Stannous chloride as a low toxicity and extremely cheap catalyst for regio-/site-selective acylation with unusually broad substrate scope

Abstract

This work reports stannous chloride (SnCl2)-catalyzed regio/site-selective acylation with unusually broad substrate scope. In addition to 1,2- and 1,3-diols and glycosides containing cis-vicinal diol, the substrate scope also includes glycosides...

SnCl2‐Catalyzed Acetalation/Selective Benzoylation Sequence for the Synthesis of Orthogonally Protected Glycosyl Acceptors

“…It is known that 4,6-O-arylidene β-galactosides and αglucosides are suitable substrates for SnCl 2 -catalyzed selective acylation. [7] Therefore, with the optimal conditions in hand, the precursors of 4,6-O-benzylidene glycosides 3-6, 10 and 16 as substrates were further evaluated in this one-pot reaction (Figure 3). As expected, 3-O-benzoyl, 4,6-O-benzylidene βgalactosides 3 a-6 a and 2-O-benzoyl, 4,6-O-benzylidene αglucoside 16 b were obtained efficiently in 76-86 % yields in two steps when DIPEA was used as the base.…”

“…It is known that 4,6‐ O ‐arylidene β ‐galactosides and α ‐glucosides are suitable substrates for SnCl 2 ‐catalyzed selective acylation [7] . Therefore, with the optimal conditions in hand, the precursors of 4,6‐ O ‐benzylidene glycosides 3 – 6 , 10 and 16 as substrates were further evaluated in this one‐pot reaction (Figure 3).…”

“…In our previous report, we had speculated that the complex of SnCl 2 with DIPEA (SnCl 2 /DIPEA: 1/2) act as the "real catalyst" for the SnCl 2 -catalyzed regioselective benzoylation. [7] The 4,6-benzylidine glycosides 4, 5, 10 and 16 as substrates were further evaluated with Ag 2 O as the base (Figure 2). Under the optimal conditions, the benzoylation of thioglycosides 4 and 10 also led to the 2-OBz products 4 b (60 %) and 10 b (79 %).…”

“…Surprisingly it was found that the Ag 2 O-mediated benzoylation of thiogalactosides led to the reverse of the acylation pattern observed in the DIPEA-mediated benzoylation. [7] Finally, a one-pot strategy was developed by combining SnCl 2 catalyzed acetalation and benzoylation (Scheme 1c). 2-OBz or 3-OBz 4,6-O-benzylidene glycosides were efficiently prepared in moderate to high yields starting from free glycosides.…”

“…[6] Just recently, we reported a regioselective acylation method using SnCl 2 as the catalyst, where SnCl 2 even could catalyze the selective acylation of 4,6-O-benzylidene glycoside trans-diols in the presence of DIPEA. [7] It has been reported that Lewis acids, such as ZnCl 2 , [8] FeCl 3 , [3b] Cu(OTf) 2 , [5a,b] and Mg(OTf) 2 , [9] could catalyze the formation of glycoside benzylidene acetals. We therefore speculated that SnCl 2 can also catalyze acetalation, and then started to conceive a one-pot strategy based on SnCl 2 catalyzed acetalation and benzoylation to obtain orthogonally protected glycoside acceptors with 2-or 3-OH.…”