Remote participation

SCHEME 33 Synthesis of 3-mannosides via remote participation. Bn, benzyl Pic, picolinyl. 

A subsequent report by Kwan Soo Kim and coworkers indicated that remote participation from an equatorial C3 position is indeed possible [16]. In this case, a tri-chloroacetimidate was used as a nucleophilic trap. Upon low-temperature activation of thioglycoside 11 using 1-benzenesulfinylpiperidine (BSP) and Tf20, bicyclic... 

Cheng, Y-P, Chen, H-T, Lin, C-C, A convenient and highly stereoselective approach for a-galactosylation performed by galactopyranosyl dibenzyl phosphite with remote participating groups. Tetrahedron Lett, 43, 7721-7723, 2002. 

An elegant deuterium labelling study of gold(I)-catalysed glycosidation of peracetyl glucopyranosyl orf/io-hexynylbenzoate revealed remote participation of the 4-0-acyl group (Scheme 85). ... 

Keywords Directing effect. Electron-withdrawing protecting groups, Glycosylation, Remote participation. Remote protecting group... 

Fig. 5 Possible dioxalynium ion intermediate by the remote participation of 3-0-acyl groups of mannosyl donors...

To obtain more support for the remote participation of 3-0-acyl and 6-0-acetyl groups in mannosyl donors, Kim and co-workers carried out experiments to trap anomeric oxocarbenium ion intermediates by the intramolecular nucleophilic attack of the trichloroacetimidoyl group at the 0-3 position of mannosyl donors the activation of 72 with 1-benzenesulfinyl piperidine (BSP) and Tf20 afforded... 

Demchenko and co-workers, on the other hand, reported that the mannosylation of 76 with donor 74, having the p-methoxybenzoyl group at the 0-4 position, gave disaccharide 77 (p/a = 3.5 1) while the mannosylation of 76 with tetra-O-benzyl-protected donor 75 provided disaccharide 78 (p/a = 2.2 1) (Scheme 13). They interpreted the slightly increased p-selectivity in the mannosylation of 76 with 74 compared to that with 75 as the result of the remote participation of the p-methox-ybenzoyl group at the 0-4 position of the donor 74 during the mannosylation [63]. 

Wong and co-workers reported that the galactosylation of trihydroxy sugar 109 with a-galactosyl phosphite donor 108 having an 0-6 acetyl group produced disaccharide 110 (p/a = 15 85) with an excess of the a-anomer, which could be formed by the remote participation of the acetyl group at 0-6 (Scheme 18) [70]. 

Corey et al. reported a successful total synthesis of pseudopterosin E (116) through a crucial coupling reaction of 2-0-benzyl-3,4-di-0-p-methoxybenzoyl-a-L-fucosyl bromide (115) and catechol 114 the fucosylation of 114 with 115 proceeded in the complete position-selective and a-stereoselective manner, providing exclusively a-fucoside 116 (Scheme 20), and the a-stereoselectivity of the reaction was ascribed to the internal 1,4-remote participation of the p-methox-ybenzoyl group (Fig. 9) [72],... 

Nifantiev and co-workers investigated the effect of acyl groups at the 0-4 position of donors on the fucosylation stereochemistry fucosylations of acceptor 121 with fucosyl donors 118—120 having p-methoxybenzoyl, benzoyl, and p-nitrobenzoyl groups, respectively, at the 0-4 position were a-selective and thus disaccharides 123 (R = Bz, p/a = 1 3.5), 124 (R = p-NOjBz, p/a = 1 2), and 125 (R = p-OMeBz, p/a = 1 5) were obtained, respectively, with an excess of a-anomers (Table 8) [73]. The a-selectivity was ascribed to the remote participation of the 4-O-acyl groups and the computational work on the difference in the total energy... 

Fig. 8 Proposed remote participation by 4- and 6-0-benzoyl groups in galactosylations...

Crich and co-workers reported that the remote participation of esters at the 0-3 position of allopyranosyl donors is possible in glycosylations based on the trapping experiment, in which the anomeric oxocarbenium ion intermediate generated by the activation of thioalloside 132 was trapped by the intramolecular nucleophilic attack of the tert-butoxycarbonyl (Boc) group at the 0-3 axial position to afford stable cyclic carbonate 133 (Scheme 21) [65],... 

However, Crich and co-workers reported that there was no evidence to support the remote participation by an 0-3 equatorial ester, by 0-4 axial or equatorial esters, or by 0-6 esters, and concluded that the remote participation by esters at these positions does not occur under typical glycosylation conditions [65]. 

Narasaka and co-workers reported a successful p-selective glycosylation, which is one of the key steps in the total synthesis of (—)-sordarin, by employing the remote participation of the 0-3 axial p-methoxybenzoyl group of glycosyl donor 134 the glycosylation of sordaricin ethyl ester 135 with 134 provided glycoside 136 (p/a = 6.5 1) with an excess of the P-anomer (Scheme 22) [75]. 

Nifantiev and co-workers reported that the glucuronylation of 121 with the donor 137 bearing the acetyl group at 0-3 produced only a-linked disaccharide 138 (Scheme 23), and assumed that the a-stereoselectivity of the donor 137 resulted from the formation of a stabiUzed glycosyl cation intermediate generated by the remote participation of the acetyl group at 0-3 (Fig. 11) [76]. 

Fig. 9 The 1,4-remote participation in the fucosylation with donor 115 bearing 4 q ybenzoyl group p-me ox...

The high p-selectivity in the reaction between 140 and 141 was attributed to the reaction of the silylated nucleoside with the iminium ion intermediate, which resulted from the remote participation of the diethylthiocarbamoyl group at the 0-3 position of 140 (Scheme 25) [77]. 

The 3-0-diethylthiocarbamoyl libofuranosyl donor 140 was also utilized for the p-stereoselective C-glycosylation by Mukaiyama and co-workers the reaction of 140 and carbon nucleophile 144 produced the coupling product 145 (p/a = 96 4) with an excellent p-selectivity, which presumably resulted from the remote participation of the thiocarbamoyl group at 0-3 of 140 (Scheme 26) [78]. 

Fig. 11 Proposed stabilized dioxonium ion intermediate resulting from the remote participation of the 3-0-acetyl group of 137...

Unlike the neighboring group participation by 0-2 protecting groups, the existence of the remote participation by potentially participating groups at remote positions of glycosyl donors has been controversial. We reviewed reports that were opposed to and in favor of the remote participation. Nevertheless, results and evidence in favor of the remote participation are quite substantial, especially for... 

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