Yield (Scheme 2). Scheme 2. 5-HT6 Receptor Agonist custom synthesis Deprotection of TMS and Bn GroupsFigure two. Preferred silyl
Yield (Scheme 2). Scheme two. Deprotection of TMS and Bn GroupsFigure two. Preferred silyl etheracetate exchange of Neu5Ac: C4 (2 C9 (1 C8 (2 C2 (anomeric).Neu5Ac ReSET revealed totally different regioselectivity than previous perform with pyranose sugars.16,17 In aldohexoses, the main C6 normally exchanges initial followed by the anomeric C1. Soon after C1 exchange, C2 is generally next to react then further exchange happens within a sequential manner about the pyranose ring. Witschi and co-workers also performed ReSET on N-acetyl glucosamine (GlcNAc), which can be an aldose sugar structurally comparable to Neu5Ac when it comes to bearing an NHAc group. In that case, the initial exchange also occurred in the key C6 instead of the anomeric position, which was proximal to the amide.16 The presence of NHAc in two presumably pulls electron density in the C4 O-Si bond, which enables for exchange to take place initially at C4 in favor of the key C9 position. In addition, the presence of methylene protons at C3 assures a significantly less sterically hindered atmosphere than what exactly is located in typical pyranose sugars. As soon as C9 is acetylated, C8 may be the next to react. Again, the electronic effect on the C9 ester group tends to make the C8 O-Si bond most susceptible to attack. The observation of C8 exchange in favor with the anomeric silyl ether group indicates that the quaternaryIn pursuit on the synthesis of Neu4,5,7,8,9(Ac)5 (15), compound four was selectively deprotected to expose the C7 and C8 diol (11, Scheme three). The anomeric silyl defending group remained in tact presumably due to steric hindrance. Subjecting 11 to 1.five equiv acetic anhydride gave selective acetylation of C7 (12), even though excess acetic anhydride gave 13 (Scheme three). Upon hydrogenolysis of 12, acyl migration in the 7-O-acetyl towards the C8 position occurred affording compound 9. Attempts to prevent migration utilizing numerous catalysts like palladium (98 ), palladium hydroxide, platinum(IV) oxide, and Raney nickel have been unsuccessful. C7 to C8 acyl migration occurred under all situations, suggesting the C-8 acetate is a thermodynamic sink. Meanwhile, 13 was subjected to hydrogenation to get rid of the anomeric silyl and benzyl groups to afford naturally occurring 15 in 92 yield. This route allowed for an option synthesis of 15, which had been previously synthesized.dx.doi.org10.1021ol502389g | Org. Lett. 2014, 16, 5044-Organic Letters Scheme 3. Option Synthetic Route to Neu4,five,7,eight,9(Ac)LetterAUTHOR INFORMATIONCorresponding Author(530) 754-6915. Tel: (530) 754-9557. E-mail: jgervayhagueucdavis.edu.NotesThe authors declare no competing economic interest.ACKNOWLEDGMENTS This perform is supported by the National Institutes of Health, NIH Grant No. R01GM090262. NSF CRIF system (CHE 9808183), NSF Grant No. OSTI 97-24412, and NIH Grant No. RR11973 offered funding for the NMR spectrometers used on this project. We thank Dr. Jerry Dallas (University of California, Davis) for help with the long-range HMBC NMR experiments and 2D NMR experiments.
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