Lythrancepine alkaloids

Lythrancine-type alkaloids have a hydroxyl or an acetoxyl group at carbons 3, 4, and 11. Lythrancines 100-103 and lythrancines 104, 105, and 106 are epimeric at C-3. Lythrancepines 107, 108, and 109 are C-4 deoxy derivatives of lythrancines I-IV. This was demonstrated by lithium aluminium hydride reduction of the O-tosylate of lythrancine 102 and acetylation of the product to lythrancepine 109 and its C-3 epimer. 

Analysis of all the above results led to only two possible structures, A and B, for lythrancines I-IV and lythrancepines I—III. Structure A is preferred because the molecular models show large interactions between the 10-methylene group and the aromatic hydrogen atoms in B and the 13-membered ring is highly strained. X-ray crystallographic studies of lythrancine 101 O-brosylate confirmed stereochemistry A. Thus, the absolute stereochemistry of seven quinolizidine alkaloids was established as 100-103 and 107-109 (104). 

Chromatography of the mother liquors of lythrancepine 102 afforded three minor alkaloids lythrancines 104, 105, and 106. The structure and stereochemistry of these bases was elucidated by analysis of NMR and mass spectra and by comparison with those of lythrancine 103. The assigned structure 104 for lythrancine-V was unequivocally confirmed by the previously described conversion of lythrancine 102 to this alkaloid via isomer 114 (61). 

Although a number of alkaloids belonging to the simple arylquinolizidine class and the lactonic type had been synthesized, no successful synthesis of cyclophane alkaloids was accomplished until that of lythranidine (94), a unique alkaloid with a 2,6-trans disubstituted piperidine structure, was reported (28, 29). Quinolizidine metacyclophane alkaloids lythrancepines II (95) and III (96) have also been synthesized recently (30, 31). A review on the synthesis of lythranidine (94) is available in Japanese (32). 

In the penultimate step of a synthesis of the unstable antibiotic Streptazolin, an iron tricarbonyl group co-ordinated to a 1.3-diene unit and two benzyl ethers were severed simultaneously using tribromoborane at -90 °C [Scheme 4.148].274 Tribromoborane has also been used to deprotect a phenolic benzyl ether in the presence of two phenolic methyl ethers in a synthesis of the alkaloid Lythrancepine [Scheme 4.149].275... 

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