Efficient conditions for conversion of 2-substituted furans into 4-oxygenated 2-enoic acids and its application to synthesis of (+)-aspicilin, (+)-patulolide A, and (-)-pyrenophorin

2-Substituted furans 1a,b,c were found to be conveniently transformed into trans 4-oxo-2-enals 2a,b,c in 62-87% yields by using NBS/pyridine in THF-acetone-H2O (<-15 degrees C then rt) or NBS/ NaHCO3 in acetone-H2O (<-15 degrees C then rt after addition of pyridine). Further oxidation of the enals 2a-c using NaClO2 led to the trans 4-oxo-2-enoic acids 3a-c in good yields. With this transformation in mind, we designed syntheses of(+)-aspicilin, (+)-patulolide, and (-)-pyrenophorin. In the synthesis of (+)-aspicilin as shown in Schemes 1 and 2, the pivotal intermediate 6 was prepared from olefin 7 in which 2-furyl group is attached. The AD reaction of 7 secured the C(5) and C(6) stereochemistry of aspicilin, and the subsequent transformation using the protocol described above afforded the ester 6. Stereocontrolled reduction of 6 followed by deprotection and the Yamaguchi macrocyclization furnished (+)-aspicilin. For the synthesis of(+)-patulolide (Scheme 3) and (-)-pyrenophorin (Scheme 4), the intermediates are the furans 38 and 44, which were prepared easily by the classical methods using furyllithium 33. The furan ring oxidations proceeded as well, furnishing acids 40 and 46 in good yields, acetalization of which afforded the known intermediates 41 and 47, respectively.