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 {HEAD ~ formation} of that sulfide group in A, competing side reactions caused by OH are avoided; for example, {TAIL ~ the pKa} of the thiophenol is below 10.0 
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 { HEAD ~ formation } of that sulfide group in A , competing side reactions caused by OH are avoided ; for example , { TAIL ~ the pKa } of 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 HEAD HEAD HEAD HEAD HEAD OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH OTH