摘要:

In multi-step syntheses involving polyhydroxylated natural products such as carbohydrates that are variously derivatized at different positions, orthogonal removal of one or another type of protecting group is of vital importance. Discrimination of different classes of protecting groups, such as ethers, esters, etc., is often possible with a great degree of success, as for example, selective removal of an 0-acetyl by catalytic transesterification in the presence of an ether protecting group, or hydrogenolytic removal of a benzyl ether protection in the presence of ester groups such as acetates.3Differentiation of different types of protecting groups within a given class of protecting groups has also been similarly achieved with great success, as for example, hydrogenolytic removal of a benzyl ether group in the presence of a methyl ether.3However, the situation becomes more challenging when the same protecting group is used to mask more than one position in a polyfunctional molecule and their preferential partial deprotection is required. Selective unmaslung of one or more of such protecting groups has been achieved in some cases.4 Of particular interest to us was the regioselective deprotection of the 2-0-benzyl group of per- 0-benzylated 1,6-anhydromannopyranose mediated by SnC14 (1) and Tic14 (2). Considering the greater susceptibility of p-methoxybenzyl (PMBn) ethers to Lewis acid catalysts5 and the complexation of benzyl ethers with 14b and 24b16 we decided to investigate the action of 1 on PMBn ethers of some carbohydrates, We expected the methoxy substituent on the phenyl group in the PMBn moiety to enhance complexation with 1, possibly resulting in a facile reaction under mild conditions. Since 1 is a strong Lewis acid, the need to use chlorotrimethylsilane and anisole, as in the tin(I1)chloride- chlorotrimethylsilane-anisole system for deprotection of PMBn ethers, can be eliminated. Moreover, the complex formation in the case of 1 presents possibilities for unusual regioselectivity in partial de-0-p-methoxybenzylation reactions, a problem that has not been addressed in reports on the oxidative cleavage of PMBn ethers by 2,3-dichloro-5,6-dicyanobenzoquinone (DDQ)7ceric ammonium nitrate (CAN),8N-bromosuccinimide (NBS)8or bromine8.

ISSN:0732-8303    

DOI:10.1080/07328309808002353

出版商:Taylor & Francis Group    

年代:1998

数据来源: Taylor

摘要:

Much attention has been paid to the synthesis of glycosides and oligosaccharides for the preparation of natural products and their analogues to investigate biological functions.2Most known glycosidation methods are based on the activation of a leaving group at the anorneric center of a glycosyl donor.3In some cases alcohol derivatives such as ROSnBu3 and ROSiMe3 are used to increase the reactivity of glycosyl acceptors.3

ISSN:0732-8303    

DOI:10.1080/07328309808002354

出版商:Taylor & Francis Group    

年代:1998

数据来源: Taylor

ISSN:0732-8303    

DOI:10.1080/07328309808002332

出版商:Taylor & Francis Group    

年代:1998

数据来源: Taylor