Acid-labile aldehyde

Furthermore, nonaqueous, especially nonprotic, solvents allow Pictet-Spengler condensation reactions to occur with acid>labile aldehydes e.g. when bearing additional acetal functions) and may significantly influence the stereoselectivity (see below). 

Inosine formed by either route is then phosphorolyzed to yield hypoxanthine. Although, as we have previously seen, much of the hypoxanthine and guanine produced in the mammalian body is converted to IMP and GMP by a phosphoribosyltransferase, about 10% is catabolized. Xanthine oxidase, an enzyme present in large amounts in liver and intestinal mucosa and in traces in other tissues, oxidizes hypoxanthine to xanthine, and xanthine to uric acid (see fig. 23.20). Xanthine oxidase contains FAD, molybdenum, iron, and acid-labile sulfur in the ratio 1 1 4 4, and in addition to forming hydrogen peroxide, it is also a strong producer of the superoxide anion 02, a very reactive species. The enzyme oxidizes a wide variety of purines, aldehydes, and pteridines. 

Notes. This test is very reliable. This test is compatible with acid-labile resins such as the Wang and chlorotrityl resins. While in the case of the Wang resin similar results were obtained, in the case of the chlorotrityl resin solutions also became colored, indicating cleavage of the aldehyde (BAL handle) from the resin. 

Ladlow and coworkers recently developed an acid-labile fluorous benz-aldehyde protecting group 43 to facilitate the parallel synthesis of sulfonamides 44 (Scheme 25) [56]. The Suzuki coupling reaction was conducted under microwave irradiation. All the intermediates and the final products were purified by F-SPE. 

While most NADH models are acid labile, quinones and aromatic aldehydes have been reduced by NMAH in the presence of perchloric acid (Fukuzumi et al., 1985a, 1986). 

Photocycloaddition reactions of aromatic aldehydes with cyclic ketene silylacetates have been investigated by Abe and coworkers [61]. Regio- and diastereoselective formation of the bicyclic 2-alkoxyoxetanes 69 was observed in high yields. Hydrolysis of these acid-labile cycloadducts with neutral water efficiently gave aldol-type adducts 70 with high threo-selectivity (Sch. 18). 

The selected example by Maclean et al. [40] reported a 240-member encoded pyrrolidine library, whose synthetic scheme is reported in Figure 9.12. Coupling of the resin bound acid labile linker FIMPB (4-hydroxymethyl-3-methoxyphenoxybutyric acid) with the first monomer set (four Fmoc amino acids, A) was followed by its coding (tags Ti-T3), and by coding of the second monomer set B (tags T4-T6) then the second monomer set (four aldehydes, B) was added and the reactive imines were immediately reacted with the third... 

Cleavage of acid-labile protective groups.1 The reaction of H202 (70%, FMC) and CI3CCOOH in CH2C12/(-butyl alcohol converts a dimethyl acetal (1) into a hydroperoxy methyl acetal (a), which can be isolated but which for convenience (and safety) is reduced to the aldehyde 2 in 80% overall yield. The same conditions can effect oxidative cleavage of tetrahydropyranyl and trityl ethers. 

Electron-rich alkenes, like the diene referred to in the first example, are very acid labile, so need a gentle catalyst. Aldehydes are the most usual dienophiles used in these syntheses. 

A recent innovation in in-situ microencapsulation is the development of acid-triggered release of pesticide from the microcapsules [12]. Diols and aldehydes are reacted to form an acid labile acetal moiety. The acetal is then reacted with isocyanate to create a prepolymer. The prepolymer is a polyisocyanate cmitaining the acid labile moiety and suitable for in-situ shellwall polymerization. The prepolymer is dissolved into a pesticide, emulsified into water, and shellwall formed in-situ. Under alkaline or neutral pH conditions in a container, the insecticide is safely contained in the microcapsules. Acid could be added to the spray tank to rapidly release capsule contents prior to application. Alternate shellwall chemistry for in-situ microencapsulation utilizes etherified urea-formaldehyde prepolymers in the oil phase that are self-condensed with acid catalyst to produce encapsulating aminoplast shellwalls [13]. This process does not have the problem of continuing CO2 evolution. Water-soluble urea-formaldehyde and melamine-formaldehyde prepolymers can be selected to microencapsulate water or aqueous solutions [14]. 

Pyridinium fluorochromate is a stable solid, which can be stored for long periods." h is as reactive as PCC, but slightly less acidic (pH of a 0.01 M solution = 2.4S compared to 1.7S for PCQ, and thus substrates with acid labile groups can be oxidized without the need to add a buffer." With 1.S equiv. of oxidant in dichloromethane at room temperature, primary and secondary alcohols are oxidized to aldehydes and ketones in high yield (Table 9). Unfortunately, ( )/(Z)-isomerization has been observed during the oxidation of allylic alcohols." ... 

Fleet and coworkers discovered that chromium(VI) oxide in 3 1 dichloromethane/diethylether, in the presence of celite, acts as an efficient oxidant for a range of alcohols, and is tolerant of a wide range of acid labile functionalities. In dichloromethane alone chromium(VI) oxide was reported to be inert due to low solubility. This oxidant is particularly effective for the preparation of ketones, but aldehydes, especially a,3-unsaturated, are found to be prone to overoxidation. The oxidizing species is unstable, and therefore it is better to add chromium(VI) oxide to a solution of the alcohol in 3 1 dichloiomethane/di-ethylether. N.b. Diethyl ether/chlorinated solvent mixtures have beat reported elsewhere to inflame spontaneously in the presence of chromium(VI). )... 

The phosphorus ylide may be obtained from a wide range of different halides, and thus ketones may be converted into a range of unsaturated compounds. A useful reaction with the methoxymethylene Wittig reagent leads, via the acid-labile enol ether, to an aldehyde (Scheme 3.48). 

Enynes which have the secondary or tertiary center at the propargylic position are isomerized to 2-alkenyl-methylenecyclopentane in moderate to high yield with a catalytic amount of [CpRu(CH3CN)3]PF6 in acetone or dimethylformamide (DMF) [25], This catalyst system is acidic, and an acid-labile group such as a dimethyl acetal 54 is hydrolyzed to aldehyde 55. 

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