Cyclopropazulenes [article]

Alan Payne
2002
At the outset of this study, cyclopropa-fused azulenes represented an unknown class of nonbenzenoid cycloproparenes. The research described in this thesis outlines the synthesis of the first examples of such hydrocarbons, namely 2-chloro-l,ldimethyl-l//-cycloprop[e]azulene (357), 1,1 -dimethyl-l//-cycloprop[e]azulene (386) and 1,1-dimethyl-l//-cycloprop[/]azulene (261). (357) R = ci (386) R = H (261) The first route towards cyclopropazulenes that was examined involved the transannular aldol
more » ... nsannular aldol condensation of a derivative of \Hbenzo[a]cyclopropa[/]cyclodecene-3,8-dione. Thus, the dione (202) gave the transannular aldol product (221) as the sole regioisomer upon acid catalysis. The aldol (221) could not be dehydrated to give the conjugated enone under a variety of conditions. Other attempts to convert the aldol (221) into \Hbenzo[/]cycloprop[o]azulene were unsuccessful. (221) In view of this, a related dione system with an epoxy bridge positioned to control the direction of the transannular aldol condensation was synthesized. Treatment of the dione (231) with base gave the enone (232) which was converted into the benzofulvene (250). However, the removal of the epoxy-bridge and the debromination required for the formation of the cyclopropazulene (258) could not be achieved. S" -#- The second route examined the potential for photochemical extrusion of nitrogen from an azulenopyrazole such as (342) to form the cyclopropazulenes. In an investigation towards the synthesis of (342), the thienopyrazoles (274) , (278) and (293) , as well as the furopyrazole (282) , were synthesized. These pyrazole-fused heterocycles showed interesting divergent photochemistry. For example, the irradiation of the furopyrazole (282) gave the fragmented product (299), while the thiophene (278) gave the 2-propenyl derivative (311). N o cyclopropa-fused heterocycles where observed in any of the photochemical reactions. (299) hv R = /-Bu X = 0 IN T-'MIIII 4j-" R = Ph, X = S (274) R = /-Bu,X = S(278) R = Me, X = S (293) R = /-Bu, X = 0 (282) hv R = /-Bu N X = S ^ \ N HJ (311) (331) (332) The pyrazole-fused a-pyrones (331) and (332) were prepared to provide access to the azulenopyrazole (342) via a Houk-Leaver azulene synthesis. However, treatment ii of the cc-pyrone (331) with A^-dimethylaminofulvene gave the azulenopyrazole (342) in disappointingly low yields, while (332) did not give any azulenic products under the same reaction conditions. The cyclopropazulenes (357) , (385) and (261) were finally synthesized from the sulfones (347) and (387) . Thus, the adducts of 2-diazopropane and the sulfones (347) and (387) were dehydrochlorinated in the presence of iV^V-dimethylaminofulvene to give the azulenopyrazoles (353) , (386) and (199) , which on irradiation yielded the cyclopropazulenes (357) , (386) and (261) respectively. The cyclopropazulenes exhibited similar n.m.r spectral properties to their benzocyclopropene counterparts; however, crystal structures of (386), (261) and the 1,3,5-trinitrobenzene complex of (357) showed that the cyclopropa-fusion did not cause marked bond alternation. o 2 CI R CI (347) R = CI (387) R = H 1. M e 2 C N 2 2. NEt 3 3. Dimethylaminofulvene (353) R = CI (385) R = H hv (199) hv (357) R = CI (386) R = H
doi:10.26182/5d088c7b733d2 fatcat:wjwopmj4xza2nimrebyn6dptgu