UNSATURATED ALIPHATIC COMPOUNDS 111,140. Allyl alcohol

Allyl alcohol may be prepared by heating glycerol with formic acid  

Laige quantities are prepai ed commercially from propylene  

Treat the combined distiUates of b.p. 195-260° with anhydrous potassium carbonate to neutralise the Uttle formic acid present and to salt out the allyl alcohol. Distil the latter through a fractionating column and collect the fraction of b.p, up to 99° separately this weighs 210 g, and consists of 70 per cent, allyl alcohol. To obtain anli5 dious allyl alcohol, use either of the following procedures — 

Acetaldehyde condenses in the presence of a little sodium sulphite or sodium hydroxide solution to aldol. The latter ehminates water upon distUlation at atmospheric pressure, but more efficiently in the presence of a trace of iodine, which acts as a catalyst, to yield crotonaldehyde  

Crotonaldehyde can be produced so cheaply on a commercial scale that its preparation in the laboratory is rarely undertaken. It is, however, included here as an illustration of the preparation of an unsaturated aliphatic aldehyde. 

Large quantities are prepared commercially from propylene  

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A reiterative application of a two-carbon elongation reaction of a chiral carbonyl compound (Homer-Emmonds reaction), reduction (DIBAL) of the obtained trans unsaturated ester, asymmetric epoxidation (SAE or MCPBA) of the resulting allylic alcohol, and then C-2 regioselective addition of a cuprate (Me2CuLi) to the corresponding chiral epoxy alcohol has been utilized for the construction of the polypropionate-derived chain ]R-CH(Me)CH(OH)CH(Me)-R ], present as a partial structure in important natural products such as polyether, ansamycin, or macro-lide antibiotics [52]. A seminal application of this procedure is offered by Kishi s synthesis of the C19-C26 polyketide-type aliphatic segment of rifamycin S, starting from aldehyde 105 (Scheme 8.29) [53]. 

The aliphatic Claisen rearrangement is simpler in that there is no rearomatisation at the end but there is an ionic step first as the vinyl ether 67 has to be made and the easiest way to do that is from the allylic alcohol 65 by acetal exchange with another vinyl ether to give 66 and elimination to give 67. All these steps, including the rearrangement occur under the same conditions16 and the product is a y,S-unsaturated carbonyl compound 68. 

Oxidation. Triphenylbismuth carbonate suspended in CH2CI2 is a heterogeneous oxidant for a variety of functional groups. Allylic alcohols are efficiently oxidized to the corresponding unsaturated aldehydes or ketones, even in the presence of a thiol, which is itself oxidized by this reagent to a disulfide, cis- and Irani-1,2-Glycols are cleaved to dialdehydes hydrazones are oxidized to diazocompounds oximes are cleaved to ketones and 1,2-dialkylhydrazines are oxidized to azo compounds. Phenylhydrazones, semicarbazones, tosylhydrazones, aromatic and aliphatic amines, enamines, and enol ethers are inert to 1. 

The aliphatic version of this rearrangement is associated with the names of Cope and Ciaisen. It also converts a C-O bond in an ether into a C-C bond in the product. An allylic alcohol (40) or (43) is converted into a vinyl ether (41) or (44) by reaction with another vinyl ether (to give aldehydes or ketones) or with an ortho ester (to give esters). The rearrangement is rapid at 100 150 C and gives y,5-unsaturated carbonyl compounds (42) and (4S). 

Oxidation of Alcohols. Alkoxymagnesium bromides of secondary alcohols and allylic and benzyhc alcohols are oxidized to the corresponding ketones or aldehydes with N-chlorosuccinimide in the presence of r-BuOLi. The method is ineffective for primary saturated and some unsaturated alcohols, but oxidations of the former substrates to aldehydes occur readily if t-butoxymagnesium bromide is substituted for f-BuOLi. Secondary alcohols (eq 2) and primary benzylic (eq 3) and allylic alcohols are oxidized to the corresponding carbonyl compounds in good yields using 2—3 equiv of a 1 1 mixture of copper(II) bromide—r-BuOLi in THF at rt. However, the reagent is not effective for the conversion of primary aliphatic alcohols to aldehydes. 

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