Compounds containing Two Fused Five-Membered Rings

Compounds containing Two Fused Five-Membered Rings (5,5) 273... 

This chapter covers bicyclic ring systems containing two five-membered rings where each ring contains one heteroatom and neither heteroatom is situated at the position of the ring junction. These 5 5 fused heterocyclic systems are represented by the general structures 1-4, wherein X and Y may be the same or different heteroatoms and represent O, NH, S, Se, and Te. The fully conjugated title compounds have a central C-C bond and are isoelectronic with the 10-7i-electron pentalene dianion 5 (Fig. 1). 

Intramolecular conjugate addition is most common with a readily enoUzable Michael donor, such as a 1,3-dicarbonyl compound. For example, the mild base K2CO3 promotes the cyclization of the p-keto-ester 36 by a 5-exo ring closure (1.48). The product 37 contains two five-membered rings fused cis to each other, as would be expected on the basis of the thermodynamic stability of such bicyclo[3.3.0]octane ring systems. 

In spite of the diverse nature of alkaloid structures, two structural units, i.e. fused pyrrolidine and piperidine rings in different oxidation states, appear as rather common denominators. We therefore chose to give several examples for four types of synthetic reactions which have frequently been used in alkaloid total synthesis and which provide generally useful routes to polycyclic compounds with five- or six-membered rings containing one nitrogen atom. These are ... 

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