Isagogics is foundational to interpretation, it sets the stage, so to speak, or provides the lens through which interpreters view and frame the text and it often determines actual points of interpretation - this is where presuppositions can creep in and distort exegetical conclusions. Let us then ask some important isagogic questions that might shed light on the nature of John 6: What is the context of John's Gospel? Who was it written for? What was its original purpose, as distinct from the purpose it might serve in the church today? That is the way we begin to get at what John was writing about, as distinct from imposing our own concerns and views upon the text. For a long time in Biblical studies it has been popular to suppose that the Gospel of John was written for a putative Johannine community that was in conflict either with the Jewish synagogue or with Gnosticism. Such a proposed setting leads to the expectation that we will find in it, as we do in the synoptics, the institution of the Lord's Supper, which expectation in turn has apparently shaped the view of many scholars that John 6 is somehow John's treament of the sacrament. I'm sure that approach seems, on the face of it, quite plausible to many of us.
I suggest, though, that this is a great example of scholarship overlooking the obvious in the pursuit of the novel (something scholars are tempted to do to get noticed in academe which we pastors are mercifully freed from). John himself (and yes, unlike many scholars, I believe he wrote the Gospel) tells us the purpose of his Gospel most clearly: it was not primarily a hortatory text for a community which already believed and was struggling under theological attack from without, rather it was evangelistic in purpose; it was written "that you may believe that Jesus is the Christ and that by believing you may have life in his name" (20:31). It was an evangelistic tract, if you will, and we can deduce from what John tells us and from the interlocutors Jesus mainly discourses with in the Gospel who it was aimed towards: Jews who needed to believe, for the sake of eternal life, that "Jesus is the Christ", the long-awaited Messiah prophesied in the OT. Thus we find all through this very Jewish flavoured Gospel the linkage of belief in Jesus as the Christ/Messiah with the reception of eternal life (1:7; 3:15-16; 3:36; 5:24; 6:40; 6:47; 10:40-42: 17:20), the reference to Jewish teachers who don't immediately "get it" (Nicodemus) and later secret Jewish believers who are afraid of the authorities and perhaps have later "back-slidden" from that initial, somewhat unsure faith. This is what the discourse in John 6 directly refers to: belief in Jesus as the Christ, expressed in the metaphorical language of eating (and drinking) through faith, which - if they'd had ears to hear - Jesus' and John's listeners would have picked up on, such metaphorical language being familiar from Rabbinic discourses where disciples are urged to "eat the Torah" and so on. This is, I point out again, the interpretation of John 6 in our confessions (SD VII 61) and Luther's sermons.
That is not to say that John 6 has no reference to the Lord's Supper, which our Lord instituted to strengthen faith in him - as our confessions also note - but only that the reference is indirect and perhaps better termed an allusion. Therefore, I urge, we need to be very careful about drawing implications for our practice of the Lord's Supper from John 6.