stored proc formatted SQL double quote issue Sybase

i have a stored proc like this

--liquibase formatted sql

--changeset author:id

create proc abc

as begin

select @var1= "insert into a1"       select @var1= @var1+ " select  c1,c2"       select @var1= @var1+ " from b2 "        exec (@var1)



liquibase throws an error column not found .

If i put single quote instead of double quote it is running fine. Please let me know, how to work with double quotes in select @variable= " something"  in a stored proc.