Three new derivatives of vitamin B,, with a pyrrole head group attached to the corrin ring have been prepared. Ligdnd-exchange reactions and reduction provided reactive Co"' and Co" complexes. Their electrochemical properties and their potential for fixation at the surface of electrodes by electropolymerization were studied.Introduction. -Model systems for enzyme-catalysed transformations play an important role for understanding the mechanism of such reactions. This is particularly important for reactions where cofactors are involved and become a conditio sine qua non for the coenzyme-B12-dependent transformations [I]. Detailed investigations in several laboratories have unravelled important features of the vitamin-BIZ-dependent reactions [ 2 ] , and many attempts have been undertaken to incorporate these results into model systems. Although many structural features have been reproduced in this way, further attempts are necessary for a more comprehensive modelling of the in vivo reactions. For example, the enantioselectivity in the methylmalonyl-succinyl rearrangement is rather low and the enantiomeric excesses (ee) of the product do not exceed 20% in reactions of racemic methylmalonyl substrates [3]. Our experiments with a homochiral methylmalonyl substrate has recently led to similar observations [4] '). Other aspects like stereospecific recognition of substrates by an appropriately modified vitamin-B catalyst deserve further attention. Whereas the mechanistic features of the in vivo methylmalonyl-succinyl rearrangement and similar rearrangements are emerging [ 6 ] , corresponding investigations in model systems have hitherto remained inconclusive [4].Our model systems, which incorporate features of molecular recognition between substrates and vitamin-B,,-derived catalysts both endowed with anchoring groups in the periphery, are driven by an electrochemical-photolytic cycle and lead to an enhanced rearrangement [7]. For further investigation of chiral recognition, transfer of chirality in the methylmalonyl-succinyl rearrangement, and other vitamin-B, ,-dependent reactions, and for a detailed investigation of the electronic features of these transformations, it is
rated without material loss by flash chromatography. The bismesylates 19 b/20 b, prepared under standard conditions in 85-90% yield, can be converted into the tetracyclic aziridine 21 b (70-85%) under the conditions used for 5 b (1 0 mmol, triphenylphosphane/THF). Cyclization of 21 b to 22 proceeds under the optimized conditions found for 1 ( 5 mmol, 1.05 equiv. nBuLi/THF, -78 + + 20 "C) without any competition by the base-induced reactions typical of bicyclob~tanecarbonitriles~' 51 and azabicyclobutanes. Trituration of the crude product with ether affords a practicaliy quantitative yield of crystalline 22 (Z-3-azapentacyclo-[5.1.0.02~4.03~5.06~8]octane-7-carbonitrile), which remains unchanged up to its melting point (128 "C). The NMR data (Fig. 1) are characteristic for cyanocyclobutane and azabicyclobutane moieties. 23 2 4 H H 25 C H * -C~H~ 26 Saponification of 23, obtained by treatment of 22 with acetic anhydride, leads "directly" to oxaazapentacycle 24: reaction of 22 with Na,S (CH,Cl,/tBuOH, reflux) and with benzylamine/BF, (CH,Cl,/tBuOH, 50 "C) gives heterocycles 25 and 26, respectively (55-60%, not optimized).
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Virtually enantiopure a-t-butyldimethylsilyl ketones (R)-1 and (S)-5 are converted regioselectively to the corresponding trimethylsilyl enol ethers (R)-2 and (S,Z)-6. Subsequent asymmetric Michael addition to nitroalkenes in the presence of SnCl 4 affords the 1,4-adducts 3 and 7 in good yields and high diastereo-and enantiomeric excesses. Removal of the t-butyldimethylsilyl group with n-Bu 4 NF, THF, NH 4 F/HF leads to the a,b-disubstituted g-nitro ketones (S,R)-4 (de = 91-92 %, ee > 98 %) and (R,S)-8 (de > 96 %, ee > 98 %) in very good overall yields.The Michael addition is one of the most important methods for C-C-bond formation in synthetic chemistry 1 and a great variety of asymmetric versions of this name reaction have been developed in recent years. 2Nitroalkenes are of special interest as excellent Michael acceptors due to their low tendency for 1,2-addition and the strong anion-stabilizing effect of the nitro group. 3 The latter is an important functional group which can be transformed into other functionalities, such as the carbonyl group via Nef reaction or the amino group by reduction. 4 We now wish to report a new efficient method for the diastereo-and enantioselective synthesis of a,b-disubstituted g-nitro ketones with high diastereomeric and enantiomeric excesses via a-t-butyldimethylsilyl ketones, which can be easily prepared based on the SAMP/RAMP-hydrazone methodology. 5 The synthetic utility of the concept of a-silyl ketone control in asymmetric synthesis has already been demonstrated by us in various other applications. 6As is depicted in Scheme 1, initially the silyl ketone 1 was converted regioselectively to the corresponding silyl enol ether 2 by deprotonation with lithium diisopropylamide and treatment with chlorotrimethylsilane. Deprotonation at the stereogenic centre and thus racemisation did not occur, because the approach of the amide is hindered by the bulky silyl group resulting in formation of the desired, enantiomerically pure silyl enol ether in almost quantitative yield (95%). Due to the cyclic structure of silyl ketone 1, the procedure leads to the (R)-configuration of 2, which was used as nucleophile in the asymmetric Michael addition to nitroalkenes. It was found necessary to employ Lewis acids in order to promote the 1,4-addition in dichloromethane. 7 Accordingly, a variety of Lewis acids were screened, with AlCl 3 and LiClO 4 resulting in no product formation, TiCl 4 affording many by-products and BF 3 ∑OEt 2 causing partial removal of the a-silyl group. Best results were obtained by the use of SnCl 4 , with Michael additions taking place with little by-product formation. After simple workup and isolation, the silyl nitro ketones 3 were obtained in good yields and excellent diastereo-and enantiomeric excesses (de,ee > 96 %). The absolute configuration of crystalline 3a could be determined by X-ray structure analysis. The (R,S,R)-configuration obtained indicates the expected trans orientation of the silyl group and the introduced nitroalkane moiety at the sixmembered ring and ...
Das fur Synthesen in der ,,C-Reihe""] bewahrte transDioxa-tris-o-homobenzolcarboni tril 17 I2] ist, als Epimerengemisch, ein gut zugangliches Edukt fur Monoazaoctabisvalene 2. Es ist an C-9 acid genug, um ohne wesentliche Beeintrichtigung der Epoxidfunktionen deprotoniert und regioselektiv unter Dreiringbildung zu 18 cyclisiert zu werden (0.15 M Ansatze, 1 Aquiv. Bis(trimethylsily1)lithiumamid/THF, 0-20"C, 50-60Y0; Fp = 135-136"C, Jl,2 21 3.5, J l , , ~4 , J2,4 N 3.5, Jz.s N 3.5, J4,5 N 3.5 Hz). Bei der Umsetzung von 18 (50 mmol) mit Natriumazid/MgSO, (5 Aquiv., H,O, 40°C, ca. 5d) ist Substitution an C-4 keine Konkurrenz; in 90-95 Yo Gesamtausbeute erhllt man ein Gemisch der Azidodiole 19a (78%)/20a (7%) und dreier daraus entstandener (charakterisierter) Diazide (in toto 5 -10 YO), aus welchem sich durch Flashchromatographie die Hauptkomponenten verlustfrei abtrennen lassen. Die unter Der in Schema 1 skizzierte Syntheseweg basiert auf folgenden Befunden und Annahmen: 1) Das Edukt scyllo-Tnpropargyltriol 7 b (siehe Tdbelle 1) resultiert aus der dreifachen Epoxidoffnung
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