Organotransition-metal-modified sugars: Part 17. Glucal-derived carbene complexes: synthesis and diastereoselective benzannulation

The addition of triisopropylsilyl and/or isopropylidene-protected 1-lithio-d-glucals prepared in situ from stannylated precursors 5 and 6 to hexacarbonyl chromium followed by methylation affords d-arabino-hex-1-enopyranosylcarbene complexes 7 and 8. They undergo a diastereoselective benzannulation upon reaction with tolan and 3-hexyne to give polyoxygenated chromans 9 to 12 in good yields and moderate diastereomeric excess values. The conformation of the glucal moiety in the carbene ligand and in the chroman skeleton is controlled by the nature of the protective groups. 1H-NMR studies and single crystal X-ray analyses indicate a 5H4-conformation of the sugar moiety for the triisopropylsilyl compounds 7, 9a,b, 10a,b and 11 and 4H5-conformation for the isopropylidene derivatives 8 and 12a–c in solution, and further for 8 and 12a in the solid state.