Carbamates vinyl, rearrangement

As a supporting evidence, it is well-known that the electron-rich 0 6-arene)Ru complex of terminal alkyne 428 rearranges easily by the treatment with NaPR, of the )/ -vinylidenc complex 429, which is a strongly electrophilic carbene complex. Attack of ROH on the carbene carbon generates the the alkoxycarbene complex 431 via 430 [166]. Formation of ketone 427 by attack of the allylic alcohol is understanable by this mechanism. Formation of Ru-vinylidene complex 429 from the terminal alkyne has been proposed as the intermediate 432 of the reaction of terminal alkyne, amine and CO2 to form the vinyl carbamate 433 [167,168]. 

Umpolung reactivity is also evident in the addition of an organolithium to a vinyl carbamate 56.34 The product 57 can be quenched with electrophiles at low temperature, but rearranges to 58 by a [l,2]-acyl transfer on warming. 

Norbomene adds to photolytically produced ethoxycarbonylnitrene specifically at the exo face the same aziridine is produced in the thermal addition of ethoxycarbonyl azide, but via the triazoline rather than the nitrene, with much imine by-product. There can be problems of selectivity and rearrangements when one reacts ethoxycarbonylnitrene with more complex substrates, e.g. alkenic steroids. Ethoxycarbonylnitrene via a-elimination) adds to vinyl chlorides to give 2-chloroaziridines, which can be rearranged thermally to yield 2-chloroallyl carbamates. This nitrene also adds to enamines, giving an array of rearranged products. A modem discussion of the reactivities of ethoxycarbonylnitrene (electrophilic) in comparison with phthalimidonitrene (nucleophilic) towards alkenes of different electronic properties has tqipeared. ... 

Rearrangement of the tricyclic system found in 148 to that of 149 was easily accomplished. When the ketone 159 was treated with concentrated hydrochloric acid or 48% HBr in glacial acetic acid the carbamate suffered hydrolysis and dehydration occmred yielding a compound to which structure 149 was assigned on the basis of its spectroscopic properties and its composition. The IR absorption at 1735 cm assigned to the ketone function is appropriate to this structure but the band at 1630 cm assigned to the enamine double bond is considerably lower than that found in fawcettidine and its analogs (Section VII, B). The chemical shift of the vinyl proton also differs considerably from that reported in the model series (Section VII, B). 

Insertion into carbon-sulphur bondscan take place at room temperature. Thus, t-butyl isocyanide inserts into the C—S bond of a-cyano sulphides, such as 140, giving thioimidates 141 which then rearrange further to a mixture of E and Z isomers of A -vinyl carbamates 142. 

Alkyl-, vinyl-, and aryl-substituted acyl azides undergo thermal 1,2-carbon-to-nitrogen migration with extrusion of dinitrogen — the Curtius rearrangement — producing isocyantes. Reaction of the isocyanate products with nucleophiles, often in situ, provides carbamates, ureas, and other A-acyl derivatives. Alternatively, hydrolysis of the isocyanates leads to primary amines. 

Ene-carbamates 499 can be prepared via a Curtius rearrangement of a hydrolyzed MBH adduct 498 followed by the reaction of intermediate vinyl isocyanates with an alcohol (methanol or f-butanol). In addition, different acyloin (a-hydroxyketones) 500 have been obtained in good overall yields based on a Curtius rearrangement in the presence of water. These methodologies have been demonstrated in the synthesis of some bioactive compounds (501and 502) (Scheme 3.219). 

Activation of the carboxylic acid as the acyl chloride permits direct reaction with azide anion to form the acyl azide substrates for Curtius rearrangement. Sodium azide is commonly used, and the reaction has been used on the process chemistry scale for the synthesis of benzyl-A-vinyl carbamate. Acryloyl chloride was combined with sodium azide in a biphasic system with phase-transfer catalysis (PTC), providing acyl azide 25. Upon heating, Curtius rearrangement provided vinyl isocyanate, which was distilled directly into benzyl alcohol containing phenothiazine (27) to inhibit polymerization of 26 and triethylamine to catalyze addition of the alcohol to the isocyanate. The vinyl carbamate product 28 was isolated by crystallization. As the autiior clearly pointed out, preparation and reaction of acyl azides, particularly on large scales, require appropriate safety precautions. 

An improved method that can be easily scaled-up has been developed for the preparation of benzyl-N-vinyl carbamate 810 (Z-vinylamine), a valuable synthetic intermediate in the synthesis of p-lactam antibiotics [589]. In this method, vinyl isocyanate 809, formed by the Curtius rearrangement of acryloyl azide 808, is codistilled with a solvent such as toluene into benzyl alcohol containing a catalyst and an inhibitor. The product thus obtained can be purified by crystallization, thereby avoiding purification by high-vacuum distillation or chromatography. Potential safety issues associated with the process are important [590, 591]. 

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