic acids. Selenonic acids (RSe(O)2OH) are analogues of sulfonic acids. Peroxyseleninic acids (RSe(O)OOH) catalyse epoxidation reactions and Baeyer–Villiger oxidations. Selenuranes are hypervalent organoselenium compounds, formally derived from the tetrahalides such as SeCl4. Examples are of the type ArSeCl3. The chlorides are obtained by chlorination of the selenenyl chloride. Seleniranes are three-membered rings (parent: C2H4Se) related to thiiranes but, unlike thiiranes, seleniranes are kinetically unstable, extruding selenium directly (without oxidation) to form alkenes. This property has been utilized in synthetic organic chemistry. Selones (R2C=Se) are the selenium analogues of ketones. They are rare due to their tendency to oligomerize. Diselenobenzoquinone is stable as a metal complex. Selenourea is an example of a stable compound containing a C=Se bond. Thioselenides (R−Se−S−R), compounds with selenium(II)–sulfur(II) bonds, analogous to disulfides. Organoselenium compounds in nature Selenium, in the form of organoselenium compounds, is an essential micronutrient whose absence from the diet causes cardiac muscle and skeletal dysfunction. Organoselenium compounds are required for cellular defense against oxidative damage and for the correct functioning of the immune system. They may also play a role in prevention of premature aging and cancer. The source of Se used in biosynthesis is selenophosphate. Glutathione oxidase is an enzyme with a selenol at its active site. Organoselenium compounds have been found in higher plants. For example, upon analysis of garlic using the technique of high-performance liquid chromatography combined with inductively coupled plasma mass spectrometry (HPLC-ICP-MS), it was found that γ-glutamyl-Se-methylselenocysteine was the major Se-containing component, along with lesser amounts of Se-methylselenocysteine. Trace quantities of dimethyl selenide and allyl methyl selenide are found in human breath after consuming raw garlic. Selenocysteine and selenomethionine 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. Selenosulfides are also proposed as biochemical intermediates. Selenomethionine is a selenide-containing amino acid that also occurs naturally, but is generated by post-transcriptional modification. Organoselenium