For simplicity, only the 24(for 22a; 68% yield, 86% for 22b; (b) KHMDS (1

For simplicity, only the 24(for 22a; 68% yield, 86% for 22b; (b) KHMDS (1.5 equiv), THF, -78 C, 3 h; then Comins reagent (1.5 equiv), THF, -78 C, 1 h; c) MeMgI (3.0 M in Et2O, 1.5 equiv), CuI (2 mol%), THF, 0 C, 15 min, 80% over the two steps. (LPs)-induced cytokine production at 10 g mL-1 [IL-10 = 0.03; IL-12 = 0.02; tumor necrosis factor- (TNF) = 0.48].2 In a recent communication we reported the total synthesis and structural revision of biyouyanagin A (2b) and its 24-epimer, 24-stereochemistry of the substituents around the cyclobutane ring, an arrangement that looked odd at the outset rather,2 given the steric congestion associated with it. Another impetus for undertaking the total synthesis of biyouyanagin A was to advance further the advent of cascade reactions4 and exploit recent developments in organocatalysis5 for total synthesis purposes. Retrosynthetic Analysis While there are myriad ways to disassemble the biyouyanagin A molecule retrosynthetically, the one made possible by a retro [2+2] cycloaddition reaction (Figure 4) is both aesthetically and practically most appealing. In the synthetic direction such a reaction can, in principle, be realized by irradiation with UV light, although no precedent existed at the outset of this work for the photoinduced [2+2] cycloaddition of substrates such as the two components defined by the proposed cyclobutane disconnection (i.e. triene 17b or its 7-epimer 17a and enone 18, Figure 4). If successful, however, this approach would consititute a highly convergent strategy for the total synthesis of the natural product and might also have implications in its biosynthesis. To be sure, however, this rather obvious hypothesis had been proposed as a plausible biosynthetic pathway towards biyouyanagin A by its discoverers.2 In considering such a scenario, inspection of the two transition states that could lead to the biyouyanagin A molecule (I: and II: structure (1a or 1b) originally proposed for biyouyanagin A2 requires the transition state II, an arrangement that suffers from severe steric congestion between the -lactone moiety of the enone and the side chain of the triene component as demonstrated by manual molecular models, and shown in Figure 4. On the other hand, the alternative arrangement of the reacting components as shown in the transition state I is free of such unfavorable interactions. This realization created a suspicion in our minds with regards to the structure of biyouyanagin A as proposed in the isolation paper.2 Specifically, we began to Entacapone sodium salt favor the stereochemistry as shown in structure 2b, although the cloud of ambiguity over the configuration of the C-24 stereocenter (see structure 2a) remained. In addition, the NOE interactions reported for biyouyanagin A,2 in conjunction with manual molecular models, did not exclude the structure 2b (or 2a), a fact that fueled our skepticism about the true structure of the natural product further. It was with this background that we embarked on the synthetic journey to biyouyanagin A, whose true structure became our immediate puzzle to solve. Open in a separate window Figure 4 Retrosynthetic analysis of biyouyanagin A and transition states of the proposed photoinduced [2+2] cycloaddition reaction. For simplicity, only the 24(for 22a; 68% yield, 86% for 22b; (b) KHMDS (1.5 equiv), THF, -78 C, 3 h; then Comins reagent (1.5 equiv), THF, -78 C, 1 h; c) MeMgI (3.0 M in Et2O, 1.5 equiv), CuI (2 mol%), THF, 0 C, 15 min, 80% over the two steps. MVK = methyl vinyl ketone; THF = tetrahydrofuran; KHMDS = potassium hexamethyldisilazanide; Tf = trifluoromethanesulfonyl Open in a separate window Scheme 2 Synthesis of Propargyl Alcohol 26.a aReagents and conditions: (a) DMP (2.0 equiv), CH2Cl2, 25 C, 5 h, 92%; (b) acetylene, 3:1 isomeric ratio). Although this mixture could not chromatographically be conveniently resolved, the desired stereoisomer could be isolated easily by fractional crystallization from CH2Cl2/hexanes (62% yield). Alternatively, the two isomers could be separated by flash column chromatography of their 4-nitrobenzoates (4-nitrobenzoyl chloride, Et3N, 4-DMAP, 95% combined yield), and then two free alcohols (26 and 4-= 0.69, CHCl3); lit.,6b []D25 = -270.7 (= 0.11, CHCl3)} as shown in Scheme 4. Similarly, {4-stereochemistry were the NOEs between H-6 and H-17,|4-stereochemistry were the NOEs between H-17 and H-6,} {H-6 and H-22,|H-22 and H-6,} {and H-17 and H-22.