Synthesis and characterisation of muramic acid 1′,2-lactam-β-(1→4)-d-glucosamine derivatives related to repeating units of bacterial spore cortex

A procedure for preparing allyl 4,6-O-benzylidene-N-phthaloyl-muramic acid methyl ester is described. Depending on the reaction conditions, a partial cleavage of the phthaloyl group and formation of the tricyclic allyl 4,6-O-benzylidene-muramic acid 1′,2-lactam also occurred; a successful route to the latter was via de-esterification (LiI) of the protected muramic acid ester group, dephthaloylation and cyclisation. This compound was characterised by X-ray structure analysis. Treatment of the protected muramic acid ester with tris(triphenylphosphine)rhodium(I) chloride produced the desired hemiacetal which was then transformed into the trichloroacetimidate donor and coupled with allyl 3,6-di-O-benzyl-2-deoxy-2-phthalimido-β-d-glucopyranoside to provide allyl O-(4,6-O-benzylidene-2-deoxy-3-O-[(R)-1-(methoxycarbonyl)ethyl]-2-phthalimido-β-d-glucopyranosyl)-(1→4)-3,6-di-O-benzyl-2-deoxy-2-phthalimido-β-d-glucopyranoside. De-esterification followed by removal of the two phthalimido groups and Ac2O/Py treatment gave allyl O-(2-amino-4,6-O-benzylidene-3-O-[(R)-1-carboxyethyl]-2-deoxy-β-d-glucopyranosyl-1′,2-lactam)-(1→4)-2-acetamido-3,6-di-O-benzyl-2-deoxy-β-d-glucopyranoside. The conformation analysis of allyl 4,6-O-benzylidene-muramic acid δ-lactam and propan-2-onyl glycoside of the protected muramic ester revealed in solid state for both compounds a chair conformation of glucopyranose and 1,3-dioxane rings with trans ring junction. The δ-lactam ring of the tricyclic muramyl lactam exhibits a conformation between sofa and half-chair.