A-Acetoxyaldehydes

A mixture of lead(iv) acetate (20 mmol) and KOAc (100 mmol) in AcOH (30 ml) was treated with the neat aldehyde-derived silyl enol ether (20mmol) at ambient temperature. After being stirred for 1 h at ambient temperature, the reaction mixture was diluted with water (30ml), and then extracted with pentane (3 x200ml). The combined pentane extracts were washed with saturated sodium hydrogen carbonate solution (2x50ml), dried, concentrated and distilled to give the product a-acetoxyaldehyde (45-78%). 

Lead tetraacetate/sodium salt a-Acetoxyaldehydes from glycidic acids... 

Reaction of the ylide, generated from the phosphonium salt 578 (from the alcohol 577 and phosphine hydrobromide 504), with the polyene aldehyde 542 gives aleuria-xanthin acetate 579218). The methyl ester of the naturally occurring bixin 586 is formed by a combination of some carbonyl olefinations 279). The acetoxyaldehyde 580 is olefinated with methoxycarbonylmethylene-triphenylphosphorane 67 to the ( )-unsaturated ester 581. The latter is converted into the phosphonium salt 582 upon treatment with triphenylphosphine hydrobromide 504. The corresponding ylide of 582 is reacted with the dialdehyde 539 to the polyene aldehyde ester 583. The latter is reduced and converted into phosphonium salt 584. The corresponding ylide is now reacted in a third carbonyl olefination with 585 to give the methyl ester 586 279> (Scheme 98). 

Dehydration occurred when 118 was treated with acetic acid. Reduction of the 3-carbonyl group of the product with phosphorus pentasulfide then gave 1,2-dihydro-3H-pyrrolo[l,2-a]indole derivative 119. This compound was converted into acetoxyaldehyde 120 by selenium dioxide oxidation followed by DDQ. An... 

The first technical routes to 73 were based on methylglyoxal dimethylacetal 18 (Scheme 9) as a key intermediate [27]. The acetoxyaldehyde 73 was prepared in a five-step synthesis from 18 by successive acetylene addition, partial hydrogenation, acetylation, allylic rearrangement and hydrolysis of the acetal function (sequence 18- 80- 81 - 82 83 73). A particularly... 

A BASF patent [30] describes the transformation of 81 into 4-chloro-2-methylbut-2-enal (88), which is an ideal precursor for the preparation of (3-methyl-4-oxobut-2-enyl)-triphenyl-phosphonium chloride (89). The phosphonium salt 89 is usually acetalized before being used in Wittig condensations [19,31]. Acetylation of 88 with potassium acetate in anhydrous ethanol furnished the acetoxyaldehyde 73 in high yield [32]. 

Isoprene (90) has also been considered as a starting material for the building block 73. Selective oxidation of isoprene (90) with 82% peroxyacetic acid gave an 80% yield of 3,4-epoxy-3-methylbut-l-ene (91), which was transformed into the chloroaldehyde 88 by treatment with cupric chloride in the presence of lithium chloride [32]. A short synthesis of 73 from isoprene (90) has been worked out [33]. 3-Methyl-4-nitrobut-2-en-l-yl acetate (92), obtained in excellent yield by nitroacetyloxylation of isoprene (90), was reduced with iron in acetic acid to give a mixture of oxime 93 and acetoxyaldehyde 73. Steam distillation at pH 2.5 in the presence of formaldehyde furnished distilled 73 in a 40% overall yield based on 90. 

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