|and H-22 and H-17.} {Note that adjacent protons of the cyclobutane ring may exhibit an NOE,|Note that adjacent protons of the cyclobutane ring might exhibit an NOE,} {even if they are to each other,|if they are to each other even,} {as it is the case here.|as it is the full case here.} In addition, the indicated NOEs between the aromatic and C-23 methyl protons (see Figure 8) revealed the orientation of these substituents. The absolute structures of 2a (mp 94-95 C, hexanes).Chadha for mass spectrometric and X-ray crystallographic assistance, respectively. plant led to the discovery of biyouyanagin A, a substance which was originally assigned structure 1a or 1b (Figure 1) on the basis of NMR spectroscopic analysis.{2 Biyouyanagin A exhibited significant and selective inhibitory activity against HIV replication in H9 lymphocytes (EC50 = 0.|2 Biyouyanagin A exhibited selective and significant inhibitory activity against HIV replication in H9 lymphocytes (EC50 = 0.}798 g mL-1) as compared to {noninfected|non-infected} H9 lymphocytes (EC50 25 g mL-1), thus demonstrating a therapeutic index (TI) of greater than 31.3.2 In addition, this substance inhibited strongly lipopolysaccharide (LPs)-induced cytokine production at 10 g mL-1 [IL-10 = 0.03; IL-12 = 0.02; tumor necrosis factor- (TNF) = 0.48].2 In a recent communication we reported the total synthesis and structural revision of biyouyanagin A (2b) and its 24-epimer, 24-stereochemistry of the substituents around the cyclobutane ring, an arrangement that looked rather odd at the outset,2 given the steric congestion associated with it. Another impetus for undertaking the total synthesis of biyouyanagin A was to advance further the advent of cascade reactions4 and exploit recent developments in organocatalysis5 for total synthesis purposes. Retrosynthetic Analysis While there are myriad ways to disassemble the biyouyanagin A molecule retrosynthetically, the one made possible by a retro [2+2] cycloaddition reaction (Figure 4) is both aesthetically and practically most appealing. In the synthetic direction such a reaction can, in principle, be realized by irradiation with UV light, although no precedent existed at the outset of this work for the photoinduced [2+2] cycloaddition of substrates such as the two components defined by the proposed cyclobutane disconnection (i.e. triene 17b or its 7-epimer 17a and enone 18, Figure 4). If successful, however, this approach would consititute a highly convergent strategy for the total synthesis of the natural product and might also have implications in its biosynthesis. To be sure, however, this rather obvious hypothesis had been proposed as a plausible biosynthetic pathway towards biyouyanagin A by its discoverers.2 In considering such a scenario, inspection of the two transition states that could lead to the biyouyanagin A molecule (I: and II: structure (1a or 1b) originally proposed for biyouyanagin A2 requires the transition state II, an arrangement that suffers from severe steric congestion between the -lactone moiety of the enone and the side chain of the triene component as demonstrated by manual molecular models, and shown in Figure 4. On the other hand, the alternative arrangement of the reacting components as shown in the transition state I is free of such unfavorable interactions. This realization created a suspicion in our minds with regards to the structure of biyouyanagin A as proposed in the isolation paper.2 Specifically, we began to favor the stereochemistry as shown in structure 2b, although the cloud of ambiguity over the configuration of the C-24 stereocenter (see structure 2a) remained. In addition, the NOE interactions reported for biyouyanagin A,2 in conjunction with manual molecular models, did not exclude the structure 2b (or 2a), a fact that fueled further our skepticism about the true structure of the natural product. Entacapone sodium salt It was with this background that we embarked on the synthetic journey to biyouyanagin A, whose true structure