Steroids carbon skeleton

Schouten S., Sephton S., Baas M., and Sinninghe Damste J. S. (1998b) Steroid carbon skeletons with unusually branched C-3 alkyl side chains in sulphur-rich sediments. Geochim. Cosmochim. Acta 62, 1127-1132. 

The key step in the enantioselective synthesis of a polycyclic target is the establishment of the first cyclic stereogenic center. Further construction can then be chrected by that initial center. This principle is nicely illustrated by a synthesis of (+)-estrone methyl ether (145), reported by Taber (Scheme 8). The specifically designed naphthylbomyl ester (141) is used to direct C—insertion selectively toward one of the two diastereotopic C—H bonds. The new ternary center so created then directs the formation of the adjacent quaternary center in the course of the alkylation. Finally, the chiral skew in the product cy-clopentanone (144) directs the relative and absolute course of the intramolecular cycloaddition, to give the steroid carbon skeleton. 

All steroids contain the steroid nucleus (steroid carbon skeleton) as shown here ... 

Figure 29. Relationships h een the carbon positions in isopentenyl pyrophosphate and their sources. In the mevalonic-acid pathway, all five caibon positions in isopentenyl pyrophosphate derive from acetate and, in turn from the C-1 + C-6 and C-2 + C5 positions of glucose. In the methyierythritol-phosphate pathway, one carbon derives from the C-3 + C-4 position in glucose. Moreover, the mapping of positions from preciu ors into products of the two pathways differs sharply, as indicated by stmctures of acyclic and steroidal carbon skeletons based on the MVA (a, c) and MEP pathways (b, d).

Farnesol pyrophosphate is an immediate precursor of squalene, the key intermediate in steroid and triterpenoid biogenesis, which arises from the coupling of two farnesol pyrophosphate molecules or of C,s units derived therefrom. The numerous types of sesquiter-penoid carbon skeletons represent various modes of cyclization of farnesol (sometimes with rearrangement) and it is probable that farnesol pyrophosphate is also the source of these compounds. 

Steroids are plant and animal lipids with a characteristic tetracyclic carbon skeleton. Like the eicosanoids, steroids occur widely in body tissues and have a large variety of physiological activities. Steroids are closely related to terpenoids and arise biosynthetically from the triterpene lanosterol. Lanosterol, in turn, arises from cationic cyclization of the acyclic hydrocarbon squalene. 

Steroid (Section 27.6) A lipid whose structure is based on a tetracyclic carbon skeleton with three 6-membered and one 5-membered ring. Steroids occur in both plants and animals and have a variety of important hormonal functions. 

The term steroid applies to compounds containing a hydrogenated cyclo-pentanophenanthrene carbon skeleton ... 

The chemical structure of vitamin D3 is closely related to its precursor, 7-dehydrocholesterol, from which it is produced by a photochemical reaction. Therefore, vitamin Do is closely related structurally to the four-ring nucleus of steroids derived from the cyclopentanoperhydro-phenanthrene ring system. No vitamin D activity is noticed until the B ring of 7-dehydrocholesterol is opened between C-9 and C-10. Thus, vitamin D3 is a 9,10-seco steroid and its carbon skeleton is numbered accordingly (Scheme I). The important aspects of its chemistry center about the 5,6,7-cis-triene structure. The formula for vitamin D3 is C27H44O and its formula weight is 384.64. 

The upper laminite sample from NR-10 (151.5 m), besides the 2-tetradecylthiophene, contains an isomer with an identical mass spectrum which according to the slightly shorter retention time may have a methyl-branched carbon skeleton (Figure 4a). Small amounts of a thienylhopane are present in all samples studied, but sulfur-bearing steroids could not be detected despite the abundance of sterenes in the saturated hydrocarbon fractions. 

Figure 5. Proposed structures of alkylthiophene moieties in kerogens and asphaltenes and their presumed flash pyrolysis products. Examples are give for alkylthiophene moieties with (a) linear, (b) isoprenoid, (c) branched and (d) steroidal side-chain carbon skeletons. Carbon skeletons are indicated with bold lines.

The resin fractions of organic sulfur-rich bitumens are for a substantial part composed of monomers with linear, isoprenoid, steroid, hopanoid and carotenoid carbon skeletons connected to each other by (poly)sulfide linkages. These structural units may contain additional intramolecular sulfur linkages. This sulfur-rich geopolymer is also formed by sulfur incorporation into functionalised lipids in an intermolecular fashion during early diagenesis. 

Steroids are compounds based on fused multi-ring carbon skeletons, each ring being referred to by a letter (Figure 1.23(a)). The rings may be saturated or... 

In accordance with chemical nomenclature the sterenes can be considered a subclass of steroids as they share with these the hydrogenated cyclopentanophenan-threne carbon skeleton. Sterenes are not naturally occurring substances, but are artefacts arising from free or esterified sterols through the elimination of either the elements of water or those of a fatty acid. 

Role of squalene in the biosynthesis of steroids. The biosynthesis of steroids starts with epoxidation of squalene to squalene-2,3-epoxide. The opening of this epoxide promotes cyclization of the carbon skeleton under the control of an enzyme. The cyclized intermediate is converted to lanosterol, then to other steroids. 

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. 

Cholesterol, the chapter-opening molecule, has the tetracyclic carbon skeleton characteristic of steroids. It also has eight stereogenic carbons (seven on rings and one on a side chain), so there are 2 = 256 possible stereoisomers. In nature, however, only the following stereoisomer exists ... 

This chapter examines the biosynthesis of three important components of biological membranes—phospholipids, sphingolipids, and cholesterol (Chapter 12). Triacylglycerols also are considered here because the pathway for their synthesis overlaps that of phospholipids. Cholesterol is of interest both as a membrane component and as a precursor of many signal molecules, including the steroid hormones progesterone, testosterone, estrogen, and cortisol. The biosynthesis of cholesterol exemplifies a fundamental mechanism for the assembly of extended carbon skeletons from five-carbon units. 

It has been shown that 7j8-fluoro-B-homo-steroids are much more stable than their 7jS-chloro-analogues. Aromatization of ring a of such 7)S-fluoro-B-homo-androstanes and -pregnanes, under various conditions, occurred without rearrangement of the carbon skeleton and without loss of fluorine. Whereas... 

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