Cyclobutanol reactivity

In a novel method of forming cyclobutanols, titanium(IV) chloride is found to promote the condensation of substituted bisketene silyl acetals 23 with more reactive carbon electrophiles such as acetic anhydride or benzoic anhydride. In each experiment, it is apparent that only one diastercomer of the substituted 2,4-dicarbomethoxycyclobutanols 24 is furnished, as can be vindicated by NMR spectral analysis.33... 

A laser flash study of the photoreactions of hexan-2-one and 5-methylhexan-2-one has provided evidence for the existence of the triplet 1,4-biradicals produced by the y-hydrogen abstraction typical of Norrish Type II reactivity. The photochemical behaviour of the alkanone, nonan-5-one, in urea inclusion compounds has been studied. In solution, irradiation of nonan-5-one yields hexan-2-one, propylene, and two cyclobutanols. In the clathrate, the fragmentation products were essentially the same but only one cyclobutanol was observed. The cyclization fragmentation ratio was established as 0.67, compared with 0.32 in methanol. The authors suggest that the CIS-cyclobutanol has less stringent rotational requirements and that it is this isomer (43) which is formed in the clathrate. 

The photocatalytic isomerization is explained by intramolecular hydrogen migration. The reactive state is an n-n triplet and the products arise from a common biradical intermediate. The cis isomer does not react. Irradiation of oxiranes not containing the phenyl group gives rise to cyclobutanol epimers (Eq. 350). ... 

O-Pivaloyl-D-galactopyranosides were shown to be efficient stereodifferentiating tools [9]. Thus, the [2-1-2] cycloaddition of dichloroketene to chiral vinyl galactoside 27 afforded the cyclobutanone 28 with reasonable stereoselectivity (dr 4 1) [26] (Scheme 10.6). The resulting 2,2-dichlorocyclobutanones are reactive and often cannot be isolated in pure form. More stable cyclobutanols were isolated after reduction of the keto group [26]. 

The main products formed are methylene cyclobutanols 162. In other words, the chief elimination path leads to a cyclohexadiene. Moreover, comparison of reactivity of two ketone enolates showed that the amount of minor cyclohexyne de-... 

The two ketones (28) and (29) are known to undergo the Norrish Type II hydrogen abstraction process, and their photochemical reactivities have now been studied in chirally modified zeolites. The zeolites were modified by stirring them with known amounts of ( —)-ephedrine. Irradiation of the ketones in the zeolites brought about some enantiomeric enhancement. However, the various zeolites studied behaved differently and the NaX zeolite favoured the (+)-isomer of the product (30) while the NaY favoured the ( —)-isomer. The other ketone (29) showed only low enantiomeric enhancement and gave both the cis and the trans cyclobutanols (31) and (32) in a ratio of 4 1. ... 

Photoreactions of Thymines, etc. - Irradiation at 254 nm of the pyrimidine derivative (153) induces a Norrish Type II hydrogen abstraction from a methyl group of the t-butyl substituent. The resultant 1,4-biradical (153a) undergoes cyclization to afford an unstable cyclobutanol. Elimination of water from this species affords the final product identified as the cyclobutane derivative (154). The structure of this product was verified by X-ray diffraction techniques. The Norrish type II reactivity of the pyrimidine derivative (155) at 254 nm in water follows the analogous path to that observed for (153) and yields the cyclized product (156) in 52 % yield. - °... 

A study of the photochemical reactivity of salts of the amino ketone (44) with enantiomerically pure carboxylates has been reported. The irradiations involved the crystalline materials using A, > 290 nm and the reactions are fairly selective which is proposed to be the result of hindered motion within the crystalline environment. Some of the many results, using (S)-(—)-malic acid, R-(+)-malic acid and (2R,3R)-(+)-tartaric acid, are shown in Scheme 1. The principal reaction in all of the examples is a Norrish Type II hydrogen abstraction and the formation of a 1,4-biradical. This leads mainly to the cis-cyclobutanol (45) by bond formation or the keto alkene (46) by fission within the biradical. A very minor path for the malate example is cyclization to the trn 5-cyclobutanol (47). A detailed examination of the photochemical behaviour of a series of large ring diketones (48) has been carried out. Irradiation in both the solid phase and solution were compared. Norrish Type II reactivity dominates and affords two cyclobutanols (49), (50) and a ring-opened product (51) via the conventional 1,4-biradical. Only the diketone (48a) is unreactive... 

