Is an acyl group at O-3 in glucosyl donors able to control α-stereoselectivity of glycosylation? The role of conformational mobility and the protecting group at O-6
作者:Bozhena S. Komarova、Maria V. Orekhova、Yury E. Tsvetkov、Nikolay E. Nifantiev
DOI:10.1016/j.carres.2013.11.016
日期:2014.1
The stereodirecting effect of a 3-O-acetyl protecting group, which is potentially capable of the remote anchimeric participation, and other protecting groups in 2-O-benzyl glucosyl donors with flexible and rigid conformations has been investigated. To this aim, an array of N-phenyltrifluoroacetimidoyl and sulfoxide donors bearing either 3-O-acetyl or 3-O-benzyl groups in combination with 4,6-di-O-benzyl, 6-O-acyl-4-O-benzyl, or 4,6-O-benzylidene protecting groups was prepared. The conformationally flexible 3-O-acetylated glucosyl donor protected at other positions with O-benzyl groups demonstrated very low or no alpha-stereoselectivity upon glycosylation of primary or secondary acceptors. On the contrary, 3,6-di-O-acylated glucosyl donors proved to be highly alpha-stereoselective as well as the donor having a single potentially participating acetyl group at O-6. The 3,6-di-O-acylated donor was shown to be the best alpha-glucosylating block for the primary acceptor, whereas the best alpha-selectivity of glycosylation of the secondary acceptor was achieved with the 6-O-acylated donor. Glycosylation of the secondary acceptor with the conformationally constrained 3-O-acetyl-4,6-O-benzylidene-protected donor displayed under standard conditions (-35 degrees C) even lower alpha-selectivity as compared to the 3-O-benzyl analogue. However, increasing the reaction temperature essentially raised the alpha-stereoselectivities of glycosylation with both 3-O-acetyl and 3-O-benzyl donors and made them almost equal. The stereodirecting effects of protecting groups observed for N-phenyltrifluoroacetimidoyl donors were also generally proven for sulfoxide donors. (C) 2013 Elsevier Ltd. All rights reserved.