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1.2.3.1.1.5.1 In Situ Generation of the Aldehyde

DOI: 10.1055/sos-SD-210-00185

Riva, R.; Banfi, L.; Basso, A.Science of Synthesis: Multicomponent Reactions, (20141339.

In many cases the aldehyde component of a Passerini reaction is synthesized through oxidation of a primary alcohol. Therefore, the possibility to perform a one-pot oxidation–Passerini reaction brings about an obvious advantage in terms of step economy. Moreover, the benefit is even greater when the required aldehyde is chemically or stereochemically unstable. However, most of the reagents commonly used for oxidation of primary alcohols are not well suited, because they can react with easily oxidizable isocyanides [Cr(VI) compounds], require low temperatures (Swern oxidation), form a carboxylic acid as byproduct (Dess–Martin periodinane), or react only with activated alcohols [manganese(IV) oxide]. For these reasons, 1-hydroxy-1,2-benziodoxol-3(1H)-one 1-oxide (IBX, often called 2-iodoxybenzoic acid) was selected as the best-suited one.[‌52‌] Although 1-hydroxy-1,2-benziodoxol-3(1H)-one 1-oxide is typically used in dimethyl sulfoxide, a solvent more compatible with the Passerini reaction was sought, with tetrahydrofuran turning out to be the best one. The reaction is carried out by simply mixing all reagents and stirring the heterogeneous mixture at 40 °C. The method is particularly well suited for labile aldehydes or aldehydes with an α-stereogenic center. No racemization is observed in these cases, although the diastereoselectivity is always, as in “normal” Passerini reactions, very low. For example, although it was not possible to prepare and isolate azirine-containing aldehydes, the 1-hydroxy-1,2-benziodoxol-3(1H)-one 1-oxide mediated oxidative Passerini reaction of an azirine-containing alcohol works well (Scheme 13).[‌53‌] Chiral α-azido aldehydes are another class of somehow unstable aldehydes, and the oxidative Passerini reaction is best suited in this case.[‌54‌,‌55‌] The reaction of these aldehydes is carried out at 100 °C, under microwave heating. These conditions seem superior to the original ones at 40 °C due to shorter reaction times and higher yields.[‌54‌]

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Me Me (M)-MM(MMe)Me MMM, MMM, 88 °M, eeeeeeeee 88 [‌88‌]
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Me (M)-MM(MMMe)Me Me MMM, MMM, 88 °M, eeeeeeeee 88 [‌88‌]
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Me Me Me M8, MeMM8, MMMMM, MeMe8, eeeeeee, ee, 88 e 88 [‌88‌]
Me Me M8, MeMM8, MMMMM, MeMe8, eeeeeee, ee, 88 e 88 [‌88‌]

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References


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