Cheap, readily available, air stable, nontoxic, and environmentally benign iron salts such as Fe(acac)(3) are excellent precatalysts for the cross-coupling of Grignard reagents with alkenyl triflates and acid chlorides. Moreover, it is shown that dichloroarene and -heteroarene derivatives as the substrates can be selectively monoalkylated by this method. All cross-coupling reactions proceed very rapidly under notably mild conditions and turned out to be compatible with a variety of functional groups in both reaction partners. A detailed analysis of the preparative results suggests that iron-catalyzed C-C bond formations can occur via different pathways. Thus, it is likely that reactions of methylmagnesium halides involve iron-ate complexes as the active components, whereas reactions of Grignard reagents with two or more carbon atoms are effected by highly reduced iron-clusters of the formal composition [Fe(MgX)(2)](n) generated in situ. Control experiments using the ate-complex [Me(4)Fe]Li(2) corroborate this interpretation.
Enoate reductases from the old yellow enzyme family were employed for the asymmetric bioreduction of methyl 2-hydroxymethylacrylate and its O-allyl, O-benzyl and O-TBDMS derivatives to furnish (R)-configurated methyl 3-hydroxy-2-methylpropionate products in up to > 99% ee Variation of the O-protective group had little influence on the stereoselectivity, but a major impact on the reaction rate.Keywords: biocatalysis; C=C-bioreduction; enoate reductase; old yellow enzyme; substrate engineeringThe asymmetric reduction of C=C bonds creates (up to) two chiral carbon centres and is thus one of the most widely employed strategies for the production of chiral materials. The biocatalytic variant, which is applicable to activated alkenes bearing an electron-withdrawing substituent is catalysed by enoate reductases [EC 1.3.1.X], [1,2] which are members of the old yellow enzyme (OYE) family.[3] Over the past few years, increasing attention has been devoted to these flavoproteins [4] in view of their substrate scope, [5] encompassing a,b-unsaturated carbonyl compounds (such as enals and enones), as well as carboxylic acids and derivatives thereof (such as esters, cyclic imides, nitriles, lactones) and nitroalkenes. As a rule of thumb, the degree of activation of the C=C-bond exerted by the electron-withdrawing effect of the activating substituent goes hand in hand with the substrate acceptance, which ensures generally fast reaction rates for enals, enones and nitroalkenes, whereas (di)carboxylic acids and esters are transformed more slowly.To illustrate the importance of this enzyme class for asymmetric synthesis, we aimed at their applicability for an industrially relevant product, that is, (R)-3-hydroxy-2-methylpropanoate, which is commonly denoted as the Roche ester. The latter is a popular chiral building block for the synthesis of vitamins (e.g., a-tocopherol [6] ), fragrance components (e.g., muscone [7] ), and antibiotics (e.g., calcimycin, [8] palinurin, [9] rapamycin, [10] 13-deoxytedanolide, [11] dictyostatin [12] ) and natural products (e.g., spiculoic acid A [13] ). Classical methods for its preparation include the diastereoselective addition of non-racemic alcohols as chiral auxiliaries, [14] the transformation of a chiral homoallylic acetate [15] or involve aldol condensation [16] and -most prominent -the transition metal-catalysed asymmetric hydrogenation of acrylate esters using Rh [17] (ee up to 99%) or Ru [18] (ee up to 94%). For the biocatalytic synthesis of the Roche ester only few examples are reported: the stereoselective oxidation of 2-methyl-1,3-propanediol by Gluconobacter and Acetobacter spp. [19] (ee up to 97%), the asymmetric reduction of ethyl 4,4-dimethoxy-3-methylcrotonate using bakers yeast [20] and the stereoseletive (formal) b-hydroxylation of isobutyric acid using Pseudomonas putida (ATCC 21244).[21] All of these biotransformations were performed using whole (fermenting) microbial cells with several enzymes being involved. Only recently, was it shown that a non-flavin NADH-dependen...
a,b-Dehydroamino acid derivatives proved to be a novel substrate class for ene-reductases from the old yellow enzyme (OYE) family. Whereas N-acylamino substituents were tolerated in the a-position, b-analogues were generally unreactive. For aspartic acid derivatives, the stereochemical outcome of the bioreduction using OYE3 could be controlled by variation of the N-acyl protective group to furnish the corresponding (S)-or (R)-amino acid derivatives. This switch of stereopreference was explained by a change in the substrate binding, by exchange of the activating ester group, which was proven by 2 H-labelling experiments.
A convergent total synthesis of the antibiotic macrolide myxovirescin A1 (1) is described that is largely based on reagent- and catalyst-controlled transformations. This includes a highly regioselective Negishi reaction of dibromo-alkene 48 with an alkynylzinc reagent, and a palladium catalyzed alkyl-Suzuki coupling of the resulting enyne derivative 12 with the 9-BBN-adduct derived from alkene 61. The latter was obtained via an asymmetric hydrogenation of the chlorinated beta-ketoester 49 and an anti-selective oxyallylation of the functionalized aldehyde 53 as the key steps. The preparation of the bis-borylated allyl-donor 57 used in the oxyallylation step, however, required careful optimization and led to important insights into the nature of the classical hydroborating agent "di(isopinocampheyl)borane (Ipc2BH)". It was unambiguously shown by X-ray crystallography that in the solid state this compound is dimeric, but it is prone to undergo an essentially quantitative mono-deborylation when dissolved in CH2Cl2 or benzene; its composition in ethereal solvents is even more complex as evident from 11B NMR data. Product 71 derived from 12 and 61 was elaborated into the enyne-yne derivative 75, which served as the substrate for an exquisitely selective ring closing alkyne metathesis reaction (RCAM) catalyzed by the molybdenum tris-amido complex 20 activated in situ with CH2Cl2. The resulting cyclic enyne 76 was subjected to a ruthenium catalyzed trans-hydrosilylation/proto-desilylation tandem. Although [Cp*Ru(MeCN)3]PF6 had previously been recommended as catalyst of choice for trans-hydrosilylation reactions of internal alkynes, this complex failed to afford the desired product, whereas its sterically less hindered congener [CpRu(MeCN)3]PF6 permitted the reaction to be performed in appreciable yield, but at the expense of a lower stereoselectivity. AgF-mediated proto-desilylation of the isomeric silanes 79 and 80 followed by cleavage of the remaining acetal protecting groups afforded myxovirescin A1 and its hitherto unknown 14Z-isomer 81, respectively.
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