Protecting groups intermediate

The protecting groups are also used to solubilize synthetic intermediates in organic solvents, e.g. methylene chloride. Chromatography is then possible on a larger scale, since silica gel can be used as adsorbent. Six synthetic strategies have been developed (H. Kdster, 1979) ... 

Another protecting group of amines is 1-isopropylallyloxycarbonyl, which can be deprotected by decarboxylation and a /3-elimination reaction of the (tt-l-isopropylallyl)palladium intermediate under neutral conditions, generating CO2 and 4-methyl-1,3-pentadiene. The method can be applied to the amino acid 674 and peptides without racemization[437]. 

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. 

To control the stereochemistry of epoxidation at the 10,11-double bond in intermediates in prostaglandin synthesis, a bulky protective group was used for the C15-OH group. Epoxidation of the tribenzylsilyl ether yielded 88% a-oxide epoxidation of the tri-/ -xylylsilyl ether was less selective. ... 

Carbamates can be used as protective groups for ammo acids to minimize race-mization in peptide synthesis. Raccmi/ation occurs during the base-catalyzed coupling reaction of an W-protected, catboxyl-uc ivated amino acid, and takes place in the intermediate oxazolone that foro S readily from an N-acyl protected amino... 

The BOC group can be cleaved with TBDMSOTf and the intermediate silyl carbamate converted to other nitrogen protective groups. 

R (reacts) indicates that the protected compound reacts readily, but that the original functional group is not restored. The protective group may be changed to a new protective group (eq. 1) or to a reactive intermediate (eq. 2), or the protective group may be unstable to the reaction conditions and react further (eq. 3). 

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). ... 

A -Dien-3-ol esters e.g., acetates) have greater utility as reaction intermediates than as protecting groups. They are prepared from A" -3-ketones by reaction with the acetic anhydride"" or by exchange with isopropenyl acetate. 

A cinnamoylpiperazine is described as an anti anginal agent. The key intermediate 1 can, in principle, be obtained by alkylation of the monobenzyl derivative of piperazine 71 with ethyl bromoacetate (72). Removal of the protecting group then affords the substituted piperazine (73). Acylation of this with 3,4,5-trimethoxycinamoyl chloride gives cinepazet (74). ... 

Hydrogenation of the product 147 removes the benzyl protecting groups and at the same time reduces the triazine to its dihydro derivative 148. A roundabout scheme is required for dehydrogenation due to the sensitivity of the intermediates. The product is thus converted to its silyl ether 149 exposure to air results in oxidation and desilylation. There is thus obtained the antineoplastic agent fazarabine (150), also known as ara-A C. 

Intermediate D-a-6 must now be converted into a form amenable to the crucial lactamization reaction. To this end, treatment of D-a-6 with hydrazine accomplishes the removal of the phthalimide protecting group and provides D-a-18 (Scheme 5) after acidification with dilute aqueous HC1. It is noteworthy that the acid-labile tert-butyl ester function withstands the latter step. Introduction of the... 

In subsequent studies,22 Sheehan et al. demonstrated that the action of diisopropylcarbodiimide on penicilloate 24, prepared by protection of the free primary amino group in 23 with trityl chloride (see Scheme 6b), results in the formation of the desired -lactam 25 in a very respectable yield of 67 %. In this most successful transformation, the competing azlactonization reaction is prevented by the use of a trityl group (Ph3C) to protect the C-6 amino function. Hydrogenolysis of the benzyl ester function in 25, followed by removal of the trityl protecting group with dilute aqueous HC1, furnishes 6-aminopenicillanic acid (26), a versatile intermediate for the synthesis of natural and unnatural penicillins. 

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