STUDIES ON THE SYNTHESIS OF THE C-GLYCOSIDIC PART OF NOGALAMYCIN, PART 2

The stereochemistry of the addition of metalloaryls 11w-z to the methy l ketones 10a-e was studied in connection with the construction of the hogalamycin C-glycoside. Excellent selectivities towards the (S)-isom er 13a were observed in the beta-chelate model B in the reaction of th e benzyl ethers 10a with the cerium reagent 11y and the titanium reage nt 11z or the alcohol 10c with the lithium compound 11w. A moderate 3: 1 selectivity in favor of the desired (R)-isomer was observed in the r eaction of the silyl ether 10d with 11w. A reversal of the addition se quence (reaction of 15a with MeMgI) led exclusively to 13a whereas the alcohol 15c gave a 5:3 mixture of 12c:13c.