As denoted in Snider and Brown’s model, the stereoselectivityof aldehyde−ene cyclization shows a preference for a chairlikeconformation if an aluminum complex was chosen.14 The lowselectivity associated in the cyclization of enal 8 may be attributedto complex reaction pathways under the promotion of SnCl4.Moreover, application of an ene cyclization in a system such as 8is not yet explored in total synthesis.12a,15 Therefore, we decidedto investigate the aldehyde−ene reaction instead of a directcationic cyclization which may result in several cyclized products.Several Lewis acids were thus examined. Moderate regioselectivitiesand isolated yields of cis-9p were obtained when TiCl4, BF3·Et2O, and EtAlCl2 were used (entries 1 to 4 in Table 1). MilderLewis acids such as Me2AlCl and Et2AlCl yielded good cisselectivityand isolated yields for the carbonyl−ene cyclization(entries 5 and 6). When Me3Al was chosen, exceptionalregioselectivity (p/o: >20/1) and trans-selectivity (c/t: <1/20)were achieved, albeit in a lower isolated yield due to thenucleophilic addition of aldehyde (trans-9p, 46%, entry 7).16 Fora synthetically useful scale (>2 g), the optimized conditions stillmaintain high stereo- and regioselectivity to deliver cis-9p in 75%yield (entry 8 vs 6).17 Since themeta-methoxyphenyl terminationin cationic cyclization has been challenging in literatureprecedents,18−20 the excellent regioselectivity achieved here isparticularly rewarding.
It was further discovered that the stereoselectivity can be
switched where trans-9p13 was isolated in 73% yield when the
reaction temperature was quickly raised to rt (entry 9, Table 1).
đang được dịch, vui lòng đợi..