Degradations and Rearrangement Reactions
2.7
399
⊡ Scheme 39
α-Hydroxyfurans expand to pyranoids via sequential epoxyalcohol rearrangements. Epoxidation of the α-hydroxyfuran 165 with meta-chloroperoxybenzoic acid (m-CPBA) induces
a cationic rearrangement, followed by dehydration, to form a pyranoid on the way to
(+)-resineferatonin [134] ( > Scheme 40). Dimethyldioxirane (DMDO), apparently more
sensitive to the steric circumstances than m-CPBA, has been used for the selective epoxidation of the furan 167 [135]. The monoepoxide rearranges followed by hemiacetalization to
afford the pyranoid intermediate 165 of the total synthesis of the eleuthesides. α-Aminofurans
similarly expand into azapyranoids [136]. Racemic 169 is kinetically resolved to give (S)-169
and the rearranged (2R, 6R)-170 with modified Sharpless epoxidation. Compound (S)-169 is
transformed into (2S, 6S)-170 on treatment with m-CPBA.
⊡ Scheme 40
The reaction of the bicyclic thiosugar 171 with N6-benzoyladenine in the presence of moist
tin(IV) chloride furnishes the ring-rearranged nucleoside product 172 instead of the normal
glycosylation product [137]. On heating mesylate 173 under reflux in the presence of a nucleophile, the thermal ring-expanding reaction occurs [138] ( > Scheme 41).
2.7
399
⊡ Scheme 39
α-Hydroxyfurans expand to pyranoids via sequential epoxyalcohol rearrangements. Epoxidation of the α-hydroxyfuran 165 with meta-chloroperoxybenzoic acid (m-CPBA) induces
a cationic rearrangement, followed by dehydration, to form a pyranoid on the way to
(+)-resineferatonin [134] ( > Scheme 40). Dimethyldioxirane (DMDO), apparently more
sensitive to the steric circumstances than m-CPBA, has been used for the selective epoxidation of the furan 167 [135]. The monoepoxide rearranges followed by hemiacetalization to
afford the pyranoid intermediate 165 of the total synthesis of the eleuthesides. α-Aminofurans
similarly expand into azapyranoids [136]. Racemic 169 is kinetically resolved to give (S)-169
and the rearranged (2R, 6R)-170 with modified Sharpless epoxidation. Compound (S)-169 is
transformed into (2S, 6S)-170 on treatment with m-CPBA.
⊡ Scheme 40
The reaction of the bicyclic thiosugar 171 with N6-benzoyladenine in the presence of moist
tin(IV) chloride furnishes the ring-rearranged nucleoside product 172 instead of the normal
glycosylation product [137]. On heating mesylate 173 under reflux in the presence of a nucleophile, the thermal ring-expanding reaction occurs [138] ( > Scheme 41).