Synthesis of dispiro hetero analogs of pyrrolizidine alkaloids

Reactions of spiro heterocyclic enamines with 1H-pyrrole-2,3-diones, including those fused at the N–C bond to nitrogen-containing heterocycles, were not studied. We have found that 3-aroyl-1H-pyrrolo[2,1-c][1,4]benzoxazine-1,2,4-triones Ia and Ib react with an equimolar amount of 2′,5′,5′-trimethyl-4′,5′-dihydro-4H-spiro[naphthalene-1,3′-pyrrol]-4-one (II) (which may be regarded as potential 1,3-C,N-binucleophile) in boiling anhydrous benzene (reaction time 2–5 min, until bright violet color typical of the initial pyrrolobenzoxazinetriones disappeared) to give 3′′-aroyl-4′′-hydroxy-1′′-(2-hydroxyphenyl)-3′,3′-dimethyl-2′,3′-dihydrodispiro[naphthalene-1,1′-pyrrolizine-6′,2′′-pyrrole]-4,5′,5′′(1′′H)-triones IIIa and IIIb in almost quantitative yield. The spectral parameters of compounds IIIa and IIIb are very similar to those found for model ethyl 4-benzoyl-3-hydroxy-1-(2-hydroxyphenyl)-8-methyl-2,6-dioxo-1,7-diazaspiro[4.4]nona-3,8-diene-9-carboxylate whose structure was proved by X-ray analysis [1]. Presumably, the first step is addition of the activated β-CH group in the enamino tautomer of II at the C atom in Ia or Ib, followed by closure of pyrrole ring via intramolecular attack by the amino group of the enamine fragment on the lactone carbonyl carbon atom in the oxazine ring and opening of the latter at the C–O bond, as was reported by us previously for the reaction of pyrrolobenzoxazinetriones with 1-methyl3,4-dihydroisoquinolines [2]. The described reaction is an example of regioselective synthesis of previously inaccessible dispiro heterocyclic system with various substituents in several positions of both heterocyclic fragments. The products may be regarded as dispiro heterocyclic analogs of pyrrolizidine alkaloids [3]. 4′′-Hydroxy-1′′-(2-hydroxyphenyl)-3′,3′-dimethyl-3′′-(4-methylbenzoyl)-2′,3′-dihydrodispiro[naph-