Synthesis and cycloaddition reactions of 1,1 '-dimenthene

The synthesis of the hitherto unknown 1,1'-dimenthene 11 from menthone has been studied in detail. The most convenient procedure entails conversion of menthone 6 into its tosylhydrazone 7, Shapiro transformation into the iodi de 9 and copper-mediated homocoupling to 11. Under optimised reaction condi tions a.ca. 55% overall yield of dimenthene 11 was obtained from menthone. The reactivity of dimenthene towards Diels-Alder cycloaddition reactions pr oved expectedly poor, as result of the difficulty of the diene to adopt a p lanar conformation. 4-Phenyltriazoline-3,5-dione (PTAD), tetracyanoethylene (TCNE) and singlet oxygen afford fair to good yields of the non-symmetrica l cycloadducts 13a-c. In the case of PTAD, the Ct symmetrical adduct 13a' i s also formed but it converts into the non-symmetrical adduct 13a on standi ng. The endoperoxide 13c obtained in the photoxygenation reaction is transf ormed into furan 14, possibly via the unstable syn-diepoxide 15. In contras t, the anti diepoxide 15, obtained from m-CPBA oxidation of dimenthene 11 i s stable towards heating and it is not transformed into the furan. (C) 1999 Published by Elsevier Science Ltd. All rights reserved.