Deprotections allyloxycarbonyl groups

O Dangles, F Guibe, G Balavoine, S Lavielle. A Marquet. Selective cleavage of the allyl and allyloxycarbonyl groups through palladium-catalyzed hydrostannolysis with tributyltin hydride. Application of the selective protection-deprotection of amino acid derivatives and in peptide synthesis. J Org Chem 52, 4984, 1987. 

The preparation of the trisaccharide 11.40 was carried out by condensation of imidate 11.45 with acceptor 11.43 in the presence of TMSOTf to afford disaccharide 11.50 in 87% yield (Scheme 14). Conversion of 11.50 into glycosyl acceptor 11.51 was achieved by de-isopropylidination and acetylation followed by removal of the allyloxycarbonyl group. Glycosyl acceptor 11.51 was subsequently condensed with imidate 11.44 in the presence of BF3-Et20 to give the ttisaccharide derivative 11.52 in 81 % yield. The deprotection, oxidation, and N-acetylation sequence was achieved as described for 11.39 to afford 11.40 in 66% yield over fom steps. 

Nevertheless, when the dimethylallyl group was replaced by the cinnamyl group, the use of 1 % of Pd(0) in homogeneous medium led to complete removal of the allyloxycarbonyl group with a certain amount of the deprotected carboxylic acid (ref. 20). In order to overcome this difficulty it was necessary to operate under biphasic conditions, in the presence of 1 % of catalyst, giving the expected cinnamyl-L-prolinate 17 in quantitative yield (entry 4). 

Another useful strategy for the activation of an amide towards hydrolysis involves intramolecular 0-alkylation of the amide carbonyl. An early rendition of this strategy entailed the use of 4-chlorobutanamides in which cleavage was initiated by treatment with silver(I) perchlorate in aqueous acetone. More re-cently the Fraser-Reid group showed that N-pent-4-enoyl derivatives are rapidly and efficiently cleaved under mild conditions by brief treatment with 3 equivalents of iodine in aqueous THF [Scheme 8.29]. These deprotection conditions do not affect oxidisable functionalities including p-methoxybenzyl ethers and alkyl sulfides though allyloxycarbonyl groups appear to be incompatible. Primary and secondary amines are readily protected as N-pent-4-enoyl derivatives by reaction with pent-4-enoic anhydride. 

The selectivity and mildness of the Pd(0)-catalyzed deprotection of allyl (All) esters and the allyloxycarbonyl (Aloe) urethanes117 181 reaction also allowed for the successful and efficient application of this blocking group technology in the synthesis of acid- and base-labile lipidated pepti-... 

Deprotection of allyl or allyloxycarbonyl derivatives of amino acids.1 This combination of reagents effects rapid reductive cleavage of allyl or allyloxycarbonyl derivatives of amino acids in CH2C12 containing a proton donor (p-N02C6H50H, acetic add, H20). The actual catalyst is probably bis(triphenylphosphine)palladium(0). Benzyl, Boc, and Cbo groups are stable to these conditions. This cleavage does not induce racemization. 

More recently Kunz and co-workers described a synthesis of 2,6-dideoxy-2,6-diaminopyra-noside libraries [73]. The scaffold 120 allows for selective deprotection of four positions in a manner that is independent of the sequence (O Scheme 11). Different functional groups have been incorporated at each position to give 121-4, which can then be cleaved from the solid support. For example, the 2-A-allyloxycarbonyl protecting group of 120 can be selec-... 

A SPOS approach to DNA synthesis was reported very recently. It involves a ten-step synthesis of a phosphoramidite building block of I -aminomethylthymidine starting from 2-deoxyribose [46]. Microwave-mediated deprotection of a corresponding N-allyloxycarbonyl (alloc-) protected nucleoside and acylation with the residue of pyrene-l-ylbutanolic acid (pyBA) were described (Scheme 16.25). Removal of the alloc protecting group was achieved on a support (controlled pore glass, cpg) under microwave conditions (80 °C, 10 min) to ensure full conversion. 

Alternatively, the C5 hydroxy group can be protected as 5-0-allyloxycarbonyl derivative instead (not shown in Scheme 29.6.1). This is done by reaction of abamectin with allylchloroformate and tetraethylendiamine in t-butyl methyl ether. In this case, the C5 hydroxy group can be deprotected in the last step by treatment with sodium borohydride in ethanol in the presence of catalytic amounts of tetrakis(triphenylphosphine)palladium. 

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