Dai et al. SCHEME 7. Claisen vs Tethered Cyclopropanation Strategy SCHEME 8. Diels-Alder Reaction and Transannular Acetal Formationa a Reagents and conditions: (a) 6a (10 equiv), methylene blue (0.01 equiv), toluene, 180 °C, 2 days; (b) Amberlite I-120 (acidic), CH2Cl2, rt, 30 min, 90%, 8:1 dr; (c) Ph3P+CH3I- (6.0 equiv), KHMDS (0.5 M in toluene, 5 equiv), -78 °C; then rt overnight, 96%; (d) TBAF (1.0 M in THF, 2.0 equiv), THF, rt, 1 h, 97%; (e) DIBAL-H (1.0 M in toluene, 2.2 equiv), CH2Cl2, -78 °C, 20 min; (f) CSA (0.36 equiv), 4 Å MS, CH2Cl2, rt, 75 min, 90%. SCHEME 9. Nonstereoselective Introduction of C9 Stereocentera a Reagents and conditions: (a) OsO4 (0.05 M in toluene, 1.1 equiv), pyridine, rt, 1.5 h; (b) MsCl (20 equiv), pyridine (40 equiv), CH2Cl2, 15 h, rt, 81%; (c) Super-H (1.0 M in THF, 4.0 equiv), THF, 0 °C, 45 min, 62%; (d) DDQ (2.0 equiv), CH2Cl2/buffer solution pH ) 7.00 (18:1), rt, 5 h, 80%. undergoes what might be viewed as an enelike carboxymethy- lation (see 13, Scheme 7). Subsequently, through formal oxylactonization, 15 is produced. In sum, there has been accomplished the equivalent of a cis-hydroxycarboxymethyla- tion. However, the relationship between the lactone and the resident OTBS group is not speciﬁed. By contrast, the tethered cyclopropanation route leads to 26. Subsequent site-speciﬁc vicinal reductive cleavage again accomplishes overall cis- hydroxycarboxymethylation (see 15) with the important proviso that the cis-bridgehead substituents are now disposed syn to the OTBS group. Completion of the Total Synthesis of Spirotenuipesines A and B. With an efﬁcient and stereoselective synthesis of dienophile 20 accomplished, we now turned our attention to the critical Diels-Alder reaction. It was anticipated that 20 and 9580 J. Org. Chem. Vol. 73, No. 24, 2008 SCHEME 10. Stereochemical Analysis at C9 6a would undergo a diastereoselective Diels-Alder cycload- dition, wherein the diene 6a would approach from the much less hindered convex face of dienophile 20. The resident TBS