Ring opening/closing

Werner 50) and his students. He was the first to recognize that racemiza-tion of complexes such as M(AA)3, where AA is a bidentate, can take place by a chelate ring opening-closing process. Such a mechanism and two others are shown in Figure 9. These are called intramolecular processes, because racemization takes place without the complete departure of a ligand from the first coordination sphere. 

Figure 9. Intramolecular racemization of [M (A A) 3] by the following mechanisms (A) chelate ring opening-closing) B) rhombic twist) and (C) trigonal...

Attempts to detect the main structural changes occurring in the ring-opening/ closing process in spiropyrans, using IR vibrational spectroscopy, were reported by Schiele and Arnold for a series of indolinonitrospiropyrans12 and later by... 

The acid phase forms carbocations whose carbon skeletons undergo modifications (ring opening/closing, cracking, isomerisation, etc.), Fig. 11. 

Nocquet, P.-A., Hazelard, D., Compain, P. (2011). Synthesis of spirocyclo-propyl yl ctams by a highly stereoselective tandem intramolecular azetidine ring-opening/closing cascade reaction. European Journal of Organic Chemistry, 2011,6619-6623. 

Scheme 27 A ring-opening-closing alternating copolymerization.

Hirai et al. [63] reported the use of this strategy for the synthesis of 5,5-benzaimulated spiroacetal 66 (Scheme 18). The major (doubly anomeric) isomer 66a arises from ring opening/closing of the hemiacetal 65, followed by oxypaUadation to give 69 (doubly stabilized by the anomeric effect). Subsequent syn elimination then provides 66a as the major isomer. 

Wang L, Liu Q B, Wang D Li X, Han X -W, Xiao W -J, Zhou Y -G. Tandem ring-opening/closing reactions of N-Ts aziridines and aryl propargyl alcohols promoted hy I-BuOK. Org. Lett. 2009 11(5) 1119-1122. 

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