Carboxylates blocking

Scheme 68 shows the conversion of the phenoxymethylpenicillin-derived disulfide (see Scheme 10) to penem derivative (91) (78JA8214). Of particular interest in this sequence is the reductive acylation step to afford (89) and the Wittig ring closure to give (90). The rate of the latter reaction was found to be greatly infiuenced by the steric and electronic character of both the thiol ester and the carboxyl blocking group. 

This carboxylate blocks access to the P site in the free T-state enzyme. Here, the positively charged Arg 569 guanidinium group has swung in to interact with the P . From Mitchell et al.133 Courtesy of Louise N. Johnson. 

The carboxylated block copolymers were redissolved in the solvent in which they were to be used (usually in toluene) immediately after reprecipitation because they crosslink when they are stored in the solid state. 

This work has demonstrated that carboxylated styrene-butadiene block copolymers are excellent dispersants for titanium dioxide particles in toluene. Combining our results with carboxylated block copolymers and homopolymers and Schechters (18) results with fatty acids, we can... 

We are currently exploring new routes to the synthesis of ionomers with controlled architecture, i.e. with control over the amount and location of ionic groups in the polymer backbone. One of our main interests is the synthesis of ion containing block copolymers. The applicability of anionic polymerization in the synthesis of block copolymers and other well defined model systems is well documented (22-24) Not as well appreciated, however, is the blocky nature that certain emulsion copolymerizations may provide. Thus, we have utilized both anionic and free radical emulsion polymerization in the preparation of model ionomers of controlled architecture. In this paper, the synthesis and characteristics of sulfonated and carboxylated block ionomers by both free radical emulsion and anionic polymerization followed by hydrolysis will be discussed. 

Figure 4. FT-IR spectra of (A) polystyrene/poly(isobutyl methacrylate) diblock copolymer and (B) carboxylated block ionomer obtained after hydrolysis with K0-.

Inhibitors represent one of the most powerful tools with which to investigate the physiological functions of a proteinase, but the carboxyl proteinases seemed to have no satisfactory low molecular weight inhibitors, until pepstatin was discovered. Like the other carboxyl proteinases, cathepsin D is inactivated by diazoacetyl amino acid esters in the presence of Cu, as well as related carboxyl-blocking reagents (see reference 4 for review). 

Ortho esters have found occasional use as carboxyl blocking groups in some syntheses [73, 74, 75, 76, 96] particularly where it is necessary to prevent reaction at a carboxyl group with a Grignard reagent at a subsequent stage in a synthesis. An example of an application of this type is in the preparation of (57) in studies allied to the synthesis of the antibiotic mycomycin (33) [96]. An alternative use is illustrated by the synthesis of the amino-sugar muramic acid (58) [74]. 

The formation of the tetrahydropyrroloindole derivatives requires the presence of a 3-(2-acetamidoethyl) side chain or, in the case of tryptophan derivatives, a carboxyl-blocked, N -acetylated derivative. 

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