Pyridinium chlorochromate derivative

A biomimetic synthesis of benzo[c]phenanthridine alkaloids from a protoberberine via the equivalent of a hypothetical aldehyde enamine intermediate has been developed (130,131). The enamide 230 derived from berberine (15) was subjected to hydroboration-oxidation to give alcohol 231, oxidation of which with pyridinium chlorochromate afforded directly oxyche-lerythrine (232) instead of the expected aldehyde enamide 233. However, the formation of oxychelerythrine can be rationalized in terms of the intermediacy of 233 as shown in Scheme 41. An alternative and more efficient... 

The newer reagent, pyridinium chlorochromate(VI), has the particular (though not unique) ability to transform furylcarbinols into pyran derivatives under remarkably mild conditions (dichloromethane at room temperature for 1 h).310 The conversion shown in Scheme 58b exemplifies the general... 

Furfuryl alcohol is oxidized directly to 2,3-dideoxy-DL-pent-2-eno-pyranosid-4-nlose (325, R = H) by treatment with m-chloroperoxy-benzoie acid.236 A variety of substituted furfuryl alcohols have thus been converted into over 60 enediulose derivatives (345) in connection with studies of their antimicrobial activity.211 It was later found that pyridinium chlorochromate may be applied in this reaction, instead of a peroxy acid.237... 

Oxidation of 8-hydroxymethyl-2-isopropyl-l 1 //-pyrido[2,l-b]quinazolin-11-one with pyridinium chlorochromate in methylene chloride gave the 8-carboxaldehyde, which was converted into its 8-aminomethyl derivatives by reacting with amines followed by reduction of the Schiff bases with sodium cyanoborohydride in acetic acid (87JOC2469). 

Pyridinium chlorochromate in dichloromethane reacts with furans to give a range of products, but they are all formed by 1,4-electrophilic attack of chlorochromate on the furan ring the fate of the resultant intermediate (57) by heterolytic cleavage of the Cr—O bond depends on the substituents at the a-positions of the substrate. 2,5-Dialkylfurans yield a, /8-unsaturated-y-dicarbonyl compounds, 5-methyl-2-furylcarbinols yield pyran derivatives, and 5-bromo-2-furylcarbinols yield 5-hydroxyfuran-2(5//)-ones (Scheme 25) (80T661). 

The work-up of PCC oxidations can be greatly facilitated by the use of the PCC polymeric derivative, poly[vinyl(pyridinium chlorochromate)].228 Filtration of the polymer and concentration of the organic solution allow an easy isolation of the product. 

This methodology was applied to the synthesis of L-streptose [91] and methyl a-D-mycaroside [95 a], The difficulty of this photochemical oxidation originates from the sensitivity of pyruvates to hydrolysis. This approach seems limited to small quantities of substrates but does not need any separation technique as in oxidation by pyridinium chlorochromate [95 b]. Nethertheless, photolysis of pyruvates of partially protected derivatives of a-D-g/ucofuranose and P-D-fructofuranose in benzene yields the corresponding oxidized products in excellent yields [96]. 

Although in principle naturally occurring (—)-galanthamine could have been prepared by an identical sequence of reactions commencing with D-tyrosine, an alternate route to 319, the enantiomer of 314, was developed. Thus, epimeriza-tion of the methyl ester group at C-6 of the A -trifluoroacetamide derived from 315 followed by oxidation of the allylic alcohol with pyridinium chlorochromate furnished 319 in 78% optical purity, albeit in low chemical yield. Since 319 could be converted to (-)-galanthamine (291) by the same sequence of reactions outlined for the transformation of 314 to (+)-galanthamine, its preparation may be considered to represent a formal total synthesis of 291 from L-tyrosine (163). 

Further processing of 57 towards the ketone 60 is readily effected by highly regioselective tosylation of the primary hydroxyl group (66), hydride reduction 58 -> 59, and oxidation with pyridinium chlorochromate (PCC) on aluminum oxide to afford 60 in a yield of 70 % over the three steps (63). Due to the now practical accessibility of these furanoid building blocks supplementary modifications, that have already been performed, become preparatively relevant, e. g. the conversion of tosylate 58 into the 5,6-epoxide (66), C-extensions (63, 66), shortening of the carbon chain via periodation of 57 (63), and transformation of the respective products into acyclic derivatives by acid hydrolysis of the 1,2-0-isopropylidene group (63, 66). 

In this reaction only the cyclopropane ring is opened to form III/66. This compound is in equilibrium with the y-lactol III/67, which, in case of R=H, was transformed oxidatively (pyridinium chlorochromate) to the furanone III/68. The synthesis of thiophene derivatives in a similar reaction is shown in Scheme III/ll, also. 

An alternative tt> the chromium trioxide-pyridine comidex is provided by pyridinium chlorochromate (PCC) and pyridinium dichromate (PDC). These reag ts, now ubiquitous for chromate-based oxidation of alcohols, overcome the hygroscofric nature of the chromium trioxide-i ridine complex and are prepared by a less hazardous procedure both are commercially available as are several other derivative reagents. 

Reactions, which are seemingly 1,3-sigmatropic rearrangements, have been carried out with various substrates. Cyclopropyl vinyl alcohols have been converted to 3-cyclopropylprop-2-enyl derivatives by treatment with acetic acid, ° pyridinium chlorochromate, thionyl chloride,benzenethiol, and diethylaminosulfur trifluoride, e.g. reaction of 3 with thionyl chloride to give Double bond migration has also been mediated by treatment... 

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