Endo face

This is again the result of steric shielding by the trichloromethyl group, which causes the bromine atom to be abstracted from the endo face of the intermediate radical. 

The nonequivalent n orbital extension or the higher electron density in the exo face pyramidizes the unsaturated carbons The -H bonds are bent in the endo face. 

The compact bicyclic lactams 15 and 16 are examples of chiral systems that show high facial selectivity. Interestingly, 15 is alkylated from the convex face. When two successive alkylations are done, both groups are added from the endo face, so the configuration of the newly formed quaternary center can be controlled. The closely related 16 shows exo stereoselectivity. 100... 

After extensive experimentation, it was discovered that exposure of silyl ether (+)-97 to potassium trithiocarbonate and trifluoroacetic acid in dichloromethane affords a 25 7 1 mixture of endolendo endo/exo exo/exo bisdithiepanethione products, reflecting a ca. 5 1 preference for nucleophilic approach from the endo-face of each diketopiperazine moiety. Resubjection of the isolated bisdithiepanethione diastereomers to the original reaction conditions did not result in their equilibration, indicating that the products were a result of kinetic trapping. 

Norbomene-type dipolarophiles offered yet another stereochemical consideration owing to the facial selectivity posed by the dipolarophile. While no examples of endo-face attack on... 

There are a few communications concerning cycloadditions of nitrile oxides to unsaturated oxa and aza cage systems. Benzo- and mesitonitrile oxides RCNO give, with five substituted 7-oxanorbomenes 106, mixtures of the corresponding exo-adducts 107 and 108 in nearly quantitative yields. No traces of compounds resulting from the endo-face attack was detected (274). Substituents at positions 5 and 6 of 106 render the process highly regioselective. 

In a more complex elaboration of the hydrogen atom abstraction/nucleo-philic cyclization route to tetrahydrofurans (Scheme 20), a carbohydrate-based N-alkoxy phthalimide was converted to a spirocyclic acetal in excellent yield and diastereoselectivity (Scheme 33) [136]. In this cyclization, nucleophilic attack takes place from the endo face of the trioxabicyclo [3.3.0] octane... 

Exclusive ring opening of the anti compound of a mixture of syn- and anti-1-methylbicyclo[2.2.1]heptene-2 in the presence of Ru-, Os-, Ir-, W-, and Re-based catalysts was observed116 proving that the exo face of norbomene is highly reactive while the endo face is inert toward metathesis. 

Comparison of 2 and 4 shows that the facial preference is for exo attack. Only when the exo face is blocked by a methyl group does the reagent attack the endo face. 

Despite its formal simplicity, the stepwise mechanism of the reaction between ketenes and imines raises a complex stereochemical situation since ketenes can be unsymmetrically substituted and imines can exist in either (E)- or -configurations [49]. As far as the first step of the reaction is concerned, the nucleophilic attack of the nitrogen atom of the imine can occur through the less hindered exo face, namely that which has the shortest substituent, or through the endo face, which incorporates the largest substituent (Scheme 8). In principle, the exo attack leads to second transition structures that exhibit the largest substituents at the 3-out position. 

Similar geometric optimization has been reported for bicyclo[3.2.2]nona-6,8-diene (BND). The double bond situated in the opposite direction to the methylene group was found to be more exo-pyramidalized than the other double bond and the electron density (qi, HOMO) of the former double bond in HOMO of the molecule higher than that of the latter double bond. The exo and endo faces of exo-pyramidalized double bonds proved not to be equal and the electron density was found to be higher on the endo faces. The endo molecular complexes with bromine have been found by the HF/321G method to be more stable than their exo congeners this was attributed to electronic and steric factors. As a result, endo-facial stereoselectivity of bromination ( ) predominates.21 A related theoretical study of facial selectivity and regioselectivity of the electrophilic addition of chlorine to exo-tricyclo[4.2.1.02,5]nona-3,7-diene (exo-TND) has also been reported.22... 

