Carbamates elimination reactions

Theoretical evidence for the thermal decomposition mechanism for ethyl oxam-ate, ethyl oxanilate, and ethyl A,A-dimethyl oxamate have been provided.27 Ethyl oxamate and ethyl oxanilate undergo rapid decarbonylation to give the corresponding carbamates. Ethyl A,lV-dimethyloxamate elimination reaction yields in one step, through a six-membered cyclic transition state, dimethyloxamic acid and ethylene gas. 

E2 elimination reactions occur preferentially when the leaving groups are in an anti copla-nar arrangement in the transition state. However, there are a few thermal, unimolecular sy -eliminations that produce alkenes. For example, pyrolysis of several closely related amine oxides, sulfoxides, selenoxides, acetates, benzoates, carbonates, carbamates and thio-carbamates gives alkenes on heating (Scheme 4.10). The syn character of these eliminations is enforced by a five- or six-membered cyclic transition states by which they take place. 

Heating of the benzene solution of 44 and K2CO3 induced the intramolecular Sn2 reaction to afford 6-deoxycodeine 45 in 53% yield (Scheme 10). The major side product in this reaction was, as anticipated, diene 46 (30% yield), which was formed by elimination reaction. It was reported that the solvent effect is significant in the Sn2 reaction when this reaction was carried out in acetonitrile, diene 46 was obtained as the major product. Treatment of 45 with methyl chloroformate provided a methyl carbamate, whose allylic oxidation with Se02 in the presence of f-BuOOH introduced the hydroxy group at C-6 to give 19. PCC oxidation of 19, followed by LiAlH4 reduction, provided racemic codeine. 

Figure 20.22) It is reasonable to admit the formation of allylic carbamates also in mitomycines A, B and C which are antimitotic drugs used in cancerology. Presumably the allyl function is created by an elimination reaction involving the departure of an acetalic hydroxy or methoxy group. 

Another elimination reaction sequence, utilizing chlorodimethyl-formiminium chloride as catalyst, involves the synthesis of isocyanates by phosgenating carbamates XVI ( ). Thus, phosgene is used to generate the iminium chloride XVII, which reacts with XVI to form the chloro-imidate XVIII. The isolated isocyanate is formed by thermolysis of XVIII. Since N,N-dimethylformamide is regenerated, only a catalytic amount is necessary. 

Lithium n butyl (phenylthio) cuprate has been used in nucleophilic substitution reactions of arenesulfonyl fluorides, al-lylic acetates, 9 BBN, propargyllc carbamates, and bromo-alkenes, as well as in nucleophilic additions to acetoxy-epoxides. It Is a good choice for 1,4-addItIon of an n-Bu group, having been used in 1,4 addition-elimination reactions of a-oxoketene dithloacetals and 3-halo-2-cycloalkenones, and in tandem vicinal dialkylation reactions of 5-methyleneoxa-zolones and alkynes. A typical example Is the use of the reagentin the stereospecific synthesis of (, -2-heptenoicacidfrom acetylene (eq 1). ... 

Amino groups can be blocked by converting them into carbamates through simple addition-elimination reactions (p. 901).Two popular methods involve the transformation of the free amine into a carbamate by reaction with either di- -butyl dicarbonate or benzyl chloroformate. Biochemists are even more addicted to acronyms than are organic chemists, and these protecting groups are called tBoc and Cbz, respectively (Fig. 23.45). 

Removal of the Fmoc group initially involves an elimination reaction promoted by piperidine to form the carbamic acid derivative of the amino acid and 9-methyhdenefluorene, which reacts further with piperidine by nucleophilic addition to form the byproduct. Spontaneous decarboxylation of the carbamic acid generates CO2 and the fiee amino acid. 

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

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