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Improved laboratory protocols for convenient and rapid transformations are highly desired in modern synthetic chemistry. Microwave irradiated reactions have received considerable attention in recent years and it is a subject of intense discussion in the scientific community. Microwave heating is more efficient in terms of the energy used, produces higher temperature homogeneity and is considerably more rapid than conventional heating methods. This technique as an alternative to conventional energy sources for introduction of energy into reactions has become a recognized practical method in various fields of chemistry. Microwave-assisted organic synthesis (MAOS) is known for the spectacular accelerations produced in many reactions as a consequence of the increased heating rate, a phenomenon that cannot be easily reproduced by classical heating means. As a result, higher yields, milder reaction conditions and shorter reaction times can often be attained. Its specific heating method attracts extensive interest not only because of rapid volumetric heating, but also for suppressed side reactions, energy saving, decreased environmental pollutions and safe operations. In this review, we will try to represent an overview on origin and fundamental features of microwave ovens and its usefulness in MAOS.

Intramolecular radical cyclization of suitably substituted epoxy ethers 4a-g using bis(cyclopentadienyl)titanium(III) chloride as the radical source resulted in trisubstituted tetrahydrofurano lignans and 2,6-diaryl-3,7-dioxabicyclo[3.3.0]octane lignans depending on the reaction conditions. The titanium(III) species was prepared in situ from commercially available titanocene dichloride and activated zinc dust in THF. Upon radical cyclization followed by acidic workup, epoxy olefinic ethers 4a-g afforded furano lignans dihydrosesamin 1a, lariciresinol dimethyl ether 1b, acuminatin methyl ether 1e, and sanshodiol methyl ether 1g directly and lariciresinol 1h, acuminatin 1i, and lariciresinol monomethyl ether 1j after removal of the benzyl protecting group by controlled hydrogenolysis of the corresponding cyclized products. The furofuran lignans sesamin 2a, eudesmin 2b, and piperitol methyl ether 2e were also prepared directly by using the same precursors 4a-f on radical cyclization followed by treatment with iodine and pinoresinol 2h, piperitol 2i, and pinoresinol monomethyl ether 2j after controlled hydrogenolysis of the benzyl protecting group of the corresponding cyclized products. Two naturally occurring acyclic lignans, secoisolariciresinol 5h and secoisolariciresinol dimethyl ether 5b, have also been prepared by exhaustive hydrogenolysis of 2h and 2b, respectively.

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Short, efficient and stereoselective synthesis of a furano lignans, (±)-Dihydrosesamin and (±)-Acuminatin Methyl Ether has been achieved in good overall yield through the radical cyclisation of epoxides using a Ti(III) reagent as the radical initiator.