The [2+2]cycloaddition of 2,3-dihydrofuran (1) to phenylisocyanate (2a
) at 100 degrees C for 20 h gave bicyclic beta-lactam, 7-phenyl-2-oxa-
7-azabicyclo[3.2.0]heptan-6-one (3a) with 86% yield. The reaction gave
a good result when the reaction was carried out in the absence of cat
alyst in nonpolar solvent. The [2+2]cycloaddition of I to various aryl
isocyanates at 100 degrees C for 20 h gave corresponding bicyclic bet
a-lactams in high yield and selectivity, however, the reactions of 1 w
ith alkyl isocyanates and of other vinyl ethers with 2a did not occur.
A concerted mechanism was supposed on the [2+2]cycloaddition of 1 to
2a.