Jenamidine A

Three new naturally occurring bicyclic alkaloids, jenamidines A (1), B (2) and C (3), were discovered and isolated from the culture broth of Streptomyces sp. (strain HKI0297) via the chemical screening approach. Fermentation, isolation, structure and biological activities of these three new secondary metabolites are reported. The jenamidines have an unusual octahydro-pyrido[1,2-a]pyrimidine skeleton. Jenamidine A (1) shows antiproliferative effects against the chronic myeloid leukaemic cell line K-562. In addition, the new tricyclic sesquiterpenoid, africantriol (4) was isolated from the same strain.

[reaction: see text] Addition of the enolate of tert-butyl acetate to cyanamide methyl ester 17 followed by treatment with LHMDS afforded vinylogous urea 19 in 27% yield. Vinylogous urea 19 was also obtained from 37 and tert-butyl cyanoacetate in 50% yield. Acylation of 19 with acid chloride 31d, followed by hydrolysis of the tert-butyl ester and decarboxylation with 9:1 CH2Cl2/TFA and very mild basic hydrolysis of the methoxyacetate ester, afforded jenamidines A1/A2 (3) in 45% yield. This first synthesis confirms our reassignment of the jenamidines A1/A2 structure.

The proposed structures of jenamidines A, B, and C (1-3) were revised to jenamidines A1/A2, B1/B2, and C (8-10). Jenamidines A1/A2 (8) were synthesized from activated proline derivative 43 by conversion to 26 in two steps and 50% overall yield. Acylation of 26 with acid chloride 38d gave 39d, which was deprotected with TFA and then mild base to give 8 in 45% yield from 26. (-)-trans-2,5-Dimethylproline ethyl ester (49) was prepared by the enantioselective Michael reaction of ethyl 2-nitropropionate (51) and methyl vinyl ketone (50) using modified dihydroquinine 60 as the catalyst. Further elaboration converted 49 to natural (+)-NP25302 (12). A Wittig reaction of proline NCA (76) with ylide 79 gave 72 as a 9/1 E/Z mixture in 27% yield, completing a one-step formal synthesis of SB-311009 analogues.