Synthesis of 2,3-Dihydro-2-Ylidene-1,4-Benzodioxins

The 2,3-dihydro-1,4-benzodioxin structure is present in compounds having some interesting biological properties. The first access to this structure, performed from 2-alkyl-1,4-benzodioxin or benzene-1,2-diol via stoechiometric cyclization, generally gave mixtures of products. Recently, the organometallic-catalyzed synthesis of this structure was performed using mercuric oxide, or more interesting, palladium as the catalyst. The palladiumcatalyzed heteroannulation of various monoprop-2-ynylated catechol and aryl iodides in the presence of PdCl2(PPh3)2 and CuI afforded the corresponding 1-alkylidene or 1-arylidene-2,3-dihydro-1,4-benzodioxins in excellent chemical yields. However the palladium-catalyzed cyclisation of substituted benzene-1,2-diols with different propargylic carbonates gave the corresponding 1-alkylidene or 1-arylidene-2,3-dihydro-1,4-benzodioxins eventually substituted at position 2 by an aryl or an alkyl group. Moreover, this later methodology seems very powerful since it is possible to perform this heteroannulation in an asymmetric way in order to obtain the 1-alkyl- 2-alkylidene-2,3-dihydro-1,4-benzodioxins in enantioselectivities up to 96%.