Chromene, structure

None of these reactions could establish whether the positive charge is located on oxygen or on a carbonium ion, although the latter possibility seems more likely. Either way, the positive charge is due to the presence of bonded oxygen. One is justified in speaking of basic surface oxides. The evidence in favor of the chromene structure is rather circumstantial, although many phenomena are explained. A more direct proof should be desirable. 

Poinsettifolin A serves as an example of a recently isolated flavonol with a C-5-unit attached to the chromene structure (Figure 12.9). Desmethylanhydroicaritin, isolated from Bursera leptophloea as 3,5,4 -trihydroxy-7,8-pyranoflavon, needs to be revised to 3,5,4 -triOH-7,6-pyranoflavone (compound 157, Table 12.4). ... 

Flavonols with this substitution pattern occur mainly in species of Moraceae. A related structure, petalopurpurenol was found in roots of Petalostemonpurpureus (Fabaceae). Cycli-zation into chromene structures may occur between 7-OH and C-6 or 7-OH and C-8, but substitution between 3 -OH and C-4 is also encountered (e.g., broussoflavonol D Figure 12.9). 

The rationale for design of proallatocidins related to chromenic structure of precocenes is discussed. Stabilization against environmental conditions and insect metabolic pathways, as well as modification of transport properties, sire some of the leads followed for the synthesis of more powerful insect growth regulators of this type. Some aspects of the chemistry of 3,4-epo-xyprecocenes are also examined. 

Precocenes I and II are natural products with a simple chromene structure (7-methoxy- and 6,7-dimethoxy-2,2-dimethylchromene) which were isolated from plant sources and exhibited powerful antijuvenile hormone activities in several types of insects (1). 

The gamma-chromene structure appears in many natural products as derivatives with a 4-keto group. The parent is known as chromone (3.56), and it is found especially in the flavone (flavonoid) family, which has a 2-phenyl substituent. Flavone itself has structure 3.57 and is a yellow solid found in plants. A flavonoid from strawberries, fisetin (3.58), is said to have weak memory-enhancing ability. ... 

Another structural type is chromenes. Centchroman [31477-60-8]is a pyrrolidinoethoxyplienyl chromane which is a potent antiestrogen with weak estrogenic activity. In India, it is used as a weekly contraceptive pih based on its reported abiUty to inhibit the uterine preparation for the attachment of the fertilized ova to the wall of the utems (see Contraceptives) (31). 

Reaction of nitro-2f/-chromene derivatives 134 with 135 in methanol at room temperature afforded a mixture of the Z-isomer 136 and tricyclic compound 137, which could be formed by denitrocyclization reaction of the corresponding primarily formed E-isomer and the following dehydrogenation (Eq. 15). The structural identification was based on the MS and H-NMR, however, it is not sufficiently documented and similar examples are not known (91IJC(B)297). 

Benzo[c]chromen-6-one derivatives, such as AM 1710 (372), were found to be CB2 selective agonists [229]. Although these compounds are structurally similar to the THC derivatives, the dimethyl group on 6-position of the... 

Psoralen, or derivatives of 9-methoxy-7H-furo[3,2-g]chromen-7-one tricyclic ring structures, are used as photoreactive groups in crosslinkers, biotinylation compounds, and nucleic acid probes. Psoralens have been used for many years as photochemotherapy agents for treatment of psoriasis and vitiligo (Smith and Barker, 2006). Psoralens react when exposed to UV light... 

Catalytic RCM and another Zr-catalyzed process, the kinetic resolution of cyclic allylic ethers, joined forces in our laboratories in 1995 to constitute a fully-cata-lytic two-step synthesis of optically pure 2-substituted chromenes. These structural units comprise a critical component of a range of medicinally important agents (see below). Our studies arose from unsuccessful attempts to effect the catalytic kinetic resolution of the corresponding chromenes [13] a representative example is illustrated in Eq. 3. 

