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 a substituent selected from {HEAD ~ alkyl} of 1 6 carbon atoms, alkoxy of 1 6 carbon atoms, {TAIL ~ hydroxy}, cyano, halo, nitro, carbalkoxy of 2 7 carbon atoms, trifluoromethyl, amino or CO2 H 
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 a substituent selected from { HEAD ~ alkyl } of 1 6 carbon atoms , alkoxy of 1 6 carbon atoms , { TAIL ~ hydroxy } , cyano , halo , nitro , carbalkoxy of 2 7 carbon atoms , trifluoromethyl , amino or CO2 H
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