followed by its esterification with methanesulfonyl chloride to give mesylate 240.
Radical debromination with Bu 3 SnH gives a radical that undergo cyclization with
the formation of allylic ether 241 in 80 % yield. The same reaction can be induced
photochemically in the presence of Et 3 N in 86 % yield. The mesylate moiety
of 241 then is reduced into the corresponding alkane with LiAlH 4 . Subsequent
oxidation of the allylic ether gives the α-methylidene-lactone 242. The acetal moiety
of 242 is oxidized with morpholine N-oxide (NMO) in the presence of tetran-propylammonium perruthenate (TPAP) catalyst, liberating rac-ethisolide.
Photoinduced reduction of bromoester 239 provides 243. The latter compound
reacts with one equivalent of n-hexyllithium in the presence of trimethylsilyl
chloride to give ketone 244. Its reduction with LiAlH 4 and then mesylation of the
alcohol mixtures and LiAlH 4 reduction of the mixture of mesylates furnishes 245.
Oxidation of allylic ether of 245 into the corresponding lactone and subsequent
Scheme 41 Cossy et al.’s syntheses of ethisolide and isoavenaciolide
Synthesis of 7-Oxabicyclo[2.2.1]heptane and Derivatives
179
Radical debromination with Bu 3 SnH gives a radical that undergo cyclization with
the formation of allylic ether 241 in 80 % yield. The same reaction can be induced
photochemically in the presence of Et 3 N in 86 % yield. The mesylate moiety
of 241 then is reduced into the corresponding alkane with LiAlH 4 . Subsequent
oxidation of the allylic ether gives the α-methylidene-lactone 242. The acetal moiety
of 242 is oxidized with morpholine N-oxide (NMO) in the presence of tetran-propylammonium perruthenate (TPAP) catalyst, liberating rac-ethisolide.
Photoinduced reduction of bromoester 239 provides 243. The latter compound
reacts with one equivalent of n-hexyllithium in the presence of trimethylsilyl
chloride to give ketone 244. Its reduction with LiAlH 4 and then mesylation of the
alcohol mixtures and LiAlH 4 reduction of the mixture of mesylates furnishes 245.
Oxidation of allylic ether of 245 into the corresponding lactone and subsequent
Scheme 41 Cossy et al.’s syntheses of ethisolide and isoavenaciolide
Synthesis of 7-Oxabicyclo[2.2.1]heptane and Derivatives
179