Cyclopropylcarbinyl-oxo-carbonium Ions. Part V. Synthesis and Chemistry of some Cyclopropyl Glycopyranosides

The synthesis of five cyclopropanated glycopyranosides, 6α, 6β, 7α, 7β, and 11 capable of providing access to the title ions has been examined. Simmons–Smith methylenation of methyl 4,6-O-benzylidene-2,3-dideoxy-α-D-eryrtro-hex-2-enopyranoside (8α) under carefully controlled conditions gives an excellent yield of the α-D-allo adduct (6α). A homoallyl iodide, 4,6-O-benzylidene-1,2,3-trideoxy-3-C-iodomethyl-D-ribo-hex-1-enopyranose, (9), is also produced and upon methanolysis undergoes ring closure to an equal mixture of α-D and β-D anomers of 6. Methylene insertion into the aglycon of the starting olefin 8α occurs giving small amounts of the ethyl hex-2-enopyranoside (10), the cyclopropanated glycoside derived therefrom, 11, being also obtained. Methylenation of the β-D-hex-2-enopyranoside, 8β, gives a complex mixture containing at least eight substances, although the expected β-D-manno adduct, 7β, is the major product. There is considerable anomerization of 8β and of the primary product, 7β; thus the anomer 7α is also produced. The 3-epimer (14) of the above-mentioned homoallyl iodide (9) is obtained in small yield.The cyclopronated glycosides are very susceptible to prolonged exposure to the Simmons–Smith medium. Particularly in the case of 8β the yields of the various products can be optimized by judicious choice of the duration of the reaction.Proton magnetic resonance data for the adducts are presented. The value of the spacing J34provides a clear basis for determining the configuration of the ring. However anisotropy due to the cyclopropane ring-current causes the equatorial H-1 in the β-D-manno-anomer (7β) to resonate at higher field than H-1 in the α-D anomer 7α.