Cyclopropylcarbinyl-oxo-carbonium Ions. Part VI. Synthesis and Chemistry of an Epimeric Pair of Homoallyl Iodides (3-Iodomethyl Glycals)

Abstract

The homoallyl iodide 4,6-O-benzylidene-1,2,3-trideoxy-D-ribo-hex-1-enopyranose, 4, is the second major product from Simmons–Smith reaction of methyl 4,6-O-benzylidene-2,3-dideoxy-α-D-erythro-hex-2-enopy-ranoside, 6α. The major product is the cyclopropyl glycoside 7α, which when treated separately with the Simmons–Smith reagents furnishes an 85% yield of 4. The mechanism of this transformation is investigated. It is not simply a Lewis-catalyzed iodinolysis since zinc iodide in a variety of solvents does not convert 7α to 4. Methylene iodide is essential, as is the organo-zincate complex, operative in Simmons–Smith reactions.The 3-epimer of the homoallyl iodide 4, i.e. 4,6-O-benzylidene-1,2,3-trideoxy-D-arbino-hex-1-enopyranose, 5, is produced in only 0.9% yield in the methylenation of the anomeric olefinic glycoside, 6β. The low yield is attributable to the extensive anomerization experienced by 6β itself, and by the primary reaction products. This leads to a plethora of compounds including both epimeric homoallyl iodides 4 and 5.In order to get it all together, the homoallyl iodide 4 is transformed into its epimer 5 by an unambiguous synthetic sequence. Some of the intermediates in this sequence are polyfunctional molecules of considerable synthetic potential.The possibility of an alternative route to compound 4 directly from the olefinic glycoside 6α is discussed. Coordination of the methoxyl oxygen of 6α to the organo-zincate complex could achieve an ideal SNi transition state in which [Formula: see text] is delivered at C-3 with synchronous departure of the methoxyl group. This mechanism, called the "reverse" allylic rearrangement for reasons outlined in the text, is discussed.