Beilstein J. Org. Chem.2009,5, No. 26, doi:10.3762/bjoc.5.26
for the cycloaddition of maleimide to anthracene in toluene (Scheme 5). Table 3 summarizes the results. A significantly higher yield of the cycloaddition product was obtained after 8 h at 40 °C in the presence of compound 2 (entry 3), if compared to the control reaction (entry 6). Upon irradiation
reaction (entry 4). Blue light irradiated flavins accelerate the anthracene maleimide cycloaddition significantly, but flavins 1 and 2 do not provide additional benefit if compared to tetraacetyl flavin 3.
Conclusion
We have prepared new flavin derivatives that bear an acyl guanidinium group, which is
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Graphical Abstract
Scheme 1:
Flavin–guanidinium ion conjugates 1 and 2 and tetraacetyl riboflavin (3).
Beilstein J. Org. Chem.2008,4, No. 28, doi:10.3762/bjoc.4.28
compounds 2 could be introduced by a Mitsunobu alkylation of maleimide [12]. Alternatively, substituted maleimides were prepared by reaction of maleic anhydride with the corresponding amines followed by ring closure [13][14].
Cycloaddition kinetics of 1a and 2a was examined first by 1H NMR in CD2Cl2 at room
yields and with moderate values of ee. A remarkable increase in enantioselectivity was observed using maleimide 2i. The steric hindrance imposed by the large 2,6-diisopropylphenyl moiety of 2i resulted in 76% ee at −70 °C but also lowered reaction rates.
Only 13% yield could be obtained under such
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Graphical Abstract
Scheme 1:
Diels-Alder reaction of anthrones 1 and maleimides 2 catalyzed by chiral Brønsted bases 4–8.