Quinonoid intermediate

Direct aromatization of the quinonoid intermediates is a photochemically allowed but thermally forbidden rearrangement (Scheme 5.6). When phenylethyl radicals are generated photochemically at 20 °C there is evidence95 of a-o coupling by way of the aromatized product 7. The products derived from these pathways can be trapped in thermal reactions by radical98 or acid1 catalyzed... 

It is of interest to speculate on the precise structure of the macroinitiator species in these polymerizations. The work of Engel et a .94 suggests the likelihood of a quinonoid intermediate (e.g. 45, Scheme 9.13), at least for the polymerizations involving triphenylmethyl radical (44). 

Doubt has been expressed as to the validity of the above mechanism by the observation that in the bromination of 2-hydroxy-4,6-methoxyacetophenone, bromine enters the 3 position and replaces the acyl group at a rate which is increased by acetylation of the hydroxy group, which should not be the case if a quinonoid intermediate is formed, as required above734. However, since the hydroxy group becomes acetylated during the course of the reaction, thereby partly changing the medium to bromine in acetic acid, this result is ambiguous... 

Kametani and co-workers used a previously demonstrated ring expansion method to construct the isopavine skeleton (144,145). The method was successfully applied to the synthesis of ( )-reframidine (27) (Scheme 23) (145). Treatment of the 3-aryl-3,4-dihydroisoquinolinium iodide 115 with diazomethane furnished aziridinium iodide 116. On standing in 6 N hydrochloric acid, crude 116 underwent a one-step ring expansion-ring closure to afford ( )-reframidine in 20% yield. The same product could be obtained via benzazepine 118 depending on the reaction conditions. It has been postulated that the aziridinium iodide 116 may have formed a transitory quinonoid intermediate 117 which is attacked... 

Cyclization. A second kind of reaction is represented by the conversion of S-adenosylmethionine to aminocyclopropanecarboxylic acid, a precursor to the plant hormone ethylene (see Chapter 24).159 The quinonoid intermediate cyclizes with elimination of methylthioadenosine to give a Schiff base of the product (Eq. 14-27).160-161a The cyclization step appears to be a simple SN2-like reaction.162... 

The quinonoid intermediate formed by cleavage at a can react in various ways (1-4)... 

Figure 14-5 Some reactions of Schiff bases of pyridoxal phosphate, (a) Formation of the quinonoid intermediate, (b) elimination of a (3 substituent, and (c) transamination. The quinonoid-carbanionic intermediate can react in four ways (1—4) if enzyme specificity and substrate structure allow.

Consider the number of different steps that must occur in about one-thousanths of a second during the action of an aminotransferase. First, the substrate binds to form the "Michaelis complex." Then the transimination (Eq. 14-26) takes place in two steps and is followed by the removal of the a-hydrogen to form the quinonoid intermediate. An additional four steps are needed to form the ketimine, to hydrolyze it, and to release the oxoacid product to give the PMP form of the enzyme. The reaction sequences in some of the other enzymes are even more complex. How can one enzyme do all this ... 

Aspartate aminotransferase 57s, 135s, 753 absorption spectra 749 active site structure 744 atomic structure 750 catalytic intermediates, models 752 NMR spectra 149 quinonoid intermediate 750 Ramachandran plot 61 sequence 57 transamination 742 Aspartate ammonia-lyase 685 Aspartate carbamoyltransferase 348s active sites 348 regulation 540... 

The binding of a symmetric chromophore to a protein or nucleic acid often induces CD in that chromophore. For example, the bands of enzyme-bound pyridoxal and pyridoxamine phosphates shown in Fig. 14-9 are positively dichroic in CD, but the band of the quinonoid intermediate at 20,400 cm-1 (490 nm) displays negative CD. When "transimination" occurs to form a substrate Schiff base (Eq. 14-26), the CD is greatly diminished. While the coenzyme ring is known to change its orientation (Eq. 14-39 Fig. 14-10), it is not obvious how the change in environment is related to the change in CD. 

A corroboration of the key role of the intermediate cation 291 is the observation of a sharp yield increase of disproportionation products 297 and 298, compared to their direct formation from salt 280 on treatment of dimer 281 with catalytic amounts of triethylammonium perchlorate, which acts as the protonating agent generating intermediate 293. The formation of unsaturated dimers 296 is possible not only by the direct hydride transfer, followed by deprotonation of dimeric salt 294 (pathway a, Scheme 16), but also by equivalent 1,5-hydrogen transfer in the ortho-quinonoid intermediate 295 formed by deprotonation of the acidic methine group in intermediate 291 (pathway b). 

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