became our immediate puzzle to solve. Open in a separate window Figure 4 Retrosynthetic analysis of biyouyanagin A and transition states of the proposed photoinduced [2+2] cycloaddition reaction. For simplicity, only the 24(for 22a; 68% yield, 86% for 22b; (b) KHMDS (1.5 equiv), THF, -78 C, 3 h; then Comins reagent (1.5 equiv), THF, -78 C, 1 h; c) MeMgI (3.0 M in Et2O, 1.5 equiv), CuI (2 mol%), THF, 0 C, 15 min, 80% over the two steps. MVK = methyl vinyl ketone; THF = tetrahydrofuran; KHMDS = potassium hexamethyldisilazanide; Tf = trifluoromethanesulfonyl Open in a separate window Scheme 2 Synthesis of Propargyl Alcohol 26.a aReagents and conditions: (a) DMP (2.0 equiv), CH2Cl2, 25 C, 5 h, 92%; (b) acetylene, 3:1 isomeric ratio). Although this mixture could not be conveniently resolved chromatographically, the desired stereoisomer could be isolated easily by fractional crystallization from CH2Cl2/hexanes (62% yield). Alternatively, the two isomers could be separated by flash column chromatography of their 4-nitrobenzoates (4-nitrobenzoyl chloride, Et3N, 4-DMAP, 95% combined yield), and then two free alcohols (26 and 4-= 0.69, CHCl3); lit.,6b []D25 = -270.7 (= 0.11, CHCl3)} as shown in Scheme 4. Similarly, 4-stereochemistry were the NOEs between H-6 and H-17, H-6 and H-22, and H-17 and H-22. Note that adjacent protons of the cyclobutane ring may exhibit an NOE, even if they are to each other, as it is the case here. In addition, the indicated NOEs between the aromatic and C-23 methyl protons (see Figure 8) revealed the orientation of these substituents..With a fluoride residue in its structure, this relatively small molecule may serve as a new lead for a drug discovery program in the anti-HIV area of pharmaceutical research. Table 2 {Anti-HIV-1 Activities and Cytotoxicities of Biyouyanagin A and Analogsa|Anti-HIV-1 Cytotoxicities and Activities of Biyouyanagin A and Analogsa} thead th align=”right” valign=”top” rowspan=”1″ colspan=”1″ Entry /th th align=”center” valign=”top” rowspan=”1″ colspan=”1″ compound /th th align=”center” valign=”top” rowspan=”1″ colspan=”1″ HIV-1 IC50 (M)b /th th align=”center” valign=”top” rowspan=”1″ colspan=”1″ MT-2 CC50 (M)c /th th align=”center” valign=”top” rowspan=”1″ colspan=”1″ TId /th /thead 1AZT0.078 50 64122a2028514.432b261987.5432544417.5542526810.8652019710.0761921311.1872333314.499312327.51010191437.511112751218.9121216130180.013131241735.6141434137740.015152345920.016163546213.317182992532.0 Open in a separate window aAverage value from quintuplicate experiments. bIC50 is the concentration at which 50% inhibition of HIV-1 replication is observed as measured by a TZM-bl/luciferase assay. cCC50 is the concentration at which 50% inhibition of cell metabolism is observed as measured by a colorimetric XTT assay. dTherapeutic index (CC50/IC50). Conclusion {Relying on a highly convergent strategy,|Relying on a convergent strategy highly,} the described total synthesis of biyouyanagin A and its 24-epimer highlights the importance of cascade reactions in total synthesis and the continuing role the later discipline plays in natural products chemistry and biology. with this plant led to the discovery of biyouyanagin A, a substance which was originally assigned structure 1a or 1b (Figure 1) on the basis of NMR spectroscopic analysis.2 Biyouyanagin A exhibited significant and selective inhibitory activity against HIV replication in H9 lymphocytes (EC50 = 0.798 g mL-1) as compared to {noninfected|non-infected} H9 lymphocytes (EC50 25 g mL-1), thus demonstrating a therapeutic index (TI) of greater than 31.3.2 In addition, this substance inhibited strongly lipopolysaccharide (LPs)-induced cytokine production at 10 g mL-1 [IL-10 = 0.03; IL-12 = 0.02; tumor necrosis factor- (TNF) = 0.48].2 In a recent communication we reported the total synthesis and structural revision of biyouyanagin A (2b) and its 24-epimer, 24-stereochemistry of the substituents around the cyclobutane ring, an arrangement that looked rather odd at the outset,2 given the steric congestion associated with it. Another impetus for undertaking the total synthesis of biyouyanagin A was to advance further the advent of cascade reactions4 and exploit recent Entacapone sodium salt developments in organocatalysis5 for total