Alkenes halolactonization

Enol lactones with a halogen at the vinylic position have been synthesized as potential mechanism-based inactivators of serine hydrolyases <81JA5459). 5-Hexynoic acids (181) can be cyclized with mercury(II) ion catalysis to y-methylenebutyrolactones (182) (Scheme 41). Cyclization of the 6-bromo and 6-chloro analogues leads stereospecifically to the (Z)-haloenol lactones (trans addition) but is quite slow. Cyclization of unsubstituted or 6-methyl or 6-trimethylsilyl substituted 5-hexynoic acids is more rapid but alkene isomerization occurs during the reaction. Direct halolactonization of the 5-hexynoic acids with bromine or iodine in a two-phase system with phase transfer catalysis was successful in the preparation of various 5-halomethylene- or 5-haloethylidene-2-phenylbutyrolactones and 6-bromo-and iodo-methylenevalerolactones (Scheme 42). 

The conversion of an alkene to a halohydrin can also be considered as an epoxidation because this can be achieved by a simple ring closure.184 Although no reagent is yet available to perform an asymmetric conversion of an isolated alkene to a halohydrin, the reaction can be controlled through diastereoselection. One such case is the halolactonization of y,8-unsaturated carboxylic acids, N,N-dialkylamides.185189... 

Lactones, cyclic esters such as compound A, are prepared by halolactonization, an addition reaction to an alkene. For... 

Bromine, of course, attacks the alkene to form the bromonium ion. If it has the r. stereochemistry, it cyclizes but, if it doesn t, it reverts to starting material. This reaction m remind you of halolactonization (p. 872). 

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

Catalytic, enantioselective halolactonization of (Z)-l,3-enynes (39) and 1,1-disubstituted alkenes with NBS has been reported to occur in the presence of chiral urea derivatives, such as (41), to produce allene derivatives (40) in <91% ee ... 

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