Rearrangements carbamates

Radical reactions with organotin hydrides have been valuable for dehalogenation of organic halides. Bu3SnH/AIBN dehalogenation of a bicyclic carbamate furnished the ring-contracted product by carbamate rearrangement (Scheme 12.126) [229]. 

Lithiated alkyl carbamates rearrange on warming to form a-Hthiooxycarboxa-mides. We noticed this and similar migrations as undesired side reactions [7, 103,104] the rearrangement has been investigated more closely by Nakai and coworkers [Eq. (43)] [105]. 

The Curtms rearrangement has been used to prepare 5-aminothiazole (11) (60.61), 4-methyl-5-aminothiazole. 2-chloro-5-aminothiazole (58), and 2.4-dimethyl-5-aminothiazole (62) (Scheme 11). Heating the corresponding azides yield carbamates that resist hydrolysis but react with acetic anhydride to give the 5-acetylaminothiazoles. 

Acylation. Reaction conditions employed to acylate an aminophenol (using acetic anhydride in alkaU or pyridine, acetyl chloride and pyridine in toluene, or ketene in ethanol) usually lead to involvement of the amino function. If an excess of reagent is used, however, especially with 2-aminophenol, 0,A/-diacylated products are formed. Aminophenol carboxylates (0-acylated aminophenols) normally are prepared by the reduction of the corresponding nitrophenyl carboxylates, which is of particular importance with the 4-aminophenol derivatives. A migration of the acyl group from the O to the N position is known to occur for some 2- and 4-aminophenol acylated products. Whereas ethyl 4-aminophenyl carbonate is relatively stable in dilute acid, the 2-derivative has been shown to rearrange slowly to give ethyl 2-hydroxyphenyl carbamate [35580-89-3] (26). 

An isocyanate 2 formed by a Curtius rearrangement can undergo various subsequent reactions, depending on the reaction conditions. In aqueous solution the isocyanate reacts with water to give a carbaminic acid 6, which immediately decarboxylates to yield an amine 3. When alcohol is used as solvent, the isocyanate reacts to a carbamate 7 ... 

Carboxylic acid, 161, also serves as starting material for a substituted pyrazine that has proven to be an important diuretic agent. As the first step in the synthesis the acid is converted to the corresponding amide (165). Treatment with a single equivalent of hypobromous acid effects Hoffmann rearrangement of only one of the amide groups. Ethanolysis of the intermediate carbamate leads directly to the amino ester (166). Exposure of the... 

The isocyanate formed on rearrangement adds water in a nucleophilic addition step to yield a carbamic acid. 

The nickel(II) dithiocarbamate complexes are neutral, water-insoluble, usually square-planar, species, and they have been studied extensively by a range of physical techniques. The usual methods for the synthesis of dithiocarbamate complexes have been employed in the case of Ni(II), Pd(II), and Pt(II). In addition, McCormick and co-workers (330,332) found that CS2 inserted into the Ni-N bonds of [Ni(aziri-dine)4P+, [Nilaziridinelgf, and [Ni(2-methylaziridine)4] to afford dithiocarbamate complexes. The diamagnetic products are probably planar, but they have properties typical of dithiocarbamate complexes, and IR- and electronic-spectral measurements suggested that they may be examples of N,S-, rather than S,S-, bonded dithiocarbamates. The S,S-bonded complexes are however, obtained, by a slow rearrangement in methanol. The optically active lV-alkyl-iV(a-phenethyl)dithio-carbamates of Ni(II), Pd(II), and Cu(II) (XXIV) have been synthesized, and the optical activity was found to be related to the anisotropy of the charge-transfer transitions (332). 

Lithiated allylic carbamates (35) (prepared as shown) react with aldehydes or ketones (R C0R ), in a reaction accompanied by an allylic rearrangement, to give (after hydrolysis) y-hydroxy aldehydes or ketones. The reaction is called the homoaldol reaction, since the product is a homolog of the product... 

Most of these reactions probably begin with a 1,3 nitrogen-to-oxygen rearrangement, followed by the actual elimination (illustrated for the carbamate) ... 

Employing this method, enantioenriched phenol esters 68, amides 69, and carbamates 70 (after Curtius rearrangement of the intermediate acyl azide) were prepared in yields often greater than 90% with ee-values reaching up to 97% (generally 80-95%, see Fig. 37). 

Direct oxidation of amides can also lead to Hofmann-type rearrangement with formation of amines or carbamates. One reagent that is used is Pb(02CCH3)4. 

Another way to obtain a carbamate via an isocyanate intermediate is the conversion of an imidazolide RCOIm into the azide RCON3, followed by the Curtius-Schmidt rearrangement and treatment with an alcohol [2241... 

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