Oxetanes asymmetric

Racemic 5-methyl-5 -(sodiomethyl)-A-(4-methylphenylsulfonyl)sulfoximine reacts with ketones to give an initial methylene transfer which produces an intermediate epoxide that is ring expanded to the oxctanc56. Application to 4-rerf-butylcyclohexanonc affords a single oxetane in 69% yield. While only achiral alkylidcne transfer reagents were utilized, in principle this reaction is amenable to the asymmetric synthesis of oxetanes. 

Asymmetric alcoholyses catalyzed by lipases have been employed for the resolution of lactones with high enantioselectivity. The racemic P-lactone (oxetan-2-one) illustrated in Figure 6.21 was resolved by a lipase-catalyzed alcoholysis to give the corresponding (2S,3 S)-hydroxy benzyl ester and the remaining (3R,4R)-lactone [68]. Tropic acid lactone was resolved by a similar procedure [69]. These reactions are promoted by releasing the strain in the four-membered ring. 

In 1985, Kwiatkowski et al. reported a tetramethyl ethylenediamine (TMEDA)-catalyzed dimerization of ketene giving the interesting compound 4-methylene-oxetane-2-one (diketene). This substance can be hydrogenated by either Pd/C to racemic p-BL as well as by asymmetric catalysis according to Takaya et al. using Ru complexes of (5)-BINAP as catalyst, with an ee of 92% [111] (Fig. 39). 

The double bonds in certain heterocyclic compounds, such as furans, Af-acylpyrroles and A-acylindoles are also susceptible to photoaddition of carbonyl compounds to form oxetanes (equation 106) (77JHC1777). A wide range of carbonyl compounds can be used, including quinones, a-diketones, acyl cyanides, perfluorinated aldehydes and ketones and esters. A remarkable case of asymmetric induction in oxetane formation has been reported from optically active menthyl phenylglyoxylate and 2,3-dimethyl-2-butene the oxetane product obtained after hydrolysis of the ester group had an optical purity of 53% (79AG(E)868). 

A chiral bis(silyl)binaphthol-modified aluminum catalyst has been successfully applied to the asymmetric [2+2] cycloaddition of ketenes to aldehydes, giving optically active 4-substi-tuted oxetan-2-ones with moderate enantioselectivity [69] (Eq. 8A.45). 

Solid-state photochemistry absolute asymmetric oxetane synthesis from an achiral acyclic imide using die chiral crystal environment, J. Org. Chem., 58, 3476-3477. (b) Sakamoto, M., Takahashi, M., Fujita, T.,... 

Isatine derivatives gave the corresponding cycloadducts with high stereoselectivity when irradiated in the presence of furan and benzofuran [72]. The reaction of furan with acyl cyanides yields the corresponding oxetanes, but both diastereoisomeric endo- and exo-oxetanes are formed (Scheme 3.36). When chiral acyl cyanides are used, low asymmetric induction is observed [73]. Furan also reacts with chiral ketones. In this case, an ot-cleavage reaction before the 2+2 cycloaddition modifies the expected products (Scheme 3.37). When (—)-menthone was used as a substrate, a chiral product was obtained as a 2 1 diastereoisomeric mixture the most abundant product has the (1/ , 3R) configuration [74]. When the reaction was performed on carbohydrate 36, a complex reaction mixture was obtained (Scheme 3.38) [74, 75]. 

Zamojski, Scharf, and Bach and colleagues have shown the PB reaction to be applicable for the asymmetric synthesis of oxetanes. In 1982, Zamojski and... 

The first report concerning an asymmetric Paterno-Buchi reaction with a chiral carbonyl component was reported in 1979 by Gotthardt and Lenz [128]. The photocycloaddition of the enantiomerically pure menthyl ester of phenylglyoxylic acid 133 with 2,3-dimethyl-2-butene gave the oxetane 134 with a diastereomeric excess of only 37% (Sch. 45). 

Chiral enamine derivatives have also been used as electron-rich alkenes. The oxazoline derivative 17 reacted with benzaldehyde to yield the two stereoiso-meric oxetanes 18a and 18b with a diastereomeric excess of 67% (Scheme 5) [12]. A significantly higher diastereoselectivity was observed in the case of the reaction of the pyrrolidine derivatives 19 where the enamine function is localized inside the five membered ring [13]. Then the oxetane 20a (R — n-Cgil g) was used in an asymmetric synthesis of the antifungal alkaloid (+ )-preussin. The approach of the 3n,7T excited ketone preferentially occurred syn with respect to... 

In the 80 years since the original report of a photochemical addition to form an oxetane, the Patemo-Btichi reaction has become increasingly familiar to chemists engaged in the synthesis of complex molecules. Existing reviews have summarized mechanistic considerations and defined the scope and limitations of the photocycloaddition reactions. In confluence with the widespread advances in synthetic methods and asymmetric synthesis, the Patemo-BQchi reaction has prospered and led to significant developments in the area of organic synthesis. 

---