Bridgehead atom polycyclic

Bridgehead atom (Section 4.9) An atom that is shared by more than one ring in a polycyclic molecule. 

This review covers nonannelated and annelated eight-membered heterocycles containing one nitrogen. However, it does not include polycyclic structures where nitrogen is the bridgehead atom (1), nor where nonadjacent ring carbons are linked by a bridge (2). 

If all of the cyclopropyl carbons in a polycycle are bridgehead atoms, one of the bonds next to the carbonyl function is cleaved, as exemplified by the hydrogenation reaction of a de-hydro[3.3.3]propellane system 6."... 

The same technique can be used with flexible macrocycles, with one significant caveat - while a polycyclic system has bridgehead atoms which act as focus points, there are no such anchor points in a monocyclic macrocycle. The construction of a superstructure requires the choice of at least three such anchor points, which have to be arbitrarily chosen. The final choice is made by generating and evaluating several alternative sets of anchor points and superstructures. 

The procedure described here is a modification of one involving the thermal fragmentation of 1-adamantyl hypoiodite and cycliza-tion of the resulting iodo ketone/ By means of this procedure, 4-protoadamantanone is obtained from 1-adamantanol with consistent yields in the range of 71 to 82% and a purity greater than 98%. This method is also applicable to the preparation of other polycyclic ketones from the related bridgehead alcohols with a-bridges of zero, one, or two carbon atoms (see Table I). 

A different class of polycyclic naphthoquinone dyes is based on the n apht h 12,361 i ndolizine -6,11 -dione system 14, which is readily accessible by condensation of 2,3-dichloro-l,4-naphthoquinone with active methylene compounds in the presence of pyridine [22], or by reaction of 2-methoxy-3-pyridino-1,4-naphthoquinone with an active methylene compound [23], In 14, the bridgehead nitrogen atom acts as an effective auxochrome, and hence orange to red colors are observed without further substitution. Derivatives of 14 (R = amide group) are of particular value as vat dyes and pigments. Related isomeric heterocyclic structures have also attracted interest, e.g., 15, a yellow disperse dye for polyester [24],... 

Instead attention is next directed to polycyclic compounds for which there is a different formal bond order than that which simplistic graph theory would imply. Unlike the subject matter introduced in Chapter 2, here traditional covalent bonds, in contradistinction to consideration of hydrogen bonds, are an integral part of the nomenclature. As a first example, note that, unlike the larger propellanes, the bond order is equal to zero between the bridgehead carbon atoms in [l,l,l]-propellane [40] (Figure 7). This "anomoly" is accounted for in the nomenclature by the name ... 

Benzo derivatives, such as the benzo[l,3]dithiolium tetrafluoroborate 35 <2004JME5265>, are also known. For polycyclic systems that are isoconjugate with dianions, neutral rings with two oxygen/sulfur atoms, such as the cyclopenta[l,2]dithiole 36 , can be made. The occurrence of bicyclic systems with a bridgehead sulfur, such as the trithiapentalenes 37, should also be noted <1971AHC(13)161>. 

Anti-Bredt alkene a polycyclic alkene that contravenes Bredt s rules by having a C=C bond attached to a bridgehead carbon atom... 

Finally, S. Arai and M. Hida review polycyclic aromatic nitrogen cations containing bridgehead (ring fusion) nitrogen atoms. The chemistry of quinolizinium salts was covered by Thyagarajan in Volume 5 of our series back in 1965, and although other partial reviews are available, we now have for the first time a modem comprehensive treatment. 

A study with the cyclopropane incorporated into a polycyclic system, e.g. 9, showed that the addition of hydrogen bromide across one of the cyclopropane bonds occurred with retention of configuration at the electrophilic bridgehead carbon atom. ... 

Centropolyindanes constitute a complete family of arylaliphatic polycyclic hydrocarbons containing several indane units. Mutual fusion of the five-membered rings leads to three-dimensional, carbon-rich molecular frameworks bearing a central carbon atom, such as benzo-annelated [3.3.3]propellanes, triquinacenes, and [5.5.5.6]- and [5.5.5.5]fenestranes. In this review, the structural concept of centropolyindanes is contrasted to other fused indane hydrocarbons. Besides the syntheses of the parent centropolyindanes and recently described related indane hydrocarbons, the preparation of a large variety of bridgehead and arene substituted centropolyindanes is presented including strained, heterocyclic, and centrohexacyclic derivatives. In appropriate cases, the particular reactivity and some structural features of these unusual, sterically rigid polycyclic compounds are pointed out. 

The review of Stanovnik (1991) concentrates on diazoalkane additions to some nitrogen containing heteroaromatic systems, namely to monocyclic pyridazines, and to bicyclic and polycyclic azolo- and azinopyridazines with a bridgehead N atom. 

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