Tropinone

Robinson's "double Mannich" reaction

The 1917 synthesis by Robinson is considered a classic in total synthesis^[8] due to its simplicity and biomimetic approach. Tropinone is a bicyclic molecule, but the reactants used in its preparation are fairly simple: succinaldehyde, methylamine and acetonedicarboxylic acid (or even acetone). The synthesis is a good example of a biomimetic reaction or biogenetic-type synthesis because biosynthesis makes use of the same building blocks. It also demonstrates a tandem reaction in a one-pot synthesis. Furthermore, the yield of the synthesis was 17% and with subsequent improvements exceeded 90%.^[5]

This reaction is described as an intramolecular "double Mannich reaction" for obvious reasons. It is not unique in this regard, as others have also attempted it in piperidine synthesis.^[9][10]

In place of acetone, acetonedicarboxylic acid is known as the "synthetic equivalent" the 1,3-dicarboxylic acid groups are so-called "activating groups" to facilitate the ring forming reactions. The calcium salt is there as a "buffer" as it is claimed that higher yields are possible if the reaction is conducted at "physiological pH".

Reaction mechanism

The main features apparent from the reaction sequence below are:

1. Nucleophilic addition of methylamine to succinaldehyde, followed by loss of water to create an imine
2. Intramolecular addition of the imine to the second aldehyde unit and first ring closure
3. Intermolecular Mannich reaction of the enolate of acetone dicarboxylate
4. New enolate formation and new imine formation with loss of water for
5. Second intramolecular Mannich reaction and second ring closure
6. Loss of 2 carboxylic groups to tropinone

Some authors have actually tried to retain one of the CO₂H groups.^[11]

CO₂R-tropinone has 4 stereoisomers, although the corresponding ecgonidine alkyl ester has only a pair of enantiomers.

From cycloheptanone

IBX dehydrogenation (oxidation) of cycloheptanone (suberone) to 2,6-cycloheptadienone [1192-93-4] followed by reaction with an amine is versatile a way of forming tropinones.^[12][13] The mechanism evoked is clearly delineated to be a double Michael reaction (i.e. conjugate addition).

Biochemistry method

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^[14]

Reduction of tropinone

The reduction of tropinone is mediated by NADPH-dependent reductase enzymes, which have been characterized in multiple plant species.^[15] These plant species all contain two types of the reductase enzymes, tropinone reductase I and tropinone reductase II. TRI produces tropine and TRII produces pseudotropine. Due to differing kinetic and pH/activity characteristics of the enzymes and by the 25-fold higher activity of TRI over TRII, the majority of the tropinone reduction is from TRI to form tropine.^[16]