Bicyclo skeleton

For bicyclic structures the von Baeyer name consists of the prefix bicyclo-, followed in square brackets by the numbers of carbon atoms separating the bridgeheads on the three possible routes from one bridgehead to the other, followed in turn by the name of the alkane (or other homogeneous hydride, or repeating unit hydride) containing the same number of atoms in the chain as the whole bicyclic skeleton (examples 55-57). Replacement nomenclature can be applied to hydrocarbon names (example 58). 

In order to obtain good yields from a Weiss reaction sequence, the H+-concentration has to be adjusted properly in the reaction mixture. The reaction is usually carried out in a buffered, weakly acidic or weakly basic solution. By the Weiss reaction simple starting materials are converted into a complex product of defined stereochemistry. There is no simpler procedure for the synthesis of the l,5-c -disubstituted bicyclo[3.3.0]octane skeleton it has for example found application in the synthesis of polyquinanes. ... 

The reaction in Entry 8 was used in the synthesis of members of the phomoidrides. The cyclohexene ring that is constructed creates a bicyclo[4.3.1]skeleton containing seven- and nine-membered rings. 

The combination of the Diels-Alder reaction of P-sulfonylnitroethylene and the Barton-Zard reaction provides a new synthesis of pyrroles fused with polycyclic skeletons (Eq. 10.31).36 Pyrroles fused with bicyclo[2.2.2]octodiene are important precursors for synthesis of isoindoles via the retro Diels-Alder reaction (Eq. 10.32).37... 

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. 

The intramolecular version of the PKR has attracted much more synthetic interest, leading to valuable bicyclic systems and being more general and viable [97]. Particularly, 1,6-enynes have been extensively used for the construction of the bicyclo[3.3.0]octane [105], cyclopenta[c]furan [106], or cyclopenta[c]pyrrole [107] skeletons in a single step. 

Two types of synthesis of this skeleton are reported depending on the positions of the sulfur introduced either at the bridge or at the bridgehead the former has been studied by Stetter s research group189-191 and the latter by Klages and Schmidt192 starting from bicyclo[3.3.1]nonane derivatives and cyclohexane derivatives, respectively. 

Dicyano-substituted triafulvenes react with enamines to produce exclusively the cross-conjugated dicyanomethylene compounds 519, whose formation can be rationalized by a methylene bicyclo(2,l,0)pentane intermediate 51879 296 Since cyclanone enamines 520 and other cyclic enamines 522 react analogously, this C-C-insertion 237) of the triafulvene ring skeleton into the enamine C=C bond represents a versatile ring expansion mode (C + C3), which makes accessible a series of unsaturated medium-ring compounds (521/523) that are otherwise difficult to synthesize. 

Benzyltriethylammonium tetrathiomolybdate, [BnNEt3]2MoS4, is a useful sulfur transfer reagent which has now been shown to effect a one-pot, three-step sequence of S-transfer— reduction—Michael addition. The intramolecular version successfully produces 1,4-oxathianes from the bromoenone 108 (Scheme 65). In like manner, the bicyclo[3.3.1]nonane skeleton incorporating a thiopyran ring can be constructed . 

The results obtained in combination with the data on the transformations of cations 9-11, led to the conclusion that 1,2-shifts of methyl groups occur readily in carbocations having a pentalene fragment. On the basis of the data obtained for rearrangements of such carbocations, an alternative mechanism has been proposed (52) for the rearrangement of structurally related carbocation 16 having a bicyclo[3.3.0]octane skeleton described in (33) (Scheme 12). 

In summary, the parent 1,3-bishomotropylium ion [43] is produced upon protonation of the bicyclo[6.1.0]nonatriene [44] and its NMR spectrum was recorded under stable ion conditions. Two different bridged 1,4-bishomo-tropylium ion skeletons [45] and [46] have been prepared from a multitude of different precursors. However, no unbridged 1,4-system has yet been characterized. 

Narasaka and co-workers found that the PKR of an allenyne proceeds at room temperature under atmospheric pressure of CO, affording a bicyclo[4.3.0]nonane skeleton (Scheme 16.46) [47]. 

The notation refers to the configuration at the junction of three cyclohexane rings with the bicyclo[2.2.2] octane skeleton. A prime indicates that the cyclohexane ring is oriented counterclockwise when viewed along the Cs-axis of bicyclooctane part otherwise the orientation is clockwise. 

Unusual amino acids include a class of unnatural a-amino acids such as phenylalanine, tyrosine, alanine, tryptophan, and glycine analogs, and f)-amino acid analogs containing 1,2,3,4-tetrahydroisoquinoline, tetraline, l,2,3,4-tetrahydro-2-carboline, cyclopentane, cyclohexane, cyclohexene, bicyclo[2.2.1]heptane or heptene skeletons. Different selectors were exploited for the separation of unusual amino acids, most of the production being made by Peter and coworkers teicoplanin [41, 56, 84, 90, 93, 124, 141-144], ristocetin A [33, 94, 145, 146], and TAG [56, 147]. Enantiomeric and diastereomeric separations of cyclic -substituted a-amino acids were reported by other authors on a teicoplanin CSP [88, 89], Ester and amide derivatives of tryptophan and phenylalanine were recently analyzed on a Me-TAG CSP [58],... 

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]. 

It has been shown" that isomerization of the exocyclic allylic system of the five-membered ring D of kaurenols depends on the orientation of the C(15) hydroxyl group. The total synthesis of methyl atis-16-en-19-oate, a tetracyclic diterpenoid possessing a bicyclo[2.2.2]octane skeleton, has been accomplished" using a homoallyl-homoallyl radical rearrangement process of methyl 12-hydroxykaur-16-en-19-oate monothioimid-azolide (280) as the pivotal step. Two plausible mechanisms have been presented" ... 

The very good yields of especially the bridgehead alkoxy derivatives 231 and the excellent regioselectivity with which they can be transformed to other skeletons plus the fact that a spirocyclopropane moiety is a mimic of and can in fact be considered as a masked gem-dimethyl substituent [28, 112] makes these products versatile precursors of certain natural products. In fact, one may conceive new approaches to the total syntheses of taxol [113-115] and of mediter-raneol [116]. Both strategies rely on the MIMIRC reaction of lithium cycloalka-dienolates [117] with the a-chloro acrylate 1-Me to produce a tricyclic precursor to the appropriate bicyclo [n.2.1]alkanedione derivative, which are key structural units of several diterpenes and their metabolites. 

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