Polycyclic system

Herradon, P. C. Hidber, J. J. Irwin, R. Locher, M. Maestro, T. Maetzge, A. Mourino, 

Pfammatter, D. A. Plattner, C. Schickli, W. B. Schweizer, P. Seiler, G. Stucky, W. Petter, 

Escalante, E. Juaristi, D. Quintana, C. Miravitlles, E. Molins, Helv. Chim. Acta 1992, 75, 913-935. 

Decalin is a bicyclic system composed of two fused six-membered rings. The smictures of c -decalin and tram-decdin are as follows  

The relationship between these compounds is stereoisomeric (as in the previous section). These two compounds are not interconvertible by ring flipping. They are two diflferent compounds with different physical properties. Many naturally occurring compoimds, such as steroids, incorporate decalin systems into their structures. Steroids are a class of compoimds comprised of four fused rings (three six-membered rings and one five-membered ring). Below are two examples of steroids  

Testosterone is an androgenic hormone (male sex hormone) produced in the testes, and estradiol is an estrogenic hormone (female sex hormone) produced from testosterone in the ovaries. Both compounds play a number of biological roles, ranging from the development of secondary sex characteristics to the promotion of tissue and muscle growth. 

Another common polycyclic system is norbornane. Norbornane is the common name for bicyclo[2.2.1]heptane. We can think of this compound as a six-membered ring locked into a boat conformation by a CH2 group that serves as a bridge. Many naturally occurring compounds are substituted norbornanes, such as camphor and camphene  

Camphor is a strongly scented solid that is isolated from evergreen trees in Asia. It is used as a spice as well as for medicinal purposes. Camphene is a minor constituent in many natural oils, such as pine oil and ginger oil. It is used in the preparation of fragrances. 

Reaction of (291) with methyl-lithium gives in 94% yield a product to which structure (292) has been assigned. Although the mechanism and in particular the explanation for the high stereoselectivity is not established, the reaction affords a useful method by subsequent solvolytic rearrangement of synthesis of (293). 

A mechanistic study of the decomposition of t-butyl hypochlorite leading to free-radical halogenation of (302) and (303) shows that in (302) attack is mainly at C-1 and C-6, but in (303) 93 % of attack is at C-8, and it is concluded that the endo-cyclopropane ring assists hydrogen abstraction at C-8. The stereochemistry of abstraction has not yet been determined removal of the anti-hydrogen is to be expected. Similar examination of (304)—(306) shows that radical addition masks any hydrogen abstraction. Alcohol (307) gives (308) with thionyl chloride.  

Reagents i, Hexachlorocyclopentadiene ii LiAlH4 iii, Bu OH-Na iv, CUCI-CH2N2 v, CrOj-py vi, Hj-Pd. 

In further studies of the tricyclo-octane system (329), Sauers reports the transformation of (330) into (331) with lithium aluminium hydride, and reactions 

Klinsmann, J. Gauthier, K. Schaffner, M. Pasternak, and B. Fuchs, Helv. Chim. Acta, 1972, 55,2643. 

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The early Escherunoser-Stork results indicated, that stereoselective cyclizations may be achieved, if monocyclic olefins with 1,5-polyene side chains are used as substrates in acid treatment. This assumption has now been justified by many syntheses of polycyclic systems. A typical example synthesis is given with the last reaction. The cyclization of a trideca-3,7-dien-11-ynyl cyclopentenol leads in 70% yield to a 17-acetyl A-norsteroid with correct stereochemistry at all ring junctions. Ozonolysis of ring A and aldol condensation gave dl-progesterone (M.B. Gravestock, 1978 see p. 279f.). 

Intramolecular reactions between donor and acceptor centres in fused ring systems provide a general route to bridged polycyclic systems. The cts-decalone mesylate given below contains two d -centres adjacent to the carbonyl function and one a -centre. Treatment of this compound with base leads to reversible enolate formation, and the C-3 carbanion substitutes the mesylate on C-7 (J. Gauthier, 1967 A. Belanger, 1968). 

In polycyclic systems the Birch reduction of C—C double bonds is also highly stereoselective, e.g. in the synthesis of the thermodynamically favored trans-fused steroidal skeletons (see p. 104 and p. 278). 

The spectra of protonated polyaza heterocycles are frequently complicated by the occurrence of covalent hydration. This is more common with polycyclic systems, e.g. pteridine. 

Building-block rings (e.g. benzenoid) which are terminal are not disconnected central benzenoid rings in a polycyclic system may be eligible for disconnection especially if adjacent rings are benzenoid or not readily disconnectible. 

Mesomerism involving polarized and nonpolarized contributing enamine forms influences the enamine s spectral properties and chemical reactivity. For mesomerism to be present, a planar arrangement is required for the three atoms of enamine grouping and the five atoms immediately bound to this system. If this condition is not fulfilled, full interaction of the tt electrons of the double bond with the free electron pair on the nitrogen atom is impossible. Enamines in which mesomerism is inhibited do not show the properties characteristic of enamines, and only the mutual electrostatic interaction of the double bond and lone electron pair of the nitrogen atom can be observed. Such steric hindrance of mesomerism occurs mainly in polycyclic systems. 

The chemistry of mononuclear isothiazoles has been developed since 1956 without the aid of thiohydroxylamine, the preparation of this very unstable substance having only recently been reported. Bicyclic and polycyclic systems involving the isothiazole (1,2-thiazole) structure have long been known and were fully reviewed in 1952, but little new work has been reported since then and the present review... 

Dihydroxyfurazan reacts with bis-electrophiles in the presence of a base and a phase transfer catalyst to form furazano fused bicyclic or polycyclic systems (Scheme 174) (92URP1715808, 92URP1715809, 92URP1752734, 97MI8). 

Thiadiazole 5-oxides and 5,5-dioxides are the least studied among the four possible isomers of thiadiazole system. The main summary is in the first and the second editions of Comprehensive Heterocyclic Chemistry (84CHEC-I(6)447, 96CHEC-II(4)289). Apparently the 5-oxide and the 5,5-dioxide derivatives have not yet been reviewed. There are very few papers in the literature treating these derivatives, and most of these are regarding polycyclic systems. 

Gratifyingly, when compound 24 is refluxed in a solution of toluene at 110°C, it undergoes quantitative [4+2] cycloaddition to polycyclic system 25. The indicated stereochemistry of 25 was anticipated on the basis of the trans,trans geometry of the phenyl-diene system in precursor 24 and the presumed preference for an exo transition state geometry. These assumptions were vindicated by the eventual conversion of 25 to endiandric acids A (1) and B (2). 

Closely related to the polyepoxide cascade procedure for the synthesis of polycyclic systems is Corey s biomimetic-type, nonenzymatic, oxirane-initiated (Lewis acid-promoted) cation-olefin polyannulation. By this strategy, compound 96, containing the tetracyclic core of scalarenedial, was constructed by exposure of the acyclic epoxy triene precursor 95 to MeAlCl2-promoted cyclization reaction conditions (Scheme 8.25) [45]. 

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