Abstract
This work describes the investigation of the Diels-Alder reaction of the electron-rich diene trans- 1-methoxy-3-trimethylsilyloxy-1,3-butadiene (Danishefskys diene) and the non-activated dienophiles 2-cyclopentenone and 2-methyl-2-cyclopentenone with respect to reactivity, regioselectivity and stereoselectivity. An observed double bond isomerization limits the practicability of 2-cyclopentenone as a dienophile in Diels-Alder reactions. 2-Methyl-2- cylopentenone could be converted quantitatively into one regioisomeric Diels-Alder adduct, however the stereochemical control turned out to be very demanding.
Highlights
The use of electron-rich dienes is a common method to improve both reactivity and regioselectivity in Diels-Alder [4+2] cycloadditions
In order to improve the amount of products, Me2AlCl in CH2Cl2 at -28 °C was tested albeit to give only a yield of 18% and a product ratio of 56:44
We described that C=C bond isomerization severely limits the utility of 2-cyclopentenone 9a as a dienophile and the following experiments were carried out with 2methyl-2-cyclopentenone 9b
Summary
The use of electron-rich dienes is a common method to improve both reactivity and regioselectivity in Diels-Alder [4+2] cycloadditions. Besides Danishefsky’s diene 2 a variety of related electron-rich dienes 5-8 have been developed.[5] Whereas the use of (-M)-substituted cyclopentenones in Diels-Alder reactions with Danishefsky’s diene 2 is well explored,[6] only few examples are known, where non-activated. Cyclopentenones have been employed as starting materials.[7] we were interested in exploring the scope of [4+2] cycloadditions employing diene 2 and non-activated 2-cyclopentenone 9a and 2-methyl-2-cyclopentenone 9b with respect to reactivity, regioselectivity and possibly stereoselectivity. The results towards this goal are reported below. Diels-Alder reactions with Danishefsky’s diene and related electron-rich dienes
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