~ ~~ Pentacarbonyl(ary1methoxycarben)-KomplexePentacarbonyl (

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Pentacarbonyl(methoxyphenylcarben)chrom(O) und -wolfram(O) reagieren mit Methyl-bnv.Phenyllithium bei tiefen Temperaturen zu Lithium-pentacarbonylalkylmetallaten( -I) 1-4. Die Umsetzung dieser Komplexe mit Bis(tripheny1phosphin)iminium-chlorid ergibt Bis(tripheny1phosphin)iminium-pentacarbonylchlorochromat(0) (5) bzw. Bis(tripheny1phosphi)iminium-akylpentacarbonylwolframatd -I) 6, 7. Die Konstitutionen der neuen Verbindungen werden durch spektroskopische Untersuchungen gesichert. Transition Metal Carbene Complexes, CVII') Synthesis of PentacnrbonyIalky1c~omat~-I) and -tungstates(--I) Pentacarbonyl(methoxyphenylcarbene)chromium(O)and -tungsten(O) react with methyl-as well as phenyllithium at low temperatures to give lithium pentacarbonylalkylmetalates(-I) 1-4. Treatment of these complexes with bis(tripheny1phosphie)iminium chloride yields bis(tripheny1phosphine)iminium pentacarbonylchlorochromate(0) (5) as well as bis(tripheny1phosphie)iminium alkylpentacarbonyltungstates( -I) 6,7. The structures of the new compounds are established by spectroscopic investigations.Aufgrund ihres ausgcpragten elektrophilen Charakters am Carbenkohlenstoffatom reagieren (Alkoxyary1carben)peptacarbonyLKomplexe von Chrom(0) und Wolfram(0)2) rasch mit Aminen, Thiolen, Selenolen und Phosphinen '). Nicht grundgtzlich anders envartet man das Reaktionsverhalten bei Umsetzungen mit Lithiumorganylen 4).So gelang bei der Reaktion von (CO)&rC(OCH3)C6H5 mit Phenyllithium 6ei Raumtemperatur die Isolierung von l,2-Dimethoxy-l,l,2,2-tetraphenylethan 5), was nit einer Addition des Nucleophils an das &arbso, Spaltung der Metall-Methylenkohlenstoff-Bindung und Dimerisierung des organischen Liganden zu erklaren ist. Folgerichtig konnte in einer ') CVI. ubergangsmetall-Carben-Komplexe, CVII 3843 ahnlichen Reaktion rnit anschlieDender Methanolat-Abspaltung mittels HCl Pentacarbonyl(diphenylcarben)wolfram(O) dargestellt werden 6). Durch schonende Behandlung der postulierten Additionsverbindungen Li[(CO),-MC(OCHJ(R)R] rnit SiOJPentan gelang die Synthese neuer, teilweise sehr empfindlicher Pentacarbonyl(diary1carben)-und Pentacarbonyl(n-olefin)-Komplexe von Chrom(0) und Wolfram(O)'_). Von diesen Ergebnissen ausgehend stellte sich nunmehr die Frage, ob eventuell diese intermediaren Metallate isolierbar sind. In diesem Fall lieDe sich dann der Reaktionsablauf unschwer beweisen. b, C. P . Casey und T J . Burkhardt. J. Am. Chem. SOC. 95,5833 (1973).

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Efficient syntheses of ( +)-N-methylpseudoconhydrine (5) and (-)-hydroxysedamine (10) are described starting from the readily available (2RS,SS)-Methyl 5-(ferf-butyldimethylsilyloxy)-2-ethoxypiperidine-l-carboxylate (1) and proceeding via homochiral acyliminium ion as an intermediate. Compounds 5 and ?b were obtained in 32 and 34%, resp., overall yield from 1. After deprotection of the OH function of 3a, b the diastereomeric mixture of 6a, b was separated and converted to pseudoconhydrine ?b and epi-pseudoconhydrine ?a, respectively. Reduction of the urethane moiety of 3a, b followed by treatment with hydrochloric acid provided enantio-and diastereopure 5. Reaction of 1-phenyl-1-(trimethylsi1oxy)ethene with 1 afforded 8a and 8b in a ratio of 1:9. Deprotection of 8a, b and separation of the diastereomers give after diastereoselective reduction of the ketone function and conversion of the urethane moiety into the methyl group natural (-)-hydroxysedamine (10). Starting from Eb, deprotection and inversion of the OH function furnished 12 as a single homochiral diastereomer which was converted to ent-10.J3-Hydroxypiperidine alkaloids are found in nature, e.g. in Conium, Prosopis, Azima, Carica, and Cassia Many of these alkaloids display significant pharmacological activities, especially those from Prosopis africana[2a] and Microcus philipinensis[2b]. Therefore, they offer attractive targets for (asymmetric) synthesis[lb1. (+)-N-Methylpseudoconhydrine 5 (-)-H ydroxysedam ine 10In this field we concentrated on enantiopure 5-hydroxypipecolic acids starting with lr3]. We anticipated that (+)-Nmethylpseudoconhydrine (5)r41 and (-)-hydroxysedamine[sl (10) should be available by addition of allyltrimethylsilane and 1 -phenyl-1 -(trimethylsilyloxy)ethene, respectively, to the homochiral acyliminium generated from 1 with Lewis acids to yield 2a, b and Sa, b. Subsequent transformations of the functional groups and separation of the diastereomers should be straightforward. This would be the first chiral pool synthesis of natural (-)-hydroxysedamine (10) by starting from 1.The a-ethoxy carbamate 1r7] was treated with allyltrimethyl~ilane[~~,~I and zinc chloride to give via N-acyliminium ion the P-hydroxypiperidine derivatives 2a, b in a ratio of 24:76 (determined by GC analysis) in 96% overall yield (Scheme 1). A preferred trans diastereoselectivity was also observed in the reaction of cyanotrimethylsilane/ZnCl~ with lr7]. 'H-NMR spectroscopy of 1 revealed an axial orientation of the tert-butyltrimethylsiloxy group, and therefore the less shielded a-side of the N-acyliminium ion is attacked. This axial orientation can be interpreted by an attractive gauche effect of the nitrogen substituent and the J3-oxygen Catalytic hydrogenation of 2a, b with Pd/C in methanol resulted in a diastereomeric mixture of 3a, b (3:7) which was separated after deprotection of the OH function with a 6 M methanolic hydrogen chloride solution by column chromatography. Interestingly, the 13C-NMR spectra of 3a, b display single resonances for each of the ca...