Carbon atom bridgehead

The bicyclic compound decahydronaphthalene, or bicyclo[4.4.0]decane, has two fused six-mem-bered rings. It exists in cis and trans forms (see Fig. 1.10), as determined by the configurations at the bridgehead carbon atoms. Both cis- and rran -decahydronaphthaiene can be constructed with two chair conformations. 

Furthermore highly strained compounds such as bicyclo[3.2.1]oct-l-ene 11, containing a double bond to a bridgehead carbon atom, have been prepared however this strained olefin could be identified only as its Diels-Alder product from subsequent reaction with an added diene." ... 

For the formation of the new double bond, the general rules for eliminations do apply. Following Bredt s rule, no double bond to a bridgehead carbon atom will be formed. If the elimination can lead to a conjugated system of unsaturated groups, this pathway will be favored. Otherwise the Hofmann rule will be followed, which favors an elimination towards the less substituted carbon center. 

Rates of detritiation of the 1 and 2 positions of biphenylene and the 5 position of benzo [bjbiphenylene by lithium cyclohexylamide at 50 °C relative to benzene have been determined as 490, 7.0, and 1,865, respectively and the enhanced reactivity of the position a to the strained 4-membered ring has been attributed to the enhanced electronegativity of the strained bridgehead carbon atom. The same... 

These compounds are much more toxic than chlordan (Table III), and yet are stable toward alkaline reagents (15), being unable to eliminate hydrogen chloride without the formation of a double bond at a bridgehead carbon atom. Thus in this type of compound the conclusion must again be reached that dehydrochlorination with alkali and insecticidal activity have no systematic relationship. 

A cyclam derivative with an amine pendent donor attached at the bridgehead carbon atom was presynthesized from diethylaminomalonate and l,9-diamino-3,7-diazanonane in refluxing MeOH, followed by BH3-THF reduction.1507 It acts as a pentadentate ligand at pH 8.5 and forms a six-coordinate complex (592) with a water molecule sitting at the remaining axial site. 

All are tertiary halides so that attack by the S mode would not be expected to occur on (16) or (17) any more than it did on (8) (cf. p. 82). Sn2 attack from the back on the carbon atom carrying Br would in any case be prevented in (16) and (17) both sterically by their cagelike structure, and also by the impossibility of forcing their fairly rigid framework through transition states with the required planar distribution of bonds to the bridgehead carbon atom (cf. p. 84). Solvolysis via rate-limiting formation of the ion pair (SN1), as happens with (8) is... 

In cyclic systems, the usual simple requirements of Saytzev or Hofmann rules may be overridden by other special requirements of the system, e.g. the preference for elimination from the truns-diaxial conformation in cyclohexane derivatives (cf. p. 255). Another such limitation is that it is not normally possible to effect an elimination so as to introduce a double bond on a bridgehead carbon atom in a fused ring system (Bredt s rule), e.g. (47) (48) ... 

Two isomers of the cyclopropene, 3c and 3c with the methyl group in 3-and 1-position, respectively, have to be considered. Although the number of IR absorptions suggests that only one isomer is formed and the experimental spectrum is better matched by the spectrum calculated for 3c, the theoretical IR spectra of the two isomers are too similar to allow for a definitive assignment. According to DFT calculations, the isomer 3c is by 3.6 kcal/mol more stable than the isomer 3c with the methyl group at the bridgehead carbon atom, which is in line with the preferentially formation of 3c (Scheme 8). 

The asymmetric synthesis of (+)-Codeine 432 devised by White and colleagues included a Beckmann rearrangement to introduce the nitrogen atom in the carbocyclic structure (equation 182). Even though two isomeric lactams 430 and 431 were obtained as a result of the rearrangement, the preferential migration of the bridgehead carbon atom produced 430 as the predominant isomer. The synthesis of the non-natural enantiomer of Codeine was completed after oxidation, olefin formation and reduction. 

The propellanes are a class of compounds with three condensed rings, either three- or four-membered, sharing a bridgehead bond. In two [1.1.1] propellane derivatives studied by Seiler et al. (1988), no peaks were observed in the deformation density of the central bridgehead bonds of lengths 1.58 A, but peaks at the apex of the inverted (i.e., pyramidal) bridgehead carbon atoms are in agreement wih electrophilic attack at these positions. 

A novel synthesis75 of, presumably, a hexahydro-l,5-naphthyridine (27) resulted as a by-product of the preparation of a dipyrrolenyl derivative by reductive cyclization of the ketone 26. The stereochemistry about the bridgehead carbon atoms was not established. 

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