The syntheses of 3-(Z-pyrrolyl)-and 3-(Z-furanylj-indoles are described via the ring-closure of 2-(Z-nitrophenylethenylj-pyrroles or -furans, respectively, with triethylphosphite.
The necessary starting compounds are obtained by Lewis acid mediated Michael type addition reactions of either pyrroles or fwans to &nitrostyrene, followed by bromination/dehydrobromination.
The ethenylpyrroles may also be prepared by similar additions of pyrroles to B-bromo-B-nitrostyrenes and dehydrobromination of the products. 3-(Z-Pyrrolyl)-and 3-(Z-furanylj-indoles (1) and (2) are relatively unknown' structural types, and are not in our experience, accessible by the obvious Fischer indole approaches. 2 A general route to such systems is desirable because of their potential further applications in synthesis. 1 2 Here we describe a convenient synthesis, firstly, of 3-[Z-(N-methylpyrrolyl)] indole (l,R=Me) in which the indole ring is formed in the last step. Thus N-methylpyrrole was reacted with Bbromo-8-nitrostyrene (3) at room temperature, in the presence of zinc iodide, to give the adduct (4,R=Me) (95%), which was dehydrobrominated by a suspension of fine'ly divided calcium carbonate in N,N-dimethylacetamide to give the nitrostyrene (S,R=Me) (67%) as a mixture of E and Z-isomers. Cyclisation to the target indole (l,R=Me) was achieved by reaction with triethylphoephite at 160°, in 40% yield. 3 The yield, although modest, is compensated for by the brevity and convenience of the route. No compounds resulting from cyclisation with the more electron rich pyrrole ring were isolated, and when the reaction was repeated with the N-dealkyl-B-nitrostyrene
(5,R=H) a dark intractable tar formed. Since the structures, (6). or (7), to be expected from the alternative cyclisations to the five-membered ring are strained it is possible that they do not survive the relatively harsh conditions and lead, at least in part, to the large amounts Of resinous material which always accompany the indoles in the reaction products.