M -The hydroxylactams Sa,b could be converted into the furo[3.4-ajpyrrolizinederivatives 1 Oa, b via theBaza-Cope rearrangement cyclization reaction of N-acyliminium ions.Previously we reported the synthesis of the pyrrolizidine derivative 8, starting from N-(2-aryi-3-butenyl)hydroxylactam Sc, using the tandem Paza-Cope rearrangement cyclization reaction of N-acyliminium ions as a key step (Scheme 2).' Searching for a new synthetic path to necine bases like macronecine3 and petasinecine4 via a similar series of reactions as mentioned above, we discovered a synthesis for the hitherto unknown furo[3.4-ajpyrrolizinerlng system 1 0 , which is the subject of this paper.The required starting hydroxylactams 5a.b were prepared according to Scheme 1 succinimide 4 'coupling ) I Ar 0 Scheme 1Upon treatment of the phenyl substituted hydroxylactam Sa, having a 2-benzyloxymethyl substituent attached to the butene moiety, with formic acid or bifluoroacetic acid for 18 h at room temperature the bicyclic ethersa and the tricyclic ether 10a were isolated in an approximately 1:l ratio.More forcing reaction conditions, trilluoroacetic acid for 18 hat 60 OC, led to sole formation of the tricyclic ether 10a (54% isolated yield).The bicyclic alkoxylactam Sa proved to be remarkably unreactive, but upon reflux for 20 h in trifluoroacetic acid H-nmr analysis revealed that it had been converted for the greater part into the tricyclic ether 108, the remainder being unchanged starting material.
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SPECKAMP et al. (1990) studied this question.