The corresponding cyclohexenyl system 56 (Scheme 16) remains relatively unreactive, however, even when the reaction is performed under an ethylene atmosphere after 24 h (10mol% lb, 1 atm ethylene, CH2Cl2), only 10-20% of chromene 47 is obtained. This persistent lack of reactivity is presumably because (1) the relatively strain-free six-membered ring is less prone (relative to cyclopentenyl and cycloheptenyl structures) to react with LnRu=CH2 [21], and (2) in case ring rupture does occur with the proper regiocontrol to afford 57... 

SCHEME 3.50 Chemical structures of chromene-based red emitters. 

The only concept able to explain all the observed phenomena was brought forward by Garten and Weiss (111). They suggested that the oxygen was bound in chromene-like structures. On oxidation in the presence of acid, carbonium ions would be formed ... 

Table 1. The key compounds in the structure optimization of 2-amino-4H-chromenes... 

Gomez-Garibay, F. et ah, Chromene chalcones from Tephrosia carrollii and the revised structure of oaxacacin, Z. Naturforsch. C, 56, 969, 2001. 

Much interest has been devoted to the photochromic behaviour of simple chromenes, especially because of practical applications of spiropyrans, particularly indolinospirans.5 Kolc and Becker30 have been able to demonstrate the o-quinoneallide structure of the colored form 4, by producing it in THF at -75° and trapping by reduction with LiAlHj. It is concluded31,32 that the same intermediates occur when spiropyrans are irradiated, because only the pyran moiety has an... 

Earlier 4-bromo structural assignments should now be reversed. The 3-bromo derivative can be transformed into the desired chromene by zinc and acid.19 137... 

Among chromenes, only the spiropyrans and their heterocyclic derivatives have found a wide practical application as photochromic substances.5 6,7-Chromenediols have been proposed as analytical reagents for the spectrophotometric assay of rare earth cations.279 Chromenes with structure and activity similar to those of hashish constituents have been prepared.280 A number of chromenes, mostly with aryl substituents in positions 2,3, and 4 have been patented as biologically active substances.126,280 290... 

Deguelin (19),32 in which rings D and E have a chromene, not a benzofuran, structure, is fully as ichthyotoxic and insecticidal as rotenone. It has been suggested33 that the toxophore in rotenoids might be the sequence =CH—CH=CH—O— in rings C and D, which is found both in (13) and in (19). The same sequence is found in... 

NMR spectroscopy has proved of great value in the structural identification of both natural and synthetic chromenes. Much information has been accumulated and it is impractical to attempt even a partial listing of references here the specialist review literature should be consulted. 

In addition to the aromatic multiplet, the H spectrum of 2//-chromene (4) exhibits a quartet at 8 4.53 (2-CH2) and peaks at 5.38 (3-H) and 6.20 (4-H) (Figure 1). The last peaks are particularly significant, appearing in a variety of multiplet structures, the nature of which depends on the substitution pattern. However, the benzylic proton always appears downfield of H-3. Alkyl-substitution at C-2 causes an upheld shift of both alkenic protons, but disubstitution results in shifts to lower field. In the widely occurring 2,2-dimethylchrom-... 

The 19F NMR spectra of several highly fluorinated derivatives of 2//-pyran (81MI22201, 83JCS(P1)1239) and 4/f-pyran <83JCS(P1)1235> have been reported, as have those of some fluorinated 2,2-dimethyl-2//-chromenes (80JHC1377). The structures of some pyran-2-ones derived from the reaction between phenylacetylene and fluorinated allenes have been ascertained with the aid of their 19F spectra (77IZV2517). 

The 2//-chromene part of the naturally occurring lonchocarpin (206) remains unaltered when the compound is subjected to oxidation. This helped to determine the structure of lonchocarpin (53MI22300). 

Hydroxyl groups in chromenes behave normally for example, they are acylated or methylated by the usual reagents. A natural 2//-chromene isoevodionol (210) is methylated by dimethyl sulfate to the dimethyl ether (211) but its inertness to diazomethane shows hydrogen bonding of the hydroxyl group (70TL3945). Such observations are useful in determination of structure. 

Chromene formation also results from the reaction of DDQ with the isomeric but-l-enyl derivatives (104). Similar structural restrictions are apparent. Thus, 2-(3-methylbut-l-enyI)phenol affords the chromene, whereas the unsubstituted compound does not cyclize. 

---