Studies on Lignin Model Compounds of the beta-O-4 Type: Crystal Structures of threo-1-(4-Hydroxy-3,5-dimethoxyphenyl)-2-(2-methoxyphenoxy)-1,3-propanediol and 3-Hydroxy-1-(4-hydroxy-3,5-dimethoxyphenyl)-2-(2-methoxyphenoxy)-1-propanone--Methanol (1/1).