A protective group

Since (A) does not contain any other functional group in addition to the formyl group, one may predict that suitable reaction conditions could be found for all conversions into (A). Many other alternative target molecules can, of course, be formulated. The reduction of (H), for example, may require introduction of a protecting group, e.g. acetal formation. The industrial synthesis of (A) is based upon the oxidation of (E) since 3-methylbutanol (isoamyl alcohol) is a cheap distillation product from alcoholic fermentation ( fusel oils ). The second step of our simple antithetic analysis — systematic disconnection — will now be exemplified with all target molecules of the scheme above. For the sake of brevity we shall omit the syn-thons and indicate only the reagents and reaction conditions. 

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. 

Oxidation of N -substituted pyrazoles to 2-substituted pyrazole-l-oxides using various peracids (30) facilitates the introduction of halogen at C, followed by selective nitration at C. The halogen atom at or is easily removed by sodium sulfite and acts as a protecting group. Formaldehyde was... 

Fig. 26. Representative biomimetic cyclization where P = a protecting group, TFA = trifluoroacetic acid, and (240) (C2H5)3SiCH2C H P(OC2H5)2...

AdeninyUiydroxypropanoic acid alkyl esters [(R,5)-AHPA esters, (30)] represent a new class of broad-spectmm antiviral agents, which are, like (3)-DHPA, targeted at SAH hydrolase (62). The free acid, (R,3)-AHPA, is only weakly active as an antiviral agent. Thus the alkyl moiety merely serves as a protecting group to faciUtate uptake of AHPA by the cells. Within the cells, the AHPA alkyl esters would be hydroly2ed to release the free acid, which should then interact with SAH hydrolase. 

Since syntheses of three-membered rings with two heteroatoms almost always proceed under mild conditions with good yields and from simple starting materials, they are useful intermediates in synthesis. The easy formation of N—O and N—N bonds offers synthetic potentialities. Furthermore, the three-membered ring may serve as a protecting group during additional operations of substitution. 

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) ° ... 

Selection of a Protective Group from This Book... 

The ability to convert a protective group to another functional group directly without first performing a deprotection is a potentially valuable transformation. Silyl-protected alcohols have been converted directly to aldehydes, ketones, bro-mides, acetates, and ethers without first liberating the alcohol in a prior deprotection step. 

For a particular phenol, the authors required a protective group that would be stable to reduction (by complex metals, catalytic hydrogenation, and Birch conditions) and that could be easily and selectively removed. 

This derivative is similar to the methylsulfonylethyl derivative. It is cleaved by 1 M NaOH, < 1 h. The related 4-chlorobenzenesulfonylethyl carbamate has also been used as a protective group that can be cleaved with DBU or tetramethyl-guanidine. ... 

The number used to designate a protective group (PG) in a Reactivity Chart is the same as that used in the body of the text in the first edition. 

The process of selecting a protective group involves a number of discrete steps. First, the proposed scheme is summarized, with reactants, reaction conditions, and products delineated for each synthetic step. Next, the relative reactivities of the functional groups in each reactant... 

However, treatment of cortisone 3,20-bissemicarbazone with acetic anhydride and pyridine removes the 20-semicarbazone group preferentially. Selective removal of a protecting group can be also achieved by a selective reaction to give a new intermediate which can be converted into the desired product ketone. Thus progesterone 20-monoenol acetate (42) is prepared from the 3,20-bisenol acetate (40) via selective electrophilic attack of iodine at C-6 followed by reductive dehalogenation of (41). ... 

Protecting group (Section 17.9) A temporary alteration in the nature of a functional group so that it is rendered inert under the conditions in which reaction occurs somewhere else in the molecule. To be synthetically useful, a protecting group must be stable under a prescribed set of reaction conditions, yet be easily introduced and removed. 

SELECTION OF A PROTECTIVE GROUP FROM THIS BOOK... 

This ester was designed as a protective group for the 2-position in glycosyl donors. It has the stability of the benzoate during glycosylation, but has the ease of removal of the chloroacetate. It is readily introduced through the acid chloride... 

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