Chemoenzymic Synthesis of Chiral Furan Derivatives: Useful Building Blocks for Optically Active Structures

60%; [a]25D -13.4O (c 0.7, MeOH);24 'H NMR (CDCld 8 0.95 (3 H, d, J = 6.0 Hz), 1.40-2.00 (3 H, m), 2.15 (2 H, br s), 3.30-3.90 (4 H, m). Synthesis of 10. This compound was synthesized by starting from cis-3Dc as reported above for cis-3Ac. The diastereomeric ratio has been obtained by capillary GC analysis (150 OC 0 min/4 OC/min/250 OC 5 min, tR = 13.72 (major), 14.10 (minor)). The yield is reported in Table 11: 'H NMR (CDCI,) 6 0.77 (3 H, d, J = 7.1 Hz), 1.16 (3 H, d, J = 7.0 Hz), 2.10-2.90 (3 H, m), 3.67 (3 H, s), 3.73 (3 H, s), 4.10-4.33 (1 H, m), 5.02 (1 H, d, J = 5.9 Hz), 5.09 (1 H, d, J = 2.4 Hz), 7.3 (5 H, s); 13C NMR (CDClJ selected data 6 16.2, 16.4, 33.0, 34.9, 51.5, 52.5, 56.5, 80.7, 92.2, 155.0, 173.6. Anal. Calcd for CI7Hz3NO5: C, 63.54; H, 7.21; N, 4.36. Found: C, 63.55; H, T.24; N, 4.38. The same product was obtained by adding Bu3P (1 equiv, entry 2, Table I1 or 4 equiv, entry 3, Table 11) prior to 3Dc addition. An analytical sample of 10 as a 1:l C1, epimeric mixture has been obtained via hydrogenation of methyl 4,4-dimethoxy-3methylcrotonate (E2 = 6:4) (H2, 1 atm, Pd/C, MeOH) and subsequent BF3-Et20-catalyzed cyclization with excess 1D: 'H NMR (CDCI,) 6 1.03 (3 H, d, J = 6.6 Hz, C1,-Me R isomer), 1.16 (3 H, d, J = 7.0 Hz, C1,-Me S isomer); 13C NMR (CDCl,) selected data 6 91.3 (R isomer) and 92.2 (S isomer).