synthesis purposes. Retrosynthetic Analysis While there are myriad ways to disassemble the biyouyanagin A molecule retrosynthetically, the one made possible by a retro [2+2] cycloaddition reaction (Figure 4) is both aesthetically and practically most appealing. In the synthetic direction such a reaction can, in principle, be realized by irradiation with UV light, although no precedent existed at the outset of this work for the photoinduced [2+2] cycloaddition of substrates such as the two components defined by the proposed cyclobutane disconnection (i.e. triene 17b or its 7-epimer 17a and enone 18, Figure 4). If successful, however, this approach would consititute a highly convergent strategy for the total synthesis of the natural product and might also have implications in its biosynthesis. To be sure, however, this rather obvious hypothesis had been proposed as a plausible biosynthetic pathway towards biyouyanagin A by its discoverers.2 In considering such a scenario, inspection of the two transition states that could lead to the biyouyanagin A molecule (I: and II: structure (1a or 1b) originally proposed for biyouyanagin A2 requires the transition state II, an arrangement that suffers from severe steric congestion Entacapone sodium salt between the -lactone moiety of the enone and the side chain of the triene component as demonstrated by manual molecular models, and shown in Figure 4. On the other hand, the alternative arrangement of the reacting components as shown in the transition state I is free of such unfavorable interactions. This realization created a suspicion in our minds with regards to the structure of biyouyanagin A as proposed in the isolation paper.2 Specifically, we began to favor the stereochemistry as shown in structure 2b, although the cloud of ambiguity over the configuration of the C-24 stereocenter (see structure 2a) remained. In addition, the NOE interactions reported for biyouyanagin A,2 in conjunction with manual molecular models, did not exclude the structure 2b (or 2a), SF1 a fact that fueled further our skepticism about the true structure of the natural product. It was with this background that we embarked on the synthetic journey to biyouyanagin A, whose true structure became our immediate puzzle to solve. Open in a separate window Figure 4 Retrosynthetic analysis of biyouyanagin A and transition states of the proposed photoinduced [2+2] cycloaddition reaction. For simplicity, only the 24(for 22a; 68% yield, 86% for 22b; (b) KHMDS (1.5 equiv), THF, -78 C, 3 h; then Comins reagent (1.5 equiv), THF, -78 C, 1 h; c) MeMgI (3.0 M in Et2O, 1.5 equiv), CuI (2 mol%), THF, 0 C, 15 min, 80% over the two steps. MVK = methyl vinyl ketone; THF = tetrahydrofuran; KHMDS = potassium hexamethyldisilazanide; Tf = trifluoromethanesulfonyl Open in a separate window Scheme 2 Synthesis of Propargyl Alcohol 26.a aReagents and conditions: (a) DMP (2.0 equiv), CH2Cl2, 25 C, 5 h, 92%; (b) acetylene, 3:1 isomeric ratio). Although this mixture could not be conveniently resolved chromatographically, the desired stereoisomer could be isolated easily by fractional crystallization from CH2Cl2/hexanes (62% yield). Alternatively, the two isomers could be separated by flash column chromatography of their 4-nitrobenzoates (4-nitrobenzoyl chloride, Et3N, 4-DMAP, 95% combined yield), and then two free alcohols (26 and 4-= 0.69, CHCl3); lit.,6b []D25 = -270.7 (= 0.11, CHCl3)} as shown in Scheme 4. Similarly, 4-stereochemistry were the NOEs between H-6 and H-17, H-6 and H-22, and H-17 and H-22. Note that adjacent protons of the cyclobutane ring may exhibit an NOE, even if they are to each other, as it is the case here. In addition, the indicated NOEs between the aromatic and C-23 methyl protons (see Figure 8) revealed the orientation of these substituents. The absolute structures of 2a (mp 94-95 C, hexanes) and 2b (mp 75-76 C, hexanes) were ultimately solved by X-ray crystallographic analysis (see ORTEP drawings, Figure ?Figure99 and ?and10,10, respectively), {which unambiguously established.|which established unambiguously.