Alkaloids with exocyclic nitrogen

Numerous UCNMR investigations on alkaloids have been reported in the literature [598, 599]. In Table 5.13 the 13C chemical shifts and structures of representative alkaloids of different types are collected Pyrrolidine, piperidine and pyridine [600-602], tropane [600, 603-605], izidine [606-612], indole [600, 603, 613-633], isoquinoline [599, 630, 634-647], quinolinic [648-656], imidazole [657], yuzurimine alkaloids [658], alkaloids with exocyclic nitrogen [659, 660], diterpenoid [661-663], steroid [664-666] and peptide alkaloids [667-671], The complete signal assignment for the alkaloids given in Table 5.13 was achieved using the correlations between 13C NMR spectral parameters and structural properties and the 13C chemical shift values of model compounds described in Chapters 3 and 4 of this monograph. 

Yttrium-catalyzed diene cyclization/hydrosilylation was applied to the synthesis of aliphatic nitrogen heterocycles such as the indolizidine alkaloid ( )-epilupinine. l-Allyl-2-vinylpiperidine 30 was synthesized in four steps in 59% overall yield from commercially available ( )-2-piperidinemethanol (Scheme 10). Treatment of 30 with phenylsilane and a catalytic amount of Gp 2YGH3(THF) gave silylated quinolizidine derivative 31 in 84% yield, resulting from selective hydrometallation of the A-allyl G=G bond in preference to the exocyclic vinylic G=G bond. Oxidation of the crude reaction mixture with tert-huVf hydrogen peroxide and potassium hydride gave (i)-epilupinine in 51-62% yield from 30 (Scheme 10). 

Iminium ion-vinylsUane cyclization. This reaction can be used to control the configuration of exocyclic double bonds /i to the nitrogen function of alkaloids. Thus, reaction of the (Z)- and (E)-vinylsilanes 1 and 2 with paraformaldehyde (excess) and tf-camphor-10-sulfonic acid proceeds with >98% retention of configuration to give 3 and 4, respectively. The latter product is identical with the natural indoloquinolizidine alkaloid deplancheine. ... 

Delnudine 020 25 03). This unusual new alkaloid was found in D. denudatum. Its empirical formula (C20H25NO3) coupled with the presence of a carbonyl and exocyclic methylene group (5 4.72, 4.96) indicated a heptacyclic structure. The proximity of the ketone and double bond was concluded from the u.v. absorption Umax 300 nm, log< 1.76) (see Section 2B). The presence of a carbinolamine was shown by formation of an N-acetyl ketone on acetylation, and that of a secondary hydroxyl near the nitrogen by the drop of two units when it was oxidised to a carbonyl group. 

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