Dimethoxyacetaldehyde

Scheme 2.2.1.3 Screening for novel carboligation activity four enantiomeric products 9-12 are shown which can be formed starting from the substrates benzaldehyde 4 and dimethoxyacetaldehyde 8.

High-Throughput Screening for Carboligation Activity with the Substrates Benzaldehyde and Dimethoxyacetaldehyde... 

The BFD mutant library generated by epPCR was expressed in microtiter plates and 8000 clones were subjected to the carboligation assay with benzaldehyde 4 and dimethoxyacetaldehyde 8 as the substrates. The reaction was incubated for 24 h at 30 °C as suggested from experiments using the positive control strain... 

Fig. 2.2.1.2 Colorimetric assay for carboligase activity. A) DMA-HPP 10 reduces 2,3,5-triphenyltetrazolium 13 chloride to the respective formazan 15, which has an intense red color. B) Formation of the red formazan 15 dye can be observed only in the presence of DMA-HPP 10 neither substrate dimethoxyacetaldehyde 8 nor benzaldehyde 4 causes any change in color.

Further studies, with the substrate ratio altered from 1 3 to 1 100, were performed in order to suppress the formation of benzoin 12 and increase the yield of DMA-HPP 10. Although this strategy was successful, the results showed that even a 100-fold excess of dimethoxyacetaldehyde 8 (500 mM) relative to benzaldehyde 4 (5 mM) could not completely supress benzoin 12 formation catalyzed by variant 55E4. Furthermore, the formation of the mixed product DMA-HPP 10 was decreased by a factor of 2 relative to the 1 3 substrate mixture. Variant 55 E4 showed a 55-fold higher productivity with respect to the formation of DMA-HPP 10 under these conditions as compared to variant L476Q. The overall carboligation... 

Fig. 2.2.1.3 HPLC diagram showing the formation of DMA-HPP catalyzed by variant BFD55E4. BFD variant enzymes were incubated with benzaldehyde 4 (20 mM) and dimethoxyacetaldehyde 8 (60 mM) for four days, and product formation was analyzed by chiral HPLC (see circles). A) BFD variant...

In contrast to wild-type BFD, BAL accepts aromatic aldehydes substituted in the ortho position as well. Only a few aromatic aldehydes, such as pyridine 3- and 4-carbaldehyde as well as sterically exceedingly demanding aldehydes, resulted either in very low yields or in no benzoin condensation at all [62]. Moreover, mono- and dimethoxyacetaldehyde are good substrates for BAL, leading to highly functionalized enantiopure hydroxypropiophenone derivatives (Scheme 2.2.7.22) [63]. 

Scheme 2.2.7.22 Application of monomethoxy- and dimethoxyacetaldehyde in BAL-catalyzed C-C bond formation reactions.

Scheme 5.4. Solid-phase parallel synthesis of tetrahydro-p-carboline alkaloid library, (a) FmocTrpOH, HATU, DMAP, DIPEA, DMF (b) 25% piperidine in DMF (c) Fmoc-protected amino acid, DIC, HOBt, DIPEA, DMF (d) 25% piperidine in DMF (e) dimethoxyacetaldehyde, NaBHaCN, MeOH/DMF (1 1) (f) r nCO, DIPEA, DCE (g) neat formic acid, 50°C, 3h.

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