1,3-Cyclopentenedione ring

Carbene complexes are versatile intermediates in ring construction [47], The insertion of alkoxycarbenechromium complexes 93 into cyclobutenediones 63 leads to the ring-expanded cyclopentenediones 94 as shown in Scheme 33 [48]. 

It is thus clear that thermal rearrangements of 2,3-diazido-l,4-quinones proceed in two discrete stages. The first involves the formation of the diacyl cyanides and 2-azido-2-cyano-l,3-cyclopentenediones the second entails the subsequent ring expansion of these cyclic azides to 2-aza-3-... 

The dipolar addition reaction between DMAD or dimethyl maleate and the diimine species [Fe(CNR)3(Pr N=C HCH=NPr>)] (R = Bu , CH2Ph, Cy, 2,6-xylyl etc.) occurs to give complexes of type (4) in the case of [Fe(CO)2(CNR)(Pr N=CHCH=NPri)] there is a competition between CO and CNR insertion into the metallacycle. Compounds containing fused pyrrole rings arc also formed which on heating release substituted 2-vinylpyrroles. -64 similar cycloaddition also occurs between [Fe(C0)3(Bu N=CHCR=0)] (R = Me, Ph) and DMAD. 5 Purther details of the insertion of alkynes into vinylketene iron tricarbonyl complexes have appeared thermolysis of the insertion products gives cyclopentenediones or phenols. -... 

Ring Contraction Reactions.—The photochemical rearrangement of the kojic acid derivative (133) via the postulated intermediate (134) gave in low yield the ring-contracted cyclopentenedione (135) which was reduced in situ to terrein (136) (Scheme 43). °°... 

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