One-pot double glycosylation with the conformationally armed thioglycoside 37. This is analogous to the effect of the very similar benzylidene group, which is deactivating the donor partially due to locking the structure in an unreactive conformation and due to the electronic effect of a trans-gauche conformation of the hydroxymethyl group [22,39]. Mydock, L. K.; Kamat, M. N.; Demchenko, A. V. Kamkhachorn, T.; Parameswar, A. R.; Demchenko, A. V. Premathilake, H. D.; Mydock, L. K.; Demchenko, A. V. Heuckendorff, M.; Premathilake, H. D.; Pornsuriyasak, P.; Madsen, A. Ø.; Pedersen, C. M.; Bols, M.; Demchenko, A. V. Wang, S.; Meng, X.; Huang, W.; Yang, J.-S. Heuckendorff, M.; Pedersen, C. M.; Bols, M. Pedersen, C. M.; Nordstrøm, L. U.; Bols, M. Jensen, H. H.; Nordstrøm, L. U.; Bols, M. Pedersen, C. M. Super Armed Glycosyl Donors – The best aid for a better reaction rate & Radical Azidonation – Novel safe methodologies. This group consists of three methyl groups bonded to a silicon atom [−Si(CH3)3], which is in turn bonded to the rest of a molecule. Trimethylsilyl acetylenes can be selectively deprotected in the presence of a catalytic amount of silver salts. This explains the comparatively large rate enhancements observed by TBS and TIPS groups compared to unprotected OH and also that TIPS, which is more bulky than TBS, but essentially has the same inductive effect, causes a greater rate enhancement. On the other hand, when the conformation is restricted by cyclic silyl protective groups (i.e., DTBS and TIPDS) and equatorial rich, comparatively unreactive donors result. Particularly in oligosaccharide synthesis where many orthogonal hydroxy protective groups are required silicon protective groups have frequently been introduced in both glycosyl donors and acceptors. TES-protected glycosylimidates were also employed in the synthesis of antitumor saponins which contained partially acylated oligosaccharides. α-Selective glycosylation with a TIPDS-protected glucal donor. In line with this, the analogue of 46 having a TIPDS group rather than a DTBS was not particularly unreactive, as it is more flexible due to the bigger ring. AgNO 3 and AgOTf proved to be the most effective catalyst in a mixture of methanol, water and dichloromethane. Register for the Beilstein Open Science Symposium 2020 - it's free and online! A.; Wink, D. J. Zhu, X.; Kawatkar, S.; Rao, Y.; Boons, G.-J. Here the selectivity is very much related to the conformational restriction and face-discrimination imposed by the cyclic silyl group upon the system. Conformational change induced by bulky vicinal protective groups such as TBS, TIPS and TBDPS. Glycosylation with thioglycoside 7. The exchange of the 2-O-benzyl with a 2-O-TIPS leads to some erosion of stereoselectivity though the donor was still highly β-selective . Scheme 9: A somewhat similar influence has been observed with the much less steric demanding 4,6-O-benzylidene protective group . The most popular and commercially available silyl-protective groups are trimethylsilyl (TMS), triethylsilyl (TES), tert -butyldimethylsilyl (TBS), tert -butyldiphenylsilyl (TBDPS), triisopropylsilyl (TIPS) as well as the diol-protective groups DTBS and TIPDS (Figure 1). The Yang group has also investigated superarmed galactothiofuranosides . This approach works for glucosides, mannosides, and galactosides and both, α- and β-thioglycosides . This was for example used for the synthesis of the serine protease inhibitor banyaside. Gervay–Hague has reported that TMS-protected glycosyl iodides are remarkably more reactive than their benzyl-protected analogues [13,27].
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