San1ariun1 triiodide catalyzed cycloaddition of epoxides with isocyanates : a facile synthesis of oxazolidinones

Oxazolidinones were synthesized in high yields via cycloaddition of epoxides with isocyanates catalyzed by samarium triiodide.

Organolanthanide· chemistry is of great interest now and the reports on using samarium(nl) in organic chemistry have rapidly increased'. For example, a-haloketones could react with aldehydes to give a,j3-unsaturated ketones promoted by Smi 3 2 . Also mediated by Sml3• 13-diketones or 13-ketoesters could conden~e with aldehydes to form benzylidene-substituted 13-diketones or 13-ketoesters 3 . Besides, catalyzed by Sml 3 • tetrahydrofuran ring could be opened with acyl chlorides or acid anhydrides to yield 4iodobutyl esters-1· 5 . Oxazolidinones are widely used in drug and polymeric materials. Among reported methods, the cycloaddition of epoxides with isocyanates seems to be a simple and convenient one for synthesis of oxazulidinones 6 . As n result. a number of catalysts have been developed for this reaction with varying degree of success. However, in the reaction using these catalysts, vigorous reaction temperature and/or reactive polar solvents are required and thus they are often accompanied by undesirable reactions, such as the trimerization of isocynnates and addition to solvents. Therefore, in some cases dropwise addition is necessary to suppress the undesirable reactions 7 . Recently n-Bu 1 Snls 9 .
Ph 3 PO' and n-Bu3Sni-Ph-1Sb have been reported to be relatively effective catalysts for this reaction. However, till date, only one document concerning the addition reaction of epoxides with heterocumulenenes catalyzed by rare earth compounds has been reportedw. Herein we wish to report Sml 3 catalyzed cycloaddition of epoxides with isocyanates to give oxazolidinones in high yields under very mild con-

Results and discussion
On treating epoxides I and isocyanates 2 with 10 mol% Sml 3 for about 3 h at room temperature, oxazolidinones were obtained in high yields. The results are summarized in Table I. In our experiment, the reaction conditions were well studied. We found that only catalytic amount of Smi 3 (10 mol% based on isocyanates) was needed in the reaction to give satisfactory yields of the products. On using higher amounts of Sml 1 (20. 50. XO%) or even stoichiometric amount of Sml 1, the yields were not increased much.
According to reported work 10 , a possible mechanism of Sml 3 catalyzed cycloaddition of epoxides with isocyanates is presented in Scheme 2.
As shown in Scheme 2, initially an intermediate 4 is produced in sitll via ring-opening of epoxide by Sml 3 . Then further addition of 4 to isocyanate 2 generates intermediate 5. Finally. substitution of iodo group by nitrogen anion leads to the cycloaddition product 3. It can be concluded that only catalytic amounts of Sml 3 is needed for the reaction. Besides. since intermediate 5 having iodo group is more easily substituted by nitrogen anion that an intermediate with a chloro group. it seems reasonable that the reaction can proceed at lower temperature than that reported earlier 10 . In conclusion, it has been found that cycloaddition of epoxides with isocyanates could be efticiently by Smi 3 to give oxazolidinones in high yields. The notable advantages of the present procedure are its mild conditions, simple operation. short reaction time. high regioselectivity and good to excellent yields.

Experimental
Tetrahydrofuran was distilled from sodium-benzophenone prior to use. All reactions were conducted under a nitrogen atmosphere. M.ps. were obtained on an electrothermal apparatus and are uncorrected. Infrared spectra (KBr) were recorded on a Shimadzu IR-408 spectrophotometer and 1 H NMR spectra (CDCI 3 ) on a Bruker AC-300 (300 MHz) spectrometer, chemical shifts being expressed in 8 ppm downtield from internal tetramethy lsi lane.
General procedure for synthesis of' oxazo/idinones : To a pale yellow suspension of Srni 3 (0.1 mmol) in THF under nitrogen atmosphere. were auded epoxide 1 (I mmol) and isocyanate 2 (I mmol) and stirred for time given in Table I (monitored by TLC, about 3 h) at room temperature. Then water was added to the mixture and the product was extracted with diethyl ether. The organic phase was collected, dried over Na 2 S0 4 and evaporated to afford the crude product. It was further purified by preparative TLC on silica gel using cyclohexane and ethyl acetate (5 : I) as eluent. All the products were identified : 3a, vmax 1737 (C=O) cm-1 ;