Structures chromonic

The fragmentation pattern of isoxazoles on electron impact has been well studied. It has been used as an important tool for the structural assignment of isoxazoles obtained from the reaction of chromones with hydroxylamine 79MI41600, 77JOC1356). For example, the structures of the isoxazoles (387) and (388) were assigned on the basis of their fragmentation patterns. Ions at mje 121 (100%) and mje 93 (19.8%) were expected, and indeed observed, for the isoxazole (388), and an ion at mje 132 (39.5%>) was similarly predicted and observed for the isoxazole (387). 

Pyran-4-one (56a) and its benzo derivative (chromone) show chemical properties in agreement with substantial jr-electron delocalization and consistent with a betaine structure 56b (Scheme 27). Experimental data have therefore generated numerous theoretical studies on the aromaticity of pyranones, which have been extensively reviewed.219 Earlier studies suggested that chemical shifts and coupling constants... 

Fig. 8.28 Structure of 6-chloro-4-0X0-10-propyl-4 H-pyrano[3,2-g]quino-line-2,8-dicarboxylate which, in contrast to many related compounds (chromone-carboxylates) lacking the chloroquinoline, is excreted as a glutathione conjugate.

The term chromone alkaloid is used to describe a structure consisting of a pyridine, piperidine, or pyrrolidine ring linked to the A ring of chromone (1). In... 

The co-occurrence of the chromone noreugenin (3) with all the alkaloids so far isolated enabled the recognition of this structure as part of the alkaloid molecule (9). Degradations, interconversions, and spectral studies have been used to determine the structure of die alkaloids. 

Cui, C-B. et al.. Constituents of a fern, Davallia mariessi Moore. IV. Isolation and structures of a novel norcarotane sesquiterpene glycoside, a chromone glucuronide, and two epicatechin glycosides, Chem. Pharm. Bull, 40, 2035, 1992. 

The chromone cromolyn sodium (5-5) was at one time considered the forerunner of a novel class of antiallergic and antiasthmatic drugs that act at one of the earliest stages of the allergic reaction. Detailed experiments, acmally conducted after the dmg s clinical effectiveness had been confirmed, suggested that the compound inhibited the release of mediators of the allergic reaction from mast cells. The dmg is not very active when taken orally and is usually applied topically to the lung by insufflation as its sodium salt. Considerable efforts to uncover additional structurally related mediator release inhibitors have had only limited success. 

A structurally unusual 3-blocker that uses a second molecule of itself as the substituent on nitrogen is included here in spite of the ubiquity of this class of compounds. Exhaustive hydrogenation of the chromone (13-1) leads to a reduction of both the double bond and the carbonyl group, as in the case of (11-2). The car-boxyhc acid is then reduced to an aldehyde (13-2) by means of diisobutylaluminum hydride. Reaction of that intermediate with the ylide from trimethylsulfonium iodide gives the oxirane (13-3) via the addition-displacement process discussed earlier (see Chapters 3 and 8). Treatment of an excess of that epoxide with benzylamine leads to the addition of two equivalents of that compound with each basic nitrogen (13-4). The product is then debenzylated by catalytic reduction over palladium to afford nebivolol (13-5) [16]. The presence of four chiral centers in the product predicts the existence of 16 chiral pairs. 

Bond angles and lengths for 3-bromoflavone have been determined <82AX(B)983>. Whilst the two rings of the chromone system are essentially coplanar, the 2-phenyl group is twisted by 45.9° relative to the rest of the molecule apparently the steric interaction between the bromine atom at C-3 and H-2 outweighs the resonance stabilization which would be associated with a completely planar structure. 

The basicity of pyran-4-one, which is significantly greater than that of simple aromatic and aliphatic ketones, is apparent from its pKa of 0.1 and has been rationalized in terms of a betaine structure (212 X = O). Benzo-fusion increases the basicity the p2sTaof chromone (20) is 2.0 (65ZOB1707). As a result of their basicity, both pyran-4-one and chromone have a great propensity to form salts with acids (63CJC505). 

Of the classes of compounds discussed in this chapter the chromones (1 R Ph), including flavones and isoflavones (1 R = 2-Ph and R = 3-Ph), and coumarins (2) are the most numerous. Many of these compounds are found in plants and there is a wide variety of structures amongst them. There appears not to be a single well-defined biological role for... 

Simple chromones are less susceptible to photolysis but 3,5,7-trimethoxy-2-methyl-chromone (369) is converted into a dimer which may have structure (370) (73TL5073). The 3-benzylchromone (371) is cyclized to the tetracycline analogue (372) (72JCS(P1)1103). 

When chromone and hydroxylamine hydrochloride are heated together in ethanol, a mixture of isomeric isoxazoles (430) is obtained, (430a) being the major product. The two isomers can be distinguished by mass spectrometry. When the reactants are brought together under anhydrous conditions, chromone oxime (431) is obtained its structure is supported... 

Pyran-4-ones and their monobenzo derivatives undergo ring opening when treated with aqueous alkali. This reaction has been widely used in the investigation of structure of chromones, flavones and isoflavones but xanthones are little affected. The nucleophile attacks the 1,2-bond in a rate-determining step whose rate constant varies with the electron density in the ground state at C-2. A kinetic study of the reaction with several chromones supports the mechanism shown in Scheme 25 (79ACH(101)73). 

One problem associated with the K-R synthesis of chromones is the simultaneous formation of coumarins which is sometimes observed, even with simple hydroxyacetoph-enones (24CB88). It is therefore necessary to exercise caution with structural assignments... 

Two disadvantages are associated with this synthesis. Yields range from low to moderate and, of more significance, cyclization of the initially formed ester can give rise to chromones or coumarins or to mixtures of the two heterocycles. With the wide range of analytical techniques now available, it is not difficult to distinguish the isomeric benzopyranones. However, some of the structural assignments in early work have been shown to be erroneous and care is therefore advised in the interpretation of results. 

In a later paper (81TL1749) the same group has shown that the lactone structure is not a prerequisite for chromone formation, since the enol ester (478) yields a chromone on irradiation. ... 

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