Synthesis of 2-(4-trifluoroacetamidophenyl)ethyl O-(spL-glycero-α-spD-manno-heptopyranosyl)-(1→7)-O-(spL-glycero-α-spD- manno-heptopyranosyl)-(1»3)-spL-glycero-α-spD-manno-heptopyranoside, coresponding to the heptose region of the Salmonella Ra core structure