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2. Results and discussion Reactions of the N-ylides derived from the pyridinium salts1, isoquinolinium salts 2, and quinolinium salts 3 with the dichloro substituted a,b-unsaturated carbonyl compounds 4–7 have been investigated (Fig. 1). Pyridinium phenancylide generated in situ by the deprotonation of phenancyl pyridinium fluoroborate 1a (1.1 mmol) with sodium carbonate (3.5 mmol) as a basewas allowed to react with 2,3-dichloro-indenone 4 (1.0 mmol) at 50?C for 24 h to lead to complete conversion of 4. Chromatographic separation of the reaction mixturegave the indeno[2,1-a]indolizin-11-one 8 directly in 42% yield. Reactions of the same ylide from 1a with 3,4-dichlorocoumarin 5 similarly afforded the corresponding 1,2-annulated indolizine 9 in 38% yield (Table 1). At the same time, reaction of pyridinium ethoxy carbonylmethylide (from 1b) with 4a,6,7,8a-tetrachloro-1,4-methanonaphtha-lene-5,8-dione 6 under similar conditions afforded the indo- lizine 10 (63%). These products are fully characterized by spectral(IR,1HNMR,andMS)andanalyticaldata.Notably,the1H NMR spectra of 10 are characterized by an unusually low field absorption (d 9.8–9.9 ppm) of the proton at C1 (see compound 10 in Fig. 1) caused by the strong anisotropic deshielding effect of the nearby carbonyl group.We have further investigated the reactions of the isoquinolinium ylides derived from 2a–2d and quinolinium ylides from 3a–3c with dichloroalkenes 4 and 6. These reactions also led to the one-pot synthesis of the corresponding 1,2-annulated indolizines 11–14 as pentacyclic compounds in moderate to high yields (Table 1). The structures of these products are further supported by an X-ray crystallographic analysis of 12a。 It is noteworthy that the reactions of the ylide from 1a with alkenes 4 and 5, and the reaction of the ylides from 2 and 3 with alkene 4 are highly regioselective to give only a single product (8, 9, 11, and 13) without their regioisomers. The regiochemistry in 13a is unambiguously established by an X-ray crystallographic analysis。 |
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