5,10-methenyl-THF 5,10-methenyltetrahydrofolate 5,10-methylenetetrahydrofolate 7444-29-3 CH-THF anhydroleucovorin methenyl-H4F methenyl-THF methenyl-tetrahydrofolate n5-n10-CH-THF n5-n10-methenyltetrahydrofolate