Exo, endo form

The topologies of various cryptands are shown in Fig. 1. Considering the macrobicyclic ligands, each bridgehead may be turned either inward or outward with respect to the molecular cavity (11,12,22). This leads to three topological isomers exo-exo, exo-endo, and endo-endo. Crystal structures of one representative of each have been described. [2.2.2] adopts the endo-endo conformation with both nitrogen lone pairs directed into the cavity, while its bis-borane derivative is exo-exo (23) and the mono-borane derivative of 11.1.11 has the exo-endo form (24) (Fig. 2), with the —BH 1 group outside the cavity. 

The endo/endo isomers show a very strong peak at m/e 118 corresponding to (LXXXVII), and this peak is very weak in the spectra of the exo/endo forms. 

Exo-endo data given, exo endo forms also observed. Faeile E-Z intereonversion. Temperature study and MO caleulations undertaken. EfFeet of metal eation also studied. 

C, b.p. 170 C (decomp.), has a characteristic odour. It is the Diels-Alder product of cyclopentadiene reacting with itself, the exo-form being formed most rapidly but the endo-form is thermodynamically favoured. At temperatures above ISO C a retro-Diels-Alder reaction occurs and cyclopentadiene monomer is regenerated see diene reactions. 

Dicyclopentadiene exists ia two stereoisomeric forms, the endo and exo isomers. Commercial DCPD, 3a,4,7,7a-tetrahydro-4,7-methano-lH-iadene, is predominandy the endo isomer (exo endo 6 953 by capillary gas chromatography). The dimer is the form ia which CPD is sold commercially. 

The next seven references are cited not because of the experimental procedures described but because they indicate diversification in the types of enamines prepared and studied. Both Paquette (25) and Kasper 26) have condensed 2,5-methylene-l,2,5,6-tetrahydrobenzaldehyde (5-nor-bornene-2-carboxyaldehyde) (2) with several cyclic and open-chain aliphatic secondary amines. Kasper studied the ratio of endo to exo aldehyde formed upon hydrolysis of these enamines and the dihydro enamines. Paquette investigated the addition of sulfene to the enamines. -Fluoro-... 

There are two possible Cs conformations of the Sy homocycle exo and endo. The exo global minimum (Fig. 2) lies 15 kJ mol below the endo-form. Both conformers undergo facile pseudorotation through C2 transition states, with barriers of less than 4 kJ mol [54]. The exo-conformer possesses the geometry found in the sulfur allotropes y-Sy and 5-Sy [72]. This Cs structure has four bonds near the length of a normal S-S bond and one rather long bond of 215 pm with a dihedral angle of 0°. 

The stereoselectivity of epoxidation with peroxycarboxylic acids has been well studied. Addition of oxygen occurs preferentially from the less hindered side of the molecule. Norbornene, for example, gives a 96 4 exo endo ratio.76 In molecules where two potential modes of approach are not very different, a mixture of products is formed. 

Sila-biperidin rac-(SiRS, C2 SR), 35, can be prepared by hydrolysis of the corresponding methoxysilane and behaves in a similar way to sila-procyclidine The exo form has centrosymmetric dimers formed via a pair of intermolecular OH---N hydrogen bonds (0---N distance 2.810 A), while the endo form has intermolecular OH---N hydrogen bonds linking the molecules into infinite chains (222). 

The reactions with quadricyclane, shown in Eq. (31), gave products identical to those formed by the same silene reacting in a [2 + 2] manner with norbornene. Mixtures of exo endo isomers were frequently observed. Again, only silenes of the Auner type have been studied with this reagent,51-53,185,188 so it is not known whether the Wiberg- or Brook-type silenes will undergo this mode of cycloaddition. 

Slee and LeGoff performed further investigations on the reaction of dimethyl acetylenedicarboxylate 4-20 with an excess of furan 4-21, as first described by Diels and Alder (Scheme 4.5) [la]. At 100 °C, 4-24 and 4-25 were not produced (as proposed), but rather 4-22 and 4-23, since at elevated temperature an equilibrium takes place and the primarily formed 4-24 and 4-25 isomerize to give a 6 1-mixture of the exo-endo and the exo-exo products 4-22 and 4-23, respectively. However, at lower temperature, in the primarily formed [4+2] cycloadduct the double bond substituted with the two carbomethoxy group acts as the dienophile to give the two products 4-24 and 4-25 in a 3 1 ratio with 96% yield within five weeks, as has been shown by Diels and Olsen [la,lc]. For a differentiation of these two types of adducts, Paquette and coworkers [7] used a domino and pincer product . The Cram group [8] described one of the first examples of a reaction of a tethered bisfuran 4-26 with dimethyl acetylenedicarboxylate 4-20a to give 4-27. 

Copper(II) triflate has also been used for the carbenoid cyclopropanation reaction of simple olefins like cyclohexene, 2-methylpropene, cis- or rran.y-2-butene and norbomene with vinyldiazomethane 2 26,27). Although the yields were low (20-38 %), this catalyst is far superior to other copper salts and chelates except for copper(II) hexafluoroacetylaeetonate [Cu(hfacac)2], which exhibits similar efficiency. However, highly nucleophilic vinyl ethers, such as dihydropyran and dihydrofuran cannot be cyclopropanated as they rapidly polymerize on contact with Cu(OTf)2. With these substrates, copper(II) trifluoroacetate or copper(II) hexafluoroacetylaeetonate have to be used. The vinylcyclopropanation is stereospecific with cis- and rra s-2-butene. The 7-vinylbicyclo[4.1.0]heptanes formed from cyclohexene are obtained with the same exo/endo ratio in both the Cu(OTf)2 and Cu(hfacac)2 catalyzed reaction. The... 

