Furyl groups substituent effects

Italian workers have reported extensive studies on the evaluation of a values for thienyl and furyl groups. These are collected in Table XII. The am, Om, and am values reveal the electron-withdrawing inductive effect of the rings, while as expected from the previous discussion of this section, these substituents fall into that small category whose op, oj>, and Op values are all different, revealing an ability to both accept and donate electrons by resonance under the appropriate incentives. 

Thirteen cyclic phosphonates IVD were prepared to test their herbicidal activity. In series IVD, when the phosphorus-containing six-member ring and furyl group as were kept, further examination was focused on the effect of substituent Yj,. As a preliminary bioassay, the cyclic phosphonates IVD were tested for inhibitory effect... 

In many cases, substituents linked to a pyrrole, furan or thiophene ring show similar reactivity to those linked to a benzenoid nucleus. This generalization is not true for amino or hydroxyl groups. Hydroxy compounds exist largely, or entirely, in an alternative nonaromatic tautomeric form. Derivatives of this type show little resemblance in their reactions to anilines or phenols. Thienyl- and especially pyrryl- and furyl-methyl halides show enhanced reactivity compared with benzyl halides because the halogen is made more labile by electron release of the type shown below. Hydroxymethyl and aminomethyl groups on heteroaromatic nuclei are activated to nucleophilic attack by a similar effect. 

The cycloaddition of diazomethane to SchifT bases from heterocyclic aldehydes and anilines provides a useful route to heterocyclic substituted triazolines. Unlike olefins bearing heterocyclic substituents, the heterocyclic imines can be obtained readily by reaction of the appropriate aldehyde and amine thus the diazomethane-imine addition has greater scope than the olefin-azide reaction. NMR spectroscopic studies of the orientation of addition are in accord with previously reported mechanistic considerations (see, e.g., Scheme 93).329 In addition to the influence of the N-aryl group, the electron-withdrawing power of the heterocyclic substituent on the Schiff-base carbon also has a substantial effect on imine reactivity, in the order 2-quinolyl 2-, 3-, or 4-pyridyl > phenyl > 2-thienyl as 2-furyl.329... 

IR spectra there is absorption in the region 1540-1620 cm-1, characteristic of the chelated cis-enol bands characteristic of a free carbonyl group are absent. The effect of substituents on the enolization of /3-di-ketones (in acetone) was studied by NMR spectroscopy. In the cu-acyl-2-acetoselenophene (20) and a> -acyl-3-acetoselenophene(21) series, the acyl groups COR increase the enolization in the following order of R methyl < a-furyl < a-thienyl < trifluoromethyl < phenyl < a-pyridyl. Thus, /S-diketones containing aroyl or heteroaroyl radicals together with selenienyl exist completely as the cis-enols.121... 

A delicate balance of steric and electronic effects at phosphorus controls the course of decomposition of the phosphonium betaines (151) that may be generated in protic solvents. Electron-withdrawing heteroaryl substituents e.g. 2-furyl and 2-thienyl) promote intramolecular collapse of the betaine to form the normal Wittig products, as also does enclosure of the phosphorus in the ring-strained dibenzophospholium system. Except in such cyclic systems, both electron-donating and bulky groups e.g. o-tolyl and t-butyl) reduce the rate of intramolecular collapse and allow a dehydra-... 

The fZj-selectivity has been attributed to be due to a propeller-like conformation of the stationary phenyl groups which, in the transition state, favours cw-oxaphosphetane formation [51]. Introducing 6>rr/io-substituents in the phenyl groups increases the steric pressure in the transition state and thus the fZj-selectivity. However, the sterically less restricting tris-furyl substituted phosphorus ylides also lead to high fZj-selectivities (see Section C.2.a) obviously, in addition to purely steric effects, polarity effects are also important. 

A recombinant Escherichia coli strain overexpressing yeast reductase Ara Ip was utilized for the reduction of racemic 3-oxo-4-phenyl-P-lactam 16 to cis-(3S,4R)-3-hydroxy-4-phenyl-p-lactam 17 [20]. Detailed studies in this case showed that only under mild optimized fermentation conditions, a DKR occurred in situ and a single enantiopure product was formed, as shown in Scheme 12.8. This reaction could also be carried out in gram scale. When the reaction was carried out in a shaking flask, the DKR was not effective and the yields of alcohols were low because of the starting material decomposition. No DKR was achieved in the case of substrates having 2-thiophenyl or 2-furyl substituents instead of the phenyl group. 

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