Exo/endo configuration

X Reactant ratio exo endo Solvent R Product ratio exo endo Configuration... 

An interesting conversion from the endo-endo to the exo-endo configuration is observed when the 2,4-di-2-pyridyl-substituted bispidinone 14 is dissolved in hot ethanol and hydrochloric or perchloric acid are added. The H-NMR spectra show a loss of symmetry of the compound, which can be explained by the formation of the hemiaminal cyclization product 105 (see Chart 19) (23, 164). This unusual product also yielded crystals for X-ray analysis (147). 

FIGURE 8.4 Examples of some diastereoisomers of different cyclic compounds. 1,2-Dimethylcyclohexane (a, b — enantiomers a, c and b, c — diastereoisomers) (e,e)-(a, a) — (e, a) (e—equatorial a—axial) — conformational diastereoisomers trans-cis — configurational diastereoisomers Decaline trans-cis — configurational diastereoisomers 2-Substituted bicyclo[2.2.1]heptane exo-endo — configurational diastereoisomers. 

Stereospecihcity manifests itself in the dimerization of a diene with a diene. The double bond that remains free may deviate from the ring formed (exo configuration) or approach it (endo configuration). Endo condensation is the predominant pathway in the case of ion-radical reactions (Scheme 7.21). 

Carbonylation of 2,3-homo-l//-azepines has been effected by means of their metal and carbonyl complexes and provides a useful route to a variety of isomeric azabicyclo-nonadienones. For example, the tricarbonyliron complex with carbon monoxide at 80 °C and 160 atm yields the 9-oxo-2-azabicyclo[3.3.l]nona-3,7-diene (167) (57%) or the 9-oxo-2-azabicyclo[3.2.2]nona-3,6-diene (168) (60%) depending on the exo or endo configuration of the tricarbonyliron complex. A third isomer, namely ethyl 7-oxo-9-azabicyclo[4.2.1]nona-2,4-diene-9-carboxylate (169), is formed on heating (125 °C) the azepine with carbon monoxide under pressure in the presence of the rhodium carbonyl complex [Rh(CO)Cl2] (78CB3927). 

In pioneering NMR work (see Section 5.21.2.2) Simmons and Park showed that uncom-plexed diazabicycloalkanes (N[(CH2)fc,i>m]N), the parents of cryptands, can exist in three different configurations in solution the exo-exo in which both nitrogen lone pairs point away from the cavity, the endo-endo in which both lone pairs point into the cavity and the intermediate exo-endo with one lone pair inside and the other outside (Figure 6). 

Stereospecificity manifests itself in the dimerization of a diene with a diene. The double bond that remains free may deviate from the ring formed (exo configuration) or may approach it (endo configuration). Endo condensation is the predominant pathway in the case of ion radical reactions (Scheme 6-17). As seen, the charge distribution in the reactants dictates the head-to-tail pathway of the reaction. For the cation radical, the position selectivity at the C(l) atom is 100%, regioselectivity being 0%, whereas at the C(4) atom the position selectivity is 0% and regioselectivity is 100%. In other words, only the addition of the D+ -C(l) + D°-C(4) type is observed (symbol D° refers to a neutral diene and D+ to a diene in cation radical form). 

In cyclizations of this type the reaction can be governed by kinetic factors or by conditions favoring the formation of a thermodynamically more stable product. For example, the formation of the kinetically favored pyr-rolo[2,3-b]quinoxalines 52 with the endo-configuration takes place under basic conditions, while in the absence of a base initially both stereoisomeric adducts 52 and 53 are formed. In the course of the reaction, however, rearrangement of 52 into the thermodynamically more favored exo-adduct 53 occurs (Scheme 44) (81DOK384). 

Helmchen and co-workers [13] independently achieved similar results with catalysts of CAB 3 derived from valine (R = /-Pr). When R is 2,4,6-trimethylphenyl, the cycloaddition of crotonaldehyde and cyclopentadiene occurs with 72 % ee exo endo = 3 97) in the presence of 0.2 equiv. chiral catalyst. More recently, the same authors systematically investigated the influence of different experimental conditions on the enantioselectivity [15]. Improved enantioselectivity was obtained in THE or by addition of THE (ee up to 86 %). A transition-state model is proposed for prediction of the absolute configuration of the adducts (Eig. 2). In this model, the R group directs the R S02 group to the opposite side of the ring, where the latter group again participates in trans attack on boron. The conformation of the complexed enal has been determined to be s-cis, as has the coordination of carbonyl to boron syn to H. This model correctly predicts the outcome of all the examples studied. 

Isomerism at the acetal carbon atom is theoretically possible in five-membered acetal rings formed when pyranose or furanose sugars are condensed with aldehydes. For furanose derivatives, the isomerism is exo-endo, and the superior stability of exo isomers ensures that one isomer only will be obtainable, and this of the exo configuration. " 1,2-0-Benzylidene-a-D-glucofuranose " therefore has the complete configuration XLV. If the... 

Vinyloxiranes are reduced very selectively by diisobutyl-aluminium hydride (Eq. 168). Without regard to the configuration (exo,endo), the reduction of norborn-ane diepoxides with LiAlH proceeds in such a manner that the norbornane moiety remains intact in the reaction. (Eq. 168). ... 

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