Retronecine esters

On the evidence available there is no reason to doubt that the necyl-necines or parent alkaloids of this group are constituted normally, in which case (VI p. 613) probably represents a typical retronecine ester. 

Hoskins and Crout have synthesized C-9 monoesters of retronecine (127) in reasonable yields by using iV,N -dicyclohexylcarbodiimide as the coupling reagent.The use of AT,JV -carbonyldiimidazole, with prior formation of the acylimidazole, was necessary with aj8-unsaturated acids and bulky a-trisubstituted acids. Subsequent esterification at C-7 of the retronecine ester with a suitable acid chloride produced unsymmetrical diesters of retronecine. 

Druce. A retronecine ester is reported to be present in S. otites Kunze ex DC. 

Edgar JA, Boppre M, Kaufmann E. Insect-synthesised retronecine ester aUcaloids precursors of the common arctiine (Lepidoptera) pheromone hydroxydanaidal. J ChemEcol 2007 33(12) 2266-80. 

Fig. 4. Isomeric 1-hydroxymethylpyrrolizidines, the specific biogenetic precursors of necine esters. Most PAs are retronecine esters...

Step or two-step, rarely three-step, reactions. The only more complex transformation is the conversion of the retronecine moiety of senecionine N-oxide into the otonecine moiety of senkirkine. Root cultures of S. vernalis transform senecionine N-oxide into senkirkine (Fig. 7 A) without occurrence of detectable intermediates [58]. In Emilia flammea retronecine is an efficient precursor of emiline (Fig. 7 A) the major PA of the plant [611. The formation of otonecine was suggested to proceed by hydroxylation at C-8 and N-methylation of a retronecine ester followed by ketone formation with concomitant cleavage of the C-N bond (Fig. 7B) [61]. The origin of the M-methyl group of senkirkine from S-adenosylmethionine has been demonstrated [62]. 

Arctiids are able to esterify retronecine with insect-derived necic acids [91, 101,103]. Creatonotine and isocreatonotine (Fig. 13,3 and 4) were isolated from adults of C. transiens which as larvae had received dietary ester alkaloids or retronecine [ 103]. These two major alkaloids were accompanied by small amounts of callimorphine and isocallimorphine (Fig. 13,1 and 2) and two further retronecine 09-esters, together with the respective 07-esters, as always (Fig. 13,5-8). None of these retronecine esters has been reported from plants so far. With the exception of callimorphine, the necic acid moieties of the new alkaloids are the a-hydroxy analogues of isoleucine, valine, and alanine, respectively. Since free... 

Mann P, Eich E, Witte L, Hartmann T (1996) GC-MS study on the alkaloid pattern of Merremia quinquefolia (L.) H.Hall. f first occurrence of retronecine esters, simple phenylethylamine derivatives, and pyrroUdides in the Convolvulaceae. Book of abstracts, 44th Annual Congress of the Society for Medicinal Plant Research and a Joint Meeting with the Czech Biotechnology Society, Prague, Czech Repubhc, p 148 (P 251)... 

Selective esterification at C-9 of retronecine (18) was also achieved by Hoskins and Crout 140) who used simple acids together with. iVA -dicyclohexylcarbodiimide as coupling reagent. iVAf -Carbonyldiimidazole was preferable when the acids to be used were ap-unsaturated or sterically hindered at the a-position. Subsequent esterification at the C-7 position of the retronecine ester then yielded unsymmetrical diesters of retronecine. 

Symphytum x uplandicum Nyman Heliotridine or retronecine esters Echimidine, symphytine, lycopsamine, intermedine, 7-acetyllycopsamine, 7-acetylindicine, symlandine, uplandicine 222) (79) ... 

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