Kurosaki Yasunobu, Hanari Taiki, Anada Masahiro, Shimada Naoyuki, Hashimoto Shunichi
Symposium on the Chemistry of Natural Products, symposium papers, (51) 247-252, Sep 1, 2009
The dirhodium(II) complex-catalyzed tandem cyclic carbonyl ylide formation-1,3-dipolar cycloaddition reaction sequence represents one of the most efficient methods for the rapid assembly of complex oxapolycyclic systems. Consequently, the development of a catalytic enantioselective version of this sequence has become a challenging objective. In this context, we previously reported that the tendem formation of keto- or ester-carbonyl ylide from α-diazoketone and intermolecular 1,3-dipolar cycloaddition under the influence of Rh_2(S-BPTV)_4 or Rh_2(S-PTTL)_4, respectively, give cycloadducts with up to 93% ee. Very recently, we also reported high levels of enantioselection (up to 99% ee) for the intermolecular cycloaddition of 2-diazo-3,5-diketoesters-derive dcarbonyl ylides with arylacetylene, alkoxyacetylene and styrene dipolarophiles using Rh_2(S-TCPTTL)_4. In order to demonstrate the synthetic potential of this methodology, we addressed catalytic asymmetric syntheses of oxabicyclic natural products. The 1,3-dipolar cycloaddition of cyclic carbonyl ylide dereved from α-diazoketone 11 with 2 equiv of aromatic aldehyde 12d using 1 mol % of Rh_2(S-BPTV)_4 in benzotrifluoride proceeded smoothly to afford exo-cycloadduct 10d in 67% yield with 87% ee. Peterson olefination of enantiomerically pure 10d followed by DIBAL-H reduction and chlorination afforded the allyl chloride Z-17. S_N2'-Substitution of Z-17 with MeMgI in the presence of CuCN・2LiCl at -40℃ gave the desired product 19 as a major product. Deprotection of silyl protecting group provided psoracorylifol C (7). The 1,3-dipoar cycloaddition of carbonyl ylide derived from α-diazo-β-ketoester 22 with 3 equiv of phenylacetylene 23d using 1 mol % of Rh_2(S-TCPTTL)_4 afforded the 8-oxabicyclo[3.2.1]octan-2-one 24d in 73% yield with 95% ee. Catalytic hydrogenation of the double bond provided 25 in 93% yield with perfect endo diastereoselectivity. Silylation of enantiomerically poure 25 followed by enol triflate formation and reductive elimination of the trifluoromethanesulfonate group furnished alkene 28. Reduction of the ester group in 28 and subsequent silylation gave 29. Allylic oxidation of 29 with SeO_2 followed by oxidation with MnO_2 and subsequent deprotection of two TBDPS groups completed the asymmetric synthesis of 8.