Homodimers enantioselective

In Ghosh s enantioselective total synthesis of the cytotoxic marine macrolide (+)-amphidinolide T1 (318) [143], the C1-C10 fragment 317 was constructed by CM of subunits 315 and 316 (Scheme 62). The reaction mediated by catalyst C (5 mol%) afforded in the first cycle an inconsequential 1 1 mixture of (E/Z)-isomeric CM products 317 in 60% yield, along with the homodimers of 315 and 316. The self-coupling products were separated by chromatography and exposed to a second metathesis reaction to provide olefins 317 in additional 36% yield [144]. 

Miiller and co-workers have developed an enantioselective enzymatic crossbenzoin reaction (Table 2) [43, 44], This is the first example of an enantioselective cross-benzoin reaction and takes advantage of the donor-acceptor concept. This transformation is catalyzed by thiamin diphosphate (ThDP) 23 in the presence of benzaldehyde lyase (BAL) or benzoylformate decarboxylase (BFD). Under these enzymatic reaction conditions the donor aldehyde 24 is the one that forms the acyl anion equivalent and subsequently attacks the acceptor aldehyde 25 to provide a variety of a-hydroxyketones 26 in good yield and excellent enantiomeric excesses without contamination of the other cross-benzoin products 27. The authors chose 2-chlorobenzaldehyde 25 as the acceptor because of its inability to form a homodimer under enzymatic reaction conditions. 

Further investigations revealed that the efficiency of this complex for the enantioselective isomerization of cyclohexene oxide under conditions similar to the catalytic process (mixture of HCLA 59, 67 and DBU) is close to that of the presumed HCLA 59 homodimer (Table 6, entries 1 and 2). These experiments also confirmed the dramatic influence of DBU on both enantioselectivity and rate (entry 3) . ... 

Using the diazadiene Fe complex 31, derived from optically active menthol, as a catalyst, enantioselective [4+4] cocycloaddition of isoprene and piperylene (30) occurred to give 3,5-dimethyl-l,5-cyclooctadine (32) with 61% ee in 89% yield without forming homodimers [10]. 

Most of the phosphinate substrates (entries 1-7) are efSciently cyclised with good to excellent enantioselectivities. Homodimers, formed by intermolecular metathesis, were also detected in the crude reaction mixtures. In entries 1 and 6, with a substrate containing an allyloxy arm, no reaction was observed with any of the catalysts studied. This can be due to the formation of an unreactive alkylidyne via chelation of the P(0) group and Lewis-base-promoted... 

The synthesis of faranal, the trail pheromone of the common ant Monomorium pharaonis, was recently reported by Minnaard, Feringa, and coworkers [4]. Hence, CM of 5 with ethyl thioacrylate furnished the unsaturated thioester 6 in good yield along with 5% of the homodimer of 5 (Scheme 10.2). The second stereocenter was installed by an enantioselective conjugate addition, which led to 7 in both high yield and high selectivity. 

Amidases catalyze the second step in die two-enzyme step conversion of nitriles to the corresponding acids, the hydrolysis of amides. Among stereoselective nitrile-converting enzymes, the highest enantioselectivities were found in amidases. Amidases have been mostly characterized as thiol-dependent homodimers (aa) sharing amino acid sequence homologies with other amidases. The functional relationship between nitrile hydratases and amidases is structurally documented by the close proximity of their genes [71,72]. In this chapter, stereospecific amidases alone or with nonselective nitrile hydratases are described. 

Pseudomonas chloraphis B23, the nitrile hydratase of which was used as the second-generation biocatalyst for the industrial production of acrylamide [83], contains additionally an enantioselective amidase that was purified and characterized as a typical homodimer [84]. The amidase exhibited activity against a broad range of ahphatic and aromatic amides and exhibited enantioselectivity for several aromatic amides including 2-phenyl-propionamide (Fig. 24), phenylalanine amide, and 2-(4-chlorophenyl)-3-methylbutyramide (Fig. 29), but not for the amide of naproxen. The enzyme resembles in a number of characteristics other enantioselective amidases. 

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