}H. 1) on the basis of NMR spectroscopic analysis.2 Biyouyanagin A exhibited significant and selective inhibitory activity against HIV replication in H9 lymphocytes (EC50 = 0.798 g mL-1) as compared to {noninfected|non-infected} H9 lymphocytes (EC50 25 g mL-1), thus demonstrating a therapeutic index (TI) of greater than 31.3.2 In addition, this substance inhibited strongly lipopolysaccharide (LPs)-induced cytokine production at 10 g mL-1 [IL-10 = 0.03; IL-12 = 0.02; tumor necrosis factor- (TNF) = 0.48].2 In a recent communication we reported the total synthesis and structural revision of biyouyanagin A (2b) and its 24-epimer, 24-stereochemistry of the substituents around the cyclobutane ring, an arrangement that looked rather odd at the outset,2 given the steric congestion associated with it. Another impetus for undertaking the total synthesis of biyouyanagin A was to advance further the advent of cascade reactions4 and exploit recent developments in organocatalysis5 for total synthesis purposes. Retrosynthetic Analysis While there are myriad ways to disassemble the biyouyanagin A molecule retrosynthetically, the one made possible by a retro [2+2] cycloaddition reaction (Figure 4) is both aesthetically and practically most appealing. In the synthetic direction such a reaction can, in principle, be realized by irradiation with UV light, although no precedent existed at the outset of this work for the photoinduced [2+2] cycloaddition of substrates such as the two components defined by the proposed cyclobutane disconnection (i.e. triene 17b or its 7-epimer 17a and enone 18, Figure 4). If successful, however, this approach would consititute a highly convergent strategy for the total synthesis of the natural product and might also have implications in its biosynthesis. To be sure, however, this rather obvious hypothesis had been proposed as a plausible biosynthetic pathway towards biyouyanagin A by its discoverers.2 In considering such a scenario, inspection of the two transition states that could lead to the biyouyanagin A molecule (I: and II: structure (1a or 1b) originally proposed for biyouyanagin A2 requires the transition state II, an arrangement that suffers from severe steric congestion between the -lactone moiety of the enone and the side chain of the triene component as demonstrated by manual molecular models, and shown in Figure 4. On the other hand, the alternative arrangement of the reacting components as shown in the transition state I is free of such unfavorable interactions. This realization created a suspicion in our minds with regards to the structure of biyouyanagin A as proposed in the isolation paper.2 Specifically, we began to favor the stereochemistry as shown in structure 2b, although the cloud of ambiguity over the configuration of the C-24 stereocenter (see structure 2a) remained. In addition, the NOE interactions reported for biyouyanagin A,2 in conjunction with manual molecular models, did not exclude the structure 2b (or 2a), a fact that fueled further our skepticism about the true structure of the natural product. It was with this background that we embarked on the synthetic journey to biyouyanagin A, whose true structure became our immediate puzzle to solve. Open in a separate window Figure 4 Retrosynthetic analysis of biyouyanagin A and transition states of the proposed photoinduced [2+2] cycloaddition reaction. For simplicity, only the 24(for 22a; 68% yield, 86% for 22b; (b) KHMDS (1.5 equiv), THF, -78 C, 3 h; then Comins reagent (1.5 equiv), THF, -78 C, 1 h; c) MeMgI (3.0 M in Et2O, 1.5 equiv), CuI (2 mol%), THF, 0 C, 15 min, 80% over the two steps. MVK = methyl vinyl ketone; THF = tetrahydrofuran; KHMDS = potassium hexamethyldisilazanide; Tf = trifluoromethanesulfonyl Open in a separate window Scheme 2 Synthesis of Propargyl Alcohol 26.a aReagents and conditions: (a) DMP (2.0 equiv), CH2Cl2, 25 C, 5 h, 92%; (b) acetylene, 3:1 isomeric ratio). Although this mixture could not be conveniently resolved chromatographically, the desired stereoisomer could be isolated easily by fractional crystallization from CH2Cl2/hexanes (62% yield). Alternatively, the two isomers could be separated by flash column chromatography of their 4-nitrobenzoates (4-nitrobenzoyl chloride, Et3N, 4-DMAP, 95% combined yield), and then two free alcohols (26 and 4-= 0.69, CHCl3); lit.,6b []D25 = -270.7 (= 0.11, CHCl3)} as shown in Scheme 4. Similarly, 4-stereochemistry were the NOEs between H-6 and.