Aldehydes aldehyde -> alkyne elongation

Aldehyde -> Alkyne Elongation via Carbene and Carbenoid Rearrangements... 

Seyferth s diazomethane phosphonic acid dimethyl ester, which enables the aldehyde —> alkyne elongation shown in Figure 14.30, is unstable. This is why Bestmann replaced it by a less sensitive synthetic equivalent, namely diazo acetone phosphonic acid dimethyl ester (Formula C in Figure 14.31), since with potassium methoxide—which is proportionately present in a solution/suspension of solid potassium carbonate in dry methanol—the dia-... 

Fig. 14.32. Aldehyde alkyne chain elongation via [l,2]-rearrangement of a vinyl carbenoid (Corey—Fuchs procedure). The aldehyde and phosphonium ylide A generated in situ undergo a Wittig olefina-tion and form the 1,1-dibro-moalkene (B). In the second stage, the dibromoalkene is reacted with two equivalents of n-BuLi and the vinyl carbenoid D is formed stereoselectively. The carbenoid undergoes H migration to form the alkyne C. The alkyne C reacts immediately with the second equivalent of n-BuLi to give the lithium acetylide and is reconstituted by reprotonation during aqueous workup.

Fig. 11.28. Aldehyde alkyne chain elongation via [1 -rearrangement of a vinyl carbene (Seyferth procedure). First, a Horner-Wadsworth-Emmons olefination of the aldehyde is carried out to prepare alkene A. Upon warming to room temperature, alkene A decomposes and gives the vinyl carbene B. From that, the alkyne is formed by way of a [1,2]-rearrangement.

Intermediate 143 having secured, the synthesis was carried on by deprotection to form the primary alcohol 144 (Scheme 25) which was oxidized to the aldehyde 145 and converted into the envisaged alkyne (corresponding to 139) via a Corey Fuchs chain elongation via dibromide 146. 

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