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Ruthenium Tetroxide (Ruo4) Oxidation of N-Alkyllactams Proceeded Regioselectively Depend- Ing on the Size of Lactam Ring, Except for the Seven-Membered Ring

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Ruthenium Tetroxide (Ruo4) Oxidation of N-Alkyllactams Proceeded Regioselectively Depend- Ing on the Size of Lactam Ring, Except for the Seven-Membered Ring No. 1 357 Chem. Pharm. Bull. 35(1) 357-363 (1987) Ruthenium Tetroxide Oxidation of N-Alkyllactams SHIGEYUKIYOSHIFUJI,* YUKIMI ARAKAWA, and YOSHIHIRONITTA Schoolof Pharmacy,Hokuriku University,Kanagawa-machi, Kanazawa920-11, Japan (ReceivedJuly 31, 1986) Ruthenium tetroxide (RuO4) oxidation of N-alkyllactams proceeded regioselectively depend- ing on the size of lactam ring, except for the seven-membered ring. Four- and eight-membered N- methyl- and N-ethyllactams were oxidized at the exocyclic ƒ¿-carbon adjacent to nitrogen to produce the N-acyllactams and NH-lactams, while five- and six-membered lactams underwent endocyclic oxidation to yield the cyclic imides. Oxidation of seven-membered lactams yielded a mixture of products arising from both exocyclic and endocyclic oxidations. These regioselectivities were confirmed in the oxidation of substrates having a tertiary carbon at the oxidation position. Keywords•\oxidation; ruthenium tetroxide oxidation; regioselective oxidation; hydroxyl- ation; imide synthesis; N-alkyllactam; N-acyllactam; imide; ruthenium tetroxide; two-phase method Ruthenium tetroxide (RuO4) is a good reagent for the conversion of N-acylated cyclic amines to the corresponding lactams,1) by oxidation of one of two carbons adjacent to nitrogen. As a common feature of the RuO4 oxidation in this conversion (la to 2 in Chart 1) and in the transformation of cyclic ethers into the corresponding lactones,2) it has been considered that RuO4 predominantly oxidizes a secondary carbon rather than a tertiary one. However, as reported previously,3) we obtained an opposite result in the RuO4 oxidation of some 1-azabicycloalkan-2-ones, such as quinolizidin-4-one (1b), which gave the hydroxylated products, such as 3, resulting from the oxidation of the tertiary carbon. At that time, we predicted that the 1-azabicycloalkan-2-ones might belong not to the category of N-acyl cyclic amines but to that of N-alkyllactams, in terms of RuO4 oxidation. However, to date, there has been no report on the latter category. Here we wish to describe a systematic study on the RuO4 oxidation of N-alkyllactams. As model compounds, simple N-methyl- and N-ethyllactams ranging in size from four- to eight-membered rings were selected for the present work. The RuO4 oxidation of these substrates was carried out at room temperature under catalytic conditions using a catalytic amount of ruthenium dioxide (RuO2) in combination with an excess of 10% aqueous sodium metaperiodate (NaIO4). In the catalytic procedure of RuO4 oxidation, a two-phase system of organic solvent-water is usually employed as a reaction medium and the organic phase is a chlorinated methane such as carbon tetrachloride (CCl4)4) or the combination of CCl4 and Chart 1 358 Vol. 35 (1987) TABLE I. RuO4 Oxidation of N-Alkyllactams I II III IV acetonitrile developed by Sharpless et al.5) Recently we developed a new solvent system of ethyl acetate (AcOEt)-water which gave short reaction times and high yields of the products in the transformation of N-acylated L-proline esters into the corresponding L-pyroglutamic acid derivatives.1c) Therefore, our new system was applied to the present work. However, in the oxidation of the four-membered lactams, the organic phase (AcOEt) was omitted due to the ready solubility of these compounds in water. The oxidation reaction was monitored by observing the disappearance of the starting lactams on thin layer chromatographic (TLC) plates; it proceeded slowly, with a yellow color which indicated the existence of active RuO4 generated in situ from RuO2 under the above conditions. The results are summarized in Table I. A highly regioselective oxidation depending on the ring size of the lactams, except for seven-membered lactams, was observed. Four- and eight-membered lactams were oxidized at the exocyclic ƒ¿-carbon to nitrogen. N-Methyl-ƒÀ-lactam afforded a low yield of the de- methylated NH-lactam. Eight-membered N-methyllactam also gave predominantly the NH- lactam together with a small amount of the N-formyllactam. N-Ethyllactams were transform- ed into the N-acetyllactams (and the NH-lactam in the case of the eight-membered lactam). Formation of the NH-lactams must be based on the elimination of aldehyde (formaldehyde or acetaldehyde) from the intermediates leading to the N-formyl or N-acetyl lactams and/or on the hydrolytic deacylation of the N-acyllactams. Five- and six-membered lactams were oxidized at the endocyclic ƒ¿-carbon to provide the corresponding cyclic imides in high yields. The precursors of these imides are the correspond- ing ƒ¿-hydroxylactams which could be