R27 is alkyl of 1 6 carbon atoms, arylalkyl of 7 10 carbon atoms, allyl, fluorenylmethyl, or {TAIL ~ phenyl} which is optionally mono , di , or tri substituted with a substituent selected from alkyl of 1 6 carbon atoms, alkoxy of 1 6 carbon atoms, hydroxy, cyano, halo, nitro, carbalkoxy of 2 7 carbon atoms, trifluoromethyl, amino or {HEAD ~ CO2 H} 
R27 is alkyl of 1 6 carbon atoms , arylalkyl of 7 10 carbon atoms , allyl , fluorenylmethyl , or { TAIL ~ phenyl } which is optionally mono , di , or tri substituted with a substituent selected from alkyl of 1 6 carbon atoms , alkoxy of 1 6 carbon atoms , hydroxy , cyano , halo , nitro , carbalkoxy of 2 7 carbon atoms , trifluoromethyl , amino or { HEAD ~ CO2 H }
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