Wiesner reaction

The Wiesner Reaction. The reaction of lignified tissue and phloroglucinol—hydrochloric acid gives a visible absorption spectmm with a maximum at 550 nm. This has been attributed to coniferaldehyde units in lignin as the groups responsible for the color formation. 

Following Adler s identification of comferaldehyde units as the structural type giving rise to color in the Wiesner reaction, many other color reactions were re-examined and found attributable to the same structural unit These include the reaction of ligmfied tissue with methanol-hydrochloric acid (Brauns... 

Davidson et al. [122] reported the difference spectra of spruce wood sections and groundwood pulp that had been irradiated and then stained with phlo-roglucinol. The irradiated and stained samples showed an absorption increase at = 550 nm, which the authors attributed to photochemical formation of coniferaldehyde, based on the well-known Wiesner reaction. However, in a subsequent pnblication [123], they found that phloroglucinol gives a similar color reaction with methoxy-p-quinone, which would be a more reasonable explanation of the earlier resnlts. 

Thermal and photochemical electrocyclic reactions are particularly useful in the synthesis of alkaloids (W. Oppolzer, 1973,1978 B K. Wiesner, 1968). A high degree of regio- and stereoselectivity can be reached, if cyclic olefin or enamine components are used in ene reactions or photochemical [2 + 2]cycloadditions. 

Pomar, F., Merino, F., and Ros Barcelo, A., 2002, CMinked coniferyl and sinapyl aldehydes in lignifying cell walls are the main targets of the Wiesner (phloroglucinol-HCl) reaction, Protoplasma 220 17-28. 

Wiesner s expression used different symbols, but this is not important.) This expression strictly holds only for a first-order reaction and Vetter [559] provides a more general expression. However, the above expression is sufficient for most simulation purposes. The equation for fi holds in practice only for rather large values of the rate constant for small values below unity, fi becomes greater than the diffusion layer thickness, which will then dominate the concentration profile. At the other end of the scale of rate constants, for very fast reactions, // can become very small. The largest rate constant possible is about 1C)1 ° s 1 (the diffusion limit) and this leads to a fx value only about 10 5 the thickness of the diffusion layer, so there must be some sample points very close to the electrode. This problem has been overcome only in recent years, first by using unequal intervals, then by the use of dynamic grids, both of which are discussed in Chap. 7. 

A sampling of the colors produced in the reaction of various phenols and amines with lignin is presented in Table 2.1.1. Phenolic compounds are applied as acidic aqueous solutions and aromatic amines as aqueous solutions of the corresponding hydrochloride or sulfate in the presence of acids. Of the phenols, phloroglucinol is the most important, and its solution in hydrochloric acid is known as the Wiesner reagent. 

Chaksine (91) was isolated in 1935 by Siddiqui and Ahmad (143) from the seeds of Cassia absus L. (Caesalpiniaceae), a plant used in folk medicine. On the basis of degradation reaction products, Wiesner et al. proposed structure 91 (144), but Singh et al. arrived at a different structure (92) that would satisfy the experimental data (145). Final proof of structure 91 was obtained... 

In addition to this incident, man no doubt was aware of the environment s degradative effect on wood since he first began using such materials. However, it was not until 1827 that the chemical phenomenon of wood weathering was reported by Berzelius (4), followed by Wiesner (5) in 1846, and Schramm (6) in 1906. However, systematic studies on weathering reactions in wood did not begin until the 1950s (J). 

The first example in which a kinetic current was observed (Wiesner, 1943) involved a reaction in parallel with the electrode process of the type given in scheme (37). A rigorous treatment is available (Koutecky, 1953) and has been applied to several inorganic systems. However, examples of straightforward organic systems are lacking. 

Fascinating rearrangements of aziridines have been applied to the synthesis of diterpene alkaloids by Wiesner and co-workers (Scheme 40). For example, the ester 217, prepared from cyclopentadiene carboxylate (215) and the benzyne precursor 216 by a Diels-Alder reaction, was converted to the aziridine 218 by treatment with benzenesulfonyl azide in 83% yield. When the aziridine 218 was heated with water for 24 h, the hydroxy ester 219 was obtained in 97% yield subsequent oxidation with the Jones reagent afforded the ketone 220. This rearrangement is analogous to that of the benzenesulfonylaziridine of norbomene. ... 

Wiesner discovered that the photoreaction of an allene to an a/J-unsaturated ketone always shows the regioselectivity illustrated in the example (487 — 488) above. The general reaction shows another interesting feature in polycyclic a/l-unsaturated ketones, the allene often adds highly selectively to one face of the double bond.402 In the case of the ketone (489), the product (490) appeared from models to be more crowded than the alternative (491) also, the lower face of the double bond in 489 appeared to be the more hindered face. On both counts, the alternative product (491) ought to have been formed and was not. 

Three syntheses of this compound have been achieved by the Wiesner group. In two of them an allene addition to a vinylogous amide was a key step while in the third and most direct synthesis a Michael addition to the vinylogous amide system was used in one of the ring-forming reactions. 

Recently Wiesner and Santroch 97) have described an elegant synthesis of compound CLXXXIX, possessing a structure closely related to CXC so that feasible modification of the functionality of both products should lead to a common intermediate. The latter compound, along with CLVIII, is formed from the aromatization reaction sequence previously described when carried out on pyrooxonitine (CXXXIV) 82). Such a... 

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