Steroids are a kind of cyclopentadienyl phenanthrene derivatives, which are widely distributed in the biological world. Also known as sterols and steroids. Include sterols (such as cholesterol, lanosterol, sitosterol, stigmasterol and ergosterol), bile acids and bile alcohols, steroid hormones (such as adrenocortical hormone, androgen and estrogen), insect ecdysone, cardiac glycosides (such as digitalis glycoside) and saponins, and toad venom. In addition, there are synthetic steroid drugs, such as anti-inflammatory drugs (prednisone, dexamethasone), steroid drugs that promote the synthesis of protein (norolone phenylpropionate) and oral contraceptives. Steroid compounds do not contain bound fatty acids and are unsaponifiable lipids; These compounds belong to isoprene-like substances, which are generated by cyclization, intramolecular recombination and chemical modification of triterpenoids. Steroids are fused tetracyclic compounds, consisting of three hexacycloethane (A, B, C) and 1 five-carbon ring (D). The six-carbon rings A, B and C in natural steroid molecules are all chair conformation (cyclohexane structure), which is the most stable conformation (the only exception is that the A ring in estrogen molecules is aromatic ring and planar conformation).