Anti-Bredt double bond

The outcome of the intramolecular reaction between vinylcar-benoids and furans depends on the structure of the vinylcarbenoid and the position of the tether. In the case of the 2-substituted fiiran 126, the triene 127 is formed exclusively (Scheme 43). Presumably, the additional strain in the intramolecular version favors unraveling of the furan ring to the triene rather than forming the [3+4] annulation product. In contrast, reaction of the 3-substituted furans 128 results in the synthesis of novel tricyclic products 129 that contain two formally anti-Bredt double bonds. " ... 

The reaction of mCPBA with the methano-bridged [5,6] open fulleroid (77) has been reported to result in anomalous selective electrophilic addition at the bridgehead anti-Bredt double bond to give (80) rather than the usual epoxidation product (79). The mechanistic preference for the unprecedented stepwise addition of mCPBA over the concerted epoxidation has been rationalized in terms of the notable ar-orbital misalignment of (78) by >30°, calculated at the B3LYP/6-31G(d) level. ... 

The chiral C2-conformer 42 of a ( )-cycloalkene can transform into the enantiomeric Cf-conformer 44 through a planar Cs-conformer 43, and this rope jump racemization can be prevented by anchoring one end of the unsaturated center onto the ring by means of an extra-bridge. This bridging creates a bicyclic anti-Bredt rule compound 45, revealing that all anti-Bredt rule compounds (45) with one double bond are necessarily asymmetric (C, symmetry) and have one asymmetric carbon atom. 

Wudl et al. [283] obtained the ring-opened TV-MEM-ketolactam 153 by self-sensitized photo-oxygenation of. V-methoxyethoxymethyl (MEM)-substituted [5,6]azafulleroid 151 (Scheme 61). The reaction is highly regioselective, most likely because the anti-Bredt carbon-carbon double bonds in 151 are more strained. 

Generally, bicyclic compounds (71) containing a bridgehead double bond and with S 5 7 may be regarded as an anti-Bredt s rule compound, which is quite unstable because of high ring strain energy (33). 

Twist as well as pyramidalization are the typical deformations of bridgehead double bonds in bicyclic compounds of type 44 (Table 4). Structures of the type 44 comprise Bredt-olefms (formerly called anti-Bredt-olefins) and bridgehead olefins. In Bredt-olefins, the double bond is located at the bridgehead of a bicyclo[m.n.o]alkane skeleton with m,n,o 1, whereas... 

More than one of the three possible oop bond angle deformations are usually apparent in the distorted olefins. Nevertheless, it is possible to detect in most cases the dominant type of distortion. The kinetic stability of olefins with distorted double bonds is enhanced by steric shielding. Hence, distorted double bonds in bicyclic and polycyclic structures may not display the reactivity to be expected if the dominant type of the oop distortion is the selectivity-controlling factor. The strong preference of syn- over anti-addition in Bredt-olefins may be due to this steric shielding. 

With this first round of simplifications complete, the majority of the cyclic bulk appended to the central core of the target molecules has been successfully excised. Of course, what still remains within 15 is the challenging [4.3.1] bicyclic system endowed with the anti-Bredt bridgehead C—C double bond. While this latter motif does seem formidable, if it is viewed within the context of the smaller six-membered ring in which it resides, then a possible course of retrosynthetic action should immediately be suggested in the form of an intramolecular Diels—Alder reaction (IMDA). Accordingly, application of this transform to 15 would lead directly to a precursor such as 17. Although the power of the Diels—Alder reaction to create complicated polycyclic architectures is rarely paralleled,"... 

Bridgehead alkenes, often referred to as anti-Bredt molecules, are indeed unstable. The problem with these compounds can be understood by inspection of orbital overlap in such systems. The p-orbital at the bridgehead is far from the necessary coplanarity with the other p-orbital. In order to achieve n-bonding, the system must be strongly twisted. In the extreme cases, where the p-orbitals of the double bond are close to orthogonality, such molecules can be considered as electronic analogues of excited states of alkenes (Figure 2.26). 

The above examples illustrate the importance of resonance that involves the amide t-system. When the electronic unity of amides is disrnpted by a structnral change (i.e. rotation), amides behave in unnsnal ways. Below, we will provide several additional examples of the anti-Bredt s amides distorted by the constraints imposed by their bicyclic structnre. The nsnally robnst N = C partial double bonds of amides become very sensitive towards hydrolysis and polymerization when the nitrogen atom is placed at a bridgehead position. 

Given the need for photoactivation, an often encountered additional requirement for substrates participating in DPM reactions is the presence of a strong chromophore as provided, for example, by the presence of a phenyl group on at least one of the double bonds. In acyclic 1,4-dienes, the central sp -hybridized carbon normally needs to be tetrasubstituted otherwise, competing 1,2-hydrogen shifts can occur. This requirement does not apply, however, to those cyclic substrates where isomerization would lead to an anti-Bredt olefin (as would be the case for the conversion of barrelene to semibullvalene, 6 7, shown in Scheme 9.2). 

Although (848) is an anti-Bredt system, stabilization of the positive charge is possible by electron donation from the strained anti double bond. The stereochemistry of epoxidation, hydroboration and osmylation of the double bond in (850), isolongifolene, and related molecules is controlled largely by the bicycloheptyl moiety and results in predominantly endo-attack. The regiospecific fragmentation of the C-1—C-12 bond in patchoulol (851) by lead tetra-acetate has been used to provide... 

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