Retraction notice to 'Design and synthesis of CHAP31, trapoxin B and HC-toxin based bicyclic tetrapeptides disulfide as potent histone deacetylase inhibitors' [Bioorg Med Chem 22 (2014) 3850–3855]. Issue 15 (1st August 2015) - Hoque, Md. Ashraful; Islam, Md. Nurul; Islam, Md. Shahidul; Kato, Tamaki; Nishino, Norikazu; Ito, Akihiro; Yoshida, Minoru