Pyridinium chlorochromate preparation

Draw the structure of the carbonyl compound(s) from which each of th< following alcohols might have been prepared, and show the products yen. would obtain by treatment of each alcohol with (i) Na metal, (ii) SOCl2, anc (iii) pyridinium chlorochromate. 

One such example is pyridinium chlorochromate, or PCC for short, which is prepared in the following way ... 

Entries 5 to 7 are examples of oxidation of boranes to the carbonyl level. In Entry 5, chromic acid was used to obtain a ketone. Entry 6 shows 5 mol % tetrapropylam-monium perruthenate with Af-methylmorpholine-lV-oxide as the stoichiometric oxidant converting the borane directly to a ketone. Aldehydes were obtained from terminal alkenes using this reagent combination. Pyridinium chlorochromate (Entry 7) can also be used to obtain aldehydes. Entries 8 and 9 illustrate methods for amination of alkenes via boranes. Entries 10 and 11 illustrate the preparation of halides. 

This complex, formerly called pyridine perchromate and now finding application as a powerful and selective oxidant, is violently explosive when dry [1], Use while moist on the day of preparation and destroy any surplus with dilute alkali [2], Preparation and use of the reagent have been detailed further [3], The analogous complexes with aniline, piperidine and quinoline may be similarly hazardous [4], The damage caused by a 1 g sample of the pyridine complex exploding during desiccation on a warm day was extensive. Desiccation of the aniline complex had to be at ice temperature to avoid violent explosion [4]. Pyridinium chlorochromate is commercially available as a safer alternative oxidant of alcohols to aldehydes [5], See Chromium trioxide Pyridine Dipyridinium dichromate See Other AMMINECHROMIUM PEROXOCOMPLEXES... 

The pyranocoumarin 105 can be prepared via a three-component Diels-Alder reaction between 4-hydroxycoumarin, ethyl vinyl ether and an a-dicarbonyl compound. Similarly to the above, upon treatment of 105 with sulfuric acid in THF, hydrolysis and rearrangement occur to give the furofurochromenone 106. The hemiacetal functionality in 106 may then be oxidized with pyridinium chlorochromate (PCC) to give the lactone 107 <2001EJ03711> (Scheme 28). 

A variety of oxidizing agents are available to prepare aldehydes from 1° alcohols such as pyridinium chlorochromate (PCC) and pyridinium dichromate (PDC). 

After the known intermediate 79 (contaminated with ca. 6 % < /.v isomer) [39] was prepared from Hajos-Parrish ketone [40] 78, the tert-butyl ether was cleaved (quant.) and the ketone protected as the acetal (96 %). The secondary alcohol was oxidized by pyridinium chlorochromate (PCC) to provide ketone 80 in good yield (71 %) and after fractional crystallization afforded material absent of any m-hydrindane (Scheme 10.6). [NOTE All compounds shown in Schemes 10.6 and 10.7 are shown in the ent-configuration, as published]. The oxidation of protected hydrindane 80 under Saegusa-Ito conditions [41, 42] gave enone 81 (82 %), confirmed by X-ray crystallography. 

Alcohols are the most important precursors in the synthesis of carbonyl compounds, being readily available. More complex alcohols are prepared by reaction of Grignard reagents with simpler carbonyl compounds. Ordinarily MnO and Cx OY in acid are used to oxidize 2° RjCHOH to RjCO. However, to oxidize 1° RCHjOH to RCHO without allowing the ready oxidation of RCHO to RCOOH, requires special reagents. These include (a) pyridinium chlorochromate (pcc),... 

Pyridinium chlorochromate [26299-14-9] M 215.6. Dry in a vacuum for Ih. It is not hygroscopic and can be stored for extended periods at room temp without change. If very suspect it can be readily prepared. [TET LETT 2647 1975 S 245 1982]. 

This method is of value when the alcohol is readily available from natural sources, or when it can be prepared, for example, by the reaction of an alkenyl-organometallic reagent with an aldehyde. An example of the former is the oxidation of the terpenoid alcohol carveol to carvone (Expt 5.88) using pyridinium chlorochromate-on-alumina reagent. 

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). 

The secondary alcohol is readily oxidized with pyridinium chlorochromate (PCC)25 (65) which is commercially available or easily prepared by addition of pyridine to a solution of chromium(VI)-oxide in hydrochloric acid. 

Step 2 Oxidation with pyridinium chlorochromate (PCC) affords the aldehyde. Step 3 Wittig reaction provides the TM. (Note The ylide reagent is prepared by treating Ph3P with CD3I to obtain Ph3P+CD3 r and followed by deprotonation with w-BuLi.)... 

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). 

Pyridinium chlorochromate, described as a safe, stable, and readily prepared alternative to the chromium trioxide-pyridine complex, is a convenient oxidant for primary and secondary alcohols84 and should find use in steroid chemistry. Dimethyl sulphide ditriflate , prepared from DMSO and trifluoromethanesulphonic anhydride at —78°C, has been used to oxidize 3-hydroxy-steroids.85 The biological... 

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. 

Pyridinium chlorochromate (PCC) Corey and Suggs prepared PCC by mixing CrOs with pyridine in HCl. PCC is used for the oxidation of primary and secondary alcohols in CH2CI2. This reagent is less efficient than Collins reagent for the oxidation of allyl alcohols. 

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