CIS-TRANS ISOMERS OF SOME l,3-dioxane DERIVATIVES * by Bogdan KĘDZIERSKI, Hanna PIOTROWSKA, Tadeusz URBAŃSKI, and Andrzej BORYS

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1 ROCZNIKI CHEMII ANN. SOC. CHIM. POLONORUM 46, 1559 (1972) CIS-TRANS ISOMERS OF SOME l,3-dioxane DERIVATIVES * by Bogdan KĘDZIERSKI, Hanna PIOTROWSKA, Tadeusz URBAŃSKI, and Andrzej BORYS Institute of Organie Chemistry and Technology, Polytechnical University, Warszawa Stwierdzono, że stosunek izomerów cis-trans pochodnych 1,3-dioksanu podstawionych w położeniu 2 grupą alkilową a w położeniu 5 grupą nitrową i atomem chlorowca (Cl lub Br) zależy od metody otrzymywania. yctailobjieho, HTO OTHOUieHMe U,UC-TpaHC M30MepOB np0i13b0flhfaix 1,3-flMO- KcaHa 33Mem;ennbix ajikmjiamk B nojiojkehmm 2 c HMTPO rpynnoii u ra- JiowflOM (Cl min Br) B nojiojkemie 5 3aBMCMT OT cnocoóa nojiynehmh. It was established that the ratio of cis-trans isomers of l,3-dioxane derivatives substituted by alkyls in position 2 and with a nitro group and a halogen (Cl or Br) in position 5 depends on the mode of preparation of the compounds. Examination of stereochemistry of l,3-dioxane derivatives substituted in positions 2, 5 have so far shown that their preferred conformation is that of chair with different positions of substituents J>. Now, we studied the conformation of l,3-dioxane derivatives with a nitro group and a halogen (chlorine and bromine) in position 5, and an alkyl group (methyl, iso-propyl and tert.-butyl) in position 2. Two isomers should be expected: that with axial and eąuatorial nitro group (isomers la and le respectively). N0 2 X X = Cl, Br, R =, tert-c 4 Conformations of isomers la and le In the present work we applied two methods of preparing the substances 1. One of them was originally used by Eckstein 2 ) to prepare 2-aryl-5-nitro-l,3-dioxanes. It consisted in reacting 2-halogeno-2-nitropro- * Part CIII on "Chemistry of Nitropraffins". Part CII: Kędzierski B., Piotrowska H., Urbański T., Chemistry a. Jndustry, 1971, 1302.

2 1560 B- Kędzierski, H. Piotrowska, T. Urbański, and A. Borys pane-l,3-dioles (2) with aromatic aldehydes in an acid medium. Now we reacted the same dioles with acetaldehyde (in form of paraldehyde): 0 2 N>. ^CHsOH C +CH3CHO K?* 1 (A) X ^ ^CH-.OH * 2 Another new method used in the present paper consists in reacting sodium salt of 2-alkyl-5-nitro-l,3-dioxanes (3) with a halogen X 2 : Na N^/\^ \0/\ R 3 ^ 1 (B) Salt 3 was obtained through the seąuence of reactions, as follows: CH 2 OH O t Nv/x H+ /\ o NaOH 0 2 N C CH 2 OH + R CHO * HOCH/j 7 3łCH : 0 ĆH 2OH ^Q/\ r The step leading to 4 was formerly described by one of the authors of the present paper 3>. The products of both reactions A and B were analyzed by gas-chromatography. Table 1 gives the results of the analysis obtained with the mixture of compounds la and le prepared by method A, with CHO as the ring closing agent (R = ). It shows a considerable predominance of isomer la with an axial nitro group over isomer le in this method of preparation. Table 2 gives the results of analysis of a number of compounds obtained by preparative method B, i.e. by halogenation of sodium salt 3. The ratio le/la of isomers was different from one in the instance of preparation of l,3-dioxane derivatives by method A. The proportion of isomers la only slighly exceeded that of isomers le. Thus, the mode of preparation of compounds greatly influenced the ratio of cis-trans isomers of 5-nitro-l,3-dioxane derivatives. By using preparatory gas-chromatography the products of halogenation were isolated in a relatively pure form. Their purity was determined by analytical gas-chromatography. The results are collected in Table 3. Table 1 Content of isomers la and le in products obtained by cyclization (reaction A) R X Temp. Content % kol. C min min le la Cl Br

