Journal of Organic Chemistry p. 1335 - 1343 (1982)
Update date:2022-08-25
Topics:
Kuehne, Martin E.
Okuniewicz, J. F.
Kirkemo, C. L.
Bohnert, J. C.
Condensation of methyl 1,2,3,4,5,6-hexahydroazepino<4,5-b>indole-5-carboxylate (6) with 5-chloro-2-ethyl-4-hydroxypentanal lactol (14) yielded the 14α- and 14β-hydroxyvincadifformines (8a,b) in an epimeric ratio which was solvent dependent.On condensation of the indoloazepine 6 with 4,5-epoxy-2-ethylpentanal (9) the same compounds were formed together wih a (hydroxymethyl)-D-norvincadifformine, 10, as the major product.Dehydration of 14β-hydroxyvincadifformine (8a) readily gave tabersonine (1), while 14α-hydroxyvincadifformine (8b) could only be dehydrated bycarbamate pyrolysis.The latter compound (8b) and the (hydroxymethyl)-D-norvincadifformine 10 could be converted to vincadifformine (18) through the chloromethyl derivative 16.Condensation of the indoloazepine 6 with the epimeric 4,5-dichloro-2-ethylpentanals (19) also gave the (chloromethyl)-D-norvincadifformine 16 and 14β-chlorvinacadifformine (20).Dehydrohalogenation of the latter compound provided a third sequence to tabersonine (1).A synthesis of the C-20 epimeric pandolines (2a,b) from 5-chloro-4-ethyl-4-hydroxypentanal lactol (22) similarly allowed control of the C-20 epimeric product ratio through its solvent dependence.
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