ability of the homoconjugated carbonyl group [260–263] which can be expressed
with limiting structures 199
0 and 199” (n(C¼O)/σ(C–C)/2p(C(6) hyperconjugative
interaction). The lithium enolate of ketone (+)-200 (does not undergo oxa ring
opening at À60
C) reacts with Eschenmoser’s salt affording exocyclic enone
(À)-201. Radical C-glycosylation of (À)-201 with acetobromogalactose and
Bu 3 SnH/AIBN provides the endo α-C-galactopyranoside (+)-203. The reaction
engenders radical intermediate 202 that is quenched exclusively onto its exo face
by the tin hydride. Oxidation of the selenide moiety with mCPBA and subsequent
seleno-Pummerer rearrangement gives (+)-204. The latter compound is reduced
with Bu 3 SnH/AIBN into all-endo trisubstituted 7-oxanorbornanone (+)-205. The
Cossy et al. photoreduction of this ketone is stopped after 60 % of conversion,
furnishing the 3-hydroxycyclohexanone (+)-206. After ketone reduction and acetylation, the α-C-pyranoside (+)-207 is obtained [264].
Interestingly, irradiation of 5-exo-bromo-5-endo-benzyloxy-7-oxabicyclo[2.2.1]
heptan-2-one in the presence of Et 3 N neither leads to the corresponding
cyclohexenol derivative nor to the reduction of the ketone into a mixture of exoand endo-7-oxanorbornan-2-ols. Only the product of hydrogenolysis of the bromide
(6-endo-benzyloxa-7-oxabicyclo[2.2.1]heptan-2-one) is formed and isolated in
60 % yield. The radical intermediate, formed by single-electron transfer (SET)
from Et 3 N to the bromide and subsequent C–Br bond cleavage, can be quenched
intramolecularly as exemplified with the next reaction [265] (Scheme 36).
The two-electron reduction of 7-oxanorbornane derivative of 208 into
cyclohexenol 209 has been realized with SmI 2 in THF using either MeOH or
ethylene glycol as the proton source [266] (Scheme 37).
5.4 Chalconide and Halide/Metal Exchange
Peduncularine is the principal alkaloid of Tasmanian shrub Aristotelia peduncularis
that presents anticancer activity [267]. In 2006, Kitamura et al. [268] have
presented a synthesis of the racemic alkaloid starting with the Diels–Alder
Scheme 36 Cossy et al. photoreductive cyclization
Scheme 37 Yoshida et al. reductive oxa ring opening
174
A.J. Moreno-Vargas and P. Vogel
with limiting structures 199
0 and 199” (n(C¼O)/σ(C–C)/2p(C(6) hyperconjugative
interaction). The lithium enolate of ketone (+)-200 (does not undergo oxa ring
opening at À60
C) reacts with Eschenmoser’s salt affording exocyclic enone
(À)-201. Radical C-glycosylation of (À)-201 with acetobromogalactose and
Bu 3 SnH/AIBN provides the endo α-C-galactopyranoside (+)-203. The reaction
engenders radical intermediate 202 that is quenched exclusively onto its exo face
by the tin hydride. Oxidation of the selenide moiety with mCPBA and subsequent
seleno-Pummerer rearrangement gives (+)-204. The latter compound is reduced
with Bu 3 SnH/AIBN into all-endo trisubstituted 7-oxanorbornanone (+)-205. The
Cossy et al. photoreduction of this ketone is stopped after 60 % of conversion,
furnishing the 3-hydroxycyclohexanone (+)-206. After ketone reduction and acetylation, the α-C-pyranoside (+)-207 is obtained [264].
Interestingly, irradiation of 5-exo-bromo-5-endo-benzyloxy-7-oxabicyclo[2.2.1]
heptan-2-one in the presence of Et 3 N neither leads to the corresponding
cyclohexenol derivative nor to the reduction of the ketone into a mixture of exoand endo-7-oxanorbornan-2-ols. Only the product of hydrogenolysis of the bromide
(6-endo-benzyloxa-7-oxabicyclo[2.2.1]heptan-2-one) is formed and isolated in
60 % yield. The radical intermediate, formed by single-electron transfer (SET)
from Et 3 N to the bromide and subsequent C–Br bond cleavage, can be quenched
intramolecularly as exemplified with the next reaction [265] (Scheme 36).
The two-electron reduction of 7-oxanorbornane derivative of 208 into
cyclohexenol 209 has been realized with SmI 2 in THF using either MeOH or
ethylene glycol as the proton source [266] (Scheme 37).
5.4 Chalconide and Halide/Metal Exchange
Peduncularine is the principal alkaloid of Tasmanian shrub Aristotelia peduncularis
that presents anticancer activity [267]. In 2006, Kitamura et al. [268] have
presented a synthesis of the racemic alkaloid starting with the Diels–Alder
Scheme 36 Cossy et al. photoreductive cyclization
Scheme 37 Yoshida et al. reductive oxa ring opening
174
A.J. Moreno-Vargas and P. Vogel