Cyclobutanols ring-opening

Dihydro-2,3-methano-l,4-naphthoquinones 15 rearrange to cyclobutanols 16 on irradiation. The reaction is. at least in part, reversible, as the excited cyclobutanol undergoes ring opening and disproportionation back to starting material.15 16... 

The 2 + 2 addition to olefins, or e.g., the double bond of the keto-enol form of BM-DBM can also occur, leading to intermediate cyclobutanols which then undergo ring opening (De Mayo reaction) to form structures such as those shown in Figure 9. (Bringhen A. Personal communication 1993). 

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... 

Electrophilic ring opening of bicyclo[l. 1.0]butanes generally leads to mixtures of cyclobutyl, cyclopropylmethyl and allylmethyl compounds, among which the first two products dominate.The parent bicyclo[1.1.0]butane (1) is converted into cyclobutanol and cyclo-propanemethanol, without formation of any but-3-en-l-ol, upon treatment with 0.001 N sulfuric acid. Acetolysis of bicyclo[1.1.0]butane affords a mixture of all three acetates. In general, the product ratio is highly dependent on the substitution pattern of the bicyclic system. 

The influence of chiral inductors on the photochemical cyclization of the adamantane-substituted ketones (48) in zeolites has been examined. Only the endo-products (49) are formed. The best ees are obtained for both derivatives (X = H or F) with (—)-pseudoephedrine as the chiral auxiliary. The cyclobutanols undergo retro-Aldol ring opening to afford the ketones (50). The study was extended to the more heavily substituted derivatives (51). ... 

Sml2 is a selective and mild reducing reagent and was used to prepare p-aminocyclopropanols and -cyclobutanols such as 89 and 91 by reductive ring opening of suitable 5-spiroisoxazolidines <04TL8375>. 

Synthetically useful methods evolve from coupling after a regioselective ring opening of cyclobutanols and capture of p-allylpalladium species that are generated from an intramolecular Heck reaction. ... 

This procedure is an adaptation of a photoannulation reaction, originally developed by de Mayo ( ref 22 ) and leads initially to the stereoselective formation of a cyclobutanol ring (intermediate 44 ) which undergoes a spontaneous ring opening to the typical iridoidic dialdehyde ( intermediate 45 ) which is theoretically in equilibrium with the lactol iridoidic moiety ( intermediate 46 ), as depicted in the scheme 9. 

In this type of reaction of an unsymmetrically substituted cyclobutanol (Eq. 16, R=Ph) with bromobenzene, a single, regioisomeric product, is obtained via cleavage of the less hindered and more easily accessible C-C bond, bond (a), as in the dehydrogenative ring opening of the similar substrate (Eq. 14). The... 

In the arylative ring opening of 3-substituted cyclobutanols, enantioselective cleavage of the C-C bond has been achieved by using a palladium catalyst with a chiral ligand [33-35]. Particularly, the use of the chiral ferrocene-containing N,P-bidentate ligand shown in Eq. 17 leads to excellent enantioselectivity. 

Other than cyclobutanols, the four-membered ring of myrtenal also undergoes the arylative ring opening to afford a monocyclic product with a moderate... 

This process is very fast with = 1, and has been observed in the photolysis of nitrite esters X = NO and the oxidation of cyclopropanols, with a regiochemistry of ring opening which has been compared in terms of frontier molecular orbital theory to the one observed with cyclopropylcarbinyl radicals (Section V.2.B). It is also a fast process with n = 2 as in the oxidation of cyclobutanols, although photolysis of a steroidal cyclobutyl nitrite gives the cyclobutanol in substantial amounts (unexpected), along with products resulting from the )5-scission of the cyclobutanoxyl radical to tertiary and also... 

The Pd-catalyzed reaction of tert-cyclobutanols in the presence of pyridine gives the ring-opening products. For example, 7-vinylbicyclo[4.2.0]octan-7-ol (17) was converted to l-(2-methylenecyclohexane-l-yl)-2-propen-l-one (18) in 56% yield upon tfeat-ment with Pd(OAc)2, pyridine, and MS3A under O2 atmosphere (Scheme 25). The reaction seems to involve /3-carbon cleavage (bond a breaking) from the palladium alkox-ide intermediate. 

Transition metal alkoxides of tert-cyclobutanols undergo ring opening by P-carbon elimination to generate ketones that are metalated at the y-position these can be utilized in many functionalization reactions. Palladium, rhodium, and nickel have been the most studied metals in these transformations. Useful transition metal cyclobutanolates are generated from either cyclobutanones or cyclobutanols. 

Palladium(0)-catalyzed asymmetric arylation of tert-cyclobutanol 33 with bromobenzene, involving enantioselective C-C bond cleavage, furnished the chiral y-arylated ketone 34 (Scheme 3.13) [24]. Good enantioselectivity was observed when a chiral ferrocenyl P,N-ligand was used. y-Alkenylated and y-allenylated ketones were also obtained by asymmetric ring opening. 

An intramolecular variant of palladium-catalyzed arylative ring opening of cyclobutanols was developed, in which P-carbon elimination occurred at the more congested quaternary carbon to produce five-membered carbocycles (Scheme 3.14) [25]. 

Scheme 3.14 Pd-catalyzed intramolecular arylative ring opening of cyclobutanols.

Monosubstituted cyclobutanones reacted with arylboronic acids in the presence of a Rh(I)-P(f-Bu)3 catalyst to afford butyrophenone derivatives by the addition of an arylrhodium(I) species to the carbonyl group, followed by ring opening of the resulting rhodium(I) cyclobutanolate by P-carbon elimination... 

Asymmetric ring-opening reorganization of cyclobutanol 42 proceeded via 1,4-rhodium migration to give 1-indanone 43 in a diastereo- and enantioselective manner (Scheme 3.24) [35]. 

The alkylrhodium(I) species formed by ring opening of 3-(2-silylphenyl)-cyclobutanol 44 underwent a 1,4-rhodium/silicon exchange to generate... 

Scheme 3.23 Rh(l)-catalyzed asymmetric ring opening of cyclobutanol. (DTBM-SEGPHOS 5,5 -bis[bis(3,5-di-ferf-butyl-4-methoxyphenyl)phosphino]-4,4 -bi-1,3-benzodioxole.)...

Under H2, the rhodium(I)-catalyzed C-C bond cleavage reaction of cyclobutanone 73 afforded ring-opened alcohol 74 (Scheme 3.42). Based on the state-of-the-art knowledge of cyclobutanone/cyclobutanol chemistry, we may assume that hydrogenolysis proceeds through ring opening of rhodium cyclobutanolates by P-carbon elimination. 

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