Insertions, group-selective

As a final illustration of the tremendous functional group compatibility of alkylzinc iodides, the preparation of a protected tso-dityrosine derivative is described in Protocol 7.23 This procedure is noteworthy in the that it illustrates the possibility of inserting zinc selectively into an aliphatic carbon-iodine bond in the presence of an aromatic carbon-iodine bond. This stands in sharp contrast to the situation described in Protocol 4 where excess zinc can react with the electrophile, and must therefore be removed. The selectivity of... 

There are two (limiting) possibilities that could explain the enantioselectivity a group-selective metal insertion distinguishing the enantiotopic allylic protons, or a face-selective addition that distinguishes the enantiotopic double bond faces. Figure... 

Scheme 8.28. Formally group-selection insertion of oxygen into enantiotopic C-H bonds, (a) An asymmetric Kharasch reaction [124], The catalyst is similar to that shown in Scheme 8.12, except that each oxazoline bears two methyl substituents at C-5. (b) Kinetic resolution of dihydronaphthalenes [125]. The reaction uses a Jacobsen epoxidation catalyst (Scheme 8.6, type A).

In this case, intron insertions are selected on the basis of acquisition of resistance to erythromycin due to the incorporation of an inactivated ermB gene, or RAM (Retrotransposition-Activated Marker), that became functional as a consequence of the successful insertion of the group II intron-encoding DNA into the genomic target [133,134]. 

For cases of dropped RCCAs or dropped banks, including inadvertent drops of the RCCAs in those groups selected to be inserted as part of the rapid power reduction system, it is shown that the DNBR remains greater than the safety analysis limit value and, therefore, the DNB Design Basis is met. 

The reason for its selectivity lies in the insertion step of the cycle. In the presence of the two bulky PPhi groups, the atiachmeni to the mcial of -CH2CH2R (anti-Markovnikov addition, leading to a straight chain product) is easier than the attachment of -CH(CHOR (Markovnikov addition, leading to a branched-chain product). 

Thereafter, molecules have been synthesised with a bicyclic ring, such as a quinoleine or an indole, inserted. Many of these compounds like zoniporide and BMS-284640 are selective NHE1 inhibitors, but some inhibit also other isoforms. Most recently, an additional group of compounds with 4-substituted (benzo[b]thiophene-2-carbonyl) guanidines has been synthesised and these are potent NHE1 inhibitors. A structurally distinct compound, S-3226, was found to be the first selective NHE3 inhibitor. 

The palladium-catalyzed stannylboration (90) [124] or silylboration (87) [109, 114] succeeds in the intramolecular carbocyclization of diynes and enynes (Scheme 1-27). It is interesting that a very strained four-membered cycUzation of hexa-l,5-diyne proceeds without any difficulties, similarly to five- or six-membered cycUzation. The boryl group is selectively introduced into the more reactive C=CH rather than C=C for enynes and into the terminal C=CH rather than the internal C=CR for diynes, again suggesting a mechanism proceeding through the first insertion into the Pd-B bond in preference to the Pd-Sn or Pd-Si bond. 

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