The triplet state of the ecdysone (152) is reactive and aRords the products shown in Scheme 5. The formation of the reduction products (153) and (154) is presumed to follow a path where loss of a hydroxyl radical yields the allyl radical (155) which then gives the products (153) and (154) by hydrogen abstraction. The ketone (156) is formed by a 1,2-bond migration and the cyclobutanol (157) arises by secondary irradiation of the diketone (156). The enone (158) shows solvent dependent photochemistry. Thus in ethyl acetate the deconjugated product (159) is formed while in methanol the reduced ketone (160) is produced. ... 

Solid-state photolysis of a salt crystal formed between a prochiral, photo-chemically reactive keto-acid of 2-benzoyladamantane 44 and a nonabsorbing optically active amine leads to enantioselective Norrish/Yang photocyclization giving the optically active cyclobutanol 45 (Scheme 11) [59-61]. Irradiation of a total of 17 salts gave moderate to near-quantitative enantiomeric excesses. One of the best results was obtained using the prolinol salts 44 formed dimorphic salt crystals with with (5)-( + )-prolinol, of which the needle-shaped crystals gave... 

Norrish Type II reactivity is often a common reaction path for ketones with available Y 7< °sens. Hydrogen abstraction by the excited carbonyl group results in the formation of a 1,4-biradical which can undergo either bond cleavage to reform the carbonyl group and an alkene or bond formation to yield a cyclobutanol derivative. The fragmentation path is followed by the ketone (13). The interest in this reaction is the control which can be exercised on the ketonization of the resultant enol (14). Apparently in the presence of (->-ephedrine asymmetric formation of the final product, (R)-2-methylindanone (15),... 

We have investigated the Norrish II reactivity of 4-methoxyvalerophenone at a single temperature (30°C) in various isotropic solvents, nematic EB, and the smectic phases of CCH-4 and CCH-2 (34). F/C and t/c cyclobutanol ratios obtained from photolysis of this compound in these solvents are shown in Table 1. Both the F/C and tic ratios are significantly larger in CCH-4 than in the nematic or isotropic phases studied. This result seems a bit surprising at first, in view of the very small difference in the shapes of the transoid and cisoid biradicals derived from this ketone. However, we have recently obtained deuterium NMR evidence that suggests that this... 

Cleavage of a cyclobutanol subunit also creates a site for CC coupling, ring strain is the cause for such reactivity. 

All these fairly similar results are, however, in sharp contrast to solvolysis of cyclopropylmethyl chloride, which is much more reactive than cyclobutyl chloride or 4-chlorobut-2-ene (Roberts and Mazur, 1951 a). There was also positive evidence for internal return in the cyclopropylmethyl chloride solvolysis, although the solvent used (50 aqueous ethanol) is known to be unfavorable for internal return. Another reaction, closely related to the protolysis of cyclopropyldiazomethane mentioned above, is the reaction of that diazoalkane with ethereal benzoic acid. The product ratio cyclopropylmethanol cyclobutanol = 5.8 indicated a strong decrease of skeletal rearrangement (Moss and Shulman, 1968). 

Norrish Type II hydrogen abstraction to afford the usual biradicals, which can cyclize into cyclobutanols. Both the c/ -(13) and the ra -isomeric forms are possible. This particular investigation has studied the influence of antibodies (12B4, 20F10 ad 21H9) on the cyclization reaction. The authors observed that the most reactive antibody, 20F10, catalyses the formation of the cw-product... 

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