Oxyallyl zwitterions

Direct interception of the initially formed bicyclic oxyallyl zwitterion derived from cyclohexadienones is difficult, due to its facile rearrangement to cyclopropyl ketone. One notable exception is the observation that 4-trichloromethyl-substituted dienone 94 gave solvent adduct 97 when irradiated in methanol, and both 97 and 98 upon irradiation in acidic methanol (Scheme 24)48. It was proposed that 97 arises either from a facile... 

Analogous olefin trapping chemistry employing 2,7-cyclooctadienone has been investigated51. With this substrate, the oxyallyl zwitterion 112 could be intercepted intermolecularly with simple alkenes in a (presumably stepwise) 3 + 2 manner (Scheme 27). Importantly, intermediate 112 differs from 2,5-cyclohexadienone-derived zwitterions in terms of its lack of a facile rearrangement pathway. An interesting side-issue... 

Only a limited number of examples are known of applications of thietanes in organic synthesis. Prominent among these examples would be electrophilic ring opening reactions leading to polyfunctional sulfur compounds (33)-(37), utilization of 3-thietanones (55) and metal complexes (87) derived therefrom as oxyallyl zwitterion equivalents in cycloaddition reactions, synthesis of dipeptide (63) with a /3-thiolactone, Raney nickel desulfurization of thietanes (e.g. 120 cf. Table 7) as a route to gem-dimethyl compounds, and desulfurization of thietanes (e.g. 17) in the synthesis of cyclopropanes (also see Table 7). 

The reaction regioselectivity can be controlled by a /3-methoxyl or /3-cyano group. In each case, the substituent appears (i on the 7t-bond of the residual enone moiety in the bicyclic photoproduct8 9. Furthermore, regioselectivity of the oxyallyl zwitterionic intermediate generated without light using the Favorskii approach cited above is the same. 

Photoisomerization of pyran-4-one 301 yields pyran-2-one 302 (Scheme 40). The postulated intermediates involved in this transformation resemble those suggested in the irradiation of pyrylium salts <1984CHEC>. In a related example, irradiation of tetramethyl pyran-4-one 303 gives cyclopentenone 304 via reaction of nucleophilic solvent with the initially formed oxyallyl zwitterion intermediate. 

Interestingly, the so-called Zimmerman-Schuster oxyallyl zwitterion may be generated independently by a Favorskii-like route, and leads to the same reaction products as obtained photochemically In addition, the stereochemistry of the rearrangement of the zwitterion to the bicyclo[3.1.0]hex-3-en-2-one could be established, since it was found that with two different aryl groups, the endo aryl group in the zwitterion appeared endo in the bicyclic photoproduct and the exo group remained exo ... 

Photogenerated oxyallyl cations undergo intramolecular cycloadditions when tethered to a furan an example is illustrated in Eq. 16 [44], The cyclo-hexadienone precursors required substituents which provide the right electronic characteristics for the photoconversion to the oxyallyl zwitterions, therefore limiting the versatility of this reaction. 

Fortunately, less electrophilic cations such as oxyallyl zwitterions were able to react with more nucleophilic dienes (Scheme 12), while reaction with less reactive dienes was more difficult.. In one of our studies, the reaction of chloroketone 39 with NEts resulted in only the elimination product 40, while chloroketone 41 afforded two diastereomers of cycloadduct (42a and 42b) in 17 1 ratio, along with chloroketone cycloadduct 43 and elimination product 44. 

We also investigated the cycloaddition of a cyclopentenyl oxyallylic zwitterion derived from a chloroketone that contained the same tethered diene. The chlorination of ketone 48 at the less substituted a-position was accomplished by treatment with LDA and trifluoromethanesulfonyl chloride. The crude product was then treated with sodium trifluoro-... 

There have been several examples of good stereocontrol in 4+3 cycloaddition reactions. For instance, it is known that cyclopentenyl and cyclohexenyl oxyallylic zwitterions bearing a stereogenic center react preferentially with dienes from the face opposite to the substituent. One of the precedents from our laboratory is the intramolecular reaction of ketones 50 and 52 (Scheme 15). Treatment of these ketones with LDA/TfCl afforded the corresponding a-chloroketones, which reacted intramolecularly with the tethered furans to yield cycloadducts 51a,b and 53a,b upon treatment with base. The endo isomers (51a, 53a) dominated in both cases. Only those products derived from the transition states having furans approach the least hindered faces of the oxyallylic cations were produced. 

Enols and Enolates.—The search for the hypothetical oxyallyl zwitterion (36) and the invocations of its intermediacy continue unabated reductive transformations of aa -dibromo-ketones, plausible precursors of the zwitterion, have been extensively studied. Reduction of such ketones with a Zn-Cu couple in DMF yields 2-dimethylamino-4-methylene-l,3-dioxolans (37), by... 

Electrochemical reduction of aa -dibromo-ketones has been studied as a potential route to cyclopropanones no such species was detected, the products obtained being rationalized in terms of the intermediacy of an oxyallyl zwitterion or an enol allylic bromide. Later, however, cyclopropa-none hemiacetals were isolated from such reductions in methanol. 

The oxyallyl zwitterion is involved in a mechanistic scheme to explain the production of cycloheptenones, cyclopentanones, and cyclopentenones by the reaction of aa -dibromo-ketones with iron carbonyl (Scheme 90). The intermediacy of organoiron halide complexes is proposed to account for... 

West, EG., Hartke-Karger, C., Koch, D.J., Kuehn, C.E., and Arif, A.M., Intramolecular [4-1-3]-cycloadditions of photochemically generated oxyallyl zwitterions a route to functionalized cyclooc-tanoid skeletons,/. Org. Chem., 58, 6795-6803, 1993. 

Formation and Rearrangement of Epoxycyclopentenone Intermediates Evidence for Oxyallyl Zwitterion Intermediates... 

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