Reaction of 4,6-0-benzylidene-3-0-mesyl-allal with allylic grignard reagents. Highly regio- and stereoselective preparation of 2,6-disubstituted dihydropyran rings

✍ Scribed by Takuo Ogihara; Oyo Mitsunobu

• Publisher: Elsevier Science
• Year: 1983
• Tongue: French
• Weight: 269 KB
• Volume: 24
• Category: Article
• ISSN: 0040-4039
• DOI: 10.1016/s0040-4039(00)86024-4

✦ Synopsis

The reaction of 4,6-0-benzylidene-l,2-dideoxy-3-O-mesyl-D-ribo-hex-lenopyranose reacted with 2-propenylmagnesium chloride or 2-methyl-2-propenylmagnesium chloride to give 1,5-anhydro-4,6-0-benzylidene-2,3-dideoxy-l-C-prop-2enyl-(or 2-methylprop-2-enyl)-p-D-erythro-hex-2-enitol in good yield. Glycals have been utilized as versatile starting materials for the synthesis of carbohydrates and, in recent years, chiral natural products. In order to introduce a carbon chain on the C-l position, glycals are generally treated with carbon nucleophiles in the presence of Lewis acids or transition metals. 2, It would be expected, however, that a glycal having a suitable leaving group at the C-3 position reacts with carbanions in the absence of acidic catalysts to give 1,5-anhydro-2,3-dideoxy-l-c-alkyl-(or alkenyl)-hex-2-enitols, In This paper, we wish to report a highly stereo-and regioselective reaction of allylic Grignard reagents with 3-0-mesylate of 4,6-0-benzylidene-l,2-dideoxy-D-ribo-hex-l-enopyranose (A). 3) P.lthough 3-0-acyl derivatives of 2 can be readily prepared, 4) attempts to isolate the mesylate or tosylate of L (2-a or 22) were unsuccessful_ Thus, when $ in tetrahydrofuran (THF) was reacted first with small excess of n-butyllithium and then with l-5-2 molar equivalents of mesyl chloride or tosyl chloride at -70"~-80 'C for 30-80 min, followed by treatment with saturated aqueous sodium bicarbonate, a mixture of 2-enopyranose (2) and a,p-unsaturated aldehyde (k) was obtained in 70~80% yield rather than expected 2," or 2b M. When sodium methoxide in methanol was added to the reaction mixture obtained by the reaction of 5 with n-butyllithium and mesyl chloride (-70'tiroom temperature, 1.5 h), compound 5was