R27 is alkyl of 1 6 carbon atoms, arylalkyl of 7 10 carbon atoms, allyl, fluorenylmethyl, or phenyl which is optionally mono , di , or tri substituted with {HEAD ~ 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, {TAIL ~ trifluoromethyl}, amino or CO2 H 
selected from
R27 is alkyl of 1 6 carbon atoms , arylalkyl of 7 10 carbon atoms , allyl , fluorenylmethyl , or phenyl which is optionally mono , di , or tri substituted with { HEAD ~ 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 , { TAIL ~ trifluoromethyl } , amino or CO2 H
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