Dimethyl acetals selective reduction

Thus two successive oxidations of 429 (Scheme 59) involving the Jones reagent and dichlorodicyano-p-benzoquinone respectively led to the enone 430. A similar reductive cleavage as above of the derived dimethyl acetal 431 with diisobutylaluminium hydride occurred from the less hindered a-side to furnish the P-methoxyether 432 as the major isomer. Selective O-debenzylation of432, achieved with iodotrimethylsilane led to ( )-coccinine (413). 

In hydrogenolyses with HAICU, the dimethyl acetals of cyclobutanone and cyclohexanone are cleaved more slowly than that of 3-pentanone, while those of cyclopentanone and cycloheptanone are cleaved more rapidly (Table 1), as would be expected for a carbonium ion process. The differences in rate are small, suggesting that carbonium ion character is not strongly developed in the transition state. With the dimethyl acetal of 4-t-butylcyclohexanone, the hydride addition step occurs with strongly predominating axial addition when HAlCh is used Zn(BH4)2 with TMS-Cl, and TMS-H with TMSO-Tf are less selective (Table 2). Equatorial attack predominates, however, in the reduction of the ketone itself with TBDMS-H and TBDMS-OTf. ... 

Step 2. Reductive cleavage of the acetal and reduction of the ester groups Step 3. Selective transacetalization of the 1,3-diol with benzaldehyde dimethyl acetal... 

JV,JV-dimethylacetamide dimethyl acetal. Under the control of the hydroxy-group, the Claisen rearrangement yields a ds-disubstituted cyclopentene. lodo-lactonisation takes place selectively with the amide function after deiodination and reduction of the lactone to the lactol, re-lactonisation and a cis-selective Wittig reaction lead to the desired intermediate, which must then merely undergo re-esterification, oxidation and epimerisation. 

The reduction of double bonds (C=X, X=C, O, N etc.) with H2 or various hydrides such as R3 SiH, R3 SnH or BH3 complexes can be facilitated or mediated by titanium Lewis acids including TiCU, Ti(O Pr)4 or its derivatives [286]. The selectivity of the reduction can be tuned by the change of Lewis acid species. For example, the reduction of a ketone and its dimethyl acetal by Bu3SnH showed the latter to be more reactive than the former in the presence of TiCU, but the reactivity was reversed by use of bis-titanium isopropoxide [287] (Scheme 14.122). 

The catalytic alcohol racemization with diruthenium catalyst 1 is based on the reversible transfer hydrogenation mechanism. Meanwhile, the problem of ketone formation in the DKR of secondary alcohols with 1 was identified due to the liberation of molecular hydrogen. Then, we envisioned a novel asymmetric reductive acetylation of ketones to circumvent the problem of ketone formation (Scheme 6). A key factor of this process was the selection of hydrogen donors compatible with the DKR conditions. 2,6-Dimethyl-4-heptanol, which cannot be acylated by lipases, was chosen as a proper hydrogen donor. Asymmetric reductive acetylation of ketones was also possible under 1 atm hydrogen in ethyl acetate, which acted as acyl donor and solvent. Ethanol formation from ethyl acetate did not cause critical problem, and various ketones were successfully transformed into the corresponding chiral acetates (Table 17). However, reaction time (96 h) was unsatisfactory. 

D-Ribonolactone is a convenient source of chiral cyclopentenones, acyclic structures, and oxacyclic systems, useful intermediates for the synthesis of biologically important molecules. Cyclopentenones derived from ribono-lactone have been employed for the synthesis of prostanoids and carbocyclic nucleosides. The cyclopentenone 280 was synthesized (265) from 2,3-0-cyclohexylidene-D-ribono-1,4-lactone (16b) by a threestep synthesis that involves successive periodate oxidation, glycosylation of the lactol with 2-propanol to give 279, and treatment of 279 with lithium dimethyl methyl-phosphonate. The enantiomer of 280 was prepared from D-mannose by converting it to the corresponding lactone, which was selectively protected at HO-2, HO-3 by acetalization. Likewise, the isopropylidene derivative 282 was obtained (266) via the intermediate unsaturated lactone 281, prepared from 16a. Reduction of 281 with di-tert-butoxy lithium aluminum hydride, followed by mesylation, gave 282. 

Methylation of carbazole phenolic oxygen has been achieved using dimethyl sulfate without reaction at nitrogen. Demethylation of carbazole methyl ethers has been achieved with hydrobromic acid-acetic acid, boron trichloride,and pyridine hydrochloride. Selective demethylation of methoxyl ortho to an aldehydo function has been achieved using boron trifluoride. ° ° Partial demethylation of 1-methoxy-3-formylcarbazole occurred during Wolff-Kischner reduction." ... 

The phenyl 1-thioparatoside 145 was activated with TV-iodosuccinimide and silver triflate and reacted with a convenient derivative of the disaccharide (1-d-GalpNAc-(l ->4)-p-D-GlcpNAc. After removal of the pivaloyl-protecting group with sodium methoxide, isomerization of paratose to tyvelose was performed in a one-pot reaction by oxidation with dimethyl sulfoxide and acetic anhydride, followed by reduction with L-Selectride. Selectivity of the reduction was better... 

Two syntheses of racemic 2,3-dihydrotriquinacen-2-one (380) have been described. The first approach consisted in the selective monoketalization of diketone 364 with 2,2-dimethylpropane-l, 3-diol and Baeyer-Villiger oxidation of 375 (Scheme 60).357 Although two lactones were produced, 376 could be freed of its isomer by selective alkaline hydrolysis. Diisobutylalumium hydride reduction of 376 afforded 377, the acetate of which was converted by acid treatment to 378. Ketaliza-tion of this isomeric mixture, followed by hydrolysis, mesylation, and treatment with potassium f-butoxide in dimethyl sulfoxide afforded diene ketal 379. Deketa-lization then liberated the desired ketone. 

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