Chemical cyclization

Divergence of Enzymatic and Biomimetic Chemical Cyclization Reactions.297... 

Polymeranalogous reactions considered above may be referred to as intramolecular condensation transformations since they are accompanied by elimination of low-molecular products. On the other hand, PCSs can be obtained via polymeranalogous transformations, principally intramolecular polymerization reactions . Thermal and chemical cyclization of poly(acrilonitrile) (PAN) is an example of processes of this type. It was demonstrated by a number of researchers216-225 that thermal transformations of PAN follow the scheme ... 

Zn2+-catalyzed cleavage of (Zn2+ HPNPP)2 in ethanol discussed above.85 For the slower reacting MNPP, the chemical cleavage step (represented by k3 and requiring a methoxide which is probably coordinated to one or both of the metal ions in 35 2Zn(II)) is relatively slow, so that both the pre-equilibrium steps are established and typical Michaelis-Menten behavior is observed with saturation at higher [35 2Zn(II)]. On the other hand, with the far more reactive HPNPP the chemical cyclization step, /c3, is proposed to be faster than the k 2 step in the concentration range of 35 2Zn(II) used here. In this event, the observed kinetics would be linear in [35 2Zn(II)] as is the case in Fig. 20, with kohs — /c, [35 2Zn(I )]k2/(k, + k2). 

Intermolecular anodic cyclizations often involve initial coupling of radical-cations followed by a chemical cyclization reaction. An alternative is cyclization by internal nucleophilic addition of some reactant to an intermediate derived by anodic oxidation. 

Simple nonconjugated nitrones have a strong absorption maximum in the UV at around 230 nm. Acyclic nitrones are, in general, photo-chemically cyclized to the corresponding oxaziridines.148 The cyclic nitrone, 5,5-dimethyl-l-pyrroline 1-oxide (164) is similarly converted into the oxaziridine (165) by photolysis in ethanol/cyclohexane solution,146 and several 2-substituted 1-pyrroline 1-oxides were subsequently found to undergo the same cyclization in cyclohexane solution.147... 

Starting polymer Products of chemical cyclization Product of thermal treatment... 

The same cyclization behavior of the p-polycarbonyl chain—not to fold in the middle, as postulated for the alkaloid biosynthesis (see Scheme 3), but to roll up from an unprotected end—was also observed for the parent compound, the free 3-pentaketone 30, itself, which led in vitro directly to doubly cyclized products like 49 (52). This type of chemical cyclization is sometimes used by nature, as well, and thus could be exploited biomimetically (50,51,53) for a first total synthesis of the aloenin aglycon (52), a natural product from Aloe ar-borescens var. natalensis, in which the carbon chain is similarly folded. The same type of cyclization, using the (3-polycarbonyl chain in its complete, non-decarboxylated form 55, could achieve a still shorter synthesis of 52 (Scheme 9), being even more closely oriented to the biosynthesis (50,51). 

Uro gen II was produced by the chemical cyclization of the head-to-head polymer. 

Similar activation methods allow the chemical cyclization of (solution or solid-phase synthesized) linear oligonucleotides (62,201-204), which recently have been studied for triple-helix formation to oligonucleotide templates (204). 

Figure 7.2 Chemical cyclization to obtain Ambrox from tetranorlabdanodiol. Reproduced with permission from Martfnez-Cuido et ai. (2014), 2014, American Chemical Society...

The synthesis of Ambrox by the chemical cyclization of tetranorlabdanodiol obtained from A. jocotepecana offers several advantages over current chemical syntheses (Castro et al., 2002 Costa et al., 2005), reducing the synthesis to only one step under mild conditions and higher conversion rates. Additionally, the implementation of this process in industrial scale might contribute to the social and economic development of rural areas in the state of Michoacan, Mexico. 

Figure 9.19 Acetylene-terminated polyimide precursors synthesized by reacting BTDA 18 with less than the stoichiometric balance of 3,3 -(1,3-phenylenedioxy) bisbenzeneamine 44 and the complement to stoichiometry of 3-ethynylbenzeneamine 45 as end-capping agent. Low-molecular-weight polyamic acid 46 is then chemically cyclized to poly (isoimide) 47 or thermally cyclodehydrated to give polyimide 48.

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