Oligomeric alkaloids

In addition to simple monomeric units with isoquinoline structures, a number of dimeric and oligomeric alkaloids of this series are known. An example of this type, pilocereine (8) (Fig. 32.3), known from Pachycereus, Pilocereus, and Lophocereus (all Cactaceae), is derived by coupling of isoquinoline alkaloid units (probably by a free-radical mechanism, although proof for this is lacking), much in the same manner as the dimerization process leading to bisbenzyliso-quinoline alkaloids, a topic discussed below. 

A proposed biogenesis for the formation of monomeric, dimeric and oligomeric 3-alkylpiperidine alkaloids. The proposal is based on Baldwin and Whitehead s proposed biogenesis for the manzamines [58]. 

The acid reaction occurs, as defined in the test, when methyl red changes from yellow to orange or red at about 5.1 pH or when bromothymol blue changes from blue to yellow at about 7.1 pH. This means that most tertiary and quaternary amines are found in the ionic acid form, and this allows the thereby cationic alkaloid to precipitate with anionic iodobismuthate. The salt is believed to be constituted not of monomeric entities, of for example one nitrogen base and one iodobismuthate, but of oligomeric cations and anions. 

Fresh cocoa beans, the seeds of cacao tree (Theobroma cacao, StercuUaceae), have a sour odour and a sour, bitter and astringent taste. The main contributors to the bitter taste are purine alkaloids. The astringent taste of cocoa products is mainly influenced by oligomeric procyanidins, flavan-3-ol-C-glycopyranosides (see Section 8.3.6.2.2) and N-phenylpropenoyl amino acids (see Section 2.2.1.2.1). 

Kaiser, A., Billot, X., Gateau-Olesker, A., Marazano, C., and Das, B,C, (1998) Selective entry to the dimeric or oligomeric pyridinium sponge macrocydes via aminopentadienal derivatives. Possible biogenetic relevance with manzamine alkaloids. /. Am. Chem. Soc., 120, 8026-8034. 

Following the pioneering work on the synthesis of [x.3.0] bicychc systems by RRM from norbornene derivatives [35], Aube et al. applied this strategy to synthesize indolizidine alkaloid 251F, where enone 76 was obtained from the rearrangement of norbornene 75 (Scheme 11.20) (36). Early attempts afforded the desired product in only poor yield (30%) mainly because of olefin oligomerization. Under an atmosphere of ethylene, it was possible to optimize the metathesis cascade and the bicyclic compound 76 was isolated in 93% yield with the use of catalyst [Ru]-I (5 mol%). 

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