[2+2] cycloaddition reaction of cycloheptatriene with dichloroketene. A novel and efficient synthetic strategy for the synthesis of 2-hydroxyazulene

The reaction of cycloheptatriene with dichloroketene, generated by the treatment of trichloroacetyl chloride with activated zinc in dry diethyl ether, affords 9.9-dichlorobicyclo[5.2.0]nona-2,4-dien-8-one 2 as a major product together with 3',3',9,9-tetrachlorospiro(bicyclo[5.2.0]nona-2,4-diene-8,2'-oxetan)-4'-one 3. Structure 3 is elucidated by the X-ray crystallographic analysis of its partially reduced product. Lactone 3 exhibits [1,5] hydrogen-migration in the seven-membered ring to give 3', 3', 9,9-tetrachlorospiro(bicyclo[5.2.0]nona-3,5 -diene-8,2'-oxetan)-4'-one 7 without any decarbonylation. The reaction of 2 with diazomethane exhibits ring expansion of the four-membered ring to give 1,1-dichloro-3,3a,4,8a-tetrahydroazulen-2(1H)-one 8. 2-Hydroxyazulene 9 is successfully derived by the reaction of 8 with lithium chloride in good yield. The reaction of 8 with triethylamine exhibits dehydrochlorination to give 3-chloro-8,8a-dihydroazulen-2(1H)-one 10 in high yield. N-(Azulen-2-yl)pyrrolidine is also derived from either 9 or 10 in good yield.