3-Chromene ring

Yet another strategy was developed by Lee et al. using a bis-condensation inactive chromene ring instead of the pyran ring so as to generate a series of chromene-based red dopants (161-163) as shown in Scheme 3.50 [211]. 

Modifications of the ring conjugated with a chromene ring at positions... 

The photochromic compounds of potential interest, based on the 2//-chromene ring system, are the 2//-benzopyrans (1.18) or the three isomeric naphthopyrans (1.19-1.21). However, 2H-naphtho[2,3-( ]pyrans (1.21) show little or no useful photochromic behaviour and can be discounted from any further discussion. Although R and can be part of a carbocyclic spiro ring, they are more commonly unconnected substituents such as gem dialkyl or aryl groups. 

The chromene ring of spirobenzopyrans such as (168) and (169) is opened by treatment with mineral add and closed again by alkali — a reaction of potential importance in the thermochromic or photochromic application of such compounds (69T5995,74JOU1516). 

The structure of SOI 1 was not affected by annellation of another potentially photochromic chromene ring at the 9- and 10-positions of the naphthalene moiety. Within the limits of experimental error, the Cspiro—0(1) [1.448(8) A] and Cspiro-N(2) [1.424(8) A] bond lengths coincide with the lengths of the analogous bonds in SO 10 and S04. 

Matolcsy et al. (1980 1981) reported that 1,2-dimethoxy-4-isopropoxy-5-vinylbenzene (77) a compound derived formally by opening the dihydropyrane ring of precocene II, possesses a precocene type anti-juvenile hormone activity. This finding indicates that compounds lacking the chromene ring structure can also be active. 

All NMR data of myrtiaphenone B (13) were in agreement with the structural characteristics of prenylated benzophenones. The resonances at 8 6.40 and 5.32 shared a J constant of 10 Hz, typical of a cis double bond, and a gem-dimethyl group indicated the presence of a chromene ring. Myrtiaphenones are very closely related from a structural point of view. Myrtiaphenone A can be easily converted in myrtiaphenone B by cyclo dehydrogenation with DDQ. 

Chromene ring formation can be effected by oxidative cyclisation of 2-prenylphenols or by reaction of pyrano compounds with DDQ (dichlorodicyanobenzoquinone), by reduction of a chromone with a hydride followed by dehydration with a toluenesulphonic acid, and directly by the use of 1,1-dimethoxy-3,3-dimethyl-3-hydroxypropane. 

Atrovenetin, (Table 12.1), herqueichrysin are related phenalones (ref. 59) and are unusual in that they both contain a trimethylfurano moiety arising from an prenyl ether since there is no 2-hydroxyl group to form a chromene ring from an adjacent C-prenyl group. Synthetic work (ref.59) has led to members of the group by the Claisen rearrangement. Thus the prenyl ether of the phenalone intermediate in DMF at 100°C directly afforded atrovenetin trimethyl ether. 

Lunularic acid (100), a natural dihydrostilbene growth inhibitor found in liverworts 74) and also in algae (75), has not yet been detected in ferns. A related compound notholaenic acid (101) occurs as a frond exudate component of two gymnogrammoid ferns 76) and is reported to have photosynthesis inhibiting activity on chloroplast (77) and also antimicrobial activity 78). Ternatin (102), in which a benzene ring is linked to a monoterpene unit to form a chromene ring, has been isolated from Sceptridium ternatum 79) and S.japonicum 80). 

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