Blocking Amine Groups

We have called these catalysts man-made but this is not strictly true. We have not violated the general principal that, if you want chiral molecules, you will have to get them with the assistance of previously formed natural products. Our asymmetry was obtained from the (-)-menthol used in the chiral phosphane synthesis, but being a catalyst, a small amount of (-)-menthol could lead to a large amount of chiral product. CAMP worked equally well for the L-dopa precursor (Fig. 2), and it made no difference whether the amine-blocking group was benzoyl or acetyl. 

This Strategy would seem too laborious for practical purposes. It requires the incorporation of each residue in blocked and activated form and removal of the amine-blocking group after each chain lengthening step. 

Blends based on polyolefins have been compatibilized by reactive extrusion where functionalized polyolefins are used to form copolymers that bridge the phases. Maleic anhydride modified polyolefins and acrylic acid modified polyolefins are the commonly used modified polymers used as the compatibilizer in polyolefin-polyamide systems. The chemical reaction involved in the formation of block copolymers by the reaction of the amine end group on nylon and anhydride groups or carboxylic groups on modified polyolefins is shown in Scheme 1. 

A succinylated casein derivative that has nearly all its amines blocked can be used as a substrate in protease assays (Hatakeyama et al., 1992). As the casein is degraded by a protease, free amines are created from a-chain cleavage and release of a-amino groups. The creation of... 

Figure 15.9 The reaction of the amine-blocked derivative of 3-hydroxypropylamine with ethylmalonyl chloride gives an ethylmalonate-protected-amine compound, which can be used in the Bingel reaction to create an amine group on a fullerene surface. Reaction with Cfl in the presence of I2 and DBU gives the cyclopropanation product that can be deprotected with TFA to yield the free amine.

These results indicate that if polydienes and similar polymers can be prepared quantitatively with tertiary amine terminal groups, then they can be combined with other halogen functional polymers using established techniques to create interesting new block copolymer systems. For example, consider the reaction between telechelic pyridine terminated polybutadiene and monofunctional bromine terminated polystyrene (equation 4) -the latter has been prepared in 95% yield. >it The product would be an ABA... 

Succinic anhydride also is a convenient extender for creating spacer arms on chromatography supports. Supports derivatized with amine-terminal spacers may be succinylated to block totally the amine functional groups and form terminal carboxylic acid linkers for coupling amine-containing affinity ligands (Cuatrecasas, 1970). 

Melphalan and the racemic analog have been prepared by two general routes (Scheme I). In Approach (A) the amino acid function is protected, and the nitrogen mustard moiety is prepared by conventional methods from aromatic nitro-derivatives. Thus, the ethyl ester of N-phthaloyl-phenylalanine was nitrated and reduced catalytically to amine I. Compound I was reacted with ethylene oxide to form the corresponding bis(2-hydroxyethyl)amino derivative II, which was then treated with phosphorus oxychloride or thionyl chloride. The blocking groups were removed by acidic hydrolysis. Melphalan was precipitated by addition of sodium acetate and was recrystallized from methanol. No racemization was detected [10,28—30]. The hydrochloride was obtained in pure form from the final hydrolysis mixture by partial neutralization to pH 0.5 [31]. Variants of this approach, used for the preparation of the racemic compound, followed the same route via the a-acylamino-a-p-aminobenzyl malonic ester III [10,28—30,32,33] or the hydantoin IV [12]. 

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