Methane sulfonic anhydride (0.09 g) was added to a stirred, cooled mixture of methyl (1aRS,7bSR)-5-{N-[methoxycarbonyl]-N-[2-((Z)-3-hydroxyprop-1-enyl)-4-fluorobenzenesulfonyl]amino}-1,1a,2,7b-tetrahydrocyclopropa[c]chromene-4-carboxylate (Intermediate 64, 0.17 g) and N,N-di-isopropyl-N-ethylamine (0.134 g) in DCM (10 mL) at 0° C. The temperature was allowed to rise to room temperature and the mixture was stirred for 2 hours. A solution of ethylamine (2M in toluene, 2 mL) was added and the resultant mixture was stirred at room temperature for 3 hours then diluted with water. The organic layer was separated, dried (Na2SO4) and filtered. The filtrate was evaporated to dryness to give methyl (1aRS,7bSR)-5-[2((Z)-3-ethylaminoprop-1-enyl)-4-fluorobenzenesulfonyl-amino]-1,1a,2,7b-tetrahydro-cyclopropa[c]chromene-4-carboxylate (0.09 g) as a white solid.
