Terpenoids polycyclic

As exemplified in the present procedure, the reaction has been optimized and extended in scope it affords functionalized benzocyclobutenes as well as substituted isoquinolines in high yields. Benzocyclobutenes have been used as intermediates in the synthesis of many naturally occurring alkaloids, - steroids,polycyclic terpenoids,and anthracycline antibiotics. The traditional routes leading to the preparation of benzocyclobutenes have been... 

SnCl4 is also superior to 22 SnCl4, and is effective for the enantioselective cyclization of 2-(polyprenyl)phenol derivatives to give polycyclic terpenoids bearing a chroman skeleton.The synthetic utility of 26 SnCl4 is demonstrated by very... 

Electrooxidative ring opening of polycyclic terpenoids 25 has been investigated in an AcOH—EtjN—(C) system in an undivided cell. The electrolysis of tricyclene 25a at 15 °C yields exo-2,2-dimethyl-3-methylenebicyclo[2.2.1]heptan-5-ol 26a, Nojigiku alcohol, in 76% yield (Scheme 3-9)S6). The results reported on the electrooxidative cleavage of terpenoids are summarized in Table 3.2. 

Squalene has been shown to assume the folding pattern of Woodward and Bloch in the cyclization process that forms sterols (Fig. 17). This was demonstrated by the labehng patterns of sterols derived from acetate and mevalonate [90,91]. There are several ways by which squalene can cyclize, consequently it is the precursor of several polycyclic terpenoids (Chapter 7). The product depends on the conformation squalene assumes in binding to the cyclase and the nature and position of the nucleophiles and bases on the enzyme. Mechanistically, squalene requires an electrophilic attack at C-3 for cyclization to occur. This can be accomplished by direct attack of a as in the cycUzation that produces tetrahymanol and femene or by... 

The Rautenstrauch rearrangement has been used in a number of total syntheses of polycyclic terpenoids. Fehr and Fiirstner have disclosed closely related approaches in the total synthesis of (-)-cubebol (Scheme 5.24), where the cycloisomerization of enantioenriched propargyl pivalates 66 occurs with good chirality transfer to give 67 in 77% yield [79-81]. Using similar strategies, various terpenoids derivatives have efficiently been obtained [80, 82]. 

Conventional multistep synthesis of natural products reduces the overall yield of the target molecules. In contrast, biomimetic enantioselective domino reactions, promoted by small-molecule artificial enzymes, are more useful for the practical synthesis of natural products and related compounds. The stereoselective formation of polycyclic isoprenoids by the cyclase-induced cyclization of polypren-oids is one of the most remarkable steps in biosynthesis because this reaction results in the formation of several new quaternary and tertiary stereocenters and new rings in a single step. The use of biomimetic polycyclization with artificial cyclase is the most ideal chemical method for the synthesis of these polycyclic terpenoids. In this chapter, biosynthesis of polycyclic terpenoids, biomimetic stereoselective polyene cyclization induced by artificial cyclases, and total synthesis of bioactive natural products using stereoselective polyene cyclization as a key step will be discussed. 

Polycyclic terpenoids are metabolite products that are commonly found in nature and have a variety of biological activities. According to the Stoik-Eschenmoser hypothesis, many polycyclic isoprenoids are biosynthesized from simple linear polyprenoids such as geraniol, famesol, geranylgeraniol, and squalene via cationic polycyclization, which is considered to one of the most complex carbon-carbon bond-forming reactions in nature [1]. The complicated polycyclic structures of isoprenoids, which have many chiral centers including quaternary carbons, are stere-oselectively constructed by cyclases in an impressive key step (Schemes 9.2 and 9.3) [2],... 

According to the Stork-Eschenmoser hypothesis [lb, c], many polycyclic terpenoids such as hopene and lanosterol are biosynthesized via the site- and enantioselective protonation or epoxidation of a terminal olefin followed by diastereoselective x-cation cyclization [16]. Corey s group performed pioneering work on the cation-induced diastereoselective polycyclization of enantiopure epoxides [17]. For example, a very direct enantioselective total synthesis of members of the p-amyrin family of pentacyclic triterpenes has been developed by diastereoselective x-cation tricycli-zation reaction as a key step (Scheme 9.9) [17g]. 

Bromine-containing polycyclic terpenoids are also considered to be biosynthesized via the site- and enantioselec-tive bromination of a carbon-carbon double bond followed... 

Cationic cyclization is a common theme for the biosynthesis of a large number of polycyclic terpenoids and is partly responsible for the structural diversity of terpenoids that share the same poly-isoprene origin. Thus, it is not surprising that cationic transannular cyclization is also prominently featured in polycyclic terpenoids synthesis. 

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