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 hexacarbo nyl chromium followed by methylation affords D-arabino-hex-1-enopyranosylca rbene complexes 7 and 8. They undergo a diastereoselective benzannulation u pon reaction with tolan and 3-hexyne to give polyoxygenated chromans 9 to 1 2 in good yields and moderate diastereomeric excess values. The conformatio n of the glucal moiety in the carbene ligand and in the chroman skeleton is controlled by the nature of the protective groups. H-1-NMR studies and sin gle crystal X-ray analyses indicate a H-5(4)-conformation of the sugar moie ty for the triisopropylsilyl compounds 7, 9a,b, 10a,b and 11 and H-4(5)-con formation for the isopropylidene derivatives 8 and 12a-c in solution, and f urther for 8 and 12a in the solid state. (C) 2001 Elsevier Science B.V. All rights reserved.