Applications of Photosensitive Protecting Groups in Carbohydrate Chemistry. Advances in Carbohydrate Chemistry and Biochemistry Volume 46. Elsevier; 1988. pp. 179-204. [DOI: 10.1016/s0065-2318(08)60166-3]

Photochemistry of 2-nitrobenzylidene acetals

Photolysis of dihydroxy compounds (diols) protected as 2-nitrobenzylidene acetals (ONBA) and subsequent acid- or base-catalyzed hydrolysis of the 2-nitrosobenzoic acid ester intermediates result in an efficient and high-yielding release of the substrates. We investigated the scope and limitations of ONBA photochemistry and expanded upon earlier described two-step procedures to show that the protected diols of many structural varieties can also be liberated in a one-pot procedure. In view of the fact that the acetals of nonsymmetrically substituted diols are converted into one of the corresponding 2-nitrosobenzoic acid ester isomers with moderate to high regioselectivity, the mechanism of their formation was studied using various experimental techniques. The experimental data were found to be in agreement with DFT-based quantum chemical calculations that showed the preferential cleavage occurs on the acetal C-O bond in the vicinity of more electron-withdrawing (or less electron-donating) groups. The study also revealed considerable complexity in the cleavage mechanism and that the structural variations in the substrate can significantly alter the reaction pathway. This deprotection strategy was found to be also applicable for 2-thioethanol when released from the corresponding monothioacetal in the presence of a reducing agent, such as ascorbic acid.

Chemoenzymic synthesis of potentially caged glycosphingolipids [correction of glycoshingolipids] (GSLs): potentially caged lyso-G(M3) and its analogue

In a previous work, a number of potentially caged sphingolipids and glycosphingolipids were chemically synthesized (Zehavi, 1997. Chem. Phys. Lipids 90, 55-61). The effects of GM3 and to a lesser extent, of lyso-GM3, are being studied. Considering that biologically inert, caged lyso-GM3 could be photolysed in the cell to release lyso-GM3, thus creating an attractive opportunity to study the subsequent sequence of events in the cell, the chemoenzymic synthesis of the potentially caged lyso-GM3, (5-acetamido-3,5-dideoxy-D-glycero-alpha-D-galacto-2-nonulopyranosylo nic acid)-(2-3)-beta-D-galactopyranosyl-(1-4)-beta-D-glucopyranosyloxy (1-1)- (2S,3R,4E)-2-(4-carboxymethyl-2-nitrobenzyloxycarbonyl-amino)-3-hy droxy-4- octadecene and of a potentially caged GM3 analogue, (5-acetamido-3,5-dideoxy-D- glycero-alpha-D-galacto-2-nonulopyranosylonic acid)-(2-3)-beta-D-galactopyranosyl-(1-4)-beta-D-glucopyranosyloxy -(1-3)- (2S, 3R, 4E)-2-(4-carboxymethyl-2-nitro-benzyloxycarbonylamino)-1- hydroxy-4-octadecene was undertaken. Both compounds, being 2-nitrobenzyloxycarbonyl derivatives, are light-sensitive and could be efficiently photolysed to the biologically active, corresponding lyso-GSLs.

Synthesis of potentially caged sphingolipids, possible precursors of cellular modulators and second messengers

An increasing number of sphingolipids, glycosphingolipids and some of their degradation products have been recognized in recent years as second messengers involved in signal transduction and as modulators of numerous cellular functions. These can be converted into inert, caged compounds, introduced into cells and tissues and subsequently photolysed to active compounds thus enabling the study of fast biological processes. The novel, potentially caged compounds synthesized here are substituted 2-nitrobenzyl urethans and 2-nitrobenzyl amines derived from sphingosine, dihydrosphingosine, N-methylsphingosine, N-methyldihydrosphingosine, psychosine and glucosylsphingosine. Upon irradiation of the afore mentioned compounds they release, or are expected to release, the free biologically active amines.

Inhibition of Escherichia coli beta-galactosidase by 2-nitro-1-(4,5-dimethoxy-2-nitrophenyl) ethyl, a photoreversible thiol label

1-Nitro-2-phenylethene (beta-nitrostyrene, 1) which is a thiol-protecting reagent (Jung, G., Fouad, H. and Heusel, G. (1975) Angew. Chem. Int. Ed. Engl. 14, 817-818), was demonstrated in this work to be an irreversible inhibitor of beta-galactosidase (EC 3.2.1.23), an enzyme known to be inhibited by some thiol reagents or through modifying a methionine residue at the active site. No reversal of the inhibition was observed upon subsequent incubation with mercaptoethanol or irradiation (350 nm). 1-(4,5-dimethoxy-2-nitrophenyl)-2-Nitroethene 2) was also shown to be an irreversible inhibitor (94% inhibition, pH 8.3) of the enzyme. Kcat values of beta-galactosidase at pH 8.3 with o-nitrophenyl beta-D-galactopyranoside (ONPG) as the substrate and at the highest inhibitor concentrations employed for compound 1 (4.06 x 10(-4) M) ranged from 1.67 x 10(4) S-1 after 30 min of preincubation to <0.07 x 10(4) S-1 after 180 min preincubation. For compound 2 (9.5 x 10(-5) M) Kcat values ranged from 2.70 x 10(4) S-1 following 30 min preincubation to 1.15 x 10(4) S-1 after 180 min of preincubation; the changes in Km(app), however, were small. The activity was not recovered following incubation with mercaptoethanol. Since compound 2 and the inhibited enzyme are 2-nitrobenzyl derivatives, they are expected to be photosensitive and indeed, irradiation of the inhibited enzyme in the presence of mercaptoethanol resulted in recovery (89%, pH 8.3) of the enzyme activity.