Allylic compounds Sodium borohydride

All that remains before the final destination is reached is the introduction of the C-l3 oxygen and attachment of the side chain. A simple oxidation of compound 4 with pyridinium chlorochro-mate (PCC) provides the desired A-ring enone in 75 % yield via a regioselective allylic oxidation. Sodium borohydride reduction of the latter compound then leads to the desired 13a-hydroxy compound 2 (83% yield). Sequential treatment of 2 with sodium bis(trimethylsilyl)amide and /(-lactam 3 according to the Ojima-Holton method36 provides taxol bis(triethylsilyl ether) (86 % yield, based on 89% conversion) from which taxol (1) can be liberated, in 80 % yield, by exposure to HF pyridine in THF at room temperature. Thus the total synthesis of (-)-taxol (1) was accomplished. 

Methylmagnesium N-cyclohexyliso-propylamide, 189 By oxidation at an allylic carbon Selenium dioxide, 272 By reduction of a,0-unsaturated carbonyl compounds Sodium borohydride, 278 Sodium dithionite, 281 Other methods r-Butyllithium, 58 Butyllithium-Potassium f-butoxide,... 

It has been shown that phenylselenyl halides easily reacted with 0-allyl oximes 221 to give cyclic iminium salts 222, which by reaction with water afforded isoxazolidines 223 in moderate to good yields (equation 96) . Compounds 222 can be reduced in situ by sodium borohydride to produce Ai-alkyl-substituted isoxazolidines 224 in 50-95% yields . ... 

With sodium borohydride and catalytic amounts of titanyl acetoacetonate, a,fi-unsaturated carbonyl compounds give allyl alcohols regioselectively, whereas a-diketones and acyloins are reduced to vicinal diols.325 Enantioselectivities in the reduction of acetophenone, catalysed by 1,3,2-oxazaborolidones, have been examined using the AM1-SCF MO method. The optimized geometries, thermal enthalpies, and entropies of R and S transition states in the stereo-controlling steps of the reduction have been obtained.326... 

Scheme 2 shows Rapoport s synthesis [15]. The cinnamic acid derivative 3 prepared from m-methoxy benzaldehyde [20] was ethylated by diethyl sulfate to give ethyl cinnamate derivative 4, followed by Michael addition with ethyl cyanoacetate to afford compound 5. Compound 5 was converted to lactam 6 by the reduction of the cyano group and subsequent cyclization. Selective reduction of the lactam moiety of 6 was achieved by treatment with trimethy-loxonium fluorob orate followed by sodium borohydride reduction. Amine 8 was obtained by the reductive methylation of amine 7. Amine 8 was converted to compound 9 by methylene lactam rearrangement [21], followed by selenium dioxide oxidation to provide compound 10. Allylic rearrangement of compound 10 and subsequent hydrolysis gave compound 12. The construction of the decahydroisoquinoline structure began with compound 12,... 

These two milestone syntheses were soon followed by others, and activity in this field continued to be driven by interest in the biologically active esters of cephalotaxine. In 1986, Hanaoka et al. (27) reported the stereoselective synthesis of ( )-cephalotaxine and its analog, as shown in Scheme 4. The amide acid 52, prepared by condensation of ethyl prolinate with 3,4-dimethoxyphenylacetyl chloride, followed by hydrolysis of the ethyl ester, was cyclized to the pyrrolobenzazepine 53 by treatment with polyphos-phoric acid, followed by selective O-alkylation with 2,3-dichloropropene (54) in the presence of sodium hydride. The resulting enol ether 55 underwent Claisen rearrangement on heating to provide C-allylated compound 56, whose reduction with sodium borohydride yielded the alcohol, which on treatment with 90% sulfuric acid underwent cationic cyclization to give the tetracyclic ketone 57. Presumably, this sequence represents the intramolecular version of the Wichterle reaction. On treatment with boron tribromide, ketone 57 afforded the free catechol, which was reacted with dibromometh-ane and potassium fluoride to give methylenedioxy derivative 58, suited for the final transformations to cephalotaxine. Oxidation of ketone 58... 

Reactions of Enols and Enolate Anions.—Several methods are described for transposition of an oxo-function to the adjacent site. They involve formation of a suitable a-substituted derivative (hydroxymethylene ° or benzylidene ) and subsequent steps which transform the substituent into an isolated oxo-group. Condensations leading to both the 2-hydroxymethylene- and the 2-arylidene-3-oxo-steroids are described for 3-ketones of the 5jS-series, and also of the 5j8,9j5,10a-( retro ) series.Condensations of aromatic aldehydes at C-2 in the 5 -series are unusually slow enolisation towards C-4 is preferred, but steric compression between C-4 and C-6 in 5/3-compounds severely hinders the condensation reaction at C-4, allowing reaction at C-2 via the 2-enol. Reduction of a 21-hydroxymethylene-pregnan-20-one (337) with sodium borohydride afforded the homopregnanediol (338), although reduction of enolised P-dicarbonyl compounds frequently proceeds via elimination to give enones, and thence allylic alcohols. 

Associated with some metals (Al, Fe, Zn), BiCl3 gives Bi(0) which is a catalyst for the allylation of aldehydes and amines (ref. 35), and for the reduction of aromatic nitro compounds to azoxy compounds (ref. 36). When associated with sodium borohydride, BiCl3 gives an efficient system for the selective reduction of nitroarenes and azomethines (ref. 37). 

A general synthesis for all diastereomeric L-hexoses, as an example for monosaccharides that often do not occur in the chiral pool, has been worked out. The epoxidation of allylic alcohols with tertiary butyl hydroperoxide in presence of titanyl tartaric ester catalysts converts the carbon-carbon double bond stereose-lectively to a diol and is thus ideally suited for the preparation of carbohydrates. The procedure is particularly useful as a repetitive two-carbon homologiza-tion in total syntheses of higher monosaccharides and other poly hydroxy compounds. It starts with a Wittig reaction of a benzylated a-hydroxy aldehyde with (triphenylphosphoran-ylidene)acetaldehyde to produce the olefinic double bond needed for epoxidation. Reduction with sodium-borohydride... 

Eremophilane sesquiterpenes are not widely distributed in Eremophila species. In fact, the only other example so far known is the aldehyde (70) which occurs in E. rotundifolia (66). The structure of 70 was deduced largely from H- and C-nmr spectral data and, the absolute, stereochemistry by correlation with eremophilone. The keto aldehyde was reduced with sodium borohydride-cerium trichloride to give a mixture of diastereomeric diols. The major compound was assigned the 9a-configuration. The derived diacetate, on treatment with lithium-diethylamine, gave mainly the 9-allylic alcohol which was oxidised to eremophilone with Collin s reagent. 

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