Five-membered rings cycloaddition/annulation

A number of other ring systems have been prepared by intramolecular bis-annulation procedures. Pyrrolothiadiazolines 174 were prepared by condensation between hydrazides and compound 173 (Equation 14) <1998JPR676>. The thermally induced intra-intermolecular criss-cross cycloaddition of azine 175 in the presence of phenyl isocyanate leads to heterocyclic compound 176 containing three fused five-membered rings (Equation 15) <2002TL6431>. 

Pauson-Khand annulation of furan 26.81 provided the two exo regioisomers 26.82 and 26.83 in a 2 1 ratio. The entire mixture was then carried through the following steps since both isomers resulted in the same final product, furanether B (18.8). Noteworthy in this approach was the demonstration of tolerance of the furan ring to organometallic cycloaddition conditions and the selective enone reduction in a five-membered ring (steps d and e, Scheme 33). 

Just as in the Diels-Alder reaction, 1,4-dipolar cycloadditions lead to six-membered rings. Their principal use in five-membered heterocycles is for ring annulations giving [5,6] ring-fused systems. 

Higher cycloaddition reactions, using VCPs as the five-carbon unit, have also been developed. Rhodium-catalyzed carbonylation of VCP-enes resulted in [5+2+1] annulation to furnish eight-membered ring enones (Scheme 2.46) [66]. 

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