The nitrone and the substituted methylenecyclopropane approach, in each of the two possible regioisomeric arrays, away from the nitrone substituent and away from the methylenecyclopropane substituent. The most favoured anti-anti Transition State produces the observed major diastereoisomers [81,65c]. The only two isomers formed, for each regioisomeric mode, must derive from anti-anti exo and anti-anti endo approaches. No exo-endo selectivity is generally observed, as a consequence of the strong preference for the anti approach towards the substituent on the cyclopropyl ring. 

On oxidation of E8 the, exo-exo conformation of the neutrals changes and an exo-endo conformer is formed in several E82+ salts (refs. S,16,70 Se,53,71,72 Te72,73). Very puzzling features of these structures are the three transannular... 

Early studies with a purified and reconstituted rabbit P450, CYP2B4 (earlier name P450 LM2) on the hydroxylation of the hydrocarbon, norbomane, found that the reaction proceeded to yield a 3.4 1 mixture of exo-endo norborneols (4). Further, hydroxylation of exo-2,3,5,6-tetradeuteronorbornane was found to proceed with a large deuterium isotope effect (11.5 0.5) and a significant amount of epimerization in forming the endo and exo metabolites (Fig. 4.3). 

The transfer of the silyl ligand onto the a-carbon of the substrate is followed by the formation of an intermediate in which the (1-phenylethyl)trichlorosilane product still weakly coordinated in a rj2-fashion. The corresponding endo and exo intermediates (11a and lib, respectively) are 3.5 and 4.2 kcal/mol, respectively, more stable than the endo 7i-complex, 8a Therefore the exo-stereoisomer is again thermodynamically more stable than the endo form. Finally, the oxidative addition of a molecule of trichlorosilane occurs with concomitant liberation of the products. The formation of both R and S products is exothermic by -20 kcal/mol. 

In a further enterprise, compounds [63] and [64] (Fig. 24) were perceived as freely rotating haptens for application as TSAs for the same Diels-Alder addition. As expected, each proved capable of inducing both endo- and exo-adduct-forming abzymes. It can be noted that [63] produced more exo-catalysts (6 out of 7) whereas [64] favoured the production of endo-catalysts (7 out of 8), though it is difficult to draw any conclusion from this observation (Appendix entry 17.5) (Yli-Kauhaluoma et al., 1995). 

The dehydration of d-2-endo- and i-2-ca o-bornanols was studied by Watanabe et al. (73) at 275° using an acidic alumina and the same alumina modified by the introduction of piperidine to the hydrocarbon solution of the bornanols. Under nonacidic conditions of dehydration, 2-exo-bornanol formed 4.3% tricyclene, 95.2% camphene, and 0.5% camphor. 2-ewdo-Bornanol under similar conditions formed 12.5% tricyclene and 86.5% camphene. 

In some situations, such as in Figure 4-20, two different cis products may form. The two products cire the endo and the exo-product. The endo-product is the major or only product of the reaction. The process leading to the endo-product is Alder endo-addition. The endo-form is more stable than the exo-form. 

The terms axo and ondb are often used to indicate the relative positions of the bridging unit (in this case an oxygen atom) and the residue of the dienophiie. When these are on the same lace the adduct is referred to as the exo form, and when the bridge and the residue are on opposite faces it called the endo form. For many pairs of adducts, formed between dienophiles and cyclic dienes, the exo product has fewer steric interactions and IS the more stable. In some cases, however, secondary electronic effects may overcome steric preferences so that the endo (kinetic product) is favoured. 

In synthetic efforts toward the DNA reactive alkaloid naphthyridinomycin (164), Gamer and Ho (41) reported a series of studies into the constmction of the diazobicyclo[3.2.1]octane section. Constmction of the five-membered ring, by the photolytic conversion of an aziridine to an azomethine ylide and subsequent alkene 1,3-dipolar cycloaddition, was deemed the best synthetic tactic. Initial studies with menthol- and isonorborneol- tethered chiral dipolarophiles gave no facial selectivity in the adducts formed (42). However, utilizing Oppolzer s sultam as the chiral controlling unit led to a dramatic improvement. Treatment of ylide precursor 165 with the chiral dipolarophile 166 under photochemical conditions led to formation of the desired cycloadducts (Scheme 3.47). The reaction proceeded with an exo/endo ratio of only 2.4 1 however, the facial selectivity was good at >25 1 in favor of the desired re products. The products derived from si attack of the ylide... 

The ortho substituents used were supposed to have no influcence on the absorption spectrum and to possess the same circular dichroism as the parent compound. The ortho substituted compound can only photocyclize in its exo-conformations, but equilibration by a rotation over the phenylethylene bond converts an M-exo into a P-endo form (or P-exo into M-endo) which cannot cyclize. Indeed, the optical yields of the 4-substituted [6]hclicenes from the ortho substituted (1+4) precursors are not lowered in comparison with the unsubstituted (2 + 3) precursor. On the other hand (4+1) precursors having a para substituent give again a low optical yield. The values found suggest that racemization in the excited state is slightly hindered, what may be due to steric hindrance. 

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