3 Cis-trans isomers of some l,3-dioxane derivatives 1561 R Table 2 Content of isomers la and le in products obtaincd by halogenation (method B) X Temp. kol. C C / r a Content % min min le la Cl Br /-C 3 Cl J-C3H7 Br , '-C 4 Cl /-C 4 Br Table 3 Purity of isomers 1 after separation by preparatory gas chromatography R X Isomer Main component % Second component % Decomposition products % Cl e Cl a 98 2 Br e 92 8 Br a /'-C 3 Cl e i-gj Cl a i-c 3 Br e ;-C 3 Br a >-C 4 Cl e 89 11?-C 4 Cl a 100 r-c 4 Br e '-Q Br a Isolated by preparative gas-chromatography substances la and le of % purity (Table 3) were examined by NMR techniąue and their conformation was established by the analysis of the NMR spectra. This was possible considering that NMR analysis does not reąuire very high purity of the substances. Here is a brief account of the analysis. The spectra of compounds 1 should be characterized as belonging to system A 2 B 2 where protons H A and H B are split into ąuartets and each peak of the ąuartet forms a triplet with constant J«1.5 Hz. However, the nearest approximation to such a relatively complicated system is that of AB, i.e. of a ąuartet, and this will be accepted as sufficiently exact. In addition to system AB the spectra contain the signals of proton H(2) and of groups R. Fig. 1 shows typical spectra of two isomers la and le of 2-methyl-5- -bromo-5-nitro-l,3-dioxane (i.e. X = Br, R = ).

4 1562 B. Kędzierski, H. Piotrowska, T. Urbański, and A. Borys <f CppmJ Fig. 1. NMR spectra of isomeric 2-methyl-5-bromo-5-nitro-l,3-dioxanes Rys. 1. Widma NMR izomerycznych 2-metylo-5-bromo-5-nitro-l,3-dioksanów H 8 Hę Fig. 2. Newman projections of carbons 5 and 6 of the isomers la and le Rys. 2. Wzory rzutowe Newmana węgli 5 i 6 izomerów la i le Newman projection of carbon atoms C(5) and C(6) in isomers la and le is given in Fig. 2. It follows from the diagrams that the nitro group has a decisive influence upon the difference &H A ~&H b whereas the change of halogen X should not influence appreciably the value of the above difference. Subseąuently the structure la was assigned to the product which showed values &H A &H b near 1.00, namely 0.99, 1.00, 1.07 ppm when X = Cl and R =, tert-c 4 ) respectively, and 0.94, 0.98, 0.99 ppm when X = Br and R =, tert-c 4 H, retrpectiyely.

5 Cis-trans isomers, oj some l,3-dioxane derivatives 1563 The influence of the nitro group on the chemical shift of both protons H A and H B should be almost identical in isomer le and therefore S H ^ H R should have Iow values but the naturę of the halogen should produce an appreciable influence upon both protons. The following were figures for 5H a -5 H b recorded: 0.10, 0.14 and 0.11 ppm when X = Cl, R = H- and tert-c 4, respectively. When X = Br, R = and tert-c,!!;,, the values of H A & H b were: 0.37, 0.35 and 0.36 ppm, respectively. The results of the NMR analysis are collected in Table 4. Table 4 NMR examination of compounds la and le (60 MHz). Solyent CC1 4 /HB HA\ O z N v. sp/ x \ c 1 HAS 1, Q / \\/ C >Cs H/ R X Isomer SH A SH B /H A H B 8H 2 SR 8H A 8H B a e Cl a Br e Br a /-Q Cl e ") 0.14 Cl a "> 1.00 Br e ") 0.35 Br a a > 0.98 /-C 4 Cl e Cl a Br e Br a On the basis of the above analysis it was possible to assign the values of retention time in gas-chromatography for both isomers la and le. Infrared spectra The isomers la and le showed a great similarity of their infrared spectra particularly in vibrations v c _ 0 between 1000 and 1200 cm -1. Four high intensity bands were present: c. 1170, 1150, 1110, and 1050 cm - 1 (the latter was of lower intensity). Asymmetric N0 2 vibrations gave the freąuencies normal to a-halogenonitro compounds, i.e cm -1. In isomers le the band in ąuestion was widened and not sharp.

