tetrahydrofuran oxygen which generates the intermediate bicyclic oxonium ion
245. The bicyclic compound 245 may then undergo a regioselective ring opening
by the nucleophilic attack of the neighboring carbonate moiety producing the
observed oxocane 246. The authors applied this strategy to the racemic formal
total syntheses of laurefucin and (Æ)-E- and (Æ)-Z-pinnatifidenyne [166].
A closely related approach has been described by Li et al. who have disclosed an
enantioselective synthesis of the Δ
4 -oxocene core present in (+)-laurencin,
(þ)-prelaureatin, and other members of the laurencin family. The synthesis was
accomplished in eight steps by novel one-pot regio- and stereoselective ring
cyclization/fragmentation/expansion cascade from the tetrahydrofuran precursors,
which were prepared by the cyclization of vinylsilanes [167]. This process is
highlighted by an intramolecular oxo-carbenoid insertion and a β-silyl fragmentation sequence. Thus, treating aldehyde 250 with a slight excess of ethyl
diazoacetate (Scheme 53) in the presence of a catalytic amount of anhydrous
SnCl 2 in CH 2 Cl 2 at room temperature cleanly afforded the β-keto ester 251
(62–70 %), accompanied with Δ
4 -oxocene 252 (20–25 %) as a single diastereomer.
The authors proposed that 252 was generated via a tricyclic oxo-ylide 255, stereoselectively formed through an oxo-carbenoid resulting from the insertion of the
oxygen atom of the tetrahydrofuran ring into the generated carbene. In this highly
organized hydrindane-type intermediate 255, the proximity of three reacting partners accounts for the observed stereoselectivity in 252. A ring expansion from the
[3.3.0]bicyclooctane-like core can occur, followed by a simultaneous β-Et 3 Si group
syn-elimination, to give 252 in a highly regio- and stereoselective manner. The
formation of 251 takes place presumably from the common intermediate 254,
which undergoes a 1,4-hydride shift.
Scheme 52 Bromonium-induced ring expansion to afford eight- and nine-membered oxacycles,
by Snyder et al.
358
E. Soriano and J. Marco-Contelles
245. The bicyclic compound 245 may then undergo a regioselective ring opening
by the nucleophilic attack of the neighboring carbonate moiety producing the
observed oxocane 246. The authors applied this strategy to the racemic formal
total syntheses of laurefucin and (Æ)-E- and (Æ)-Z-pinnatifidenyne [166].
A closely related approach has been described by Li et al. who have disclosed an
enantioselective synthesis of the Δ
4 -oxocene core present in (+)-laurencin,
(þ)-prelaureatin, and other members of the laurencin family. The synthesis was
accomplished in eight steps by novel one-pot regio- and stereoselective ring
cyclization/fragmentation/expansion cascade from the tetrahydrofuran precursors,
which were prepared by the cyclization of vinylsilanes [167]. This process is
highlighted by an intramolecular oxo-carbenoid insertion and a β-silyl fragmentation sequence. Thus, treating aldehyde 250 with a slight excess of ethyl
diazoacetate (Scheme 53) in the presence of a catalytic amount of anhydrous
SnCl 2 in CH 2 Cl 2 at room temperature cleanly afforded the β-keto ester 251
(62–70 %), accompanied with Δ
4 -oxocene 252 (20–25 %) as a single diastereomer.
The authors proposed that 252 was generated via a tricyclic oxo-ylide 255, stereoselectively formed through an oxo-carbenoid resulting from the insertion of the
oxygen atom of the tetrahydrofuran ring into the generated carbene. In this highly
organized hydrindane-type intermediate 255, the proximity of three reacting partners accounts for the observed stereoselectivity in 252. A ring expansion from the
[3.3.0]bicyclooctane-like core can occur, followed by a simultaneous β-Et 3 Si group
syn-elimination, to give 252 in a highly regio- and stereoselective manner. The
formation of 251 takes place presumably from the common intermediate 254,
which undergoes a 1,4-hydride shift.
Scheme 52 Bromonium-induced ring expansion to afford eight- and nine-membered oxacycles,
by Snyder et al.
358
E. Soriano and J. Marco-Contelles