Esters, formation reduction

The cz5-aziridine substrate shows, as expected on the basis of this model, predominant formation of the trans-cyclopropane product. The starting materials for this MIRC reaction can readily be obtained from the aziridine esters by reduction to the corresponding aldehyde and a subsequent Knoevenagel reaction with malonate ester (Scheme 25) [34]. 

Scheme 7.27. Domino ester reduction/epoxide formation/reductive epoxide-opening reaction.

Secondary Alkyl Alcohols. Treatment of secondary alkyl alcohols with tri-fluoroacetic acid and organosilicon hydrides results only in the formation of the trifluoroacetate esters no reduction is reported to occur.1,2 Reduction of secondary alkyl alcohols does take place when very strong Lewis acids such as boron trifluoride126 129 or aluminum chloride136,146 are used. For example, treatment of a dichlo-romethane solution of 2-adamantanol and triethy lsilane (1.3 equivalents) with boron trifluoride gas at room temperature for 15 minutes gives upon workup a 98% yield of the hydrocarbon adamantane along with fluorotriethylsilane (Eq. 10).129... 

When trifluoroacetic acid is used as the source of protons, it is known that rapid formation of trifluoroacetate esters precedes reduction to hydrocarbons.134,204 206 Use of acetic acid in place of trifluoroacetic acid, for example, would be expected to fail to produce good conversion to reduced product because of the combination of decreased acidity and increased nucleophilicity of acetic acid relative to... 

The numerous synthetic approaches relied principally on the formation of a 5-substituted proline ester by reductive amination followed by cyclization of the lactam ring. 

The last possibility for ester formation (20, Figure 12.15) comprises the reductive elimination of esters from acyl-alkoxy-palladium complexes 17, formed by deprotonation of the alcohol adducts 16. Clearly, it requires cis coordination of the alkoxide and acyl fragment. Since monodentates have a preference for ester formation, it was thought that this mechanism was very unlikely. 

The same basic strategy was applied to the synthesis of the smaller fragment benzyl ester 28 as well (Scheme 4). In this case, aldehyde 22 prepared from (S)-2-hydroxypentanoic acid [9] was allylated with ent-10 and tin(IV) chloride, and the resulting alcohol 23 was converted to epimer 24 via Mitsunobu inversion prior to phenylselenenyl-induced tetrahydrofuran formation. Reductive cleavage of the phenylselanyl group, hydrogenolysis of the benzyl ether, oxidation, carboxylate benzylation, and desilylation then furnished ester 28. 

Perlmutter used an oxymercuration/demercuration of a y-hydroxy alkene as the key transformation in an enantioselective synthesis of the C(8 ) epimeric smaller fragment of lb (and many more pamamycin homologs cf. Fig. 1) [36]. Preparation of substrate 164 for the crucial cyclization event commenced with silylation and reduction of hydroxy ester 158 (85-89% ee) [37] to give aldehyde 159, which was converted to alkenal 162 by (Z)-selective olefination with ylide 160 (dr=89 l 1) and another diisobutylaluminum hydride reduction (Scheme 22). An Oppolzer aldol reaction with boron enolate 163 then provided 164 as the major product. Upon successive treatment of 164 with mercury(II) acetate and sodium chloride, organomercurial compound 165 and a second minor diastereomer (dr=6 l) were formed, which could be easily separated. Reductive demercuration, hydrolytic cleavage of the chiral auxiliary, methyl ester formation, and desilylation eventually led to 166, the C(8 ) epimer of the... 

The chemistry and procedures for modification of the - CO2H groups of PAA hyperbranched grafts on PE powder were analogous to those used for PAA grafts on PE or PP films and wafers. For example, a 90% yield in ester formation was possible using acid-catalyzed Fisher esterification. Likewise, quantitative reduction (ethyl chloroformate activation, borane-dimethyl sulfide reduction) to hyperbranched poly(allyl alcohol)s and amidation all could be carried out using procedures like those used for PAA/Au surfaces. 

Formation of cycloalkane-1,2-dicarboxylate esters by reduction of dibromoalkanedicarboxylate esters at a platinum cathode in tetrahy-drofuran. Data from ref. [109]... 

In addition, the reduction of a-11 and (3-11 was s tudied in dimethyl sulfoxide as solvent, in the presence of pyridiniump-toluensulfonate. This medium makes mutarota-tion slower than the redox process.73 The two anomeric forms could reduce Cr(VI) and Cr(V) by formation of a Cr(VI) and Cr(V) ester intermediate. The equilibrium constant for this step and the rate of the redox step were different for each anomer for a-11, the equilibrium constant for ester formation is higher than for (3-11, but the redox process within this complex is faster for the latter anomer. These differences can be explained by the better chelating capacity of the 1,2-cri-diol moiety of a-11. Room-temperature CW-EPR spectra of these mixtures revealed for the a anomer several five-coordinated Cr(V)-bischelates (giso= 1.9820 [crilso 15.9xl(r4cm 1 (=47.7MHz)],... 

To achieve synthesis of (-)-arisugacin A [1], (7 J-89 was obtained readily from 2-methyl-l,3-cyclohexanedione 88 in 4 steps with an overall yield of 46%, featuring vinylogous ester formation, Stork-Danheiser double alpha methylation,52,68,69 vinyl Grignard addition followed by acidic work-up,68 and an asymmetric CBS reduction [Scheme 21].70,71... 

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