Retro-Prins reaction

Lithium aluminum hydride reduction of epiquinamine, as expected, afforded 3-epicinchonamine, mp 168°, [a]D +48° (EtOH), also obtainable along with cinchonamine by the sodium-ethanol reduction of apoquinamine (23). Finally, it was shown in 1945 that, on heating quinamine or dihydroquinamine above its melting point, formaldehyde was evolved, and this was taken as evidence for the presence of a 2-hydroxymethyl on an indole a-carbon (15). In the light of the true structure, the writer would like to suggest that this aldehyde is formed by pyrolysis of a 1,3-glycol, that is, a retro Prins reaction (25). 

Amaryllidaceae Alkaloids.—O-Methylnorbelladine (162) and vittatine (163) are implicated as intermediates in the biosynthesis of narciclasine (164). " Loss of the two-carbon bridge from the latter could plausibly occur by a retro-Prins reaction on 11-hydroxyvittatine (165) to give (166), as shown in Scheme 18, analogy for hydroxylation of (163) coming from the biosynthesis of the related alkaloid haeman-thamine.Strong support for this pathway to narciclasine (164) was obtained when it was shown that tritiated (165) was an efficient precursor for this alkaloid.It is to be noted that collapse of a similar j8-hydroxy-amine is proposed for the biosynthesis of colchicine. 

Two new sesquiterpenoid metabolites, dihydroilludin M (186) and illudaceta-lic acid (187), have been isolated from Clitocybe illudens. Treatment of illudin S (188) with 40" sulphuric acid at 0°C gives the dimeric product (189) which is believed to arise by a retro-Prins reaction to generate formaldehyde and the acyl fulvene (190) followed by electrophilic recombination of the liberated formaldehyde and two molecules of the fulvene. ... 

In the course of the early work it was observed that quinamine heated above its melting point evolved formaldehyde, at the time this was taken to mean that the alkaloid had a hydroxymethyl on the indole a-carbon. In the light of the accepted formula it is more likely that this aldehyde is a special case of a pyrolysis of a 1,3-glycol that is, a retro-Prins reaction. 

Reviews have appeared on the use of the Wittig reaction in industrial practice, the Claisen rearrangement, synthetic applications of the retro-Diels-Alder reaction, organo-palladium intermediates for the alkylation and arylation of olefins, the Prins reaction to give 1,3-dienes, and intramolecular [4 + 2] (Diels-Alder) and [3 + 2] cycloadditions.An interesting discussion of the regiospecificity of the Diels-Alder reaction in terms of frontier orbital overlap favours the Woodward-Katz concept. Useful alkyne and polyene coupling reactions are described in reviews on the chemistry of vitamin the synthesis of insect sex... 

Under similar conditions, the pendant double bond attached to a 2,3-epoxy alcohol acts as an internal nucleophile attacking at C-3, resulting, after proton loss, in a mixture of cyclized products (eq 12). The cyclopropane-containing products are believed to arise via a retro-homo Prins reaction. Pendant triple bonds can also participate in related cyclization reactions and cyclic allenes result (eq 13). The observation that the threo isomer of the substrate shown in eq 12 is stable to Ti(0-/-Pr)4 has led to the suggestion that an intramolecular metal alkoxide is the active catalyst in successful cyclization reactions. 

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