First various conditions were screened for the cycloadditions. The results are summarized in
Scheme 2 and Table 1. Followi ng a procedure by Ishikawa, 6a,b a solution of 2-cyclopentenone 9a
in CH2Cl2 was treated with 10 mol% of ZnCl2 for 15 min at room temper ature and then cooled to
0 °C. Later 1.2 equiv. of Danishefsky’s diene 2 were added and the mixture was hydrolyzed after
2 h with 1 N HCl and submitted to aqueous workup. Although GC of the reaction mixture
indicated 99% conversion of the starting material 9a, only 49% of a 63:37 mixture of two
inseparable products with the desired m/z 150 (entry 1) was obtained. In order to improve the
amount of products, Me
2 AlCl in CH2
Cl 2
at -28 °C was tested next, al beit to give only a yield of
18% and a product ratio of 56:44 (entry 5). When SnCl4
in THF at -78 °C was employed, the GC
yield improved to 54% but again with an almost equimolar mixture (43:57) of the two products
(entry 7). Unfortunately, regardless of the Lewis acid or solven t tested, the product ratio and
yield could not be improved very much. The cleanest reaction without accompanying
decomposition products was observed for BF3
·OEt
2 in CH
2
Cl 2
at 0 °C. After 5 min quantitative
conversion was achieved with 59% of the two produc ts (43:57), which could be isolated in 20%
combined yield.