chloromethanesulfonyl chloride (McCl) and a base generated oxocane 240 and
tetrahydrofurans 241 and 242. Presumably, the reaction proceeds via the formation
of the meso-bicyclo[3.3.0]oxonium ion 239 through displacement of the leaving
group by the tetrahydrofuran oxygen. The stereochemically well-defined nature of
bicyclic compound 239 was apparently transferred to the products, as only one
single diastereoisomer of 240 was obtained. The results with diastereomeric pairs
revealed that the present rearrangement ring expansion takes place stereoselectively
and stereospecifically.
Recently, Snyder et al. have disclosed a novel procedure for bromonium-induced
ring expansion of tetrahydrofurans effected by bromodiethylsulfonium bromopentachloroantimonate (BDSB), affording medium-sized cyclic bromoethers resembling those of the Laurencia C15 acetogenins. This process is fast, regio- and
stereoselective and has been demonstrated to produce diverse stereochemically
eight-membered bromoethers and nine-membered derivatives [165]. This reaction
was initiated by the formation of the bromonium ion 244 (Scheme 52). The more
reactive bromonium diastereoisomer was opened according to a 5-exo attack of the
Scheme 50 Construction of the benzoxocane ring system by a selective central bond cleavage of
a cyclopropane ring
Scheme 51 Formation of oxocines by ring expansion of five-membered ethers
Synthesis of Eight- to Ten-Membered Ring Ethers
357
tetrahydrofurans 241 and 242. Presumably, the reaction proceeds via the formation
of the meso-bicyclo[3.3.0]oxonium ion 239 through displacement of the leaving
group by the tetrahydrofuran oxygen. The stereochemically well-defined nature of
bicyclic compound 239 was apparently transferred to the products, as only one
single diastereoisomer of 240 was obtained. The results with diastereomeric pairs
revealed that the present rearrangement ring expansion takes place stereoselectively
and stereospecifically.
Recently, Snyder et al. have disclosed a novel procedure for bromonium-induced
ring expansion of tetrahydrofurans effected by bromodiethylsulfonium bromopentachloroantimonate (BDSB), affording medium-sized cyclic bromoethers resembling those of the Laurencia C15 acetogenins. This process is fast, regio- and
stereoselective and has been demonstrated to produce diverse stereochemically
eight-membered bromoethers and nine-membered derivatives [165]. This reaction
was initiated by the formation of the bromonium ion 244 (Scheme 52). The more
reactive bromonium diastereoisomer was opened according to a 5-exo attack of the
Scheme 50 Construction of the benzoxocane ring system by a selective central bond cleavage of
a cyclopropane ring
Scheme 51 Formation of oxocines by ring expansion of five-membered ethers
Synthesis of Eight- to Ten-Membered Ring Ethers
357