6-O-Picolinyl and 6-O-Picoloyl Building Blocks As Glycosyl Donors with Switchable Stereoselectivity

Abstract

Remote 6-O-picolinyl or 6-O-picoloyl substituents often provide high β-selectivity due to H-bond-mediated aglycone delivery (HAD). Herein it has been demonstrated that if the nitrogen atom of the 6-O-picolinyl or picoloyl moiety is temporarily blocked by coordination to a metal center (Pd), it cannot engage in HAD-mediated β-glycosylation. Hence, the stereoselectivity of 6-O-picolinyl/picoloyl-assisted glycosylations can be "switched" to α-selectivity.

Discrimination of chiral copper(ii) complexes upon binding of galactonoamidine ligands

“…[12][13][14][15] Further studies reveal formation of carbohydrate platinum(IV) complexes with thioglycoside ligands and their use to switch the selectivity of carbohydrate building blocks. 16,17 More details and additional reports including complex formation between carbohydrates and transition metal or lanthanide ions may be found elsewhere. 11,[18][19][20][21][22][23][24][25][26][27][28][29][30] In this context, we disclose here for the first time a study to evaluate the coordination between a putative transition state analog of the glycoside hydrolysis and metal complexes.…”

Unusually Stable Picoloyl‐Protected Trimethylsilyl Glycosides for Nonsymmetrical 1,1′‐Glycosylation and Synthesis of 1,1′‐Disaccharides with Diverse Configurations

“…Although the Pico-protecting group has been regarded as as tereodirecting group in other glycosylation studies, [32,33,42] its role is clearly different in the present case. To clarify the role of the Pico group, we conducted as tructure-selectivity relationship study.F or this purpose, various TMS glucosides with 1) a nicoloyl (Nico; 15), iso-nicoloyl (iNico; 16), or Bz (8)p rotecting group at C4;2 )a b-TMS acetal at C1 (i.e., 7' and 17), and 3) with aP ico-protecting group at C3 (i.e., 17)w ere prepared for glycosylation with the donor 10.P reparation of the TMS a-/b-glycosides followed the protocols shown in Scheme 2.…”

“…Unlike the participated thioglycoside donors with a d-gluco scaffold (i.e., [42][43][44], the glycosylation of Pico-protected a-glucosidea cceptor 7 with 2-N-phthalamide (Phth)-2-deoxy-thiogalactosyl donor 45 at 0 8Cp roduced a2 :1 mixture of 1b!1'aand 1a!1'a-diastereomers 50.T he result might be due to competition of the dioxalenium ion [48] and oxacarbenium ion [49] for coupling with the TMS a-acceptor 7,although another possibility is the postglycosylation anomerization of the 1b!1'ato 1a!1'a-anomer.F or clarification, the pure 1b!1'a-anomer of 50 in CDCl 3 was treatedw ith 0.5 equiv of TMSOTf at 0 8Cf or 24 ht osimulate the glycosylationc onditions. From the 1 HNMR spectroscopy, no sight of anomerization of the 1b!…”

Intramolecular glycosylation

“…The latter approach was extended to the synthesis of a β-manno-trisaccharide, wherein complete β-manno selectivity was obtained at room temperature [ 117 ]. A useful extension of this method to glycosyl donors with switchable selectivity has also been disclosed by the Demchenko group [ 118 – 119 ].…”