Caffeoyltransferase involved in montbretin A (MbA) biosynthesis (PubMed:29967287). Catalyzes the caffeoylation of myricetin 3-O-beta-D-glucosyl 1,2-alpha-L-rhamnoside (MRG) to produce myricetin 3-O-(6'-O-caffeoyl)-beta-D-glucosyl 1,2-alpha-L-rhamnoside (mini-MbA), a precursor of MbA (PubMed:29967287). Mini-MbA and MbA are potent inhibitors of human pancreatic alpha-amylase and are being developed as drug candidates to treat type-2 diabetes (PubMed:29967287). In vitro, is able to catalyze the caffeoylation of quercetin 3-O-sophoroside (QGG), although QGG may not be a physiological substrate in vivo (PubMed:29967287). In vitro, can use coumaryl-CoA, feruloyl-CoA and acetyl-CoA, although these three acyl donors may not be physiological in vivo (PubMed:29967287). (E)-caffeoyl-CoA + myricetin 3-O-[beta-D-glucosyl-(1->2)-alpha-L-rhamnoside] = CoA + myricetin 3-O-[(6-O-(E)-caffeoyl-beta-D-glucosyl)-(1->2)-alpha-L-rhamnoside] Flavonoid metabolism. Expressed in young cromes. Belongs to the plant acyltransferase family.