In such compounds R4 can be hydrogen, {TAIL ~ a lower alkyl group}, such as {HEAD ~ a methyl or ethyl group}; or generally any other group such as amino which does not generate steric hindrance or interference when coupled to the nitrogen atom along with the R3 group 
In such compounds R4 can be hydrogen , { TAIL ~ a lower alkyl group } , such as { HEAD ~ a methyl or ethyl group } ; or generally any other group such as amino which does not generate steric hindrance or interference when coupled to the nitrogen atom along with the R3 group
OTH OTH OTH OTH OTH OTH OTH OTH TAIL TAIL TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD HEAD HEAD HEAD OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH