Solvent-free deprotection

The protection-deprotection reaction sequences constitute an integral part of organic syntheses such as the preparation of monomers, fine chemicals, and reaction intermediates or precursors for pharmaceuticals. These reactions often involve the use of acidic, basic or hazardous reagents and toxic metal salts [30], The solvent-free MW-accelerated protection/deprotection of functional groups, developed during the last decade, provides an attractive alternative to the conventional cleavage reactions. 

This approach may find application in peptide bond formation that would eliminate the use of irritating and corrosive chemicals such as trifluoroacetic acid and piperidine as has been demonstrated recently for the deprotection of N-boc groups (Scheme 6.7) a solvent-free deprotection of N-tert-butoxycarbonyl group occurs upon exposure to microwave irradiation in the presence of neutral alumina doped with aluminum chloride (Scheme 6.7) [41]. 

The quest for a solvent-free deprotection procedure has led to the use of relatively benign reagent, ammonium persulfate on silica, for regeneration of carbonyl compounds (Scheme 6.10) [48]. Neat oximes are simply mixed with solid supported reagent and the contents are irradiated in a MW oven to regenerate free aldehydes or ketones in a process that is applicable to both, aldoximes and ketoximes. The critical role of surface needs to be emphasized since the same reagent supported on clay surface delivers predominantly the Beckmann rearrangement products, the amides [49]. 

Using day supported ammonium nitrate (dayan), selective deprotection of methoxyphenyl methyl (MPM) ether has been achieved recently using microwave irradiation in solvent-free conditions (Scheme 6.15) [56]. The same reagent has been used for the cleavage of tetrahydropyranyl (THP) ethers. A similar selective preparation and deavage of THP ethers has been achieved under microwave irradiation catalyzed by iodine [57] or neat reaction in an ionic liquid [28],... 

T pe of reaction deprotection Reaction condition solvent-free... 

Solvent-Free Deprotection under Microwave Irradiation... 

The tetrahydropyranylation of alcohols under solvent-free conditions is efficiently catalyzed by bismuth triflate (0.1 mol%). The experimental procedure is simple and works well with a variety of alcohols and phenols. The catalyst is insensitive to air and small amounts of moisture, easy to handle and relatively non-toxic. The deprotection of THE ethers is also catalyzed by bismuth triflate (1.0 mol%). 

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