Bridged bicyclic system

Methods for synthesis of saturated seven-member 0-heterocycles and bicyclic systems with 0-bridge 98ACR603. 

It has been stated that bridged bicyclic systems fail to yield any cyclic acetal upon photolysis,[Pg.382]

With a synthesis of 58 completed, the key intramolecular diketone aldol cyclization was investigated. Precedent for this type of 1,8-dicarbonyl aldol reaction is rare, although an aldol reaction has been proposed in the biosynthetic pathway to the hypocrellins. The only reported examples of such diketone aldol cyclizations involve multicyclic or bridged bicyclic systems, and of these no examples exist for 1,8-diketones forming 7-membered rings. MM2 calculations indicated that a... 

Photo- and thermal decarbonylation of cyclic unsaturated ketones leads to the formation of cyclic 1,3-dienes. Such decarbonylations are commonly observed in 7-ketonorbomenes and related bridged bicyclic systems to give cyclohexadienes (equation 84)132. 

The products of these [4 + 2 + 2]-reactions have a high level of molecular complexity, which has been used to advantage in exploring the conversion of the [4 + 2 + 2]-cycloadducts into commonly encountered ring systems. Zeise s dimer, for example, has been employed to carry out a skeletal rearrangement, as illustrated in Scheme 61, producing fused and bridged bicyclic systems found in several natural product families. 

Intramolecular electron transfer initiated cyclization reaction of 7V-allyliminium salt systems may also generate 3-pyrrolidinyl ethers or alcohols in monocyclic and bridged or fused bicyclic systems (e.g. equations 63 - 65)122,123. 

In contrast, in the formation of the hydroazulene ring of bulnesol (23) Marshall and Partridge [11] started from a bridged system (21) to arrive, through an intermediate carbocation 22, at the fused bicyclic system 23 (Scheme 7.10). 

A similar desymmetrization approach was applied to the synthesis of bridged bicyclic systems. Reaction of 2-diazoacetyl-l,3-dioxane in the presence of a chiral rhodium catalyst allow the construction of the bicycle in 27-54% yield, but poor ee (4-12%) (Equation 51) <2003TA929>. 

The intramolecular cycloadditions of cychc nitronates have received much more attention. The cyclic nitronate structure provides three basic modes of intramolecular cycloaddition (Fig. 2.15). Attachment of the tether to the C(3) position of the nitronate results in the formation of a sprro system (sprro mode). However, if the tether is appended to the C(4) position of the nitronate, the dipolar cycloaddition yields a fused ring system (fused mode). Finally, if the tether is attached at any other point of the cyclic nitronate, the cycloadducts obtained will consist of bicyclic structures (bridged mode). 

Designation of relative ligand location in the shortest (i.e. highest numbered) bridge of a bicyclic system the anti ligand points away from, the syn ligand towards the lowest numbered bridge. 

Some interesting synthetic applications have been found by Strauss for the interactions of 3,5-dinitropyridine with carbanions, as generated in situ by base-promoted CH ionization, to yield bicyclic systems by meta bridging,12 under conditions favoring a similar reaction with 1,3,5-trinitrobenzene.9 3,5-Dinitropyridine reacts with 1,3-diphenyl-2-propanone and triethylamine to give the bridged ion 76, according to Scheme 5. The reaction consists... 

The bridged bicyclic system 133 represents the earliest example of an anion that was claimed to be homoaromatic272. Studies of this system actively continue and the importance of homoconjugation in accounting for the properties of the anion has been a matter of some controversy and debate. 

The main ring and the main bridge form a bicyclic system whose numbering is made in compliance with Rule A-31. 

Hemispherands (11) are by nature only partially preorganized for complexation, depending on their structural characteristics.30 129 132 Addition of hydrocarbon-based bridges, which essentially converts the monocycles to bicyclic systems, serves to provide a more rigid structural framework as in the cryptands.132... 

Exo-endo additions of organic azides to the norbornene skeleton and other bridged bicyclic systems can be easily differentiated from the NMR spectra of the adducts. The endo protons (exo adducts) of the norbornane do not couple with the bridgehead hydrogens.386 The endo protons in position 3a and 7a of adduct 65 exhibit an AB spectrum. The lack of coupling with the bridgehead protons is a result of a dihedral angle of 82°.25,96 Based on this, the exo form of cycloadduct 66 exhibits symmetrical doublets centered at 8 4.60 and... 

The distinction between regioisomeric cycloadducts of nonsymmetrical, bridged bicyclic systems and organic azides has been achieved by observing the NMR chemical shifts, the coupling constants, and the nuclear Overhauser effect. For example, the exo regioisomers 73 and 74 show the following characteristics ... 

Bridged and spirocyclic bicycloalkenones. The conversion of silyl enol ethers to a,/J-enones by Pd(OAc)2 (8, 378) can result in cyclization to bicyclic systems when applied to silyl enol ethers of cyclohexanones bearing an alkenyl side chain a or y to the carbonyl group. Although the factors favoring cyclization are not fully defined, this cyclization offers a route to a variety of bridged and spirocyclic systems.1... 

This reaction is further applied to the construction of a bridged bicyclic system (239), through 7-endo-trig ring closure of the vinyl radical, 3-exo-trig ring closure, and subsequent p-cleavage of the fused cyclopropyloxyl radical as shown in eq. 3.95. 

The reaction of terf-butyltrichlorosilane with an excess of ammonia leads to the formation of a tetrasilabicyclo[3.3.1]nonane (149), an NH-bridged cyclotetrasilazane, presumably via 148 (equation 43)110. The bicyclic system contains two six-membered rings in boat conformations. The r-Bu groups of the ClSiN2 unit are in cw-position110. 

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