No abstract

Thermal Cyclization of α‐Alkynones to 2‐Cyclopentenones Gas phase thermolysis at 600–740° of substituted 1‐pentyn‐3‐ones (α‐alkynones), which are easily prepared by acylation of trimethylsilyl acetylenes, leads to substituted 2‐cyclopentenones. The intramolecular formation of a new C, C‐bond between an acetylenic and a non‐activated carbon atom is accompanied by a [1,2]‐migration of one of the substituents at the triple bond. This novel ‘‐alkynone cyclization’ reaction may be explained by postulating an alkylidene carbene intermediate which inserts into a C,H‐bond five carbon atoms away at the non‐acetylenic part of the ketone. Several examples demonstrate that the α‐alkynone cyclization offers a simple tool for the preparation of certain monocyclic, bicyclic and spiro compounds containing a 2‐cyclopentenone moiety.

Earlier work has shown that the cyclo-addition of a ketene to a conjugated diene is always (a) 2 + 2, (b) polarily directed, and (c) suprafacial with respect to the diene C=C. The adducts of ketenes and cyclopentadiene are thus always 7-substituted bicyclo [3.2.0] hept-2-ene-6-ones. New evidence is presented to show that unsymmetrically substituted ketenes add to cyclopentadiene in such a manner that the larger substituent has a greater tendency to take up the endoposition in the adduct. This is interpreted to mean that a ketene participates in such reactions antarafacially. Thus the ketene approaches cyclopentadiene (a) with its functional plane perpendicular to that of the ring, (b) with the carbonyl carbon over the middle of the ring, and (c) with the larger of the two substituents oriented preferentially away from the ring (transition state 11).This endo-specificity for the larger ketene substituent is demonstrated by the indicated endo/ exo ratios observed in the cyclo-adducts from ketenes with the following substituent pairs :C,H,/CH, = >95/<5, CH,/Cl = SO/ZO, CH,/CH=CH, = -65/35, C,H,/CH, = -60/40, n-C,H,/CH, = -60/40, CH,/Br = 56/44. These ratios enable a list to be compiled indicating the endo-specificity of the ketene substituents. The order closely parallels the space filling capacity as derived by other methods. The establishment of such ratios required reliable configurational assignments at carbon 7. These were derived by five methods based upon the following effects: (1) Both H-C7 and CH3-C7 cause nmr. signals at higher field in endo-position (compared with e m ) . (2) The CH,-C7 group in exo-position gives rise to a nuclear Overhauser effect with the vicinal H-C1, and in one case also with the trans-annular H-C5. (3) The nmr.-coupling constants of H-C7-exo (observed at H-C7) with H-C1 is always iarger than of H-C7-endo. (4) The coupling constant of H-C7-exo with H-C6(known to be exo-) of the LiAlH, reduction products of the cyclo-adducts (observed at H-C6) is always larger than that of H-C7-endo. (5) The nmr. signals of most protons in the cyclo-adducts are a t higher field in benzene than in chloroform solution; this "benzene shift" is larger for H-C7 or for CH,-C7 when in exo-than when in endo-position.Ketene (1) reagieren mit Olefinen (2) gewohnlich unter Bildung von Cyclobutanonen (3) [l] [a]. Die kinetischen Parameter und Losungsmitteleffekte zeigen [3], dass es sich -I -A-)=c=o .+ )={ + 4-4 1 2 3 dabei um eine Mehrzentren-Cycloaddition handelt . Zur Erklarung der Beschleunigung und der sehr wirksamen konstitutione118n Spezifitat bei Olefinen mit Donator-Substituenten (4) nimmt man einen polaren ubergangszustand (5) an [3]. Sorgfaltige x 1 4 5 Analysen der Cycloaddukte weisen auf eine ebenfalls sehr wirksame Stereospezifitat im Sinne einer syn-(= suprafacialen) Addition an das Olefin (6) [4] [5] hin. Besonders ---_ _---: f 6 interessant, und bis vor kurzem immer wieder zu Widerspruchen Anlass gebend, ist die offenbar stark bevorzugte ( 2 + 2)-Cycloaddition (7) auch in Fallen (8) wo eine (2 + ...

No abstract

It is shown, in the case ofthe diketone 2, that chromatography with achiral phases of a non-racemic mixture of enantiomers can furnish fractions which differ in enantiomeric excess. Such chromatography may, therefore, be used to further enrich a sample in one enantiomer. By thc same token, chromatography is not a generally safe method for the purification of the product of an enantio-differentiating process, if the enantiomeric excess of a purified portion of that product is taken to be a measure of the efficiency of the process. The described effect represents an enantiomer differentiation induced solely by an alredy existing enantiomeric excess during chromatography. It thus belongs to a class of effects where the relative amounts of two enantiomers induce an observable difference between them. Such effects are called EE effects. The coinmon principle underlying EE effects is explained by a simple symmetry argument. Since EE effects can also occur during reactions with achiral reagents, further transformations of an enantionier-enriched product may furnish false information on its enantiomeric excess.