Blocking-groups

K > Na > Li. After neutralization or removal of the catalyst, fluid polymers are used as is or to formulate grease compounds. If a reactive end blocking group was incorporated during polymerization, the fluid polymer is then useful for making a coating, a sealant, or a Hquid mbber product. 

The presence of water in the carbonylation mixture makes it possible to halt the reaction after the second group migration (step 2). Oxidation of the intermediate boraglycol yields the corresponding ketone, and alkaline hydrolysis affords the secondary alcohol, R HOH. A blocking group of low... 

Miscellaneous Disazo Dyes. Another group of disazo dyes is prepared by condensation of two identical or different aminoazo compounds commonly with phosgene, cyanuric chloride, or fumaryl dichloride, the fragments of which act as blocking groups between chromophores. 

Fig. 4. Dyeing chaiacteiistics as affected by various blocking groups. Water solubility decreases washfastness and affinity increase.

Ester, ether, and cycHc acetal groups are used as blocking groups to allow regiospecific reactions to take place, ie, reaction at specific unblocked hydroxyl groups. 

The trivial name bisnorisopenicillin has been coined for the 6-acylamino-3-thia-l-a2abicyclo[3.2.0]heptane-2-carboxylic acid (85) structural class. While not extensively investigated, an example of this class has been prepared as shown in Scheme 66 (78JMC413). The following features of this sequence are noteworthy (a) the exclusively cis cycloaddition to form intermediate (86) and (b) the use of the 2,4-dimethoxybenzyl amide blocking group. 

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. 

The A -6-ketone functionality in polyhydroxy cholestanes is common to all the insect moulting hormones of the ecdysone type. Therefore, there has recently been considerable interest in the introduction of A -double bonds into 6-ketones. The different approaches that follow illustrate the utility of isomerization, rearrangement, and blocking groups. 

No blocking group was utilized in the Schering-Hoffman-La Roche approach instead, advantage was taken of the fact that the 7a-bromo-5a-6-ketone is the thermodynamic product ... 

Normally, phenylhydrazine reacts with the enol form of 1,1,1-trifluorometh-ylpentane-2,4-dione to give 5-methyl-l-phenyl-3-tnfluoromethylpyrazole as the major product. However, the use of pyrrolidine as a transient carbonyl-blocking group can completely reverse the regiochemistry of the addition and leads to 3-methyl-l-phenyl-5-trifluoromethylpyrazole [102] (equation 88)... 

The double a substitution of enamines with propane-1,3-dithiol ditosylate has provided a route for the introduction of a thioketal, which has found use as a blocking group in modified steroid 476) and terpene 477) syntheses. 

Cleavage of oligonucleotide from solid support and removal of A benzoyl and A isobutyryl blocking groups. 

The choice of what protective groups to use in the synthesis of each segment was based on three aspects (a) the specific steps chosen to achieve the synthesis of each segment (b) the methods to be used in coupling the various segments, and (c) the conditions needed to deprotect the 42 blocked groups in order to... 

Dobbs then modified the approach to prepare indoles devoid of substituents at position 7 by employing a blocking group. Treatment of a range of o-... 

Structurally rather similar to cefatrizine is cefaparole ). It is prepared in quite an analogous manner by active ester condensation of and 7-aminocephalosporanic acid analogue The blocking group is removed with trifluoro-... 

Block one side of the ketone by introducing a removable group. Alkylation takes place on the other side the blocking group is then removed. A common reaction for this purpose is formylation with ethyl formate (10-119) this generally blocks the less hindered side. The formyl group is easily removed by alkaline hydrolysis (12-41). 

---