Protective groups properties

Since the thiol group has favorable protective group properties for phosphates (ref. 4), and since electron poor amido phosphates are reactive towards hydroxyl groups and are converted into systems that are fairly inert towards phosphorylation, we devoted our attention primarily on those of the 278 candidates that have X = S and Y = N. 

These oxazolines have cationic surface-active properties and are emulsifying agents of the water-in-oil type. They ate acid acceptors and, in some cases, corrosion inhibitors (see Corrosion). Reaction to oxazoline also is useful as a tool for determination of double-bond location in fatty acids (2), or for use as a protective group in synthesis (3). The oxazolines from AEPD and TRIS AMINO contain hydroxyl groups that can be esterified easily, giving waxes (qv) with saturated acids and drying oils (qv) with unsaturated acids. 

This substantial group was developed as a fluorescent, acid-labile protective group for oligonucleotide synthesis. It has properties very similar to those of the DMTr group except that it can be detected down to 10 M on TLC plates with 360-nm ultraviolet light. 

These 108 reagents are used in the Reactivity Charts that have been prepared for each class of protective groups. The reagents and some of their properties are described on the following pages. 

Two new sections on the protection for indoles, imidazoles, and pyrroles, and protection for the amide — NH are included. They are separated from the regular amines because their chemical properties are sufficienth different to affect the chemistry of protection and deprotection. The Reactivity Charts in Chapter 8 are identical to those in the first edition. The chart number appears beside the name of each protective group when it is first discussed. 

Greene reported the preparation of monoaza-18-crown-6 (i) in 1972. His approach was direct, involving condensation of N-trityldiethanolamine with tetraethylene glycol ditosylate. Removal of the protecting group could be achieved by acid cleavage. Unfortunately, neither details of the synthesis nor physical properties were included in this report. 

Silyl groups have found broad appeal as protective groups because their reactivity and stability can be tailored by varying the nature of the substituents on the silicon. Their ability to migrate from one hydroxyl to another is a property that can be used to advantage, but more often, it is a nuisance. The migratory apti-... 

The B SE group was used for intemucleotide protection and is removed with ammonia, also used to remove A -acyl protective groups. Compared with the methylsulfonylethyl group, the B SE group has better solubility properties for solution phase synthesis. ... 

The completion of the total synthesis only requires a few deprotection steps. It was gratifying to find that the final deprotections could be conducted smoothly and without compromising the newly introduced and potentially labile trisulfide residue. In particular, exposure of intermediate 101 to the action of HF pyridine results in the cleavage of all five triethylsilyl ethers, providing 102 in 90% yield (Scheme 23). Finally, hydrolytic cleavage of the ethylene ketal with aqueous para-toluenesulfonic acid in THF, followed by removal of the FMOC protecting group with diethylamine furnishes calicheamicin y (1) (see Scheme 24). Synthetic calicheami-cin y, produced in this manner, exhibited physical and spectroscopic properties identical to those of an authentic sample. 

FIGURE 6 Molecular structures of poIy(CTTE), poly(CTTH), and poly(CTTP), a homologous series of tyrosine-derived polymers used in a study of the effect of the C-terminus protecting group on the materials properties of the resulting polymers. Cbz" stands for the benzyloxycarbonyl group (47). 

In order to test the influence of the C-terminus protecting groups on the properties of the resulting polymer, the ethyl, hexyl, and palmityl esters of N-benzyloxycarbonyl-L-tyrosyl-L-tyrosine were synthesized and the corresponding polymers (poly(CTTE),... 

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