Synthesis and Reactions of New Tellurium-substituted Terpenyl Compounds

In recent years, chalcogenides were widely used as efficient reagents and catalysts to create new carbon-oxygen, carbon-nitrogen and carbon-carbon bonds. Our previous research was focused on developing effective methods for the synthesis of new optically active organosulfur and organoselenium compounds derived from monoterpenes. Satisfying results prompted us to synthesize tellurium analogues. We have developed new methodologies for the synthesis of terpene ditellurides, tellurides, methyl terpenyl tellurides, and phenyl terpenyl tellurides from p-menthane, carane and pinane systems. The synthesis was based on the reaction of sodium telluride, sodium ditelluride, sodium benzenetellurolate, and sodium methanetellurolate with terpene tosylates, and chlorides. Additionally, terpene tellurides were converted in situ to telluronium ylides, and were used as reagents in asymmetric epoxydation. The best selectivity of epoxidation reaction was achieved for dicaranyl telluride.


Introduction
Mono and dichalcogenides are very useful classes of organic compounds. More often they are considered as standard reagents or catalysts in organic synthesis, e.g. dichalcogenides can be simply converted into nucleophilic, electrophilic or radical reagents [1]. In our previous investigations we have shown a new methodology for the synthesis of dialkyl diselenides based on the reaction of sodium diselenide with alkyl tosylates and chlorides [2]. Particularly interesting were the applications of our methodology for the synthesis of optically active selenides and diselenides derived from monoterpenes. We have obtained a series of selenium derivatives from p-menthane, carane, and pinane systems [3][4][5][6][7]. Analogously we have prepared a sulphur terpenyl derivatives [8]. We have demonstrated that diterpenyl diselenides e.g. dineomenthyl diselenide 1 can be simply converted into electrophilic selenium reagents 2 by the reaction with bromine, than exchange with silver salts, or to nucleophilic reagents 3 by the reaction with sodium borohydride (Scheme 1). Electrophilic reagents were tested in the addition to the double bonds (path a) [3][4][5][6][7] or selenocyclization reaction with the formation of new carbon-oxygen bonds [9], but nucleophilic reagents were used for the synthesis of allylic selenides than via oxidation for the synthesis of chiral allylic alcohols (path b) [10].

Scheme 1. Applications of dineomenthyl diselenide 1
Now we present our results for the synthesis of symmetrical and unsymmetrical ditellurides and tellurides obtained by analogical methodology based on the reaction of terpenyl tosylates and chlorides with corresponding tellurium reagents. We have decided to use tellurides for the tellurium-mediated asymmetric epoxidation.

Results and Discussion
We have started our research from the synthesis of sodium telluride 4 and sodium ditelluride 5. We have developed new two step methodology based on the reduction of tellurium with NaBH 4, than addition of one equivalent of tellurium (Scheme 2). Sodium telluride and sodium ditelluride were used for further reactions without isolation from the reaction mixture.

Scheme 2. Synthesis of sodium telluride 4 and sodium ditelluride 5
Sodium ditelluride was used for the synthesis of optically active diterpenyl ditellurides. We have started our investigation from the synthesis of dineomenthyl ditelluride 6 by the reaction of menthyl tosylate 7 with sodium ditelluride 5. Epimeric dimenthyl ditelluride 10 was obtained using neomenthyl chloride 9 for the reaction. Corresponding tosylate 7 and chloride 9 were prepared by standard methodologies described in previous papers [3]. The reactions were carried out in 100 o C in a mixture of EtOH/DMF, by 1 h for menthyl tosylate and 2h for neomenthyl chloride. Better yield was obtained using corresponding tosylate than chloride.

Fig.2. Structures of ditellurides 11-13
In the second part of our work we have prepared symmetrical and unsymmetrical tellurides. Neomenthyl telluride 14 has been prepared by the reaction of mentyl tosylate 6 with sodium telluride 4. This structure was confirmed by X-ray analysis (Scheme 4).

Scheme 4. Synthesis and structure of dineomenthyl ditelluride 14
Methyl neomenthyl telluride 15 and phenyl neomentyl telluride 16 have been prepared by the reaction of sodium methanetellurolate, and sodium benzenetellurolate, generated in situ from dimethyl ditelluride or diphenyl ditelluride and NaBH 4 , with tosylate 6 (Scheme 5).

Scheme 6.
Epoxidation with the use of tellurides14-22 The best result of asymmetric epoxidation with the use of telluronium ylides was obtained for the reaction of dicaranyl telluride 17.

Conclusions
In this communication we have presented a convenient methodology for the synthesis of sodium telluride and ditelluride by the simple reduction of tellurium with NaBH 4 and addition of another equivalent of tellurium. The reaction of sodium telluride and ditelluride with terpenyl tosylates gave with good yield diterpenyl telluride and diterpenyl ditellurides. Additionally the reaction of sodium methanetellurolate, and sodium benzenetellurolate with terpenyl tosylates gave methyl and phenyl terpenyl tellurides. All tellurides were tested in asymmetric epoxidation. The best result was obtained for bicyclic telluride from carane series.