Endocyclic effect

The first set of structures compares endo and exocyclic double bonds, but the degree of substitution on the alkene is also different, so this comparison is not valid—we are not isolating simply the exo or endocyclic effect. 

Isoxazolines with alkyl substituents are also all liquids (or low melting solids) and incorporation of aryl substituents results in crystallinity. Introduction of carboxy substituents and endocyclic carbonyl or imino groups also has the anticipated effect, with crystalline products being isolated. These trends are illustrated by the data compiled in Table 2. 

The computation of furoxans (l,2,5-oxadiazole-2-oxides) is very demanding. Very strong electron correlation effects hamper a proper treatment of this class of molecules. With respect to the geometric parameters, it is the endocyclic N—O bond that can be treated reliably either at the B3-LYP or at the MP4(SDQ) level [99MI1 ]. Table II demonstrates the problems associated with the exact determination of this bond length. 

The "acyl effect" proves crucial in the formation of the perhydroazulene systems cyclization can only take place with the presence of an acyl group on the TMM portion whereas the parent hydrocarbon fails. For example, treatment of substrate (51) with the palladium catalyst gave a mixture of the bicyclic compounds (52) and (53) in 51% yield. The formation of endocyclic olefin (52) is presumed to occur when the first formed (53) was exposed to silica gel during purification [22]. This intramolecular cycloaddition strategy was utilized in a highly diastereoselec-tive preparation of a key intermediate (54) in the total synthesis of (-)-isoclavuker-in A (55) (Scheme 2.16) [21]. 

For a number of reactions of cyclic di- and triesters of phosphoric acid, there are exchange data which can be rationalized on the assumption of trigonal bipyrami-dal intermediates which readily interconvert by pseudorotation. This constitutes a strong argument that at least these cyclic esters react by an associative mechanism and is suggestive evidence that simple trialkyl phosphates also react by this mechanism. The pH dependence of exocyclic versus endocyclic cleavage of methyl ethylene phosphate is readily interpreted in terms of the effect of ionization of the intermediate on the pseudorotation of these pentacoordinate intermediates. ... 

Gas phase reactivity toward allyltrimethylsilane was used to compare the reactivity of several cyclic A-acyliminium ions and related iminium ions.203 Compounds with endocyclic acyl groups were found to be more reactive than compounds with exocyclic acyl substituents. Five-membered ring compounds are somewhat more reactive than six-membered ones. The higher reactivity of the endocyclic acyl derivatives is believed to be due to geometric constraints that maximize the polar effect of the carbonyl group. 

Quantum calculations on metal-assisted tautomerization indicate a substantial stabilization to protonation of the endocyclic nitrogen atoms (99). In the case of HgCHg, adducts are formed by binding at either A-N6 or C-N4, which increases the stability of the respective N1H+ or N3H+ protonated species by 10-14 kcal/mol. For Pt11 binding at C-N4 the effect is much greater at 30-34 kcal/mol. 

Exocyclic reactions for aromatic carboxylic esters 174 Exocyclic reactions for aliphatic carboxylic esters 187 Endocyclic reactions for carboxylic esters 191 Carbon acid participation for carboxylic esters 195 Effective molarities 198 Ring size 199 Initiating nucleophile 200 Phosphate and sulphonate esters 200... 

A comment is appropriate here about a-effects of endocyclic substituents. Such effects are not defined according to eq. [11], since they replace carbon instead of hydrogen atoms in the parent compound. Thus, these SCSs represent differences between the actual shift effects and that of a carbon fragment in the same position. 

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