Crystal structures of sulfonamide protected bicyclic guanidines: ( S )-8-{[( tert -butyl-dimethyl-sil-yl)-oxy]meth-yl}-1-[(2,2,4,6,7-penta-methyl-2,3-di-hydro-benzo-furan-5-yl)sulfon-yl]-1,3,4,6,7,8-hexa-hydro-2 H -pyrimido[1,2- a ]pyrimidin-1-ium tri-fluoro-methane-sulfonate and ( S )-8-(iodo-meth-yl)-1-tosyl-1,3,4,6,7,8-hexa-hydro-2 H -pyrimido[1,2- a ]pyrimidin-1-ium iodide.

Two compounds, ( S )-8-{[( tert -butyl-dimethyl-sil-yl)-oxy]meth-yl}-1-[(2,2,4,6,7-penta-methyl-2,3-di-hydro-benzo-furan-5-yl)sulfon-yl]-1,3,4,6,7,8-hexa-hydro-2 H -pyrimido[1,2- a ]pyrimidin-1-ium tri-fluoro-methane-sulfonate, C 27 H 46 N 3 O 4 SSi + ·CF 3 O 3 S - , ( 1 ) and ( S )-8-(iodo-meth-yl)-1-tosyl-1,3,4,6,7,8-hexa-hydro-2 H -pyrimido[1,2- a ]pyrimidin-1-ium iodide, C 15 H 21 IN 3 O 2 S + ·I - , ( 2 ), have been synthesized and characterized. They are bicyclic guanidinium salts and were synthesized from N -( tert -but-oxy-carbon-yl)-l-me-thio-nine (Boc-l-Met-OH). The guanidine is protected by a 2,2,4,6,7-penta-methyl-dihydro-benzo-furan-5-sulfonyl (Pbf, 1 ) or a tosyl ( 2 ) group. In the crystals of both compounds, the guanidinium group is almost planar and the N-H forms an intra-molecular hydrogen bond in a six-membered ring to the oxygen atom of the sulfonamide protecting group.