Bicyclo ring systems

A very comprehensive report on the nomenclature of terpenes has been approved by the lUPAC and the American Chemical Society (4). Some of the significant changes in nomenclature recommended are the replacement of the names camphane and bomylane by bomane. Norbomane replaces norcamphane and norbomylane. Thujane is preferred over sabinane. A definite system of numbering the bicyclo ring systems is recommended. The application of these terpene rules will eliminate many trivial names which are a tax on the memory. 

O-P-0 found for these two phosphoranes were apical, 162.5° and 163.3° and radial, 107.7° and 112.7° respectively. Probably, compression of the equatorial O-P-O bond angle in the 6-membered ring of (114 RR = CH2CH(Bu )CH2) and related phosphoranes allows the ring to assume a chair conformation more easily than when phosphorus is not part of a bicyclo ring system. 

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... 

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. ... 

Another common ring system is the norbornane, or bicyclo[2.2.1.)heptane, structure. Like decalin, norbornane is a bicycloalkane, so called because two rings would have to be broken open to generate an acyclic structure. Its systematic name, bicyclo[2.2.1 heptane, reflects the fact that the molecule has seven carbons, is bicyclic, and has three "bridges" of 2, 2, and 1 carbon atoms connecting the two bridgehead carbons. 

A bicyclo[3.3.0]octane ring system 164 can be conveniently prepared by refluxing an acetonitrile solution of the azo compound 163 in the presence of excess of phenyl vinyl... 

In the same area, these authors have also tested a p-amino thiol ligand containing a bicyclo[2.2.1] ring system, the 2-azanorbornylmethanethiol, which gave... 

Intramolecular cycloadditions are among the most efficient methods for the synthesis of fused bicyclic ring systems [30]. From this perspective, the hetisine skeleton encompasses two key retro-cycloaddition key elements. (1) a bridging pyrrolidine ring accessible via a [3+2] azomethine dipolar cycloaddition and (2) a [2.2.2] bicyclo-octane accessible via a [4+2] Diels-Alder carbocyclic cycloaddition (Chart 1.4). While intramolecular [4+2] Diels—Alder cycloadditions to form [2.2.2] bicycle-octane systems have extensive precedence [3+2], azomethine dipolar cycloadditions to form highly fused aza systems are rare [31-33]. The staging of these two operations in sequence is critical to a unified synthetic plan. As the proposed [3+2] dipolar cycloaddition is expected to be the more challenging of the two transformations, it should be conducted in an early phase in the forward synthetic direction. As a result, a retrosynthetic analysis would entail initial consideration of the [4+2] cycloaddition to arrive at the optimal retrosynthetic C-C bond disconnections for this transformation. 

Two possible intramolecular disconnections are available for the [2.2.2] bicyclo-octane ring system (path A and path B, Scheme 1.4). The choice between the initial [4+2] disconnections A and B at first appears inconsequential leading to idealized intermediates of comparable complexity (54 and 57). However, when the [4+2] and [3+2] disconnections are considered in sequence, the difference becomes clear. For path A, retrosynthetic [3+2] disconnection of intermediate 54 leads to the conceptual precursor 56, which embodies a considerable simplification. In contrast, path B reveals a retrosynthetic [3+2] disconnection of intermediate 57 to provide the precursor 59, a considerably less simplified medium-ring bridged macrocycle. Thus, unification of the [3+2]/[4+2] dual cycloaddition strategy, using the staging... 

In Method 2, described by Rule A-31, the saturated cyclic parent is named as a polycyclo-[.. jalkane. The alkane name identifies the total number of carbon atoms in the ring system. The names bicyclo, tricyclo, tetracyclo, etc. identify parent compounds of two, three, four, etc. rings. Double bonds in the ring are identified by names such as w-alkenc, ,/w-alkadicne, where the n and m give the position of the double bond. Substituents consist of alkyl, cycloalkyl, phenyl, etc. radicals. 

The [4+ 4]-cycloaddition reaction of tethered bis-dienes has been used by Wender and co-workers in total synthesis as exemplified in syntheses of ( )-salsolene oxide and (-l-)-asteriscanolide (Scheme 28). In the synthesis of ( )-salsolene oxide, a nickel(0)-catalyst cleanly effects the cycloaddition of the two conjugated dienes in compound 93 to afford the bicyclo[5.3.1]undecadiene in a good yield and with moderate selectivity.99 The first synthesis of (-l-)-asteriscanolide was accomplished in only 13 steps. The key [4+ 4]-cycloaddition reaction efficiently set the requisite eight-membered ring of (-l-)-asteriscanolide in good yield and with excellent diastereoselectivity.100 The diastereoselective [4 + 4]-cycloaddition has also been applied to the synthesis of the core ring system found in several sesterterpenes such as the ophiobolins (Scheme 28).101... 

The double bond of butenolides reacts under Diels-Alder conditions and the resulting chiral bicycles have served as precursors of prostacycline analogs and chrysanthemic acids (250,251). The butenolide 248 was obtained by the procedure described by Ireland et al. (237). A bicyclo[4.3.0] ring system (254) was prepared by Diels-Alder reaction of 248 with butadiene in the presence of aluminum trichloride. Reduction of 254 (LiBH4) yielded the... 

As indicated by the conversion of 70 to 71, the electroreductive cyclization reaction provides as excellent method for the assembly of the bicyclo[3.2.1]oc-tane ring system. Several additional examples are portrayed in the following equations. In general, the use of an unsaturated nitrile rather than the corresponding ester is preferred, as this precludes lactone formation, and therefore... 

Previous work by Holton [15] on the synthesis of taxane ring systems, indicated that the preparation of the bicyclo[5.3.1]undecane skeleton lies basically on the work of Biichi on patchouli alcohol [16], which led him to the development of the so-called "epoxy alcohol fragmentation" [15a]. 

Scheme 13.6.8 shows how compound 38. which has the basic bicyclo[5.3.1]undecane ring system of taxol, can be prepared by this procedure from a taxusin intermediate readily available from camphor in either enantiomeric form [15b]. Compare the structure of 40 with a-patchoulene 5, in Scheme 1.1 (pag. 10). 

The chemistry of the bicyclo[2.2.1]heptane ring system, with and without double bonds in the two-carbon bridges, includes many well-known and interesting features which illustrate important stereochemical aspects of some organic reaction mechanisms. Among derivatives of... 

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