Dihydro model reaction

Dihydro Addition - First-order Model Reaction... 

Among ketoesters, tremendous efforts have been devoted to the hydrogenation of dihydro-4,4-dimethyl-2,3-furandione (KPL), not only as a model reaction but also because the product R(-)-pantolactone is a key intermediate in the synthesis of vitamin B5 and coenzyme A (Scheme 33.1). 

Dihydro- nicotin- Enzymatic Reaction Model Reaction Second-Order Rate Constant k... 

There is no easy understanding of the spectral properties of these compounds in general, which may or may not have a built-in chromophoric system responsible for a long-wavelength absorption like 7,8-dihydropteridin-4-one or a blue-shifted excitation like its 5,6-dihydro isomer. More important than the simple dihydropteridine model substances are the dihydropterins and dihydrolumazines, which are naturally occurring pteridine derivatives and reactive intermediates in redox reactions. 

To illustrate how this applies in the present circumstances we consider a passible group transfer reaction between A2 dihydro-naphthalene, (gQ) > a hydrogen donor, and phenanthrene,(g gr > a substrate (hydrogen acceptor) which models a polynuclear aromatic moiety commonly found in coal. In the overall group transfer reaction ... 

At typical coal liquefaction conditions, namely temperatures from 300 to 400 C and reaction times on the order of 1 hr, hydrogen transfer from model CIO donors, the A1- and A2-dialins, to model C14 acceptors, anthracene and phenanthrene, occurs in the sense allowed by the Woodward-Hoffman rules for supra-supra group transfer reactions. Thus, in the conversion of the C14 substrates to their 9, 10 dihydro derivatives the dialins exhibited a striking reversal of donor activity, the A dialin causing about twice as much conversion of phenanthrene but only one-tenth as much conversion of anthracene as did A2-dialin. 

Response Surface Methodology (RSM) was used to investigate the effects of temperature, pH and relative concentration on the quantity of selected volatiles produced from rhamnose and proline. These quantities were expressed as descriptive mathematical models, computed via regression analysis, in the form of the reaction condition variables. The prevalence and importance of variable interaction terms to the computed models was assessed. Interaction terms were not important for models of compounds such as 2,5-dimethyl-4-hydroxy-3(2H)-furanone which are formed and degraded through simple mechanistic pathways. The explaining power of mathematical models for compounds formed by more complex routes such as 2,3-dihydro-(lH)-pyrrolizines suffered when variable interaction terms were not included. 

Rhamnose and proline were reacted under a wide range of reaction conditions with the expectation of producing volatiles of differing type and ratio. Such large differences were desired to give the best opportunity for the empirical models to account for and explain the variation. If valid, the model terms would be expected to account for differences in product composition and perhaps provide insight into the reaction pathways. Some of the 23 volatiles modeled including 2,3-dimethyl-4-hydroxy-3(2H)-furanone (DMHF), 2-acetoxy-3-pentanone, and four 2,3-dihydro-(lH)-pyrrolizines will be discussed below. 

The reaction catalysed by alcohol dehydrogenases is a transfer of hydride ion from the alcohol to the 4-position of the pyridinium ring of the coenzyme NAD+ (Scheme 6), [For a review of hydride transfer in model systems, see Watt (1988).] The two hydrogen atoms at the 4-position of the dihydro-pyridine ring of NADH are diastereotopic, and over the years it has become apparent that some alcohol dehydrogenases transfer the pro-/ ... 

The alkaloid behaved similarly to voaeangine, since on the one hand its dihydro derivative was readily decarbomethoxylated to 4-epi-ibogamine (XLIII), mp 162°-164°, [a]D + 86° (hydrochloride in MeOH), and on the other its lithium aluminum hydride reduction product, catharanthinol, afforded an acetonide (XXXVII), mp 188°-191°. To account for the difficulty with which catharanthine eliminated the carbomethoxy group, it has been suggested that an intermediate in this reaction (XLIV) is highly strained (40). Since XLIV can be readily constructed from Dreiding Atomic Models, this explanation may not be correct. In actual fact, it is probable that the acid-catalyzed decomposition of catharanthine takes a different course. It has more recently... 

The enantioselective step is the oxidative addition of H2 to the square diastereomeric substrate complexes that are in rapid dissociative equilibrium. The major enantiomer of the product arises from the minor substrate-catalyst diastereomer, this isomer cannot always be detected since it reacts much more rapidly with H2 than the major diastereomer. Molecular modelling suggests that the principal enantiodifferentiating interactions are between the enamide ester function and the nearest arene substituent of the chiral diphosphine.12 The large increase in reaction rate for the minor diastereomer arises from the increased stability of the corresponding dihydro intermediate, that is, the enantioselective step is under product control. 

