Glycosyl disarmed

Transfer of the Armed-Disarmed Concept to Thioglycoside Glycosylation... 

Disarmed glycosyl trichloro- and /V-(phenyI)trifluoroacetimidates were efficiently activated by the system I2/Et3SiH for the rapid and high-yield glycosylation of primary and secondary saccharide acceptors, and it was observed that the presence of AW 4 A MS was decisive for achieving high reaction yields.62... 

M. Adinolfi, B. Gaspare, A. Iadonisi, and M. Schiattarella, Iodine/triethylsilane as a convenient promoter system for the activation of disarmed glycosyl tri-chloro- and /V-(phenyl)trifluoroacetimidates, Synlett, 2 (2002) 269-270. 

B. Mukhopadhyay, B. Collet, and R. A. Field, Glycosylation reactions with disarmed thioglycoside donors promoted by /V-iodosuccinimide and HC104-silica, Tetrahedron Lett., 46 (2005) 5923-5925. 

Per-O-acylated glycosyl iodides are stable at room temperature and can be purified on a silica gel column and stored at 0 °C. Stachulski and coworkers [202] synthesized methyl 2,3,4-tri-O-pivaloyl-glucopyranuroate iodide, which is a stable solid at 20 °C and can be stored for months at room temperature or for more than a year at 0 °C. The X-ray crystal structure of this compound, the first one of this class, shows a typical chair structure. Importantly, such a disarmed and stable iodide can be coupled with primary and secondary steroidal alcohols using I2 as a promoter, as demonstrated by the synthesis of morphine-6-glucuronide, an analgesic [202], The glycosyl donor ability... 

Although glycosyl triflates have been demonstrated to be intermediates with a number of armed donors, and even with disarmed donors not capable of neighboring-group participation, such as the sulfonate esters, typical disarmed donors with esters in the 2-position function in the anticipated manner through anchimeric... 

Diol 159 was chosen as a model for probing regioselective glycosylation with armed and disarmed thioglycosides 160 and 163, and NPOE 108. The results obtained were not anticipated (Scheme 5.35b-d). 

Remarkably, armed glycosyl donor 160 preferred the secondary-OH of 159 (Scheme 5.37b). Disarmed donor 163 and NPO E108 both preferred the primary-0 H, but the NPOE furnished a single regioisomer 176 (Scheme 5.35c and d). 

The electron-withdrawing capability of the disarming substituent at the 6-position also controls the stereochemical outcome of the Crich glycosylation method. Thus, 6-mono- and difluoro substituted 5-phenyl 2,3,4-tri-0-benzyl-D-mannopyranosides 27 and 28 and the trifluoro analogue of 5-phenyl... 

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