N-pentenyl orthoesters

Although the first published n-pentenyl orthoester (NPOE) 89 appeared shortly after (Scheme 5.22b) [72], the initial link between NPGs and 1,2-orthoesters could be... 

General Procedure for Glycosidation with n-Pentenyl Orthoesters... 

C. Roberts, R. Madsen, and B. Fraser-Reid, Studies related to synthesis of glycophosphatidyl inositol membrane bound protein anchors Part V. n-Pentenyl orthoesters for mannan components, J. Am. Chem. Soc. 777 1546 (1995). 

Jayaprakash, KN, Radhakrishnan, K V, Fraser-Reid, B, Ytterbium(III) trifluoromethanesulfonate for specific activation of n-pentenyl orthoesters in the presence of acid sensitive functionahties. Tetrahedron Lett., 43, 6953-6955, 2002. 

Mach, M, Schlueter, U, Mathew, F, Fraser-Reid, B, Hazen, K C, Comparing n-pentenyl orthoesters and n-pentenyl glycosides as alternative glycosyl donors. Tetrahedron, 58, 7345-7354, 2002. 

K. N. Jayaprakash and B. Fraser-Reid, A study of n-pentenyl orthoesters having manno, gluco, and galacto configurations in regioselective glycosylations, Carbohydr. Res., 342 (2007) 490-498. 

B. Fraser-Reid, 1. Lu, K. N. layaprakash, and I. C. Lopez, Synthesis of a 28-mer oligosaccharide core of mycobacterial lipoarabinomannan (LAM) requires only two n-pentenyl orthoester progenitors, Tetrahedron Asymmetry, 17 (2006) 2449-2463. 

Scheme 11 n-Pentenyl orthoesters (NPOEs) and n-pentenyl glycosides (NPGs)... 

The Fraser-Reid group has disclosed the synthesis of the Plasmodium falciparum GPI (30, Scheme 12.4) based on the use of n-pentenyl orthoester building blocks [30, 31, 64], In addition to the conserved pseudopentasaccharide, the target GPI features a myristoyl ester at the C2 position of the inositol, diacylglycerol phosphate at the Cl of inositol, and the mandatory phosphoethanolamine at Manlll. 

The synthesis commenced with the assembly of the glycan. Glycosylation of myo-inositol 16 with n-pentenyl orthoester 17 using ytterbium(III) triflate and... 

For the C2-acylated galactoside acceptor, a simple route begins with galactosyl bromide 41 (Scheme 8a). Orthoester formation required the use excess pentenyl alcohol and tetrabutylammonium iodide as promoter to obtain 42a and thence 42b. Acid-catalyzed reaction of the latter in the presence of excess pentenyl alcohol, then benzylidination gave acceptor 43a. The xylosyl acceptor 46b (Scheme 8b) was obtained similarly from bromide 44 via the n-pentenyl orthoester 45. Diol 46a was subjected to selective silylation at C4, paving the way to 46b. 

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