22019
Reaction of 17 alpha - bromo - 21 - iodo - 3 beta - acetoxypregn - 5 - en - 20 - one with either ( L ) - serine methyl ester , or ( D ) - serine methyl ester resulted in methyl N - [ 3 beta - acetoxy - 21 - oxopregna - 5 , 17 ( 20 ) - dien - 21 - yl ] - ( L ) - serinate and methyl N - [ 3 beta - acetoxy - 21 - oxopregna - 5 , 17 ( 20 ) - dien - 21 - yl ] - ( D ) - serinate ( as mixtures of related [ 17 ( 20 ) E ] - and [ 17 ( 20 ) Z ] - isomers ) .
 17 alpha - bromo - 21 - iodo - 3 beta - acetoxypregn - 5 - en - 20 - one, ( L ) - serine methyl ester, ( D ) - serine methyl ester, methyl N - [ 3 beta - acetoxy - 21 - oxopregna - 5 , 17 ( 20 ) - dien - 21 - yl ] - ( L ) - serinate, methyl N - [ 3 beta - acetoxy - 21 - oxopregna - 5 , 17 ( 20 ) - dien - 21 - yl ] - ( D ) - serinate
Please list all entity words in the text that fit the category. Output format is "word1, word2". 
Option: Chemical 
Text: {0} 
Answer:
 17 alpha - bromo - 21 - iodo - 3 beta - acetoxypregn - 5 - en - 20 - one, ( L ) - serine methyl ester, ( D ) - serine methyl ester, methyl N - [ 3 beta - acetoxy - 21 - oxopregna - 5 , 17 ( 20 ) - dien - 21 - yl ] - ( L ) - serinate, methyl N - [ 3 beta - acetoxy - 21 - oxopregna - 5 , 17 ( 20 ) - dien - 21 - yl ] - ( D ) - serinate