Halolactonization approach

The asymmetric halolactonization reactions of unsaturated L-proline amides, developed by Terashima and coworkers,184 has been extended to a-alkyl acrylic acid derivatives (equation 75 and Table 21).185 This allows for the synthesis of either enantiomer of an a-methyl-a-hydroxy acid using L-proline as the auxiliary. Less successful approaches to asymmetric induction with a chiral auxiliary include iodolac-... 

Early attempts to extend the halolactonization procedure to yield lactams gave cyclic imidates instead, but several approaches favor lactam products. These include working with silyl imidates, imidate esters or oxazolines, using sulfonylcarbamates or other acidic amides or by using hydroxyl-amine derivatives with increased nucleophilicity due to the a-effect. Lactams can also be favored as a consequence of steric requirements. In a few cases, amines can be cyclized to cyclic amines many lead references are given in a recent report on cyclic hydroxylamines such as (64). Veiy recent work has provided a fairly general iodolactamization procedure from unsaturated amides, trimethylsilyl triflate and iodine (Scheme 89). ... 

The electrophile-promoted cyclization in cyclic systems is a highly regio- and stereoselective reaction that can be carried out under various conditions and with a range of electrophiles. The bicyclic lactone products have the cfs-ring fusion, indicating that the electrophile approaches from the face of the molecule anti to the carboxyalkyl side chain. Such halolactonizations constitute a useful approach to bicyclic and polycyclic compounds. 

New catalytic approaches towards the enantioselective halolactonization of alkenes 12CA(456. 

Intramolecular cyclization of unsaturated carboxylic acid derivatives using strong electrophiles is one of the most effective and common approaches to y-lactones. Not to mention, reactions of the unsaturated carboxylic acids with halonium sources such as X2 and NXS, the so-called halolactonization, produce the synthetically useful halolactones [4], N-Halosuccinimide (NXS) reaction is also commonly utilized for the construction of y-lactone structures. These days, development of effective catalysts, especially for enantioselective cyclization of unsaturated carboxylic acids including halolactonization reaction, is a hot topic in synthetic organic chemistry. 

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