Enolic Ortho Esters. VII Involvement of Magnesium Halides as Lewis Acids in the Reaction of Grignard Reagents with 1,6-Dideoxy- 1,1-ethylenedioxy-2,3,4-tri-O-methyl-D-xylo-hex-5- enopyranose and its 6-Phenyl Derivative: a Correction

Abstract

The known aldehyde methyl 2,3,4-tri-O-methyl-α-D-gluco-hexodialdo-1,5-pyranoside (9) was converted in eight steps into the 6-phenyl glucose-derived enolic ortho ester (Z)-1,6-dideoxy-1,1-ethylenedioxy- 2,3,4-tri-O-methyl-6-phenyl-D-xylo-hex-5-enopyranose (22), the geometry of which was established by a single-crystal X-ray study. Treatment of the 6-phenyl enolic ortho ester (22) with titanium tetrachloride at –78° effected clean rearrangement into (2R/S,4R,5R,6S)-3,3-ethylenedioxy-4,5,6-trimethoxy-2-phenylcyclohexanone (26). Reaction of (22) with methylmagnesium iodide gave (1R,2S,4R,5S,6S)-3,3-ethylenedioxy-4,5,6-trimethoxy-1-methyl-2-phenylcyclohexanol (24), the structure and stereochemistry of which were established by an X-ray study. Reaction of (22) with phenylmagnesium bromide gave (25), the 1-phenyl analogue of (24). The firmly established structure of (24) led to proof both chemically and by X-ray means that the product from reaction of 1,6-dideoxy-1,1-ethylenedioxy-2,3,4-tri-O-methyl-D-xylo-hex-5-enopyranose (5) with methylmagnesium iodide has the hydroxy acetal structure (7) rather than the originally assigned hemiacetal structure (3).