Phenanthroindolizidine and related alkaloids: synthesis of tylophorine, septicine, and deoxytylophorinine

Abstract

Tylophorine (1) is synthesized in two ways. The first method begins with the synthesis of the amide-ester (7), which on reaction, successively, with triethyloxonium fluoroborate and sodium borohydride, gives the aminoester (8) selectively; hydrolysis and polyphosphoric acid-catalysed ring-closure of compound (9) gives the ketone (10) which yields tylophorine (1) on Clemmensen reduction. An alternative approach is by a biogenetically patterned sequence which involves condensation of (3,4-dimethoxybenzoyl)acetic acid (26) with 1-pyrroline (24) generated either in situ from putrescine by pea-seedling diamine oxidase or from ornithine by oxidation with N-bromosuccinimide; the product (16) condenses with 3,4-dimethoxyphenylacetaldehyde in benzene to give an enamine [as (12)]. This undergoes cyclisation and dehydration in methanol; sodium borohydride reduction then gives the alkaloid septicine (19) which, on oxidation with thallium(III) trifluoroacetate, yields tylophorine (1). Deoxytylophorinine (34) is made in an exactly analogous manner to the latter method for tylophorine.