R3 R6 and B are selected so that the sulfur attached to 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, {HEAD ~ the pKa} of {TAIL ~ the thiophenol} is below 10.0 
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R3 R6 and B are selected so that the sulfur attached to 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 , { HEAD ~ the pKa } of { TAIL ~ the thiophenol } is below 10.0
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 OTH OTH OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD REL TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH