Benzyl derivatives Carbonyl, determination

Despite its high selectivity in the oxidation of hexoses, GOase displays a rather broad substrate range for nearly all primary alcohols. In Table I the relative activities of alcohol substrates are given. As shown in the table, a-carbonyl alcohols as a class are the best substrates. Unsaturated or aromatic substrates constitute another reasonable good group of substrates. The inherent chemical reactivity of the alcohol is not a determinant in itself since substrates with comparable electronic properties differ markedly as substrates. For pyridine carbinols, the m- and p-derivatives are more reactive than the o-derivative. Remarkably m-substituted benzyl alcohols display larger reactivities than p-substituted benzyl alcohols. 

The sites of deprotonation of a series of A -benzyl lactams have been determined. For five- and six-membered lactams, kinetic deprotonation occurred exclusively a to the carbonyl, while seven-and eight-membered lactams gave exclusively the products arising from deprotonation at the benzylic position. The alkylation of the anion derived from (121), however, gave an approximately 3 1 ratio of (122) to (123) (R = Bu", allyl and Bn) (Equation (5)) <87JA4405>. By contrast, the deprotonation of A -(BOC)caprylolactam with lithium hexamethyldisilazide in THF at — 78°C gave the expected enolate, which could be alkylated with iodomethane (81% yield) or phenylselenyl chloride (65% yield) <90SL63>. 

In contrast to the diastereoselectivity of the reaction with benzyloxyacetaldehyde derivatives, the Sc(III)-(5, 5)-PyBOX-catalyzed cycloadditions of 2-benzopyrylium-4-olate with methyl and benzyl pyruvate showed high exoselectivity (Scheme 7.26 and Table 7.20). This is probably attributed to the unfavorable dipolar interactions between the carbonyl groups of 2-benzopyrylium-4-olate and the ester in the eniio-approach. However, the maximum enantiomeric excess of the exo-adduct was only 56% ee when (S,S)-PyBOX-TPSm was used as a ligand (Figure 7.3 and Table 7.20, entry 3). After several attempts to increase the enantioselectivity, both diastereo- (up to exo endo = 96 4) and enantioselectivities (up to 87% ee (exo)) were determined to improve in the Sc(III)-(5,5)-PyBOX-i-Pr-catalyzed reaction (up to 94% yield) when pyruvic acid was used as an additive (entries 5, 6, 8, and 9). By the examinations of some... 

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