Comment by SergeyHack

Not quite about creating synonyms, but in the same area there is Semantic Vectors https://github.com/semanticvectors/semanticvectors.

They process Lucene index and create embedded representation of it. Then you can search over that representation for "semantic" matches.

Last time I checked it about a year ago the embedded collection of documents was kept in the memory and the search was implemented by a linear scan. So I suspect it can be slow on very large collection of documents.