Arylmethylene Meldrum acid

Another synthon to 4-oxocarboxylic acids has been recently described and utilized for the synthesis of pyridazinones of potential antihypertensive activity. Thus, arylmethylene Meldrum acid derivatives 97 (prepared by reacting Meldrum acid 96 with aldehydes and subsequent reduction of the formed arylidene derivative using triethylammonium formate, TEAF) could be alkylated with 4-bromophenacyl bromide 98 to yield 99 that then reacted with hydrazine hydrate to yield 100 (2004SC783 Scheme 17). 

Methylbenzofuran-3-carbaldehydes undergo ready condensation with Meldrum s acid (isopropylidene malonate) to afford arylmethylene derivatives 83. These on flash vacuum pyrolysis at 500-600 C give 3-dibenzofuranols 84 (Scheme 21). The arylmethylene derivative, e.g., 85, presumably undergoes conversion to a methylene ketene (86, Scheme 22) on pyrolysis, which undergoes a [1,5-H] shift and subsequent cyclization and tautomerization, yielding the dibenzofuranol 87. The derived methyl ether 88 has been converted by mild acetylation with acetyl chloride and aluminum chloride and subsequent boron trichloride-induced demethylation to the natural product ruscodibenzofuran (8). A limitation is imposed on this method because 3-acetyl-2-methyldibenzofurans fail to condense with Meldrum s acid so that l-methyl-3-dibenzofuranols are not available by this method. ... 

The benzofuran-3-carbaldehyde (398) is the key intermediate in the synthesis of rus-codibenzofuran. Condensation of (398) with Meldrum s acid in the presence of piperidine acetate afforded the arylmethylene derivative (399), which on flash vacuum pyrolysis gave the benzofuranol (400) (Scheme 107) (80AJC1817). 

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