The protected galactose 6-phosphorane 7 has been generated from the phosphonium salt 6d with n-BuLi in 2:1 THF-HMPA at -60 °C. Condensation of 7 with both aliphatic and aromatic aldehydes proceeded in good yield to produce chain-extended compounds. It was also possible to condense 7 with two protected five-carbon carbohydrate aldehydes (10b and 12b) to produce two undecoses (11a and 13a) of Z configuration. Isomerization of 11a and 13a to the more stable E isomers lib and 13b was possible by irradiation in the presence of diphenyl disulfide. In all cases the cv-D-galacto configuration was maintained during the condensation. The tetra-Q-methyl-substituted galactose phosphorane derived from 15e was also prepared and demonstrated to condense with benzaldehyde while maintaining the α-D-galacto configuration. © 1979, American Chemical Society. All rights reserved.