P-74 Rh触媒によるアルキニルエーテルの異性化反応と(±)-centrolobineの合成(ポスター発表の部)
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When ether 1 and a catalytic amount of Rh_2(tfa)_4 (tfa=CF_3CO_2-) were heated in toluene, dihydropyran 2 was produced in good yield. The ring closure proceeded in a highly regioselective manner, and the isomeric five-membered product 3 was not detected at all. This reaction most likely involves the cleavage of benzylic C-H bond in the substrate and the subsequent ring closure between the resulting benzylic cation and vinyl-Rh species, which was supported by an experiment with D-labled substrate 4 (Scheme 1). Table 1 shows more examples of this transformation. Benzyl ethers having an electron-rich aromatic ring appear to gain better product yields (entries 1-5). Sulfonylacetylenes having a Ms or Tf group entered the reaction as well, to give the desired products (entries 6 and 7). Another synthetically useful aspect of this cyclization is that highly diastereoselective ring closure is viable as can be seen in entries 8-12. The above stereoselective synthesis of substituted dihydropyrans was readily applied to that of (±)-centrolobine (11), a fundamental member of naturally occurring cis-2,6-disubstituted tetrahydropyrans, as shown in Scheme 2. The synthesis started with aldehyde 5, which was propargylated with a titanium reagent and then p-methoxybenzylated under acidic conditions to give 7. The sulfonylation of 7 was carried out by a two-step sequence to give 8, the Rh-catalyzed cyclization of which afforded the desired product 9 in good yield and with high diastereoselectivity. After the sulfone moiety in 9 was removed, the reduction of the olefin and hydrogenolysis of the benzyl ether at the same time completed the synthesis of (±)-centrolobine (11).
- 天然有機化合物討論会の論文
- 2008-09-01
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