detectable in the early stage of the oxidation on TLC analysis. In an incomplete run with 1-methylpyrrolidin-2-one (5a) using a traditional two- phase system of CCl4—H2O, 2-hydroxy-1-methylpyrrolidine-5-one was isolated from the aqueous phase in 16% yield. When the hydroxy compound was oxidized with RuO4 according to our standard procedure, N-methylsuccinimide (5c) was obtained in 97% yield. Alternatively, treatment of seven-membered lactams with RuO4 provided a mixture of products arising from endocyclic and exocyclic oxidations. Next, in order to confirm the above mentioned regioselectivity depending on the size of the lactam ring, further RuO4 oxidation was done under the same conditions with some N- No. 1 359 alkyllactams bearing a tertiary carbon at the oxidation position. These compounds were chosen because a tertiary carbon is less susceptible to RuO, than a primary or secondary carbon.1,2,4) The results are shown in Chart 2. N-Isopropyl-ƒÀ-lactam (9) was oxidized with Chart 2 360 Vol. 35 (1987) long reaction times to afford two products, an NH-lactam (4c) and a N-acetyllactam (4d), in 35% and 42% yields, respectively. Apparently, this oxidation occurred at the exocyclic tertiary carbon and these products (4c, d) were formed from a common intermediate, the hy- droxylactam (10), which could not be isolated. Presumably 10 was decomposed directly to 4c by the loss of acetone and indirectly to the acetyl derivative (4d) via dehydration to the N- isopropenyllactam (11) and oxidative cleavage of the double bond of 11. RuO4 oxidation of the eight-membered N-isopropyllactam (12) proceeded very sluggishly and was not completed within 30d; a little of the starting lactam (7%) was recovered. This must be due to the difficulty of access of RuO4 to the exocyclic ƒ¿-position. Only a low yield of the exocyclic oxidized product (8c, 25%) was obtained together with the cyclic imide (13, 25%) generated in the kinetically controlled reaction. In this regard, the analogous oxidation of the seven- membered lactams gave reasonable results. N-Isopropylcaprolactam (14) was oxidized exclusively at the endocyclic ƒ¿-position to produce the cyclic imide (15) in 89% yield. On the other hand, the reaction of 1,2-dimethylcaprolactam (16) with RuO4 provided a mixture of products resulting from the exocyclic and endocyclic oxidations: four compounds, 17 (41%), 18 (18%), 19 (19%), and 20 (9%), were obtained on chromatographic separation of the mixture. Although regioselective endocyclic oxidation in the five- and six-membered lactams having a tertiary carbon at the oxidation position has already been observed with 1- azabicycloalkan-2-ones such as 1b (Chart 1),3) 1,2-diethylpiperidin-6-one (21) was examined as a model compound. Thus, treatment of 21 with RuO4 as described above using the CCl4-H2O system gave a 94% yeild of the ring-opened product (23), which is equivalent to the hydroxylactam (22). When the AcOEt-H2O system was used in the above oxidation of 21, the major product was the ring-opened imide (24) as a result of further oxidation of the initially formed amide (23).6) On the basis of the results described above, it is possible to conclude that the RuO4 oxidation of N-alkyllactams proceeds regioselectively depending on the size of the lactam ring, except for the seven-membered ring, even when a saturated tertiary carbon (which has not previously been reported to be oxidized by RuO4) occupies the oxidation position in the four-, five-, and six-membered N-alkyllactams. Recently, a similar type of reaction has been reported in the direct or indirect anodic oxidation of N-alkyllactams.7) Our RuO4 oxidation can be regarded as superior to these oxidations in respect of the high regioselectivity of the reaction and the high yield of the products. Experimental Melting points were taken on a Yanagimoto melting point apparatus. All melting points and boiling points are uncorrected. Infrared (IR) spectra were recorded on a JASCO IRA-2 or a Hitachi 270-30 spectrophotometer. Mass spectra (MS) were measured on a JEOL JMS D-100 or a JEOL JMS D-300 spectrometer. Nuclear magnetic resonance (1H-NMR) spectra were obtained at 23•Ž on a JEOL JNM-MH-100 or a JEOL FX-100 spectrometer with tetramethylsilane as an internal standard. Starting Materials All starting materials were obtained from commercial suppliers (5a, 5b, 6a) or prepared according to the established methods.8). New compounds were characterized as described below. 1-Isopropylazetidin-2-one (9)•\A colorless oil, by 83-85•Ž (17mmHg). MS m/z: 113 (M+). IRcm-1: 1737 (C=O). 1H-NMR (CDCl3) ƒÂ: 1.18 (6H, d, J=7Hz, 2•~CH3), 2.81 (2H, t, J=4Hz, C3-H2), 3.16 (2H, t, J= 4 Hz, C4-H2), 3.75-4.05 (1H, m, CH). 1-Isopropyloctahydroazocin-2-one (12) A colorless oil, by 76•Ž (2 mmHg). MS m/z: 169 (M+). IR v film maxcm-1: 1610 (C=O). 1H-NMR (CDCl3) ƒÂ: 1.14 (6H, d, J= 7 Hz, 2•~CH3), 1.35-1.93 (8H, m, C4-H2, C5-H2, H6- H2, and C7-H2), 2.4-2.61 (2H, m, C3-H2), 3.32-3.47 (2H, m, C8-H2), 4.44-4.92 (1H, m, CH).
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