reaction of furan with α-bromocrolein catalyzed by the Corey’s oxazaborolidine
catalyst 218 [270]. Aldehyde 219 so-obtained is not isolated but reacted directly
with 1-ethoxyvinyloxytrimethylsilane in a Mukaiyama condensation also catalyzed
by 218. This gives a 5:1 mixture of diastereoisomeric alcohols 220 (ee ¼ 72 %).
Treatment of the latter with t-BuOK induces the formation of intermediate epoxide
221 that undergoes a subsequent based-induced isomerization into the exo-alcohol
222. The allylic alcohol 222 is then converted into carbamate 223 upon reaction
with trichloroacetyl isocyanate followed by treatment with activated basic alumina.
Aziridination of the acrylic moiety of 223 applies the protocol developed by De
Bois et al. [271]. This produces an aziridine that is opened readily with the
conjugate base of 2-nitrobenzenesulfonamide affording a mixture of diastereoisomers from which 224 is isolated in 75 % yield after column chromatography.
Deprotection of 224 with K 2 CO 3 and thiophenol gives the primary amine 225
which is converted as benzyl amine 226. endo-Iodoamination can be carried out
by reaction of 226 with either N-succinimide or N-iodophthalimide under photochemical conditions. The endo-iodide 227 so-obtained leads to intractable mixtures
on treatment with t-BuLi or lithium naphthalenide. With Zn in AcOH, the undesired
C–N cleavage is favored. However, with SmI 2 , 227 is converted into cyclohexenol
228. The oxidative silylation of the cyclohexene moiety of 228 and subsequent
hydrogenolysis of the benzylamine gives aminodiol 229, the core of banyasides.
5.5 Cleavage of Carbon–Carbon Bonds of 7-Oxabicyclo
[2.2.1]heptan-2-ones
The Baeyer–Villiger oxidation of 7-oxabicyclo[2.2.1]heptan-2-ones is the most
used reaction to cleave a C–C bond of the 7-oxabicyclo[2.2.1]heptanes. Other
routes have used retro-Claisen, retro-Diekmann, and Grob fragmentations. They
have been reviewed elsewhere [11]. Enoxysilanes derived from 7-oxanorbornan2-ones can be ozonolyzed into 2,5-anhydrouronic acid derivatives, precursors for
C-nucleosides [225].
Baeyer–Villiger lactonization of 7-oxanorbornan-2-ones inserts the oxygen
atom between the carbonyl and the nearby bridgehead center C1 (migration of
the σ(C1–C2) bond). This is due to the electron-releasing ability of the 7-oxa
ethereal group (2p(O) HOMO) that makes the C1–C2 bond electron rich and favors
its migration. However, when the 7-oxanobornan-2-one is substituted at C3 by a
OSiR 3 , OBn, or OMe group, competitive insertion of the oxygen atom between C2
and C3 is observed (migration of the σ(C3–C2) bond) [271]. The highly
regioselective Baeyer–Villiger oxidation of 3-exo-methyl-7-oxabicyclo[2.2.1]
heptan-2-one, (+)-230 (derived from the “naked sugar” (+)-172 by catalytic hydrogenation and ketone α-monomethylation) gives lactone (À)-231. Its reaction with
acetone trimethylsilyl enol ether is induced by TiCl 4 in CH 2 Cl 2 and provides a
mixture of (+)-232 (major) and (+)-233 (minor). The minor ketone (+)-233 can be
Synthesis of 7-Oxabicyclo[2.2.1]heptane and Derivatives
177
catalyst 218 [270]. Aldehyde 219 so-obtained is not isolated but reacted directly
with 1-ethoxyvinyloxytrimethylsilane in a Mukaiyama condensation also catalyzed
by 218. This gives a 5:1 mixture of diastereoisomeric alcohols 220 (ee ¼ 72 %).
Treatment of the latter with t-BuOK induces the formation of intermediate epoxide
221 that undergoes a subsequent based-induced isomerization into the exo-alcohol
222. The allylic alcohol 222 is then converted into carbamate 223 upon reaction
with trichloroacetyl isocyanate followed by treatment with activated basic alumina.
Aziridination of the acrylic moiety of 223 applies the protocol developed by De
Bois et al. [271]. This produces an aziridine that is opened readily with the
conjugate base of 2-nitrobenzenesulfonamide affording a mixture of diastereoisomers from which 224 is isolated in 75 % yield after column chromatography.
Deprotection of 224 with K 2 CO 3 and thiophenol gives the primary amine 225
which is converted as benzyl amine 226. endo-Iodoamination can be carried out
by reaction of 226 with either N-succinimide or N-iodophthalimide under photochemical conditions. The endo-iodide 227 so-obtained leads to intractable mixtures
on treatment with t-BuLi or lithium naphthalenide. With Zn in AcOH, the undesired
C–N cleavage is favored. However, with SmI 2 , 227 is converted into cyclohexenol
228. The oxidative silylation of the cyclohexene moiety of 228 and subsequent
hydrogenolysis of the benzylamine gives aminodiol 229, the core of banyasides.
5.5 Cleavage of Carbon–Carbon Bonds of 7-Oxabicyclo
[2.2.1]heptan-2-ones
The Baeyer–Villiger oxidation of 7-oxabicyclo[2.2.1]heptan-2-ones is the most
used reaction to cleave a C–C bond of the 7-oxabicyclo[2.2.1]heptanes. Other
routes have used retro-Claisen, retro-Diekmann, and Grob fragmentations. They
have been reviewed elsewhere [11]. Enoxysilanes derived from 7-oxanorbornan2-ones can be ozonolyzed into 2,5-anhydrouronic acid derivatives, precursors for
C-nucleosides [225].
Baeyer–Villiger lactonization of 7-oxanorbornan-2-ones inserts the oxygen
atom between the carbonyl and the nearby bridgehead center C1 (migration of
the σ(C1–C2) bond). This is due to the electron-releasing ability of the 7-oxa
ethereal group (2p(O) HOMO) that makes the C1–C2 bond electron rich and favors
its migration. However, when the 7-oxanobornan-2-one is substituted at C3 by a
OSiR 3 , OBn, or OMe group, competitive insertion of the oxygen atom between C2
and C3 is observed (migration of the σ(C3–C2) bond) [271]. The highly
regioselective Baeyer–Villiger oxidation of 3-exo-methyl-7-oxabicyclo[2.2.1]
heptan-2-one, (+)-230 (derived from the “naked sugar” (+)-172 by catalytic hydrogenation and ketone α-monomethylation) gives lactone (À)-231. Its reaction with
acetone trimethylsilyl enol ether is induced by TiCl 4 in CH 2 Cl 2 and provides a
mixture of (+)-232 (major) and (+)-233 (minor). The minor ketone (+)-233 can be
Synthesis of 7-Oxabicyclo[2.2.1]heptane and Derivatives
177