Overman: Ring- Enlarging THF annulations PTSA or + R,CHO° R 一m Lewis Acid Overman: Ring-Enlarging THF annulations PTsa In reactions with ketones, only substrates with nucleophilic alkene substituents rearrange successfuly. Acetals derived from the cis-12-cyclobutandiols do not undergo ring-enlarging furan annulation under a vanety of conditions F or So.R 32C-063/22999:31AMOverman: Ring-Enlarging THF annulations JACS, 1991, 113, 5365 OH OH PTSA or BF3•OEt2 40 -70% O O + R1CHO SnCl4, -70 °C → rt 80% O O O O O O R1 O O Me Ph Lewis Acid no desired product some epimerization of acetal carbon X R1 R1 Me Ph O O OH R HO R + + cis-only cis-fused trans-fused Overman: Ring-Enlarging THF annulations JACS, 1991, 113, 5365 R3 OH OH O R1 R2 BF3•OEt2 or PTSA 45-74% O O R2 R1 R3 • In reactions with ketones, only substrates with nucleophilic alkene substituents rearrange successfully. • Acetals derived from the cis-1,2-cyclobutandiols do not undergo ring-enlarging furan annulation under a variety of conditions. O R R3 HO + O R R3 HO + O HO R R3 X O O R3 or R X = F or SO2R 32C-06 3/22/99 9:31 AM