An Unexpected Formation of a 14‐Membered Cyclodepsipeptide

The treatment of diluted solutions of the hydroxy diamides 6a and 6b in toluene with HCl gas at 100° gave the dimeric, 14‐membered cyclodepsipeptide 10 in up to 72% yield ( Scheme 3 ). The same product was formed from the linear dimer of 6b , the depsipeptide 11 , under the same conditions ( cf. Scheme 4 ). All attempts to prepare the cyclic seven‐membered monomer 9 , starting with different precursors and using different lactonization methods failed, and 10 was the only product which was isolated ( cf. Scheme 6 ). For example, the reaction of the ester 20 with NaH in toluene at 80° led exclusively to the cyclodimer 10 . On the other hand, the base‐catalyzed cyclization of the hydroxy diester 22 , which is the ‘O‐analogue' of 20 , yielded neither the seven‐membered dilactone, nor the 14‐membered tetralactone, but only the known trimer 23 and tetramer 24 of 2,2‐dimethylpropano‐3‐lactone ( cf. Scheme 7 ).