Fused polycyclic systems

When (62) was treated with ethyl vinyl ether the cycloadduct (63) is afforded. If (62) is reacted with electron donating dienophiles such as allyl alcohols, transesterification and intramolecular cycloaddition occurs in the presence of a catalytic amount of distannoxane catalyst to give cis-fused polycyclic systems such as (64) <96T733>. 

With fused polycyclic systems and cycles bearing double bonds, in order to obtain simple "bond graphs", a careful distribution of points must be carried out. 

It is worth noting that in this synthesis of Cecropia juvenile hormone a strategy which is the reverse of the one developed by W.S. Johnson [8] for the synthesis of steroids and other fused polycyclic systems bearing cyclohexane rings is used. This method involves a non-enzymatic cyclisation of a polyunsaturated intermediate with the appropriate stereochemistry (all-trans) (Scheme 13.3.6). Such cyclisation occurs with a really amazing stereoselectivity and several new chiral centres with the correct stereochemistry are created in one single step ... 

The idea that such cyclisations should be highly stereoselective derives from the Stork-Eschenmoser hypothesis, according to which the double bonds in polyalkenes of the squalene type are properly arranged to undergo cyclisation to fused polycyclic systems with the natural trans-anti-trans configuration. 

Free-radical cyclizations using ethyl radicals generated by EtsB/air system or stannyl radicals systems provide a range of carbocyclic and heterocyclic hydroxylamines (equation 77). Stereoselectivity in these reactions is variable but can be semiquaUtatively predicted by Beckwith-Houk models . Depending on the substitution pattern of the emerging cyclic system, stereoselectivity can be very high, especially in fused polycyclic systems (equation... 

Polycyclic parent hydrides. These are classified as bridged polyalkanes (also known as von Baeyer bridged systems, from the nomenclature system developed to name them), spiro compounds, fused polycyclic systems and assemblies of identical rings. The four systems may be either carbocyclic or heterocyclic. In developing their names, the following principles are used. 

Fused polycyclic systems are very numerous and diverse. They are named using the structures and names of their smaller components and the concept of ortho-fusion, which is purely a formal operation encountered in nomenclature. This concept is essential to the naming of larger systems, and is the formation of one bond by the condensation of two bonds belonging to two different cyclic systems, one of them being a mancude ring. 

About 60 fused polycyclic systems have trivial names that are retained for present use. 

The term bridge , when used in connection with an ortho-fused or ortho- and peri-fused polycyclic system as defined in the note to Rule A-31.1 also includes bivalent cyclic... 

If one or both components of the monospiro compound are fused polycyclic systems, spiro is placed before the names of the components arranged in alphabetical order and enclosed in brackets. Established numbering of the individual components is retained. The lowest possible number is given to the spiro atom, and the numbers of the second component are marked with primes. The position of the spiro atom is indicated by placing the appropriate numbers between the names of the two components. 

If at least one component of a mono- or poly-spiro compound is a fused polycyclic system, the spiro compound is named according to Rule A-41.4 or A-41.7, giving the spiro atom as low a number as possible consistent with the fixed numberings of the component systems. 

Many aromatic compounds have considerable resonance stabilization but do not possess a benzene nucleus, or in the case of a fused polycyclic system, the molecular skeleton contains at least one ring that is not a benzene ring. The cyclopentadienyl anion C5HJ, the cycloheptatrienyl cation C7H+, the aromatic annulenes (except for [6]annulene, which is benzene), azulene, biphenylene and acenaphthylene (see Fig. 14.2.2(b)) are common examples of non-benzenoid aromatic hydrocarbons. The cyclic oxocarbon dianions C Of (n = 3,4,5,6) constitute a class of non-benzenoid aromatic compounds stabilized by two delocalized n electrons. Further details are given in Section 20.4.4. 

Anionic-anionic cascades are also effective for the rapid assembly of carbon skeletons. For example, double Barbier cyclisations were used by Cook for the synthesis of tetracyclic polyquinenes.21 Molander has shown that such sequences can be used to access a range of fused polycyclic systems (Scheme 6.17).13... 

Access to 6,6-/trai,s -fused polycyclic systems is shown in the cyclization of bicyclo Lodolactones, which are derived from benzoic acid79. The 6-endo ring closure selectively affords 5,6-cr. s-6,6-truns-fused tricyclic compounds, the smaller ring being the dominant factor in the formation of the cw-ring fusion. 

By far the most common cycloalkene generated using Martin s sulfurane is cyclohexene. Isolated six-membered ring alkenes and cyclohexenes in fused polycyclic systems are all generated with regularity using the title reagent. 

Modification of fluoroquinolones by annelation of carbo- or heterocyclic rings leads to fused polycyclic systems (Scheme 26). 

Couladouros EA, Vidali VP. Novel stereocontrolled approach to syn- and awO -oxepene-cyclogeranyl trans-fused polycyclic systems asymmetric total synthesis of (—)-aplysistatin, (+)-palisadin A, (+)-palisadin B, (+)-12-hydroxy-palisadin B, and the AB ring system of adociasulfate-2 and toxicol A. Chem. Eur. J. 2004 10(15) 3822-3835. 

With the exception of benzo, fusion prefixes for monocyclic systems are treated as exemplified for cyclopenta, cyclohepta etc. It should be noted here that for fused polycyclic systems the ending. ..ene always indicates the maximum number of non-cumulative double bonds, that is, a man-cude system ... 

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