Rigid polycycles

The synthetic route to the prostaglandins developed by Kelly and his colleagues at Upjohn similarly depends on a rigid polycyclic framework for establishment of the stereochemistry. The synthesis differs significantly from that above in that a rearrangement step is used to attain the stage comparable to 23. 

With the help of the steepest descent process usually final absolute d E/3 x(--values of about 0.1 kcal mole-1 A-1 may be reached. By means of the Newton-Raphson procedure these values may in most cases easily be lowered to about 10-6 kcal mole-1 A-1. Only with a few rigid polycyclic systems and with some small molecules were we able to ap-... 

Since carbocations are structurally similar to ketones, they are discussed here. Schleyer s force field incorporates carbocation parameters, and Harris is exploring their application in conformational analysis (188). The calculated angles in a series of rigid polycyclic carbocations correlated well with ketone infrared frequencies (188a). The calculated relative stabilities among various conformers of tertiary cations of methylcyclohexane, methylcycloheptane, and methyl-cyclooctane do not contradict the limited MNR observations of these species at low temperature (188b). 

DeAngelis, G.G. and Wildman, W.C., Circular dichroism studies. 1. A quadrant rule for the optically active aromatic chromophore in rigid polycyclic systems. Tetrahedron, 25, 5099, 1969. 

This holds only if the ring fragment is conformational ly rigid, for example, due to the presence of voluminous substituents ( steric anchors ) or incorporation into a rigid polycyclic system such as /raws-bicyclo[4.4.0]decanes (decalins) or steroids. 

Grigor ev, Pospelov 1965 No. of axis of rotation, mechanical rigidity Polycyclic, aromatic cpds. 

Rigid Polycycles and Peptidomimetics from Carbohydrate Synthons... 

The N—Li bond of azaenolate D lies outside the plane of the enolate. The structure created via chelation is a rigid polycyclic species. In this structure, the 4 and 5 carbons of the pyrollidine ring block one side of the azaenolate, resulting in facial selectivity during alkylation. The alkylation product E is formed preferentially with the -configuration shown. Only traces of the 3 -configured product are formed. 

In the reaction of compounds 22, which have diene moieties embedded in rigid polycyclic frames, the carbonyl-face selectivity is completely reversed through protection of the carbonyl groups as mono- or fc-acetals. This suggests the use of protecting groups as stereodirectors (Sch. 10) [25],... 

In 1988, Ikegami et al. reported an efficient alkenylation reaction involving a vinyllead triacetate reagent containing a protected allylic alcohol (Equation (64)).86 When this reagent was used with a more rigid polycyclic system toward the synthesis of benzocarbacyclines, only a modest yield (29%) of the expected alkenylated product was isolated (Equation (65)).87... 

Relatively weak acids such as acetic acid are sufficient to catalyse the reaction of rigid polycyclic ketones to the corresponding lactones, as in the case of adamantanone.239 Stronger protic acids have been used with substrates which are more resistant to oxidation, e.g. octan-2-one (Figure 3.50).240... 

Unfortunately the bicyclic triazolium salt that had successfully been used in our research group for the enantioselective intermolecular benzoin condensation (Enders and Kallfass 2002) did not show any catalytic activity in the intramolecular reaction. We thus searched for alternative, easily accessible enantiopure polycyclic y-lactams as precursors for the synthesis of novel triazolium salts (Enders et al. 2006c for a related study see Takikawa et al. 2006). The rigid polycyclic structure of the catalysts should allow high asymmetric inductions. A first tar-... 

The pentacoordinate organotin compounds 416a-c also contain a tridentate dianionic N,N,0-chelate ligand . The structure of 416c was determined by X-ray diffraction which conformed the rigid polycyclic structure proposed on the basis of H, B, C, and Sn NMR data in solution (S Sn —150.4 ppm) . In contrast, the dimethyltin compound 416a displays a fluxional structure in solution. 

Studies of the Polonovski reaction in conformationally rigid polycyclic systems reveal the stereochemical requirements for the elimination step. For example, the 6p,7p-dideutero derivative (22) of nu-pharidine is converted on reaction with acetic anhydride into A -dehydrodeoxynupharidine (24) containing a single deuterium atom at Ca (Scheme 4). ... 

Intramolecular [27t + 2tr] cycloadditions are restricted to rigid polycyclic molecular frameworks where the reacting cyclopropane tr-bond and the 7r-bond are aligned in a distance of 2.4 to 2.9 A. Photochemical and thermochemical reactions, as well as transition metal catalyzed in-... 

Non-conjugated dienes constrained within a rigid polycyclic system (e.g. norbomadiene) react with Na-pPdCU or PdCl2(PhCN)2 to give the corresponding (diene)PdCl2... 

Galanthamine is a natural alkaloid and a potent acetylcholinesterase inhibitor. Recognizing the potential to access this rigid polycyclic core through a biomimetic oxidative coupling/Michael addition, Shair and coworkers developed a highly efficient diversity-oriented synthesis based on this scaffold (Scheme 21.16) [96]. A library of 2527 compounds was prepared on the aforementioned macrobeads... 

Centropolyindanes constitute a complete family of arylaliphatic polycyclic hydrocarbons containing several indane units. Mutual fusion of the five-membered rings leads to three-dimensional, carbon-rich molecular frameworks bearing a central carbon atom, such as benzo-annelated [3.3.3]propellanes, triquinacenes, and [5.5.5.6]- and [5.5.5.5]fenestranes. In this review, the structural concept of centropolyindanes is contrasted to other fused indane hydrocarbons. Besides the syntheses of the parent centropolyindanes and recently described related indane hydrocarbons, the preparation of a large variety of bridgehead and arene substituted centropolyindanes is presented including strained, heterocyclic, and centrohexacyclic derivatives. In appropriate cases, the particular reactivity and some structural features of these unusual, sterically rigid polycyclic compounds are pointed out. 

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