Cyclopentenediones

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]. 

With sodium methoxide, arylidene derivatives 209 undergo a rearrangement into photochromic dihetaryl-substituted cyclopentenediones 211. They are easily alkylated to form geminally substituted products 212 in 91-94% yields (07MC301). Their photochemical properties were investigated (07OS975, 08IZV853). 

Difluoromethylene groups in fluorinated cyclopentenes and cyclohexenes are hydrolyzed to oxo groups on heating with oleum and sulfur trioxide. 1,2-Dichlorohexafluorocyclopentene (17) is converted into cyclopentenone 18 and cyclopentenedione 19.146... 

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-... 

Another group of unstable carbanions are those with antiaromatic character (Scheme 5.71). Thus, cyclopropenyl anions or oxycyclobutadienes, generated by deprotonation of cyclopropenes or cyclobutenones, respectively, will be highly reactive and will tend to undergo unexpected side reactions. Similarly, cyclopentenediones are difficult to deprotonate and alkylate, because the intermediate enolates are electronically related to cyclopentadienone and thus to the antiaromatic cyclopenta-dienyl cation. 

Vinylcyclopentenediones.1 The Fischer carbene 1 reacts with terminal alkynes (excess) in benzene (70°) to give as the major product a 2-vinylcyclopentene-l,3-dione (2) with incorporation of two molecules of carbon monoxide as well as the alkyne and formation of six C-C bonds. Reaction of an internal alkyne is possible, but the yield of cyclopentenediones is lower (27%, one example). 

Furthermore, cyclopentenedione (62) reacts with 1,3-dithiolylium tetrafluoroborate (1) in aqueous ethanol with formation of compound (63) which can be easily oxidized using 2,3-dichloro-5,6-dicyanobenzoquinone (DDQ) to produce the dithiafulvalene derivative (64) (77CL77). 

Triptycene cyclopentenedione 47 underwent 1,3-DC with nitrile oxides affording polycyclic isoxazolines 48 bearing the triptycene moiety. The adducts 48 exist in solution in their two enolic forms 49 and 50, in equimolecular amounts <02JOC4612>. 

Many reactions afford, in addition to the naphthalene derivatives, mixtures of products. One of the major side reactions is the formation of indene derivatives directly from the metathesis intermediate without insertion of carbon monoxide. Other products frequently isolated as minor components are furans, cyclobutanones, and cyclopentenediones. The product distribution is dependent on numerous factors, including solvent, reaction temperature, concentration of the alkyne, and the nature of the aryl substituent. Molybdenum carbenes give, depending on the alkyne, either naphthols or indenes exclusively. The molybdenum Fischer carbenes can be tuned to give naphthoquinones by replacing one of the carbon monoxide ligands with a phosphine (Scheme 45). 

The reaction of cyclopropenone with ketene in the presence of Ni(CO)4 affords cyclopentenedione as a mixture of regioisomers [111]. 

Noble, M., Noble, D., and Fletton, R A. (1978) G2201-C, a new cyclopentenedione antibiotic isolated from the fermentation broth of Streptomy-ces cattleya. J Antibiot 31,15—18... 

Figure 5.10. Proposed mechanism for formation of a cyclopentenedione product from an orfho-quinone derived from tetrachlorocatechol. From McKague et al. (1987). Reprinted by permission of Walter de Gruyter Co.

F. tert-Butyhyanoketene. Tj-pically the ketene is prepared by dissolving 1 g. of 2,5-diazido-3,6-di-starting material as well as the intermediate cyclopentenedione is followed by thin layer chromatography v(Note 17). When the cyclopentenedione is no longer detectable, after approximately 90 minutes, the heating is stopped. The solution contains ferf-butylcyanoketene in amounts equivalent to at least a 95% yield (Note 18). 

Michael/Henry/dehydration/aromatisation reaction of 2-(2-oxoethyl) benzal-dehydes and nitroalkenes mediated by pyrrolidine to obtain polysubstituted naphthalene derivatives. DBU catalysed the conjugate additions of alcohols to ot,p-unsaturated nitriles, esters and ketones. Perhaps more important are the aza-Michael addition reactions of amines to a,p-unsaturated ketones, nitriles and esters. Recently, Costa, Vilarrasa and coworkers described the addition of lactams, imides, 2-pyridone, pyrimidine-2,4-diones and inosines to methyl propiolate and other similar compounds, DABCO and DMAP being the best catalysts. As mentioned before, tertiary amines give zwitterionic species with activated allynes. It was as early as 1932 when Diels and Alder used the reaction of pyridine with dimethyl acetylenedicarbojylate (DMAD) for the synthesis of heterocycles. The interception of the corresponding intermediate with Al-tosylimines and activated olefins provided access to l-azadienes ° and highly substituted butadienes (Scheme 2.6). When the quenching species of the zwitterionic intermediate is a 1,2-diketone, dibenzoyl maleates or cyclopentenedione derivatives could be obtained (Scheme 2.6). The interception of the zwitterionic species of N-methyl imidazole (NMI) and DMAD with ketenes to obtain unsaturated esters has also been shown. ... 

The effects of rt-donors and acceptors on the can be seen in Figure 12.60. Relative to ethylene, values for 1,1-difluoroethylene, ketenes, and cyclopentenedione are more negative, while the effects of a lone pairs (ji-donors) give more positive values. 

The 1-oxaspiropentane (425) is transformed into cyclopentenones (426a) and (426b) by aluminium chloride, and the ketone (427) is rearranged to cyclopentenedione (428) with acid. ... 

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. -... 

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