Intramolecular ring-opening of cyclopropanones by enolate anions
Magnetic Resonance Imaging
Treatment of cis-4a-methyldecalin-2,7-dione with 2 equiv of bromine gave cis-3,6(dieq)-dibromo-4a-methyldecalin-2,7-dione (9). Dehydrobromination of 9 with LiBr/Li2CO3 in DMF gave cis-6-methylbicyclo[4.4.0]deca-4,7-diene-3,9-dione (3). However, dehydrobromination of 9 with DBU in THF gave 1-bromo-5-methyltricyclo[4.4.0.01,7]decane-2,8-dione (11). Bromination of cis-1-carbomethoxy-2-hydroxy-4a-methylbicyclo[4.4.0]-1-decen-7-one (7), with cupric bromide in CHCl3/ EtOAc provided cis-6(eq)-bromo-1-carbomethoxy-2-hydroxy-4a-methylbicyclo[4.4.0]-1-decen-7-one (13), whereas bromination of 7 with 2 equiv of bromine in CHCl3 provided cis-6(eq)-bromo-8(eq)-bromo-1-carbomethoxy-2-hydroxy-4a-methylbicyclo[4.4.0]-1- decen-7-one(14). Dehydrobromination of 13 with LiBr/Li2CO3 in DMF resulted in the loss of HBr with the formation of the methyl cis-3,4,4a,7,8,8a-hexahydro-2-hydroxy-4a-methyl-7-oxo-1-naphthoate (16). However, dehydrobromination of 13 with DBU in THF gave 16 (17%), 7-carbomethoxy-5-methyltricyclo[4.4.0.01,7]decane-2,8-dione (17) (41%), and 2-carbomethoxy-1-hydroxy-8-methyltetracyclo[4.4.0.01,6.0 2,4]decan-5-one (18) (40%). Dehydrobromination of the dibromo keto ester 14 gave only the tetracyclic ketol ester 20. Bromination of keto ester 7 with NBS and triethylamine gave the two epimers of methyl cis-3,4,4a,5,6,7,8,8a-octahydro-1-bromo-4a-methyl-2,7-dioxo-1(2H)-naphthoate (15). Dehydrobromination of 15a/b with a variety of bases did not give the tricyclic diketo ester 22 but instead resulted in reductive debromination to regenerate starting material 7. The formation of 17 and 18 from 13 and of 20 from 14 are interpreted as occurring by novel intramolecular nucleophilic ring-openings of cyclopropanone-Favorskii intermediates by enolate anions. © 1995 American Chemical Society.
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