Phenyl group displacement selectivity

Figure 4.6. Displacer designed for chiral selective stationary phases (Cyclobond II). The tail section R can vary in length and serves to ad-j ust thesolubiliiy. The phenyl group fits into the cavity of the a-cyclodextrine, and the carboxyl and carbonyl groups serve to form multiple hydrogen bonds with the secondary hydroxyl groups of a-cyclodextrin [reproduced with permission in modified from reference 28].

Three selective methods to remove protective groups are receiving much attention assisted, electrolytic, and photolytic removal. Four examples illustrate assisted removal of a protective group. A stable allyl group can be converted to a labile vinyl ether group (eq. 4) a /3-haloethoxy (eq. 5) or a /3-silylethoxy (eq. 6) derivative is cleaved by attack at the /3-substituent and a stable o-nitro-phenyl derivative can be reduced to the o-amino compound, which undergoes cleavage by nucleophilic displacement (eq. 7) ° ... 

Unusual selectivities have sometimes been observed in the displacement reactions leading to zwitterions equation 17 shows a case of inverse selectivity , in which the aminoalkyl group is displaced, rather than the phenyl, leading to an anionic spirosilicate28. 

At present, we are studying the optimal conditions for the displacement reactions by using different solvents, temperatures, and acids. We have found that variation in the aryl groups also increases the selectivity of dearylation. For example, the p-methoxyphenyl group is displaced by triflic acid much more rapidly than the unsubstituted phenyl ring. 

The a-methyltoluene-2,o -sultam auxiliary is also displaced by a variety of dilithiated alkyl phenyl sulfones. This unique procedure provides direct access to synthetically useful fi-o o sul-fones which may he further functionalized or simply subjected to reductive desulfonation to give alkyl ketones. A particularly striking use of this method is the preparation of p-oxo sulfone 8, a key intermediate in a concise synthesis of (—)-semicorole (eq 5). Remarkably, the MeCLi2S02Ph reagent attacks selectively the C(4)-imide C=0 group in preference to the C(6)-ester C=0 group and no epimerization occurs at C(3) or C(F). 

Selectivity for one functional group in preference to another can be used to good advantage in reactions with malonate anions. One example is the reaction of diethyl 2-phenyl diethyl malonate and 2-chloroacetonitrile under basic conditions to give 4.53.29 In this case, the malonate anion displaced chlorine rather than reacting with... 

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