Acetate, alkaloids derived from

Recently, Mukaiyama and co-workers prepared cinchona alkaloid-derived chiral quaternary ammonium phenoxide-phenol complex 23 and used it as an efficient organocatalyst for the tandem Michael addition and lactonization between oc,f-unsaturated ketones and a ketene silyl acetal 24 derived from phenyl isobutyrate. This approach permits the highly enantioselective synthesis of a series of 3,4-dihydropyran-2-ones (25), as shown in Scheme 4.11 [17]. 

Fig. 46 Some alkaloids derived from acetate and histidine.

Indaconitine, isolated (59) from A. chasmanihum Stapf, is the fourth naturally occurring ester-alkaloid derived from pseudaconine. It is the acetate benzoate, as shown by its hydrolysis to pseudaconine and acetic and benzoic acids. Pyrolysis of indaconitine proceeds, as usual, with elimination of acetic acid, but two isomeric pyroindaconitines have been described (59), the a-isomer derived from the alkaloid itself, and the iS-isomer from indaconitine hydrochloride. No oxidative studies are on record. 

The ahphatic side chains of Lobelia alkaloids derived from acetate. The 3-oxohexanoic acid formed from three units of acetate reacted with A -piperideine to yield intermediate-1 and its imine, in which subsequent condensation of another molecule of 3-oxohexanoic acid generated the dipenthyldione derivate. Demethyl-ation of the side chains, then W-methylation, and reduction of the carbonyl functions provided 8,10-diethyllobehdiol (Fig. 11.20). 

C n.m.r. of the alkaloid derived from [l,2- c]acetate showed six bonded pairs arising from the carbons of ring C. This... 

Dicentrine, CgoHjjOjN. (Items 36, 37, 39, 40 list, pp. 172-3.) This alkaloid crystallises, from ether, alcohol, or ethyl acetate in prisms, m.p. 168-9° [a]i) + 62-1° (CHCI3), and yields well-crystallised salts. It contains two methoxyl groups and yields a monoacetyl derivative, colourless leaflets, m.p. 202°, which is not hydrolysed even by boiling alcoholic potash. 1 The methiodide, B. CH3I. HjO, has m.p. 224°, and according to Manske, yields a methine base, m.p. 158-9°, the methiodide of which with potassium hydroxide solution decomposes into trimethylamine and a crystalline substance, presumably a substituted phenanthrenyl-ethylene, which polymerises on recrystallisation. 

These alkaloids can also be derived from non-aminoacid precursors. The N atom is inserted into the molecule at a relatively late stage, for example, in the case of steroidal or terpenoid skeletons. Certainly, the N atom can also be donated by an amino acid source across a transamination reaction, if there is a suitable aldehyde or ketone. Pseudoalkaloids can be acetate and phenylalanine-derived or terpenoid, as well as steroidal alkaloids. Examples of pseudoalkaloids include such compounds as coniine, capsaicin, ephedrine, solanidine, caffeine, theobromine and pinidine (Figure 6). More examples appear in Table 1. 

Alkaloid biosynthesis needs the substrate. Substrates are derivatives of the secondary metabolism building blocks the acetyl coenzyme A (acetyl-CoA), shikimic acid, mevalonic acid and 1-deoxyxylulose 5-phosphate (Figure 21). The synthesis of alkaloids starts from the acetate, shikimate, mevalonate and deoxyxylulose pathways. The acetyl coenzyme A pathway (acetate pathway) is the source of some alkaloids and their precursors (e.g., piperidine alkaloids or anthraniUc acid as aromatized CoA ester (antraniloyl-CoA)). Shikimic acid is a product of the glycolytic and pentose phosphate pathways, a construction facilitated by parts of phosphoenolpyruvate and erythrose 4-phosphate (Figure 21). The shikimic acid pathway is the source of such alkaloids as quinazoline, quinoline and acridine. 

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