Bicyclo-systems

In the Weiss reaction (Scheme 4), an 7-dicarbonyl compound (38) condenses with two molecules of dimethyl 3-oxoglutarate (39 E = CC Me) to give a c w-bicyclo [3.3.0] oct-ane-3,7-dione tetraester (40) the one-pot reaction produces considerable complexity, with the sequential formation of four C—C bonds. Simple acid treatment removes the carbomethoxy groups, if deshed. While die reaction involves aldol and Michael sequences, die intermediacy of a cyclopentenone [4-hydroxycyclopent-2-enone (41)] has up to now been unproven. A series of such 1 1 adducts has now been reported for a variety of diketones, together with evidence diat diey are indeed intermediates en route to the bicyclo system.62 Electronic and steric effects on the reaction are also discussed in detail. 

The norbornyl system is a [2.2.1] bicyclo system, and so the S number is five. Bridgehead double bonds only form if the S number is seven or greater. There is an additional requirement, in that the double bond may exist only in a ring that contains eight or more atoms, i.e. a derivative of trans-cyclooctene or larger. 

In the nickel(II) and cobalt(II)complexes of phosphabicyclo[3.3.1]nonanes 282 (246) and 283 (247), respectively, the bicyclo systems exist in a CC conformation according to X-ray data. Some complex compounds incorporating the bicyclo[3.3.1]nonadienes as ligands have been studied for example, X-ray data were reported for the platinum(II) complex 284 (248). 

Reaction of the oxa-bicyclo system was shown to follow the same trend. Thus, migration of the methylene group was preferred in a 2 1 ratio for the exo-methylamine oxabicyclo compound 64, while the endo-methylamine provided a 12 1 mixture of 65 and 66. ... 

Various bicyclic and polycyclic compounds are produced by intramolecular reactions] 127]. In the syntheses of the decalin systems 157 [38] and 158 [128], cis ring Junctions are selectively generated. In the formation of 158, allyhc silyl ether remains intact. A bridged bicyclo[3.3. l]nonane ring 159 was constructed... 

A interesting and useful reaetion is the intramolecular polycyclization reaction of polyalkenes by tandem or domino insertions of alkenes to give polycyclic compounds[l 38]. In the tandem cyclization. an intermediate in many cases is a neopentylpalladium formed by the insertion of 1,1-disubstituted alkenes, which has no possibility of /3-elimination. The key step in the total synthesis of scopadulcic acid is the Pd-catalyzed construction of the tricyclic system 202 containing the bicyclo[3.2. Ijoctane substructure. The single tricyclic product 202 was obtained in 82% yield from 201 [20,164). The benzyl chloride 203 undergoes oxidative addition and alkene insertion. Formation of the spiro compound 204 by the intramolecular double insertion of alkenes is an exam-ple[165]. 

The decalin (bicyclo[4.4.0]decane) ring system provides another important system for study of conformational effects in cyclohexane rings. Equilibration of the cis and trans isomers reveals that the trans isomer is favored by about 2.8 kcal/mol. Note that this represents a change in configuration, not conformation. The energy difference can be analyzed by noting that the cis isomer has three more gauche butane interactions that are... 

Another system in which the factors constrolling the direction of reagent approach has been studied systematically is the bicyclo[2.2.1]heptane ring system. The stereochemistry of a number of reactions of the parent system and the 7,7-dimethyl derivative have been examined.Some of the results are given in Table 3.13. These reactions reveal... 

Another structure with a 10-7r-electron conjugated system is bicyclo[6.2.0]deca-1,3,5,7,9-pentaene. The crystal structure of the 9,10-diphenyl derivative (Fig. 9.6) shows the conjugated system to be nearly planar. " ... 

The bicyclo[2.2.0]hexa-2,5-diene ring system is a valence isomer of the benzene ring and is often referred to as Dewar benzene. After many attempts to prepare Dewar benzene derivatives failed, a pessimistic opinion existed that all such efforts would be finitless because Dewar benzene would be so unstable as to immediately revert to benzene. Then, in 1962, van Tamelen and Pappas isolated a stable Dewar benzene derivative by photolysis of 1,2,4-tri(/-butyl)benzene. ... 

The prediction on the basis of orbital symmetry analysis that cyclization of eight-n-electron systems will be connotatoiy has been confirmed by study of isomeric 2,4,6,8-decatetraenes. Electrocyclic reaction occurs near room temperature and establishes an equilibrium that favors the cyclooctatriene product. At slightly more elevated temperatures, the hexatriene system undergoes a subsequent disrotatory cyclization, establishing equilibrium with the corresponding bicyclo[4.2.0]octa-2,4-diene ... 

Studies of solvolysis of similar polyfluonnated polycyclic aromatic systems, such as 2,3-(tetrafluorobenzo)bicyclo[2 2 2]octadienes and related compounds, proved the ionic mechanism of this rearrangement [55, 36, 37] (equation 9) Possible nonclassical carbonium ion involvement has been discussed [5S, 39, 40, 41]... 

Exercise 3.8 Bicyclo[2.2.2]octane optimizations with different coordinate systems 1 53 28.5... 

In an approach to the AB rings of rubrolone 65, Boger examined the use of oxazinones as a replacement for triazines. Reaction of l,3-oxazin-6-one 66 with enamines 67 produced the corresponding pyridines 70. The reaction proceeds in a manner analogous to the triazines however, instead of losing nitrogen, these systems lose CO2 via the intermediate bicyclo[2.2.2]octanes 68. The resultant 69 then loses pyrrolidine as in the triazine example. 

Scheme 2.12 Intramolecular TMM [3+2 cycloaddition to bicyclo[3.3.0 octyl systems...

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