This is related to issue26188,

Using await in a simple statement (outside of an async def method) raises SyntaxError with the unhelpful message "invalid syntax".

It seems obvious once you've read PEP492 in detail, but I think that as more and more developers use async/await this will stump lots of people.

I've been trying to pick apart where this constraint is raised to see whether I can help with a PR, I've been through Grammar, Parser and then the AST and Compiler. 

Looking at https://github.com/python/cpython/blob/master/Python/compile.c#L4307-L4319 I can see there are helpful error messages, but during the tokenizer phase it checks that the syntax cannot be used unless you're within an async def method 
https://github.com/python/cpython/blob/6f0eb93183519024cb360162bdd81b9faec97ba6/Parser/tokenizer.c#L1574-L1583

I can't reproduce this in a REPL for 3.7.0a0, it never gets any further than the grammar or tokenizer phase.