Bridgehead position

Azonia substitution at a naphthalene bridgehead position gives the quinolizinium ion (16). Oxonia substitution, elsewhere, forms the 1- and 2-benzopyrylium ions (17) and (18). The two most well-known monoaza systems with three aromatie fused rings are aeridine (19), derived structurally from anthraeene, and phenanthridine (20), an azaphenanthrene. The better-known diaza systems inelude phenazine (21) and 1,10-phenanthroline (22), while systems with three linearly fused pyridine rings are ealled anthyridines, e.g. the 1,9,10-isomer (23). 

In contrast, the 6,8-dichloro 7 and 6,8-dimethyl derivatives, in which [6 + 4] dimerization would locate a substituent at the bridgehead position, form the symmetrical [6+6] adducts, e.g. 8.154... 

The cycloadducts formed from ethenetetracarbonitrile and the three isomerically pure methyl methyl-1//-azepine-l-carboxylates, 4, 5 and 6, have been subjected to a rigorous structural examination, from which it was concluded that adducts bearing the methyl groups at the bridgehead positions are not formed.251 Thus, the 2-methyl and the 4-methyl isomers, 4 and 5, each yield only one adduct, 7a and 7b, respectively, whereas methyl 3-methyl-1//-azepine-l-car-boxyate (6) gives 66 % yield of a 45 55 mixture of the 4-methyl 7 c and 7-methyl 7 d cycloadducts. 

Suitably Substituted Adamantanes. Adamantanes bearing four different substituents at the bridgehead positions are chiral and optically active, and 14, for example, has been resolved. This type of molecule is a kind of expanded tetrahedron and has the same symmetry properties as any other tetrahedron. 

Further evidence for the SnI mechanism is that reactions run under SnI conditions fail or proceed very slowly at the bridgehead positions of [2.2.1] (norbomyl) systems (e.g. 1-chloroapocamphane, 8). 

Radical cyclization of dihydropyridones 108 provided piperidine derivatives 109 containing a trifluoromethyl group at the bridgehead position adjacent to nitrogen <96JOC(61)8826>. 

Selective oxidations are possible for certain bicyclic hydrocarbons.285 Here, the bridgehead position is the preferred site of initial attack because of the order of reactivity of C—H bonds, which is 3° > 2° > 1°. The tertiary alcohols that are the initial oxidation products are not easily further oxidized. The geometry of the bicyclic rings (Bredt s rule) prevents both dehydration of the tertiary bridgehead alcohols and further oxidation to ketones. Therefore, oxidation that begins at a bridgehead position... 

The synthesis begins with the halogenation of adamantane (74) with bromine to give 76 or chlorine and AlClg to give 75. The four bridgehead positions... 

It is possible to force the reaction to give products bearing two alkyl-groups at bridgehead positions since 1,4-cyclo-adducts were obtained from hexamethylbenzene in 48% yield using tetrafluorobenzyne 54>, and in 30% yield using tetrachlorobenzyne 57>. 

Evidence in support of a preferred sp3 configuration is provided by the observation that reactions which involve the formation of carbanion intermediates at bridgehead positions often take place readily while those that would have involved the corresponding carbo-cation (sp2) intermediates do not (cf. p. 86). 

There are no bicyclic systems known with one tellurium atom at the bridgehead position. 

In addition the structure of the 1,2-azathiabenzene 78 was also confirmed by chemical evidence as shown in Scheme 10. Protonation of 78a (R1 = R2 = Me) with 70% perchloric acid yielded the corresponding cyclic amino sulfonium salt 82a in 87% yield, but not the starting sulfonium compound 76a, suggesting predominance of sulfilimine structure 78a rather than cyclic sulfonium ylide stmcture 80a. Thus, compound 78 could be recognized as the first example of a 1,2-azathiabenzene having sulfur at a bridgehead position. A proposed mechanism for the formation of 78 and 79 is shown in Scheme 9. The most acidic proton adjacent to sulfur in 76 is deprotonated with... 

Rearrangement of the 3,5-dimethylated carbene Id would yield the destabilized cyclopropene 3d with a methyl group in the bridgehead position 1, and consequently no detectable amount of cyclopropene 3d is formed during irradiation of Id. Indeed, whereas the 3,5-dimethyl substituted carbene Id is 3.4 kcal mol-1 more stable than the 2,6-dimethyl isomer lb, the stability is reversed for the cyclopropenes, as 3d is found to be 6.5 kcal mol-1 higher in energy than 3b at the B3LYP/6-31G(d) level of theory (Table 3). 

Similarly, in a 1,3-dipolar cycloaddition of DMAD to the conformationally locked cyclic a-alkoxycarbonylnitrone (727), bicyclic ring systems, containing a nitrogen atom at the bridgehead position have been synthesized. A mechanistic interpretation of the origin of the fused pyrroles (729) includes the intermediate formation of the aziridine ring in (728) (Scheme 2.303) (820). 

The azaadamantanes with nitrogen atoms in bridgehead positions present an interesting series of compounds with 1,3-n/n interactions in a rigid cage. The PE spectrum of... 

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