Isocyanates, acyl, derivatives

Isobutylene, reaction with chlorosul fonyl isocyanate, 46, 52 Isobutyryl fluoride, 45, 6 Isocyanates, acyl, derivatives of, 46, 17... 

Azoles containing a free NH group react comparatively readily with acyl halides. N-Acyl-pyrazoles, -imidazoles, etc. can be prepared by reaction sequences of either type (66) -> (67) or type (70)->(71) or (72). Such reactions have been carried out with benzoyl halides, sulfonyl halides, isocyanates, isothiocyanates and chloroformates. Reactions occur under Schotten-Baumann conditions or in inert solvents. When two isomeric products could result, only the thermodynamically stable one is usually obtained because the acylation reactions are reversible and the products interconvert readily. Thus benzotriazole forms 1-acyl derivatives (99) which preserve the Kekule resonance of the benzene ring and are therefore more stable than the isomeric 2-acyl derivatives. Acylation of pyrazoles also usually gives the more stable isomer as the sole product (66AHCi6)347). The imidazole-catalyzed hydrolysis of esters can be classified as an electrophilic attack on the multiply bonded imidazole nitrogen. 

In the Lossen reaction a hydroxamic acid derivative (usually an 0-acyl derivative) is deprotonated by base, and rearranges via migration of the group R to give an isocyanate 2. Under the usual reaction conditions—i.e. aqueous alkaline solution—the isocyanate reacts further to yield the amine 3. The Lossen reaction is closely related to the Hofmann rearrangement and the Curtins reaction. 

The 0-acyl derivatives of hydroxamic acids give isocyanates when treated with bases or sometimes even just on heating, in a reaction known as the Lossen rearrangement. The mechanism is similar to that of 18-13 and 18-14 ... 

The pyrido[l,2-tf][l,3,5]triazine-2,4(3//)-dione derivative 89 was obtained in a cycloaddition reaction of diphenyl-methyl isocyanate 90 with 2-pyridyl isocyanate 91 derived from the corresponding acyl azide via Curtius rearrangement <2002ARK438>. Compound 89 was also synthesized by the reaction of diphenylacetyl chloride 118 and picolinyl azide 116a in the presence of triethylamine (Scheme 11) <2002ARK438>. ... 

Acylation reactions can be carried out by the action of acid anhydrides [66, 67], chloroanhydrides [47, 61], isocyanates [68], isothiocyanates [68, 69] and thioamides [70]. It was shown that the reaction of aziridinyl ketone 49 with chloroacetyl chloride 50 in the presence of Et3N [61] leads to the formation of A-acyl derivative 51, whereas the alkylation product was not isolated. 

All these substances are very sensitive to hydrolysis (89). Functionalization at the nitrogen also takes place in the reaction of the silver salt with CISO2NCO. The sulfonyl isocyanate obtained does not show typical isocyanate reactions. It solvolyzes with H20 or CH3OH without forming intermediates to (CF3SC)4NH. However, with moist air partial hydrolysis leads to the corresponding sulfonamide according to Eq. (8). A number of acyl derivatives were synthesized via Eq. (9). The oxalyl... 

In the presence of base, acyl derivatives of hydroxamic acids undergo the Lessen rearrangement to yield isocyanates or amines. 

A further example is the Lossen rearrangement, in which an O-acyl derivative of hydroxamic acid, RCONHOCOR, gives an isocyanate on treatment with hydroxide ion, which in turn may be hydrolysed to the amine. Illustrate this reaction pathway. 

Lossen rearrangement The O-acyl derivative of a hydroxamic acid gives an isocyanate when treated with a base or, sometimes, just heat. 

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