Reductive Aldol Reactions

Aldol reaction. Reductive aldol reaction of conjugated amides, according to an intramolecular version as previously reported, is fully applicable to the preparation of iyn-3-hydroxyalkanamides. ... 

Aldol reactions. Reductive aldolization of methyl acrylate furnishes predominantly methyl jyn-2-methyl-3-hydroxyalkanoates in moderate yields. The catalyst system, consists of [(cod)RhCl]2, Me— DuPhos and Cl2Si(H)Me. For this reaction, significant interdependence of the metal, ligand, and hydride source for reactivity and selectivity has been witnessed. 

Aldol reaction of campholenic aldehyde with propionic aldehyde yields the intermediate conjugated aldehyde, which can be selectively reduced to the saturated alcohol with a sandalwood odor. If the double bond in the cyclopentene ring is also reduced, the resulting product does not have a sandalwood odor (161). Reaction of campholenic aldehyde with -butyraldehyde followed by reduction of the aldehyde group gives the aHyUc alcohol known commercially by one manufacturer as Bacdanol [28219-61 -6] (82). 

The last isomer, the so-called aldotripiperideine (185), is obtained by the action of acid catalysts on a-tripiperideine at its boiling point (298,299), or in aqueous solution at pH 9.2 and 100°C. Further aldol reaction between tetrahydroanabasine and A -piperideine obviously occurs. Hydrogenolysis of this compound gives dihydroaldotripiperideine (186) which is convertible into matridine (187), a reduction product of the alkaloid matrine. 

The dimer of 1-methyl- -pyrroline (39) was obtained by reduction of N-methylpyrrole with zinc and hydrochloric acid (132) and, together with the trimer, by mercuric acetate dehydrogenation of N-methylpyrrolidine (131). J -Pyrroline-N-oxides form dimers in a similar manner (302). Treatment of 1,2-dimethyl-zl -piperideine with formaldehyde, producing l-methyl-3-acetylpiperidine (603), serves as an example of a mixed aldol reaction (Scheme 18). 

A syn-selective asymmetiic nih o-aldol reaction has been reported for structurally simple aldehydes using a new catalyst generated from 6,6-bis[(tiiethylsilyl)ethynyl]BINOL (g in Scheme 3.18). The syn selectivity in the nitro-aldol reaction can be explained by steric hindrance in the bicyclic transition state as can be seen in Newman projection. In the favored h ansition state, the catalyst acts as a Lewis acid and as a Lewis base at different sites. In conbast, the nonchelation-controlled transition state affords anti product with lower ee. This stereoselective nitro-aldol reaction has been applied to simple synthesis of t/ireo-dihydrosphingosine by the reduction of the nitro-aldol product with H2 and Pd-C (Eq. 3.79). 

A two-step sequence of nitrile oxide-olehn cycloaddition and reduction of the resulting A -isoxazolines offers a unique and attractive alternative to the classical aldol reaction and its many variants (2J). The procedure bypasses traditional problems, including enolate equilibrium and cross condensation (20). 

Jacobsen epoxidation 359 -, Katsuki epoxidation 361 -, Mukaiyama-aldol reaction 367 f. -, oxime ether reduction 363 -, Sharpless asymmetric dihydroxyla-tion 361... 

In his first independent work, Helferich prepared y-hydroxyvaleral-dehyde (4-hydroxypentanal) by reduction and ozonolysis of methylhep-tenone, readily available from citral by a retro-aldol reaction. He was able to show that, similarly to the saccharides, this hydroxyaldehyde exists in... 

Aldol reaction of the aldehyde with itself In some instances, dehydration of the aldol and subsequent hydrogenation of the double bond was also observed, as was reductive alkylation of the monoalkylglucamine by aldol-derived aldehydes. 

The key cyclization in Step B-2 was followed by a sequence of steps that effected a ring expansion via a carbene addition and cyclopropyl halide solvolysis. The products of Steps E and F are interesting in that the tricyclic structures are largely converted to tetracyclic derivatives by intramolecular aldol reactions. The extraneous bond was broken in Step G. First a diol was formed by NaBH4 reduction and this was converted via the lithium alkoxide to a monomesylate. The resulting (3-hydroxy mesylate is capable of a concerted fragmentation, which occurred on treatment with potassium f-butoxide. 

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