Caracurine dimetho salt, structure

The names of C-toxiferine (38), C-alkaloid H (49), and C-dihydrotoxiferine (46) have been assigned to the three distinct families of curare alkaloids, of which each of these is the major representative (see Chart). The individual diquaternary members of these families differ only in the oxidation level of the central dia-zacyclo-octane ring. Thus, various oxidation processes result in the conversion of C-toxiferine (38) to caracurine II dimetho-salt (50), C-alkaloid E (51), and C-alkaloid A (52). The partial structures of these alkaloids are given in Scheme 2. 

The Chart indicates those members of the three families which with respect to structure, chemical behaviour, u.v., i.r., and n.m.r. spectra, and colour reactions are extremely similar. The alkaloids thus depicted differ only in the oxidation states of C(18) and C(18 ) C-alkaloid A (52) possesses a hydroxy-group at each of these two centres, C-alkaloid F (57) at only one, and C-calebassine (55) at neither. The same relationship holds in the same order for C-alkaloid E (51), C-alkaloid G (56), and C-curarine (54). C-Alkaloid D (53) [no OH group at C(18) or C(18 )] has hydroxy-functions at C(17) and C(17 ). In caracurine II dimetho-salt (50) these are displaced by the C(18)- and C(18 )-hydroxy-groups, resulting in two seven-membered ether rings which are also present in caracurine V dimetho-salt... 

Acid-catalysed Isomerisations of Calabash-curare Alkaloids.— As has been described, C-toxiferine (38), C-dihydrotoxiferine (46), and C-alkaloid H (49), under the influence of aqueous acid, are readily cleaved via a retro-aldehyde-ammonia addition, into their two halves . This ready cleavage was a key reaction for the elucidation of the structures of these alkaloids. In contrast, the more highly oxidised forms [caracurine II dimetho-salt (50) and C-alkaloid D... 

Scheme 1.1) 4d gives rise to caracurine II dimetho salt, C-alkaloid E, and C-alkaloid A, while 4f yields C-alkaloid D, curarine, and calebassine, as well as a number of further oxidation products of as yet unknown structure. 4e, the hybrid C-alkaloid H, forms C-alkaloids G and F. Although, as indicated above, the parent dimers can be cleaved with acid into the corresponding monomers, their more highly oxidized derivatives are found to undergo a series of isomerizations instead. 

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See also in sourсe #XX -- [ ]

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