How does Winston withdraw from the society around him, like when talking to Syme? Quotes on his withdrawal.Quotes on alienation, loneliness and...

In Chapter 5, Orwell's imagery regarding Syme (whose name is a root for "Time" and "With") is characterized by existential death and torture, which--as you point out--makes Winston sound alienated, lonely, and withdrawn.  The narration reads like the Munch painting "The Scream":

One of these days, thought Winston with sudden deep conviction, Syme will be vaporized. He is too intelligent. He sees too clearly and speaks too plainly. The Party does not like such people. One day he will disappear. It is written in his face.

Syme's fate obviously foreshadows Winston, as he is a doppelganger (a ghostly twin).  It is ironic that Syme, an expert at Newspeak, should also be "censored" himself:

Syme's fate was not difficult to foresee. And yet it was a fact that if Syme grasped, even for three seconds, the nature of his, Winston's, secret opinions, he would betray him instantly to the Thought police. So would anybody else, for that matter: but Syme more than most. Zeal was not enough. Orthodoxy was unconsciousness.

Winston's thoughts are filled with angst, alienation, and morbidity: he dare not get too close to these men.  They are all on the brink of becoming unpersons, non-existent, vaporized by the secret police.

By Chapter 9, Syme will be missing (dead):

Syme had vanished...Syme had ceased to exist: he had never existed.

Later, in Chapter 14, when Winston finally gets a message from O'Brien, Syme is resurrected in his thoughts.  O'Brien writes:

'But you write it very elegantly,' said O'Brien. 'That is not only my own opinion. I was talking recently to a friend of yours who is certainly an expert. His name has slipped my memory for the moment.'

Winston surmises:

But Syme was not only dead, he was abolished, an unperson. Any identifiable reference to him would have been mortally dangerous.

Winston should have known that O'Brien had indeed talked to Syme during his torutue in the Ministry of Love.  This should have been a sufficient omen to not make contact with the double-agent O'Brien.  Instead, Winston unwisely follows the bait.