Carbamates, benzyl, cleavage hydrolysis

H2/Pd-C, 10 h, 87% yield. A nitrobenzyl carbamate is more readily cleaved by hydrogenolysis than a benzyl carbamate it is more stable to acid-catalyzed hydrolysis than is a benzyl carbamate, and therefore selective cleavage is possible. 

Alkyl esters are efficiently dealkylated to trimethylsilyl esters with high concentrations of iodotrimethylsilane either in chloroform or sulfolane solutions at 25-80° or without solvent at 100-110°.Hydrolysis of the trimethylsilyl esters serves to release the carboxylic acid. Amines may be recovered from O-methyl, O-ethyl, and O-benzyl carbamates after reaction with iodotrimethylsilane in chloroform or sulfolane at 50—60° and subsequent methanolysis. The conversion of dimethyl, diethyl, and ethylene acetals and ketals to the parent aldehydes and ketones under aprotic conditions has been accomplished with this reagent. The reactions of alcohols (or the corresponding trimethylsilyl ethers) and aldehydes with iodotrimethylsilane give alkyl iodides and a-iodosilyl ethers,respectively. lodomethyl methyl ether is obtained from cleavage of dimethoxymethane with iodotrimethylsilane. 

Reductive cleavage of the thiazoline C-S bond in heterocycle 193 with -Bu3SnH <1986T3537> followed by in situ hydrolysis of the resulting hemiaminal and protection of nitrogen as its benzyl carbamate gave 194 in 64% overall yield. This was then converted in several steps to potent marine neurotoxin Kainic acid 195 in racemic form (Scheme 27) <1994JOC2773>. 

Cleavage of Carbamates. Since strongly acidic conditions are typically required for the deprotection of carbamates, use of TMS-I provides a very mild alternative. Benzyl and f-butyl carbamates are readily cleaved at rt, whereas complete cleavage of methyl or ethyl carbamates may require higher temperatures (reflux). The intermediate silyl carbamate is decomposed by the addition of methanol or water (eq 8). Since amides are stable to TMS-I-promoted hydrolysis, this procedure can be used to deprotect carbamates of amino acids and peptides. 

The deprotection process illustrated in Figs. 5 and 6 was followed by solid-state CP/MAS NMR spectroscopy. As illustrated in Fig. 7, the carbonyl (158.2 ppm), aromatic (128.2 and 137.4 ppm), and benzylic (67.4 ppm) resonances decreased due to cleavage of those functional groups by the carbamate deprotection. However, the propyl tether resonances at 10.5, 23.1, and 43.2 ppm remained intact (Fig. 7b). Incomplete hydrolysis left a small amount of residual ethoxy moieties at 54.1 and 17.5 ppm. The degree of deprotection achieved in the material resulted in an amine number density of approximately 0.25mmol/g, as ascertained by nonaqueous titration of the amines with benzoic acid [42]. 

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