Ring expansion cyclobutanones

Similarly, a bicyclic vinylcyclopropanol 18 underwent a regioselective hydroxylation with concomitant cyclopropane to cyclobutanone ring expansion when treated with vanadyl acetyl-acetonate and ter/-butyl hydroperoxide. 

This chapter begins by classifying the combinations of oxidation/reduction processes with subsequent cationic transformations, though to date the details of only two examples have been published. The first example comprises an asymmetric epoxidation/ring expansion domino process of aryl-substituted cyclopropyl-idenes (e. g., 7-1) to provide chiral cyclobutanones 7-3 via 7-2, which was first described by Fukumoto and coworkers (Scheme 7.1) [2]. 

The same epoxide 335 was easily obtained in mild conditions (0°C, 5 min) by m-ehloroperbenzoic acid oxidation [13b]. Epoxidation of alkylidenecyclo-propanes by m-chloroperbenzoie acid has been greatly exploited as a route to the synthesis of cyclobutanones 638 via the well known ring expansion of oxaspiropentanes 637 (Scheme 98) [176,177,8]. 

Treatment of the adducts of 30 and ketones with excess HC1 in the presence of a Lewis acid leads directly to an acyclic system as in 85 (Eq. 105) 122). It is likely that the thiophenol liberated in the ring expansion to the cyclobutanone 86... 

The ring expansion of cyclobutane derivatives to other carbocycles remains to be one of the most powerful tool in synthetic organic chemistry. Cyclobutanones are exceptionally facile starting materials for the preparations of y-lactones as well as cyclopentanones. 

Enantioselective [2 + 2 cycloaddition.2 The chiral allylic ether (1), prepared from (lS,2R)-( + )-2-phenylcyclohexanol, undergoes enantioselective cycloaddition with dichloroketene to furnish, after one crystallization, optically pure (-)-2. This cyclobutanone after ring expansion and exposure to chromium(II) perchlorate gives... 

In work which remains unpublished, Wenkert has succeeded in cleverly transforming 2-methylcyclopentanone into isocomene The key elements of his strategy (Scheme LXXXIII) are the acid-catalyzed ring expansion of methoxycyclopropane 747 to 748 and the regiospecific homologation of the cyclobutanone to 749. Unfortunately, the Wolff-Kishner reduction of this penultimate intermediate affords both 731 and its epimer. 

Proof that the intermediate cyclopropanones are involved in these reactions was shown by spectroscopic observation, isolation and trapping with various dipolarophiles.1 3 Yields of the double ring expansion products are quite good ranging up to 90% for the parent cyclobutanone,1 but the separation of the regioisomeric mixture can be tedious. 

A similar example is the ring expansion of cyclobutanone via l-tris(methylsulfanyl)methyl-cyclobutanol (6) to 2,2-bis(methylsulfanyl)cyclopentanone (7).43 The use of Af,A-diisopropyl-ethylamine is unneccessary in this reaction as the hydroxy group has already been deprotonated with butyllithium. Further examples of this type of reaction can be found in refs 43 and 44. 

Ring Expansion of Cyclobutanones 3-Methyl-3-phenylcyclopentanone (16) Typical Procedure 51... 

Cyclobutanone has been prepared by (1) reaction of diazomethane with ketene,4 (2) treatment of methylenecyclobutane with performic acid, followed by cleavage of the resulting glycol with lead tetraacetate,5 (3) ozonolysis of methylenecyclobutane, (4) epoxidalion of methylene-cyclopropane followed by acid-catalyzed ring expansion,7 and (5) oxidative cleavage of cyclobutane trimethylene thioketal, which in turn is prepared from 2-(cu-chloropropyl)-l,3-dithiane.8... 

The key steps in the ring expansion of the cyclobutanones (186) and (787) are the Baeyer-Villiger oxidation effected by H202—HOAc. It is noteworthy to point out that the Baeyer-Villiger oxidation is regiospecific and serves to be an excellent method for the preparation of y-lactone from cyclobutanones. 

There have been three reported syntheses of 1,2-diazocines by means of ring-expansive cycloaddition reactions. In the first case, Sasaki and coworkers reacted 4,4-dimethyl-3,5-diphenylisopyrazole 25 with diphe-nylcyclopropenone to obtain diazocine 26 (73SC249). The second cycloaddition was achieved by Haddadin et al., who condensed cyclobutanone with tetrazine 27 (Ar = Ph) to afford diazocine 28 (Ar = Ph, X = O) or 29, depending on whether methanolic base or diethylamine was used as catalyst (84TL2577). A similar reaction was used to prepare a triazocine see Section IV,A,1. 

The first recorded cyclopropyl acyl silane (69) was generated by vapour phase pyrolysis of a pyrazoline derived from a,/)-unsaturated acyl silane by 1,3-dipolar cycloaddition of diazomethane (vide supra, Section DTE)141. Exposure of 69 to titanium tetrachloride induced ring expansion to give the cyclobutanone in 75% yield (Scheme 113). 

Quinkert has proposed a concerted mechanism for the photochemical ring expansion of cyclobutanones Stohrer, W. D., Jacobs, P., Kaiser, K. H., Weich, G., Quinkert, G. Top. 

Cyclobutanones 50 can be synthesized by a lithium iodide induced ring expansion of oxaspiropentanes 51. Treatment of the cyclobutanones 50 with />-toluenesulfonic acid (p-TSA) in benzene under reflux affords 2//-chromenes via a tosylate induced ring fission of the intermediate cyclobutyl cation 52 (Scheme 16). The oxaspiropentane 51 (R1 = Me, R2 = H, R3 = 4-OMe) can be directly transformed into the corresponding 277-chromene upon treatment with/>-TSA in 55% yield <2004T449>. 

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