A procedure for the transformation of (+)-tazettine (397) to (+)-pretazettine (395) has been developed (212). The reaction of 397 with LiAlH4 provided a mixture of the diols 536 and 537 in an approximately 9 2 ratio. Thus, it is again apparent that the preferred stereochemical pathway for the delivery of hydride to the neopentyl carbonyl function is from the endo face of the cis-3a-arylhydroin-dole whenever the angular aryl group possesses an ortho substituent. Oxidation of 537 with manganese dioxide gave a mixture (approximately 3 2 1) of (+)-pretazettine (395), (+)-3-epimacronine (400), and (+)-tazettine (397). In a sim-... 

The oxyanion must open the epoxide to give a trans diaxial product 48a by attacking the less hindered end of the epoxide. This is inevitably from the endo face but attack at the other end of the epoxide would have to be from right inside the fold. Being a cis decalin, the product can equilibrate to the equatorial conformer 48b and the arrangement for cyclisation is perfect 51. 

Reagents, whether electrophilic or nucleophilic, prefer to approach the outside or exo face of folded molecules. These are cis -fused four-, five-, or six-membered rings. If a substituent is needed on the inside or endo face, it must be added first, that is disconnected second 54, as in the synthesis of the ketone 54. If there is only one substituent and it is endo, R2 can be H. 

The polymerisation of norbornene occurs in the presence of cationic Pd(II) complexes with weakly nucleophilic ligands via the cis insertion involving predominantly the exo face of the monomer (the diastereotopic endo face of the monomer is much less reactive) [10] ... 

The syn addition of BH3 across the double bond of norbornene takes place mostly from the more accessible outside (exo) face of the double bond. Oxidation gives a product with both the hydrogen atom and the hydroxyl group in exo positions. (The less accessible inner face of the double bond is called the endo face.)... 

Like LiAlH4 reduction, addition of a Grignard reagent to camphor occurs almost entirely from the endo face, but almost entirely from the exo face with norbornanone. 

This synthesis involves some stereochemistry. Biotin carries the alkyl chain next to sulfur on the more hindered endo face of the molecule, and any successful synthesis has to address this particular problem. Here, the chemists decided to use the fact that alkylations of cyclic sulfoxides result in tram stereochemistry between the new alkyl group and the sulfoxide oxygen atom. As expected, oxidation of the sulfide proceeded faster from the exo face, giving an 8 1 ratio of exoiendo sulfoxides. Alkylation trans to the exo oxygen gave the desired (endo) product. 

Fig. 9. A schematic representation of the interaction between the ns orbital (SHOMO) of the s-m-butadiene unit in 37 and the rr-orbital (HOMO) of the dienophilic component of 38, assuming the formation of syn-adducts [82, 84, 85]. The four center/four electron destabilization of exo-face approach (exo-2) is characterized by a larger net antibonding energy [87] than endo-face approach (endo-2) which is consequently favored. The orbitals show the effects of the disrotatory tilts which they are believed to experience. Note that in the terms endo-2 and exo-2, the 2 refers to the diene unit. Subsequently, the terms endo-1 and exo-1 are also employed where the 1 refers to the ene portions of the dienophilic unit...

Bond-formation towards the exo-face of dienophilic units such as those in 38 may also receive a degree of stereoelectronic assistance [106-108] as a result of antiperiplanar ct - (a) hyperconjugative donation from the strained crl -9 a and o4-4a carbon-carbon single bonds (Scheme 9) to the a-orbitals under construction on the exo-face (see Fig. 9). The corresponding situation for endo-face approach is less favorable, the endoxide (C-O) bonds are not able to assume such an antiperiplanar relationship. 

In both 37 and 38 (Schemes 8 and 9), the higher frontier electron densities are associated with the exo-faces of the respective 7r-systems. Intuitively, one would expect [110] reaction to occur between a face of low electron density and one of high electron density [111], resulting in either an exo-l/endo-2 or an endo-l/exo-2 interaction. However, recalling the strong preference for approach to the endo-faces... 

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