Stereoselectivity condensation

Triflates of titanium and tin are effective catalysts for various condensations of carbonyl compounds [I2I, 122, 123, 124, 125] Claisen and Dieckmann type condensations between ester functions proceed under mild conditions in the presence of dichlorobis(trifluoromethanesulfonyloxy)titaiiiuin(rV) and a tertiary amine (equations 59 and 60) These highly regio- and stereoselective condensations were used successfully m the synthesis of carbohydrates [122]... 

Chiral N/O-acetals may racemize in the solid state when water of crystallization is present. Examples are the epimerizations of the oxazolidines 97 that contain water from their preparation by stereoselective condensation. Thus, the kinetically preferred products 97a,b (which are admixed to the thermodynamically more stable products 98a,b) epimerize within some weeks in the solid state to give enantiopure 98a,b [661 (Scheme 9). It appears that the N/O-acetal hydrolyses and recloses. Solid-state racemizations are quantitative if the 1 1 equilibrium between the enantiomers is obtained. Therefore they do not really fulfill the criterion of only one product. Numerous examples in the organo-metallic field are listed in [671 and [681. 

The formation of C-C bonds by aldol condensation is a very useful method in synthesis. Besides the chemical synthesis, aldolases can be used to perform this reaction. The reaction yields a stereoselective condensation of an aldehyde with a ketone donor. 

The thiazolium and, particularly, triazolium catalysts discussed above have been developed to the extent that they perform remarkably well in the asymmetric benzoin condensation of aromatic aldehydes. Triazolium catalysts are also very effective in the (non-stereoselective) condensation of aliphatic aldehydes [250]. It seems, however, that no catalyst is yet available that enables condensation of aliphatic aldehydes with synthetically useful enantioselectivity. The best ee yet obtained are in the range 20-25%, e.g. in the dimerization of the straight-chain C2-C7 aldehydes [251]. 

Stereoselective condensation with imines. The diethylaluminum enolate of 1 reacts with a wide variety of imines with high stereofacial bias, which is ascribed to a cyclic transition state in an aluminum chelate. 

Trilluoroacetaldehyde (fluoral) undergoes facile highly stereoselective condensation with silyl enol ethers under titanium(ll) chloride binaphthol catalysis to give 2. ... 

It has also been reported that 4-substituted-5,5-dimethyloxazoIidin-2-ones can be prepared as illustrated in eq 3. Initially, stereoselective condensation of an N-acyloxazolidinone enolate with acetone affords a functionalized acyl fragment, which is then hydrolyzed to the carboxylic acid. Reaction of the hydroxy acid with DPPA at elevated temperatures yields the target via formation of the acyl azide, Curtius rearrangement and trapping of the isocyanate intermediate by the hydroxyl group (eq 3). 

Chiral 2-substituted benzaldehyde chromium tricarbonyl complexes have been reacted with chloroacetophenone in the presence of KO-fert-Bu [544], After decomplexation, the E-epoxyketone is obtained with a high selectivity (Figure 6.88). This Darzens reaction with ClCF COO-tert-Bu is poorly stereoselective. Condensation of the same aldehydes with methyl aaylate or acrylonitrile in the presence of DABCO, followed by decomplexation, also leads highly selectively to P-hydroxyesters or -nitriles 6.105 (Y = COOMe or CN) [547] (Figure 6.88). An anti aldol product is also obtained with a high selectivity from a chromium complex and the titanium enolate of PhCF OCF COS-tert-Bu at -78°C [1281, 1282], Chiral aminals of a-ketoaldehydes react with lithium or sodium enolates of ethyl acetate. After treatment with acid, compounds 6.106 are obtained with a high enantiomeric excess (Figure 6.88). 

Table 29 Stereoselective Condensations of Isothiocyanoacetate Enolates With 3-Thiazoline (254) ...

Flavans. - New tannins have been synthesized by stereoselective condensation of flavan-3,4-diols with resorcinols, and their stereochemistry has been correlated, using circular dichroism, but the general rule thus demonstrated had exceptions among some 2,3-c/s-3,4-c/s-diastereoisomers, e.g. (163), which... 

Photolysis of solutions of C6o(OH)ig at low solute concentration leads to [C6o(OH)i8] by electron transfer from Me2C(OH) radicals or from hydrated electrons, and this has enabled the reduction potential of the C6o(OH)ig/ [C6o(OH)ig] couple to be estimated. The kinetics of the photoreduction of hexanal using RhCl(PMe3)2CO as catalyst have been measured and the feasibility of a photocatalytic synthesis of hexanol from pentane, CO, and H2 in the presence of rhodium complexes has been demonstrated. Irradiation of a chiral bimolecular crystal of acridine and R-(-)- or S-(+)-2-phenylpropionic acid induces photodecarboxylation followed by stereoselective condensation to give a mixture of three optically active products, and the 3-0-S-methyl dithiocarbo-nate derivatives of oleanolic and ursolic methyl esters have been used as models for the photodeoxygenation of alcohols. ... 

The stepwise synthesis of methyl 4-(9-[3,4-di-0-( 3-D-xylopyranosyl)-j3-D-xylopyranosyl]-/3-D-xylopyranoside (17), a methyl xylotetraoside related to branched xylans, has been achieved by stereoselective condensation of 2,4-di-(9-acetyl-2-O-benzyl-a-D-xylopyranosyl bromide with methyl 2,3-anhydro- -D-ribopyranoside and reaction of the deacetylated product with 2,3,4-tri-C>-acetyl-a-D-xylopyranosyl bromide. 

In 1991, the Danishefsky group disclosed the synthesis of the C-28-C-49 subunit of rapamycin utilizing the combination of the Perrier carbocyclization reaction and an Ireland-Claisen rearrangement (see Section 12.3.3. Scheme 12.21T The Perrier carbocyclization of 5-enopyranoside 86, prepared from 2-deoxy-d-glucose derivative (Section 12., Scheme 12.2ST followed by elimination of the p-hydro group gave cyclohexenone 152 tScheme 12.40T Luche reduction of 152 afforded cyclohexenol 83 stereoselectively. Condensation of 83 with carboxylic acid 84, prepared from (i )-3-(benzylojg )-2-methylpropanal, provided ester 82 in 75% yield. 

Stereoselective condensation of an a-silyl ester anion with an a-keto- acetal leads to a mixture of E and Z-olefins, which is converted into a A -butenolide on hydrolysis and reduction (Scheme 73). Clearly only the Z-olefin will cyclize to the butenolide, and the high yields obtained reflect the high selectivity of the addition-elimination sequence. 

The integrated isolation and identification approach advocated here supported the purification and identification of two major by-products resulting from an aldolase enzymatic process used to catalyze the stereoselective condensation of a functionalized aldehyde with acetaldehyde. SFC and FTICR-MS were two technologies critical to fast turnaround time on this problem and increased confidence on the correctness of the results. 

The maca olactone leucascandrolide A 4, isolated from the calcareous sponge L. caveolata, has both cytotoxic and antifungal activity. The key step in the synthesis of 4 reported (/. Org. Chem. 2007, 72, 5784) by Scott D. Rychnovsky of the University of California, Irvine, was the stereoselective condensation of the aldehyde 1 with the allyl vinyl ether 2 to give 3. 

A possible way to induce selectivity in the photodecarboxylation process could be through photosensitized reactions in the soHd state. In fact, when a two-component molecular crystal of phenanthridine and 3-indoleacetic acid is irradiated at low temperature (-70°C), 3-methyHndole is formed in high yield as the sole product by contrast, when the same reaction is carried out in acetonitrile solution, four products are obtained.Furthermore, irradiation of two-component molecular crystals of arylalkyl carboxylic acids with stoichiometric amounts of electron acceptor causes decarboxylative condensation between the two components with important selectivities. " Thus, irradiation of (S)-naproxen in a chiral crystal with 1,2,4,5-tetracyanobenzene produces a decarboxylated condensation product retaining the initial chirality." Photolysis of an enantiomorphous bimolecular crystal of acridine with the R or S enantiomer of 2-phenylpropionic acid causes stereoselective condensation to give three optically active products. An absolute asymmetric synthesis has also been achieved by the enantioselective decarboxylative condensation of a chiral molecular crystal formed from achiral diphenylacetic acid and acridine (Scheme 9). ... 

Irradiation of a chiral bimolecular crystals formed from acridine with diphenylacetic acid and R/S-2-phenylpropionic acid (2-PPA) results in enantioselective photodecarboxylation, which is followed by stereoselective condensation between the CHMePh radical and the hydroacridine radical species (Scheme 4, enantioselective photodecarboxylation in a chiral bimolecular crystal). The radical coupling occurs in the crystal lattice to give the optically active products II to IV. On the other hand, photolysis in solution phase results in the formation of the optically inactive II and biacridine IV. 

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