Precocenes

The precocenes, e,g. (39), are the opposite of juvenile hormones in that they stimulate a juvenile Insect to become an adult. There is some hope that premature application of precocenes might lead to sterile adults and hence control of that species. 

Sariaslani FS, LR McGee, DW Ovenall (1987) Microbial transformation of precocene II oxidative reactions by Streptomyces griseus. Appl Environ Microbiol 53 1780-1784. 

Other chemicals evaluated but not yet adopted commercially include organophosphorus compounds, triphenyltin compounds, quaternary ammonium salts, imidazoles, benzimidazoles, carbamates and the precocene anti-juvenile hormones [517]. Although none of the above has found use as an insect-resist agent, several have been used as antimicrobial agents for textiles. 

A photochemical approach to the synthesis of precocenes /, II, and III, interesting as juvenile hormome inhibitors [177,178], has been deviced starting from the phenolic esters of 3-methylcrotonic acid. This is shown in Scheme 61 for the synthesis of precocene I (235) [179] and in Scheme 62 for the synthesis of precocenes II (238) and III (239) [180]. The rate of the cyclization process leading to chromanones, like 234 or 237, has been found to depend on the stereochemistry of the double bond. Thus, ring closure is more rapid for the trans than for the cis isomer, although no clear-cut trend can be established [181],... 

Dimethylaminobenzoates have also been used to establish the absolute configuration of diols in the presence of an aromatic chromophore, including metabolites of benzo[a]pyrene163 and precocene I164, as well as the cis-dihydrodiol derived from 12-methylbenz[a]anthracene165 and diastereomeric 2,2 -spirobiindane-l,l -diols166. 

Although considerable information has been gathered on cytotoxic and antijuvenile action of the precocenes, little information is available on the distribution of chromenes and... 

Information relating to the 13C spectra of chromenes is sparse, being restricted to some 2,2-disubstituted derivatives (cf. Figure 5). Substituents in the aromatic ring of chromenes affect the electron density at the double bond and an attempt has been made to relate the antijuvenile hormone activity of the precocenes (76 R = H or OMe) to the electron density at C-3 and C-4, using I3C chemical shifts as a measure of the latter property. Whilst C-4 is essentially unaffected by substitution, it appears that a 13C shift of C-3 at less than 130 p.p.m. is a necessary condition for activity (81MI22200). 

Recently, the biological role of precocene I (132) and II (133) and their conversion into the epoxides has been studied. Precocene II suppresses juvenile hormone (79MI22301, 79CC920) and this has stimulated interest in synthetic analogues (80JHC1377). 

Chromenes result from the dehydration of chromanols. The ready availability of the hydroxy compounds by the reduction of chroman-4-ones and through their reaction with Grignard and related reagents makes this an attractive route. Amongst many available examples, the syntheses of ageratochromene (precocene II 125) (58BSB22), fluorinated... 

Sahota T. S. and Farris S. H. (1980) Inhibition of flight muscle degradation by precocene II in the spruce bark beetle, Dendroctonus rufipennis (Kirby) (Coleoptera Scolytidae). Can. J. Zool. 58, 378-381. 

Consequently, it will be important for future research to examine at least two aspects of this work to elucidate the mechanism of action of precocenes on larvae of bugs, and to find whether the reported sterilization of adult female insects and the reported indue tion of diapause in Colorado potato beetles (31) involve a similar mechanism of action. 

The negative implications for pest control by precocenes themselves are clear, but it remains to be seen whether the expansion of their spectrum of activity is limited merely by the chemical structural features of precocenes or, more problematically, by the hormonal mechanisms which control insect development. In either case the JH antagonist approach to the control of larval insect pests presents a major challenge to chemical and physiological research. 

Isolation and identification of the two natural antijuvenile hormones revealed two simple chromenes 7-methoxy-2,2-dimethyl chromene and 6,7-dimethoxy-2,2-dimethyl chromene (Figure 3). Since these compounds induced precocious metamorphosis, we called them Precocene I and II respectively. Subsequently we found that both compounds had been previously identified and synthesized ( 15, 16, 17). We developed an efficient synthesis for these compounds shown in Figure 4. 

In additional biological work we found that virgin female American cockroaches, Periplaneta americana, stopped producing their sex attractant following treatment with Precocene II, while milkweed bug and Mexican bean beetle eggs treated with Precocene II were unable to hatch. Normal non-diapausing Colorado potato beetles treated with Precocene II promptly left their food plants, burrowed into the soil and entered diapause. 

All of these biological effects of the precocenes indicated that the secretion of the juvenile hormones had been prevented. 

We tested this hypothesis by treating insects with both Precocene II and juvenile hormone. We found that, when these compounds... 

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