Reaction blocking

E. Block, Reactions of Organosulfur Compounds, Academic Press, Inc., New York, 1978, p. 278. 

This type of addition process is particularly likely to be observed when the electrophile attacks a position that is already substituted, since facile rearomatization by deprotonation is then blocked. Reaction at a substituted position is called ipso attack. Addition products have also been isolated, however, when initial electrophilic attack has occurred at an unsubstituted position. The extent of addition in competition with substitution tends to increase on going to naphthalene and the larger polycyclic aromatic ring systems. ... 

Semicarbazide hydrochloride [705] and amidrazones [106] react with trifluo-ropyruvates to give six-membered heterocycles A variety of trifliioromethyl substituted heterocyclic systems is available, starting from the hydrate of trifluoropy-ruvicacid, a versatile 1,2-bielectrophihc building block (reaction type 2, equation 10) [107] (equation 24). 

Is the stability of 8Ad due to unfavorable kinetics, i.e., the bulky adamantyl groups blocking reaction, or to unfavorable thermochemistry, i.e., loss of aromaticity of the imidazole ring as a result of reaction, or both The distinction is potentially important as understanding could assist in designing stable carbenes. To decide, compare the kinetics and thermodynamics of the insertion of 8Ad into the central CH bond in propane with reactions of 8Me, which should also be aromatic but lacks shielding groups , and 9, which is neither aromatic nor crowded. 

In order to determine how many ether molecules are favored by the Schlenk dimer, MeMgCliMgMe in Scheme 10, the geometry of 9b (four ethers) is compared to that of 9a (two ethers). In 9a, two Mg-O distances (2.103 A and 2.109 A) are close to that (2.104 A) of the MgO ionic crysral. In 9b, they are 2.265 A, 2.261 A, 2.272 A and 2.272 A and are larger than those in 9a. In spite of the large Mg to O affinity, two Mg atoms do not favor the coordination of four ether molecules. Thus, 9a is a saturated complex, although there seems to be room on the two Mg atoms for further nucleophilic coordination. Mg atoms seem to persist in tetra-coordination. Ether solvation of the Schlenk equilibrium species does not block reaction channels completely. 

The best known inhibitors of glycosylation of proteins interfere with the lipid-dependent steps.35,228 Substances that specifically block reactions taking place after the transfer of the oligosaccharide to the protein are little known. As several, incompletely (or differently) glycosylated, viral glycoproteins arc still biologically active (see Section IV), these substances would escape the screening procedure based on... 

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