Very simple and straightforward alkylation of NH-heterocycles (NaH, Cl(CH2) SMe, = 2 or 3, 68-86%) permits one-pot preparation of pyrrole and carbazole sulfide models for the reaction with triflic anhydride <2003S1191>. Cyclization of the (l/7-pyrrol-l-yl)alkyl sulfides 1470 obtained leads to 2,3-dihydropyrrolo[2,l- ][l,3]thiazole 1472 and 3,4-dihydro-2/7-pyrrolo[2,T ][l,3]thiazine 1473 via intermediate l-methyl-2,3-dihydropyrrolo[2,l-/ ][l,3]thiazol-1-ium or l-methyl-3,4-dihydro-27/-pyrrolo[2,T ][l,3]thiazin-Tium salts 1471 (n = Z or 3), respectively, that were isolated in high yields in most cases (Scheme 281) <2003S1191>. 

Inspection of models shows that this internal oxygen transfer is energetically favorable in the case of the 14,15-double bond by way of a 15-membered strain-free cyclic structure. The same sequence of reactions was also reported for the 5,6-dihydro derivative of 1. Internal epoxidation does not appear to be favorable in the case of a double bond in the 9,10-position of a peroxy acid (peroxyoleic acid). 

An additional aromatic zone OZ 6 is also present in some wood extracts and in models of Maillard reactions with proline and glucose. The molecule responsible for the interesting odor of jam and burnt sugar has been identified to 3,5-dihydroxy-2-methyl-2,3-dihydro-4(H)-pyranone or hydroxymaltol [8]. 

The first successful introduction of a C(i8>-substituent was achieved independently by Corey and by Jeger, and their co-workers, in 1958. They applied the Hofmann-Loeffier Freytag reaction, an efficient method for preparing pyrrolidines by cyclisation of iV-haloamines, to the synthesis of dihydro-conessine 3) [2,3]. The process comprises irradiation of an Ar-chloro 20a-amine (i), in acid solution, when the chlorine atom is exchanged with a hydrogen atom at the unactivated C(i8) position. Later work on simple model compounds [4]... 

An autorecycling system for the specific 1,4-reduction of a,p-unsaturated ketones and aldehydes was based on 1,5-dihydro-5-deazaflavin, which can be regarded as an NADH model. The reaction occurs on heating the substrate with catalytic amounts of 5-deazaflavin in 98% formic acid, typically at 120 "C for 24 h (Scheme 80). 

The reactions of (Vl,a i.e. the dihydro-derivative (norbomane) of trieiK VI) with NQ is very slow at T < 80 C and only a few experiments were carrfed out wth this model compound because of difficulties in realizing spectroscopic measurements at hi er temperatures. 

As appropriate model compounds for these reactions " the bridgehead substituted dihydro-4-methyleneazulenes 474 were employed. Allyl-, crotyl- and propargyl-substituted dihydroazulenes 474 and 476 can be easily rearranged to the 4-substituted azulenes 475 and 477 (equations 179 and 180) whereas all attempts to obtain 4-benzylazulene 479 by rearrangement of precursor 478 gave only polymeric products (equation 181). Undoubtedly, this failure can be explained by the fact that the Cope rearrangement becomes very... 

The primary process for the oxidation by O2 - of dihydrophenazine and dihydro-lumiflavin must be analogous to that for PhNHNHPh (Scheme 7-14) to give the anion radicals of phenazine (Phen ) and lumiflavin (Fl ). These in turn react with O2 to give O2 - plus phenazine and lumiflavin, respectively the process is analogous to that for the anion radical of azobenzene (PhN NPh), The oxidation potentials (Ep,a) for PhN NPh, Phen - and Fl - in Me2SO are -1.1 V versus NHE, -0.9 V, and -0.6 V, respectively. Each value is sufficiently negative to reduce O2 to O2 - (-0.5 V versus NHE in Me2SO). Hence, the (O2 )-induced auto-oxidation of PhNHNHPh also is thermodynamically feasible for dihydrophenazine and dihydro-lumiflavin and does occur for these two model substrates of reduced flavoproteins (Table 7-3). Such an auto-oxidation reaction sequence may be relevant to the fractional yield of 02 - from the flavin-mediated activation of 02 ° and the auto-oxidation of xanthine [catalyzed by xanthine oxidase (XO), a flavoprotein]. ... 

THF models transfer their C-2 fragment in between 1,4- or 1,5-binucleophilic sites to generate five- or six-membered rings. The acid-catalyzed reactions of 2-aryloxazolidines and oxazinanes with o-aminobenzamide, o-aminothiophenol, and o-phenylenediamine give 2-aryl-4( 1 //)-quinazolinones 69, 2-arylthiazoles, and a mixture of 2-arylbenzimidazole and l-benzyl-2-arylbenzimidazole 68, respectively. Whereas benzothiazole and benzimidazole could be formed by air oxidation of their initially formed dihydro derivatives, l-benzyl-2-phenylbenzimidazole, as formulated below, could arise from initially formed diimine, its cyclization, and subsequent 1,3-hydride shift (88JCR(S)322, 89IJC(B)802). It is the sole product when o-phenyl-enediamine and the model are used in 1 2 stoichiometry. 

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