Cyclizations, hypervalent iodine-catalyzed

Based on Ochiai s procedure for a-acetoxylation of ketones, Ishihara and coworkers have developed the hypervalent iodine-catalyzed oxylactonization of ketocarboxylic acids to ketolactones [18], Optimized reaction conditions consist of the treatment of a ketocarboxylic acid with iodobenzene (10 mol%), p-toluenesulfonic acid monohydrate (20 mol%) and mCPBA as a stoichiometric oxidant in nitromethane solution Scheme 4.3 shows as a representative example the cyclization of ketocarboxylic acid 4 to ketolactone 5. 

Based on the hypervalent iodine-catalyzed bromocarbocyclization of appropriate alkenoic precursors 21, Gulder and coworkers have developed an efficient synthetic approach to 3,3-disubstituted oxoindoles 22 (Scheme 4.10) [36], These cyclizations are catalyzed by 2-iodobenzamide 20 at room temperature using NBS as the source of electrophilic bromine. Alternatively, KBr can be used as the source of bromine in the presence of Oxone as a terminal oxidant. The synthetic utility of this cyclization has been demonstrated by the... 

Within his seminal work on catalytic oxidative spiro-cyclization reactions, Kita reported on hypervalent iodine catalyzed lactamization reactions [18, 19]. These reactions lead to de-aromatization throughout the process, but appear to provide valuable entries into C-N bond formation involving congested carbon frameworks. 

Several examples of cyclizations through intramolecular C-N bond formation catalyzed by hypervalent iodine species have been reported. Antonchick and coworkers developed an efficient organocatalytic method for the preparation of carbazoles through catalytic oxidative C-N bond formation [48]. The best yields of products were obtained in hexafluoro-2-propanol using 2,2 -diiodo-4,4, 6,6 -tetramethylbiphenyl (42) as the catalyst and peracetic acid as the oxidant, as illustrated by a representative example shown in Scheme 4.23. 

Alkyl-4-oxy-3,4-dihydroisocoumarins are enantioselectively prepared by oxylactonization ofo-(alk-l-enyl)benzoates promoted by the in situ-generated chiral lactate-based hypervalent iodine(III) catalysts (13EJ07128). Chemoenzymatic synthesis of 3,4-dialkyl-3,4-dihydroisocoumarins involves one-pot dynamic kinetic reductive resolution processes catalyzed by E. co/i/alcohol desidrogenase. This strategy consists in the bioreduction of various racemic ketones to the corresponding enantiopure alcohols followed by intramolecular acidic cyclization (Scheme 71) (130L3872). 

For direct alkynylation with TIPS-acetylene, a hypervalent iodine TIPS-acetylene reagent can also be used as demonstrated in Waser s recent works. Alkynylation on C-3 of indole was catalyzed by AuCl (eq 26) and the Wacker cyclization s intermediate was intercepted by the hypervalent iodine TIPS-acetylene (eq 27).2 ... 

W. Liu, P. Zhou, C. Chen, Q. Zhang, Z. Zhu, Direct construction of 5-methyl-2-phenylisoxazol-3(2H)-ones via hypervalent iodine mediated sequential tandem oxidative cyclization of 3-oxo-N-phenylbutanamides catalyzed by zinc oxide, Org. Biomol. Chem. 11 (2013) 542-544. 

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