Cycloaddition of thiourea- and guanidine- substituted furans to dienophiles. A Comparison of the environment friendly methods

Cycloaddition reaction represent an excellent tool for constructing cyclic systems in a highly regioselective and stereoselective manner. Many biologically and technologically important systems were built upon rigid scaffolds made form one or more norbornene and/or its oxa and aza analogues to mention only the beta-turn mimics and bisporphyrine tweezer -like receptors. It has been claimed that cycloaddition could be a good approach toward chiral diamines. Indeed, one or more superbasic groups attached to such skeleton would be an interesting target molecules.

In this work, we employed cycloaddition strategy to obtain oxanorbornene framework substituted with guanidine moiety or its precursor functional group: protected amine or thiourea. To optimize condition for the cycloaddition, several environmentally friendly methods: microwave assisted organic synthesis (MAOS), high pressure synthesis (HP), high speed vibrational milling (HSVM) and ultrasound assisted (US) synthesis were employed.

In general, our results indicate HP and HSVM approaches as the methods of choice giving good yields and conversions.