R3 R6 and B are selected so that {HEAD ~ the sulfur} attached to {TAIL ~ the ring} of unit A has a pKa low enough in the corresponding thiophenol compound such that, during formation of that sulfide group in A, competing side reactions caused by OH are avoided; for example, the pKa of the thiophenol is below 10.0 
attached to
R3 R6 and B are selected so that { HEAD ~ the sulfur } attached to { TAIL ~ the ring } of unit A has a pKa low enough in the corresponding thiophenol compound such that , during formation of that sulfide group in A , competing side reactions caused by OH are avoided ; for example , the pKa of the thiophenol is below 10.0
OTH OTH OTH OTH OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD REL REL TAIL TAIL TAIL TAIL TAIL TAIL 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 OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH