Retroaldol reaction, ring

Retroaldol reaction, ring opening by - 26, 227 Retro-benzilic acid-type rearrangement imidazoles by - 28, 959 Retrodiene scission... 

Photochemical cyclobutane annealings are much more promising and find their way into synthesis to a greater extent. Photochemical cycloaddition reactions to enolized 1,3-diketones and consecutive ring openings via retroaldol reactions have been applied in ring expansion reactions numerous times. 

The desired cyclization leading to the six-membered ring was achieved by intramolecular photocycloaddition of dioxolenone 107 as shown in Scheme 26. Consecutive cleavage of the ketal followed by retroaldol reaction augmented the initial ring system by a C2 unit to trans-bicycle 108 (90). Attempts to apply this reaction principle to a trans-decalin system in order to reach a taxane skeleton failed (97). 

Phenyl-substituted oxiranes may be reduced in DMF akin to benzyl ethers by direct and indirect methods the regioselectivity is slightly higher in the indirect reduction [76]. Benzoyloxiranes can be reduced in MeCN to aldoles addition of acetic acid prevents a retroaldol reaction catalyzed by the EGB [77]. The ring opening is analogous to the reduction of 2-alkoxyacetophenone to acetophenone. 

In summary, from the results of the retroaldol reaction of natamycin, a number of its degradation products (with or without a mycosamine-group), of borohydrid-reduced natamycin and of mycosamine itself, in particular the presence or absence of ethanal in the volatile fractions, it is evident that the ethanal originates both from the lactone-ring (Cl 1-Cl2) and from the aminosugar (most likely C5 -C6 ) [12]. 

In 1966 Dunathan (18) proposed that, in PLP-mediated reactions, the bond to be broken in the substrate-cofactor compound should be perpendicular to the plane of the extended conjugated system so that there would be maximum a-n overlap between the breaking bond and the ring-imine n system. Thus 3a, 3b, and 3c represent the conformations of the imine 3 best suited to achieve transamination reactions, decarboxylation reactions, and retroaldol reactions, respectively. The enzyme will be responsible for the orientation of the amino acid-PLP complex and thus dictate the nature of the resultant reaction. An example of an enzyme catalyzing two distinct reactions was found for serine hydroxymethyltransferase (EC 2.1.2.1), which normally catalyzes the retroaldol process outlined in 3c when L-serine or L-threonine are substrates. When D-alanine was used as substrate, however, a slow transamination was observed (19). Comparison of the conformations of the amino acid-PLP complexes, 3c and 17, respectively, for the retroaldol and transamination reactions shows that both the proton removed from the... 

Figure 5 lists the aldehydes which were made in quantities ranging from 10 g to 10 kg. Figure 6 shown a representative NMR spectrum of the aldehyde taken from the unpurified residue after lypophilization of the reaction mixture. This spectrum illustrates the specificity of the enzyme. Initially, we were concerned about the possible formation of uronic acids reported by some authors (25-27). However, we have no evidence for the presence of these by-products from the carbon-13 spectra of the oxidation products. On the other hand, the small signals between 90 ppm and 103 ppm may belong to dimeric products described by Maradufu and Perlin (28), All the aldehydes are hydrated as indicated by the presence of a peak at 88.7 ppm and the absence of peaks below 200 ppm. These aldehydes are relatively stable in the presence of base as evidenced by preparation of 5-C-hydroxymethyl-L-arabino-hexopyranoses (Equation 1). Even though the synthesis was carried out in 1 N NaOH, very little by-products resulted from elimination or retroaldol reactions at the pyranose ring. 

Ozone (s. a. under (CH COO)2Co) Ketones from phenol ring viaa,/ -ethylene-y-hydroperoxy-7-lactones with retroaldol reaction... 

N Heterocyclic oxo compounds by base-catalyzed retroaldol reaction 2-Pyridone ring opening... 

Aldol condensation. The final step in a synthesis of the tricyclic taxane ring system involves an intramolecular aldol condensation of 2. Treatment with the usual bases results in a retro Michael reaction, but the desired cyclization to 3 can be effected in 90% yield by use of bromomagnesium diisopropylamide (1) or isopropylcyclohexylamide. The hydroxy ketone undergoes retroaldolization in the presence of mild acids or bases, but can be reduced and stored as the corresponding stable diol. 

During the synthesis of the Woodward reserpine precursor (17 Scheme 4), Peailman used the protected acylacetal (13) to control the stereochemistry of an intramolecular photochemical cycloaddition to (14). The strategy for opening the cyclobutane ring employed the Baeyer-Villiger reaction to convert the 7-keto ester (15) to a 3-hydroxy ester (16), which underwent retroaldolization to (17). 

The importance of the ethylene ketal described above with respect to stereocontrol of the de-Mayo reaction is emphasized by later published works of two other teams. Fetizon and co-workers obviously followed a similar concept and carried out photocycloaddition reactions with 96a in model studies (Scheme 23) (Si). As can be seen from retroaldol product 98, exclusive a attack of the cyclohexene has taken place. Thus, the relative stereochemistry of the BC ring connection is opposite to that of taxane. Totally comparable results were obtained by Berkowitz et al. in the course of cycloaddition reactions of cyclopentene, cyclohexene, or those of a cyclohexenone ketal to the camphor derivative 96b (.84). 

Redox equations, balancing, 265 Reduction of aromatic compounds, 200 Reductive amination, 404 Reductive hydroboration, 95 Reformatsky reaction, 321 Regioselective reactions, 97 Reimer-Tiemann reaction, 438 Resonance, 23 in benzene, 192 energy, 24 structures, 29 Resorcinol, 327, 432 Retroaldol condensation, 399 Ring activation, 2(fiff Robinson inaction, 398 Rosenmund reduction, 306... 

The C-acyl compounds 320 underwent hydration on being heated in aqueous alcohol, but the resulting unstable azacyclols 321 underwent retroaldol fission and afforded the ring-opened derivatives 322. ° When the reaction was carried out in deuterium oxide or in labeled water, the incorporation of deuterium in the position a to the RCO group or of 0 into the ureido carbonyl group was observed. ... 

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