Relative reactivities RRVs

Competition Experiments of Ley Ley examined product distributions to obtain relative reactivity values (RRVs) for both rhamnose and mannose sugars with benzyl, 3,4-cyclic diacetals, and benzoyl groups. Using a limiting amount of acceptor (1 equiv), two donors were added in excess (2 equiv each), followed by the addition of activator (NIS/TfOH, 2 equiv) (Scheme 11.13). After quenching the reaction, the product distribution was analyzed by -NMR. 

NMR is predictive of reactivity within a series Within a set of common donors (i.e., galactose) in which the C2 position is constant, the H chemical shift of the anomeric carbon appears to be a good predictor of relative reactivity. Little correlation is found when the C2 position is varied, or between different donor pyranoses. The extent to which this correlation will prove useful remains in question. We envision that its primary applicability may be in trouble shooting a failed synthesis when a complex donor does not behave as predicted. Checking the H NMR of the advanced intermediate may quickly reveal that the RRV value for the pyranose of interest is not as expected, and hence the reactivity is not as expected. 

Unlike the other N5 modifications described previously, a 5-azido thioglycoside was found to be less reactive than the simple amide as is clear from the measured relative reactivity values (RRVs) (Fig. 5.4) [59], However, it was subsequently reported by Teruaki Mukaiyama and coworkers that a 5-azido sialyl fluoride was more reactive than both the corresponding acetamide and the V,V-diacctyl derivatives toward activation with the combination of SnCl4 and AgB(C6F5)4, although this may be a consequence of the differing anomeric stereochemistries employed [60],... 

SCHEME 12.12 Wong s synthesis of Globo-H. CIBn, o-chlorobenzyl NBz, p-nitroben-zoyl PMP, p-methoxyphenyl RRV, relative reactivity value. 

SCHEME 14.2 Synthesis of lipid IV by Wong, Cheng, and coworkers. ClAc, chloroacetyl NIS, /V-iodosucci n i mide Phth, phthaloyl PyBOP, benzotriazol-l-yl-oxytripyrrolidinophos-phonium hexafluorophosphate RRV, relative reactivity value TBAF, tetrabutylammonium fluoride TBS, fert-butyldimethylsilyl Tf, trillyl TMS, trimethylsilyl Tol, 4-tolyl. 

Relative Reactivity Values (RRVs). which are the experimentally determined values relating the reactivities of the BB s to the stamdard sugar... 

Figure 5. Wong s One-Pot synthesis of Globo H. Numbers in parentheses are relative reactivity values (RRVs).

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