6 1564 B. Kędzierski, H. Piotrowska, T. Urbański, and A. Borys EXPERIMENTAL NMR spectra were measured on Jeolco C-60, IR spectra were measured on UR-10, Zeiss, Jena. Gas chromatography was performed on columns filled with PE 6 20M + + 2»/o phosphoric acid on Diathomite C 100/120 mesh. Apparatus PYA 104 was used for analytical gas chromatography and PYA 105 for preparatory gas chromatography. 2-AIkyl-5-hydroxymcthyl-5-nitro-l,3-dioxanes (4) To 31.7 g (0.21 mole) of trihydroxymethylnitromethane dissolved in dioxane (acetonitrile, tetrahydrofuran) there was added 0.2 mole of the corresponding aldehyde, a catalytic amount of p-toluenesulphonic acid and 20 g of anhydrous copper sulphate. The mixture was then left for 1 week at room temperaturę. Then the copper sulphate was filtered off, the filtrate poured into 100 cm 3 of 2%> aąueous Na 2 C0 3, and the whole extracted with benzene. The benzene extracts were dried over anh. MgSGi. After evaporation of benzene the crude compounds 4 were obtained with ca. 90 /o yield. There were used for subsequent reactions without further purification. 2-Alkyl-5-halogcn-5-nitro-l,3-dioxane (1) M e t h o d A. The reaction was carried out using eąuimolar amounts of the corresponding 2-halogen-2-nitropropanediol-l,3 and the aldehyde in benzene solution in presence of p-toluenesulphonic acid. Water was removed azeotropically. The benzene solution was neutralized with 2"/o aąueous Na 2 CO:j solution, washed with water, dried over ahh. MgSO), and benzene evaporatcd. The crude products were then analyzed by means of gas-chromatography. Method B. 0.1 mole of the corresponding crude 2-alkyl-5-hydroxymethyl-5- -nitro-l,3-dioxane (4) was converted into sodium salt 3 by dissolving in 200 cm 3 of 20 /o aąueous NaOH. The solution was cooled to 0 C, and then 0.1 mole of bromine was added to or gaseous chlorine was passed through the solution in such a way that the temperaturę did not rise above 10 C. The mixture was then stirred for 1/2 hour and extracted several times with benzene. The benzene extracts were washed with water, dried over anh. MgSOi, and benzene evaporated. The crude residue was analyzed by means of gas chromatography and separated into isomers by preparatory chromatography. Received February 14th, REFERENCES 1. Eliel E. L., Accounts of Chemical Research, 3, 1 (1970). 2. Eckstein Z., Roczniki Chem., 27, 246 (1953). 3. Malinowski S., Urbański T., ibid., 25, 183 (1951). IZOMERY CIS-TRANS POCHODNYCH 1,3-DIOKSANU B. KĘDZIERSKI, H. PIOTROWSKA, T. URBAŃSKI i A. BORYS Instytut Chemii i Technologii Organicznej Politechniki, Warszawa Otrzymano serię pochodnych 1,3-dioksanu ł ) zawierających w położeniu 2 grupę metylową, izopropylową i tert.-butylową, a w położeniu 5 grupę nitrową i atom chlorowca (chlor lub brom). Związki te otrzymano przez cyklizację w 2-chlorowco-2-

7 Cis-trans isomers of some l,3-dioxane derivatives nitropropanodiolu-1,3 odpowiednim aldehydem (metoda A) albo w drodze chlorowcowania soli sodowej odpowiedniego 2-alkilo-5-nitro-l,3-dioksanu (3) (metoda B). W surgwych produktach reakcji oznaczono zawartość izomerów cis-trans związków 1 metodą chromatografii gazowej, a następnie rozdzielono te izomery za pomocą preparatywnej chromatografii gazowej otrzymując poszczególne izomery o czystości powyżej 80 /o (tablica 3). Zbadano widma NMR otrzymanych związków (tablica 4). Na podstawie analizy tych widm przypisano izomerom o krótszych czasach retencji budowę le z ekwatorialnym położeniem grupy nitrowej i alkilu w położeniu 2, a aksjalnym położeniu atomu chlorowca, a izomerom o dłuższych czasach retencji budowę la, w której grupa nitrowa zajmuje położenie aksjalne. Stwierdzono, że proporcje izomerów zależą od stosowanej metody syntezy. W przypadku metody A izomer la tworzy się w przeważającej ilości (tablica 1), w przypadku metody B ilość izomeru le znacznie się zwiększa (tablica 2). Widma absorpcji w podczerwieni omawianych związków potwierdziły ich budowę.