Four Fused Rings

Steroid (Section 26.11) Type of lipid present in both plants and animals characterized by a nucleus of four fused rings (three are six-membered, one is five-membered). Cholesterol is the most abundant steroid in animals. 

From squalene, lanosterol, the first cyclic precursor, is created by a remarkable set of enzyme-catalyzed addition reactions and rearrangements that create four fused rings and seven stereocenters. 

The steroids are unified structurally in having a basic molecular skeleton containing four fused rings, three having six members and the fourth having just five ... 

Steroid one of a class of biologically active molecules built on a specific molecular framework consisting of four fused rings. 

First examples of stable carbocations were reported from several classes of thia-PAHs with four fused rings, namely, benzo[Z ]naphtho[2,l-J thiophene (40) and its 3-methoxy derivative (41), phenanthro[4,3-Z ]thiophene (42) and its 7-methoxy (43), 10-methoxy (44), and 9-methoxy (45) derivatives, phenanthro[3,4-Z ]thiophene (46) and its 7-methoxy (47) and 9-methoxy (48) derivatives, and 3-methoxybenzo[Z ]-naphtha[l,2-J thiophene (50) (Fig. 25). Regioselectivity issues and charge... 

Polycyclic Aromatic Hydrocarbons with Four Fused Rings Pyrene... 

Polycyclic Aromatic Hydrocarbons with Two or Three Fused Rings Polycyclic Aromatic Hydrocarbons PAH PNA POM Polycyclic Hydrocarbons, Nonalternant Compounds with Four Fused Rings Fluoranthene... 

POLYCYCLIC HYDROCARBONS, NONALTERNANT COMPOUNDS WITH FOUR FUSED RINGS... 

Four-Ring Fused Nonaltemant Hydrocarbons under Polycyclic Hydrocarbons, Nonaltemant Compounds with Four Fused Rings Freon 111 under Saturated Alkyl Halides Freon 113 under Saturated Alkyl Halides Fused-Ring Hydroxy Compounds... 

Sterols have four fused rings and a hydroxyl group. Cholesterol, the major sterol in animals, is both a structural component of membranes and precursor to a wide variety of steroids. 

The second major type of lipid found in some biological membranes is cholesterol. Cholesterol (fig. 17.5) is an isoprenoid compound with four fused rings, a short aliphatic chain, and a single hydroxyl group. It occurs in membranes both in its free form and esterified with long-chain fatty acids. Table 17.3 compares the lipid compositions of mem-... 

The overall style of the index follows that used in Chemical Abstracts and in The Ring Index by Patterson et al., but in order to keep the size of the index within reasonable bounds, only systems containing up to four fused rings are included. For the same reason a limit on individual ring size in any system has been placed at 10 atoms. 

Predict the color of the molecule tetracene, which has four fused rings and thus is intermediate in size between anthracene and pentacene. Also predict the color of the molecule s fluorescence. 

Enyne systems are also capable of impressive multiple RRM transformations. For the first such example see Ref. [119]. The reaction of a ruthenium alkyli-dene with an alkyne produces a new vinyl alkylidene, which can participate in further intramolecular or intermolecular metathesis reactions to form fused ring systems. This has led Grubbs to designate alkynes in such systems as relays . In a noteworthy example, Zuercher et al. [ 120] constructed the four fused rings of the steroid backbone 68 in one efficient step using tandem enyne re-... 

All share the skeleton of four fused rings, three six-membered and one five-m ember ed and conventionally lettered A-D. Beyond the ring stereochemistry and some common oxygenation patterns they share little else. Some (such as the female sex hormones) have an aromatic A ring some have side-chains on the five-membered ring. 

Steroids are naturally occurring terpenoids that are based on a system consisting of four fused rings. In 2006, Chung and co-workers published the synthesis of the basic steroidal skeleton (54->56) and employed an intramolecular PK reaction as the key step (55 —> 56 - Scheme 21).69... 

Bile acids are steroids, characterised by a carbon skeleton with four fused rings, generally arranged in a 6-6-6-S fashion. Primary bile acids are cholic acid and chenodeoxycholic acid (Figure 6.2). Within the intestines, bacteria convert primary bile acids to secondary bile acids, for example deoxycholate (from cholate) and lithocholate (from chenodeoxycholate). Both primary and secondary bile acids are re-absorbed by the intestines and delivered back to the liver via the portal circulation. 

The characteristic structure of sterol is their steroid nucleus consisting of four fused rings, three with six carbons (Phenanthrene) and one with five carbons (cyclopentane). This parent structure is known as perhydro cyclopentano phenanthrene. Cholesterol is the most abundant sterol in animals. Cholesterol... 

---